UKOOA Drill Cuttings JIP
Task 6
Drill Cuttings Recovery Project
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Document Title:
Drill Cuttings JIP Task 6 Final Report
Drill Cuttings JIP Task 6 Final Report
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CONTENTS
1. Executive Summary ................................................................................................................. 5
1.1.
Objectives ...................................................................................................................... 5
1.2.
Cuttings Recovery Equipment.................................................................................... 5
1.3.
Onshore Trial................................................................................................................. 5
1.4.
Delft Study and Further Development ....................................................................... 5
1.5.
Offshore Trial................................................................................................................. 6
1.6.
Dredging Operations .................................................................................................... 6
1.7.
Environmental Monitoring ........................................................................................... 6
1.8.
Practical Applications ................................................................................................... 7
1.9.
Conclusions ................................................................................................................... 7
2. Introduction................................................................................................................................ 8
3. Background................................................................................................................................ 9
3.1.
Previous BP Work into Drill Cuttings Removal at NW HUTTON .......................... 9
3.1.1. BP 1999 Drill Cuttings Removal Study ......................................................... 9
3.1.2. Overview of BP 1999 Drill Cuttings Removal Study................................... 9
3.2.
JIP Phase 1 ................................................................................................................... 10
3.2.1. JIP Phase 1 Research Area 6.1 – Removal Solutions ............................... 10
3.2.2. Differences between BP study and JIP Phase 1 study results ................. 10
3.3.
JIP Phase 2 Task 6 Overview .................................................................................... 11
4. Equipment Selection & Development.................................................................................... 12
4.1.
Subsea Dredging System ........................................................................................... 12
4.1.1. Initial Equipment Selection.............................................................................. 12
4.1.2. Tender Process................................................................................................. 13
4.1.3. Selected System ............................................................................................... 14
4.1.4. Onshore Testing / Trials .................................................................................. 17
4.1.5. Post Onshore Trial Development ................................................................... 22
4.1.6. Offshore Trial System ...................................................................................... 24
4.2.
Environmental Monitoring ........................................................................................... 27
4.2.1. Initial Equipment Selection.............................................................................. 27
4.2.2. Equipment Development ................................................................................. 30
4.2.3. Final System Description................................................................................. 30
4.3.
Topsides Cuttings Processing and Disposal System ............................................. 32
4.3.1. Background and Introduction.......................................................................... 32
4.3.2. Initial Equipment Selection.............................................................................. 33
4.3.3. Equipment Development ................................................................................. 33
4.3.4. Final System Description................................................................................. 34
5. Offshore trials ............................................................................................................................ 36
5.1.
Equipment Mobilisation and Installation ................................................................... 36
5.1.1. Subsea Dredging System ............................................................................... 36
5.1.2. Topsides Cuttings Processing and Disposal System ................................. 37
5.1.3. Environmental Monitoring ............................................................................... 37
5.2.
Equipment Operation ................................................................................................... 39
5.2.1. Subsea Dredging System ............................................................................... 39
5.2.2. Topsides Cuttings Processing and Disposal System ................................. 40
5.2.3. Environmental Monitoring ............................................................................... 41
5.3.
Results and Discussion ............................................................................................... 43
5.3.1. Safety.................................................................................................................. 43
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5.3.2. Overall ................................................................................................................ 43
5.3.3. Subsea Dredging System ............................................................................... 45
5.3.4. Topsides Cuttings Processing and Disposal System ................................. 50
5.3.5. Environmental Monitoring ............................................................................... 50
5.4.
Lessons Learned .......................................................................................................... 54
6. Answers to JIP Need To Know Questions ........................................................................... 60
7. Application of Subsea System to Full scale removal.......................................................... 62
7.1.
Existing System ............................................................................................................ 62
7.2.
Optimum System .......................................................................................................... 63
7.2.1. System Capacity............................................................................................... 63
7.2.2. Dredging System .............................................................................................. 63
7.2.3. Deployment System ......................................................................................... 64
7.2.4. Environmental Monitoring ............................................................................... 65
7.2.5. Manpower .......................................................................................................... 66
7.3.
Technical Implications for other pile types ............................................................... 66
7.4.
Schedule ........................................................................................................................ 67
7.5.
Financial......................................................................................................................... 67
8. Conclusions ............................................................................................................................... 72
8.1.
Overall ............................................................................................................................ 72
8.2.
Subsea Dredging System ........................................................................................... 72
8.3.
Topsides Cuttings Processing and Disposal System ............................................. 73
8.4.
Environmental Monitoring ........................................................................................... 73
9. References ................................................................................................................................ 75
9.1.
BP.................................................................................................................................... 75
9.2.
CEFAS............................................................................................................................ 75
9.3.
Delft Hydraulics............................................................................................................. 75
9.4.
Euro-Seas Engineering Solutions & Testing Ltd (Blyth Dry Docks) ..................... 75
9.5.
Halliburton Subsea ....................................................................................................... 75
9.6.
PSL ................................................................................................................................. 75
9.7.
SWACO.......................................................................................................................... 75
9.8.
Wood Group Engineering / JP Kenny ....................................................................... 75
10. Abbreviations ............................................................................................................................. 76
11. Appendices................................................................................................................................ 77
12. Appendix 1 – Offshore trial drawings .................................................................................... 77
12.1. NW Hutton – Topside Production / Drilling Deck Layout SSA-0322-D-0001-00 77
12.2. WGE Flow Diagram for Offshore Trials SK-37E-34G003-010 rev D ................... 77
12.3. SWACO Rig Plan Layout Cuttings Mound Processing and Disposal System -
Drawing ARQ 1143 012............................................................................................... 77
13. Appendix 2 – Schedule ............................................................................................................ 78
13.1. Key Project Dates ......................................................................................................... 78
13.2. Offshore Trial Schedule ............................................................................................... 79
14. Appendix 3 – Project Admin ................................................................................................... 80
14.1. JIP Task 6 CTR – ‘Agreed Scope of Work’ .............................................................. 80
14.2. Project Organisation..................................................................................................... 82
15. Appendix 4 – supporting Information..................................................................................... 83
15.1. The Key Elements of the Tender Scope of Work .................................................... 83
15.2. Success Criteria for Phase 1 Trials at Blyth............................................................. 86
15.3. Post Blyth Trial Review of System with Dredging Research Ltd.......................... 91
15.4. Delft Hydraulics Study of Dredge Head Performance and ROV Compatibility... 94
15.5. DRL Guidance Notes on Test Materials ................................................................... 96
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15.6. Environmental Monitoring Workshop Minutes ......................................................... 100
15.7. Flow Chart of Disposal Options .................................................................................. 101
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1.
EXECUTIVE SUMMARY
Task 6 of the UKOOA Drill Cuttings initiative phase II comprised development and field
deployment of cuttings recovery equipment with an extensive environmental monitoring
programme to evaluate the impacts of the operation.
Management of the project involved significant development and testing of components
and systems, a major onshore test and a full scale mobilisation to the offshore testing
location (The North West Hutton platform) located 130 km north east of the Shetland
Islands.
1.1. Objectives
The main aim of the project was to develop an in-depth understanding of the
practicalities, physical parameters and environmental impacts of drill cuttings recovery in
the marine environment. The project also provided important information regarding the
physical properties of a cuttings pile in-situ and implications for disposal of the material.
(Reference JIP Task 7).
1.2. Cuttings Recovery Equipment
A major tender exercise was implemented to identify a dredging system suitable for
achieving the trial objectives. The exercise confirmed many of the findings of the phase I
study but also indicated that many systems required more development time and funding
to reach maturity than had been previously indicated.
The system selected consisted of a heavy-duty work class ROV with a variety of dredge
heads and a standard centrifugal dredging pump that was electrically driven and located
on the seabed. Cuttings were transferred to the platform via a fixed hose. Design flow
rate was 100m
3
per hour with a theoretical maximum water: solids ratio of 3:1.
1.3. Onshore Trial
A significant onshore trial was implemented at the EEST dry dock facility in Blyth,
Northumberland. This exercise proved invaluable and demonstrated that all components
of the system could meet the design criteria. In particular, the ability to pump the
material to the delivery point 65m above sea level at the platform and the ability to
deploy, connect and manoeuvre the subsea hoses with the ROV whilst dredging. The
areas of concern from the onshore trial were the low ratio of solids achieved (worse than
20:1). This was determined as being due to a combination of synthetic samples, dredge-
head design, and operator skill. Extremely poor visibility was a major problem.
1.4. Delft Study and Further Development
Following the onshore trial a series of modifications were made to improve the system.
A cutter type dredge head was manufactured to handle material of a cohesive nature and
improvements were made to the deployment system. The other major outcomes were
the need for the dredge head to maintain contact with the material at all times and
confirmation that the system should be able to achieve the desired ratios.
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1.5. Offshore Trial
Aside from the operation itself, the offshore trial indicated the major impact that such an
operation would have on the day to day running of an offshore production platform. In
addition to the recovery equipment a large amount of equipment and personnel were
required for the treatment and disposal operations. In all 40 people were involved in
running the operation with major operations at four separate work sites on the platform of
which approximately 20 personnel were dedicated to the subsea equipment. The
topside processing system included options for disposal by re-injection or ship to shore
by skips.
Apart from the scale of the work, the deployment and operation of the equipment was
relatively straightforward with all aspects operating largely as planned.
1.6. Dredging Operations
Limitations in the disposal system limited the maximum running time but it was sufficient
to gain confidence in all the major areas tested. The rotating cutter head created a
visible but relatively small plume during operation. The majority of the trial was
implemented with a suction dredge head that proved effective. No significant plume was
observed from this dredge head. The ROV thrusters and the ROV hull moving through
the pile created small plumes.
The most effective dredging technique appeared to result from driving the ROV forward
thrusting the dredge-head deeply into the pile. The system appeared to cope well with
debris. Stopping the pump and or backflushing cleared several blockages during the
operation. Shutting down the operation and backflushing did result in significant visible
re-suspension of cuttings material from the delivery hose.
Water to solids ratios improved with operator experience and several samples below
10:1 were noted with 6:1 (by volume of wet cuttings) being the best instantaneous result.
Steady operational ratios were between 10:1 and 20:1 with an overall trial average of
approximately 4% of wet cuttings including stopping and starting the dredging operation.
Frequent starting and stopping the system significantly increased the volume of water
recovered.
1.7. Environmental Monitoring
Results from the environmental monitoring arrays confirm that plume generation and
drifting of re-suspended material was low during the operations. There were no visible
indications of an oil sheen being created at surface.
Water and solids analysis for oil content indicates oil contamination of the cuttings at the
levels that would have been expected from generic oil on cuttings sampling. It appears
that the majority of the oil remains bound to the cuttings. The level of oil in the
associated dredged water is low which may be associated with the high ratio of
recovered water to solids.
Contaminant analysis confirmed that the background levels of barium and total
hydrocarbons were not significantly increased in the Aqualander seawater samples as a
result of the dredging operation. Levels of APEs and barium were higher in the recovered
water at the platform topsides. LSA levels in the recovered cuttings material were low
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and similar to background levels. There was no detectable oil in the plumes generated
during the trial.
1.8. Practical Applications
The trial results provide a significant insight into the practical aspects of pile recovery
should this be undertaken. The issues are many, varied and complex but can be divided
into two main groups, removal of the pile prior to platform removal and removal of the
pile following platform removal. In general, it is clear that the cuttings volume recovered
will be accompanied by a minimum of ten times the volume of water.
Recovery on an operational installation would allow re-injection of the recovered
material, but in reality the operational duration of up to one year (for a 25,000m
3
pile) is
likely to be difficult to manage with a significant cost. High energy consumption is also
an issue for this case.
Recovery after platform removal could be implemented in a significantly shorter
timeframe, but the transport and safe disposal of large quantities of contaminated solids
and liquid would present major cost and operational problems.
1.9. Conclusions
Recovery of contaminated drill cuttings from the seabed to the surface appears to be
operationally feasible and indications are that secondary pollution is relatively low based
on the equipment utilised in the trial operation.
The levels of oil recorded in the material suggest that previous estimates of
contamination were representative of the trial site selected. The oil in general remains
bound to the drill cuttings with a relatively small amount migrating into the dredged water.
The practical application of cuttings recovery on an operational platform would appear to
be limited to a few installations capable of supporting the operation. The operation would
be challenging with a significant duration, significant cost implications and a major impact
on routine platform operations. In the event that a platform had ceased operating then
the operation would have to address the additional cost of maintaining platform
operations for the duration of the operation.
The direct cost of the dredging operation from an existing operational platform is likely to
be in the range of £200/m
3
to £300/m
3
to lift the material to the surface (for a 25,000m
3
pile) and possibly higher for smaller piles. This equates to at least £4000/m
3
of oil
removed assuming 5% contamination. Note that this does not include offshore
processing, storage, transport or disposal costs which will be significant.
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2.
INTRODUCTION
This report records and interprets the findings of the UKOOA drill cuttings JIP task 6 –
drill cuttings pilot recovery operation, conducted between Jan 2000 and Aug 2001.
Concept of a drill cuttings removal trial.
The output from the cuttings JIP phase 1 (task 6) suggested that several concepts
existed for physically removing cuttings from the seabed. The systems were based on
concepts ranging from deployment of pre-existing equipment to novel applications to
remove the material. The desktop studies of removal left some significant uncertainties,
specifically:
•
The practical implications of dredging a cuttings pile at significant water depth in
the offshore environment.
•
The macroscopic nature and behaviour of the pile during recovery.
•
The environmental impacts of the recovery operation in the water column and at
surface.
The risks and merits of implementing a recovery trial were discussed at length and a
decision was taken to proceed based on the perceived value that the trial would provide
to the overall understanding and completeness of the project.
From the outset, the objectives of the trial were to focus on understanding the mechanics
and environmental impacts of the recovery operation and not necessarily to develop the
most efficient operational system. It was realised that the trial would only be able to
focus on a single site but every effort was made to make the findings as widely
applicable as possible.
The disposal operation for the trial was not considered to be within the boundary of the
main operation. Nevertheless, significant practical information was gathered from this
aspect of the work and will provide useful input to other study areas within the JIP.
NW Hutton was selected as the preferred location for the trial mainly on the basis of a
low number of alternatives. It did provide the benefit of having a well-studied cuttings
pile and also being in relatively deep water, thus ensuring that the results would be
widely applicable.
Significant efforts were made to ensure that wide ranging input was built into the trial at
every stage. An active steering committee, fully independent of the project team,
managed this aspect and ensured appropriate external considerations were included.
Overall, it is considered that the project achieved the main objectives set at the outset.
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3.
BACKGROUND
3.1. Previous BP Work into Drill Cuttings Removal at NW HUTTON
3.1.1. BP 1999 Drill Cuttings Removal Study
BP has undertaken a number of reviews and studies into the removal of the drill cuttings
from the seabed beneath NW Hutton. The most recent and complete review of the
available and possible options for removal of the drill cuttings was undertaken in 1999 on
behalf of BP by JP Kenny. This study took place at the same time and in parallel to the
JIP Phase 1 study into drill cuttings removal technology. This study was part of a wider
BP project to look at the decommissioning of the NW Hutton facilities.
It should be noted that this study was focussed on removal of the NW Hutton drill
cuttings with the jacket and topsides in place.
The study report is reference No. 26 Drill Cuttings Mound Recovery to Topsides
Facilities, Rev A 29.12.99, Document No. JP Kenny No: 06.1781.U.3.002./ WGE No.
37E034F0003/SS/RE/0003
3.1.2. Overview of BP 1999 Drill Cuttings Removal Study
The study primarily consisted of a market survey to a wide range of industry
providers. The companies approached operate in the following sectors: marine
dredging, diving and subsea, marine wreck recovery and excavation systems.
Companies that expressed an interest to participate completed a data pack and
were invited to give a presentation on their proposed system.
The submitted information and presentation were scored considering the following
elements of the proposed systems: Completeness, operational viability,
operational productivity, estimated recovery time, development risk, cost, and
environmental acceptability. Using these criteria a comparative assessment was
undertaken on the proposed systems to rank them and produce a short list of
possible solutions for NW Hutton
Presentations on proposed systems were received from 18 of the 32 companies
approached. Considerable variety, innovation and application were shown.
Systems proposed, in the main, were based on known dredging techniques.
Availability ranged from applications of existing equipment through to new build
with associated small development costs, with some companies suggesting full
development funding. Certain companies could only offer part systems and were
looking for various support collaborations.
The study met its objective and concluded that the removal and pumping of drill
cuttings is feasible. There were a variety of systems that could undertake the task
and the task can be performed diverless.
The main findings were:
•
Systems could be deployed either from the platform or from a boat. For the
base case, platform deployment was considered favourable due to the
expected duration of the operation.
•
The system vehicle could be tracked or free swimming ROV.
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•
There were suitable dredge head designs available.
•
There were five possible pump types available of which the assessment
nominated the top three to be air venturi, water venturi and centrifugal
vortex.
•
The cost and schedule information provided by the presentations show a
wide range, reflecting the levels of uncertainty. The middle range shows
costs of between £3.0m to £5.0m and duration 3 to 5 months.
The study showed that due to a lack of track record and the location of the mound
within the jacket framing and its debris interface, an ongoing desk based selection
for the final solution would not be conclusive. As a result, significant uncertainty
remained as to the best selection of system components and the environmental
impacts of any potential recovery operation.
3.2. JIP Phase 1
3.2.1. JIP Phase 1 Research Area 6.1 – Removal Solutions
Research Area 6.1 of the UKOOA Drill Cuttings Initiative comprised an evaluation
of methods that are available, or under development, that could be used to
recover drill cuttings from the seabed.
Potential system promoters were identified from in-house data and contact lists
provided by the Research Sponsors and Coordinators. Twenty promoters
provided details of 22 recovery systems or key components. These included
systems employing trailing suction hopper dredgers, reverse circulation drilling,
grabs, seabed crawlers and a variety of suspended pumps with remotely operated
vehicles (ROVs) to deploy the material inlet.
Although most systems (as a whole) were still under development and unproven,
it was concluded that there exists the means to effect substantial recovery of
cuttings piles. Most proposed systems were based either on components which
had already been proven in other relevant applications or environments, or were
based on new technology that was considered to be realistic to implement in the
near future. Quantitative evaluation of the capabilities of the proposed systems
was hindered by a lack of prior experience of controlled cuttings recovery.
Task 6.1 recommended that it was necessary to undertake closely monitored trials
of different systems in order to understand more fully the difficulties of cuttings
recovery, to quantify more confidently the sediment losses during recovery, and to
derive reliable estimates of cuttings dilution, particularly when working in cuttings
piles containing large amounts of debris.
3.2.2. Differences between BP study and JIP Phase 1 study results
A number of differences exist in the results of the BP and JIP study reports.
Some differences are due to the differences in scope (BP considered options for
NW Hutton only, while the JIP is looking at options for all large drill cuttings
accumulations). However the main differences are probably down to the fact that
the operation had not been undertaken before and therefore there was a lack of
experience with the systems dredging drill cuttings and lifting them to the surface
for disposal. As a result the studies results were to a large extent views on the
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systems claimed performance rather than factual information. A further area for
uncertainty was due to the lack of firm knowledge on the properties of the cuttings
and their ability to be dredged.
There was general consensus that the only way forward to remove the uncertainty
around the ability of systems to dredge and lift the cuttings was by means of an
equipment trial.
3.3. JIP Phase 2 Task 6 Overview
On completion of the JIP Phase 1 it was agreed by the partners that a phase 2 was
required. Phase 2 was designed to fill the gaps in the scientific knowledge regarding the
four management options for drill cuttings piles: in-situ bio-remediation, leave inplace and
monitor, lift and reinject, lift and dispose onshore.
The lifting trial was task 6 of Phase 2 of the JIP. The CTR and objectives issued by the
JIP to BP for execution of task 6 is included in Appendix 3.
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4.
EQUIPMENT SELECTION & DEVELOPMENT
4.1. Subsea Dredging System
4.1.1. Initial Equipment Selection
Following on from completion of the drill cuttings removal studies in 1999, there
was agreement that the way to remove the uncertainty around the ability of
systems to dredge and lift the cuttings with acceptable performance and to better
estimate the cost of removal was by means of some form of equipment selection
and trial.
The previous study results included significant uncertainty and historically ‘budget’
cost estimates are often significantly different to tendered costs. The other major
area of uncertainty was the equipment’s ability to efficiently dredge drill cuttings
and lift them to surface. There was also uncertainty around the equipments
readiness and the duration taken to manufacture, modify or prepare the
equipment for trials.
The preliminary design parameters for the subsea recovery system were
developed at this stage. They were based on the market survey study results, a
review of the topsides facilities on NW Hutton for mounting the subsea equipment
and also a review of potential methods for disposal of the cuttings and associated
dredge water. The ‘optimum’ design parameters at this stage were:-
•
Recovery rate - 20m
3
/hr solids with an optimum ratio of 3:1 water to solids
and a maximum ratio of 10:1 water to solids. Based on the preferred slurry
mix for re-injection and rates.
•
Complete Operation safely, remotely and diverless
•
Produce minimum plume (minimise environmental impact).
•
Dredge head to remove up to a maximum of 20 m3/hr of solids at an
optimum ratio 3:1 water to solids.
•
Dredge head and hoses to handle debris up to 8” diameter and
handle/recover from blockages.
•
Dredge head to be capable of operating 50 metres from pump or vehicle.
•
Pump to lift cuttings and Debris from -144 metres LAT to head height of
+57 metres (subsequently increased to 70m).
•
Capability to remove and handle large debris for recovery at a later date
•
Reliability during operation.
•
Ability to monitor operation in zero visibility.
•
Compatibility / Operability of equipment.
•
Manoeuvrability of vehicle with dredge head in operation.
A preliminary equipment selection was made on the systems identified in the
studies. The equipment selection identified those systems that had the potential
to remove the NW Hutton drill cuttings. Systems clearly unsuited to a trial at NW
Hutton in the summer of 2000 were not considered further. The system types
rejected included all those that required the topsides to be removed prior to
cuttings removal and systems that were insufficiently developed to meet the
schedule. Where there was doubt with respect to these parameters the system
was included.
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Equipment selection was not made on the basis of the ranking in the studies due
to the uncertainties mentioned above and it was considered that the trial process
would be far better at identifying workable technologies.
It was proposed to undertake a final equipment selection by tendering for
complete systems to test. The selected system(s) would initially be subjected to
an onshore trial and on satisfactory comple tion of this an offshore trial would be
undertaken of the equipment on NW Hutton.
Companies / systems selected / included on tender list:
Tenderer
Vehicle
Pump
Coflexip Stena
To be defined
To be defined
DSND
Tractor
Centrifugal
Halliburton
ROV
Cent rifugal-Vortex
Stolt Comex Seaway
Tractor
Centrifugal
AEA Technology
Tractor - Extended boom
Centrifugal (water)
Boskalis
Tractor (vessel & ROV)
Centrifugal (water)
Ham Dredging
Tractor/ROV unit
Centrifugal Vortex
Hitec
Tractor / ROV
Airlift
Sonsub
Tractor
Centrifugal-Vortex
Tideway
Vessel / ROV
Air lift
Van Oord
Vessel / ROV
Centrifugal (water)
Systems rejected included those that:
•
Required diver assistance
•
Created uncontrolled plumes
•
Were at concept stage
•
Provider had no operational track record
•
Unable to be deployed from a platform or work inside a jacket
4.1.2. Tender Process
Throughout the tender process it was ensured that it maintained flexibility to
change the number of systems selected at each stage (of which there may be
none, dependent on results) and kept the option open to select elements from
systems for a mix and match approach if no suitable system was offered.
Particular attention was paid to the potential of SMEs to provide novel and
innovative solutions. The tender list only included companies that had indicated
that they could provide complete solutions, however to ensure that SMEs
technology was considered it was ensured that the names of the interested SMEs
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were supplied to all tenderers. To facilitate this and also to encourage possible
combination of companies where this might provide enhanced solutions BP invited
all the tenderers and the interested SMEs to a pre-tender presentation.
The pre-tender presentation introduced the contractors to the tender process, the
requirements and also highlighted the importance to consider all available
technologies including those offered by SMEs not on the tender list.
The competitive tender package was issued to 11 companies (see list in section
4.1.1) on 18 Jan 2000 at a pre tender briefing meeting attended by all 11
companies invited to tender. Tender submissions were received from 6 bidders
on 13th March 2000. 3 companies declined to tender and 2 pairs of companies
formed Joint Ventures, 2 tenders included alternative offers.
The rationale for such a large tender list was to ensure that any promising
technologies were evaluated and to ensure that a wide range of industry
participants had an opportunity to participate. It should be noted that a number of
technologies are still on the drawing board and these were not included on the
tender list, as they could not provide systems for trials in 2000.
The Key Elements of the Tender Scope of Work are included in Appendix 4:
Summary of Tenders and Alternative Offers:
Tenderer 1
Tracked vehicle with centrifugal pump
Tenderer 1 Alt 1.
Alternative including cuttings disposal by injection
Tenderer 2
Tracked vehicle with centrifugal pump
Tenderer 3
ROV with centrifugal pump
Tenderer 3 Alt 1.
Alternative trial location
Tenderer 4
Tracked ROV
Tenderer 5
Tracked vehicle with centrifugal pump
Tenderer 5 Alt 1.
Alternative with vessel based phase 3
Tenderer 5 Alt 2.
Alternative with different pump option
Tenderer 5 Alt 3.
Alternative with subsea skip system
Tenderer 6
Various deployment options with centrifugal or rotary
displacement pump
The evaluation was performed following the instructions set out in the approved
evaluation plan, which followed the equipment evaluation proposal included in the
Phase II Programme, Draft Invitation to Participate dated 22 Feb. 2000.
The Tender Evaluation exercise was split into three areas: HSE, Service Provision
(Technical, People / Competence & Scope of Work) and Commercial
(Remuneration, Contractual) with previously agreed overall weightings.
4.1.3. Selected System
In general the tenderers offered similar systems to those proposed in the earlier
studies. However the cost and amount of development work required and the
subsequent time to mobilise was quite different to some of the indications
provided in earlier studies. The types of pump offered were limited compared to
the studies where a number of novel or new pump types were offered. In the
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tenders the majority of pump solutions were proven dredging centrifugal pumps,
with only two different options, namely a hydro pneumatic pump and a Discflo
pump. All of the tenders included significant provision for development and
equipment purchase prior to any trials and in general the costs were higher than
the budget estimates derived from earlier study work.
The JIP approved the recommendation to award a contract to Halliburton Subsea
(HS) for Phase 1 onshore trials. Phase 2 was to be awarded on satisfactory
completion of Phase 1 and JIP agreement to proceed to Phase 2.
HS ranked highest in the three main criteria of evaluation and are clear leaders
overall. They could meet the proposed timeline for implementation with a full
system. Key aspects of the HS tender:
•
Work class ROV deployed dredging system; Warman centrifugal dredging
pump located in separate skid, dredging by suction with additional
mechanical agitation, hose connections between units made subsea by
ROV.
•
ROV and deployment system are existing, other items based on existing
technology / designs and require to be manufactured.
•
HS had e xtensive experience of designing, building and operating subsea
engineering solutions.
•
Good HSE management system and record.
The agreed recommendation had associated risks to the satisfactory outcome of
phase 1 and phase 2 trials that were identified at the time as:
1. Use of a single system may result in additional trial requirement if the trial is
unsatisfactory.
2. Standard, dredging centrifugal pumps may not be the optimum pumping
solution for cuttings recovery.
3. ROV’s may be limited in their ability to dredge at the required rates.
To minimise the risk of working with a single contractor / solution additional work
was undertaken to include an alternative pump in phases 1 and 2. The selected
pump was the PSL Discflo pump.
Technical reasons for selecting the Discflo pump included:-
•
Pump, motor, umbilical, suction hose and variable speed control container
all exist.
•
Discflo pumps are a patented design utilising a non-impingement pumping
principle that utilises boundary layer and viscous drag effects inside a
rotating discpac.
•
Discflo pumps are claimed to perform well on viscous, abrasive and high
solid content fluids, with lower damage to the fluid and longer pump life.
•
PSL equipment was suitable for integration into HS system to avoid any
duplication of equipment. Existing PSL equipment (motor, umbilical,
suction hose and control container) would replace HS equipment that
required to be purchased/built for the trial.
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A more detailed description of the HS system is contained in references 16, 17
and 18, and for the PSL Equipment in reference 20.
During the evaluation it became clear that whilst the majority of solutions were
platform based in most cases there was a lack of knowledge/experience of the
likely difficulty of deploying the systems from a producing platform. Where the
tenderer lacked experience the assistance of BP’s engineering support
contractor’s would be used to bridge any gaps. For most of the tracked vehicles it
was clear that there would be significant difficulty and cost involved in engineering
a deployment system for a tracked vehicle due to vehicle weights of typically 10
tonnes, and that these difficulties were not factored into the tendered prices and
would have to be included in the overall cost estimates.
Tenders included options for vessel based systems and the best vessel based
option was more competitive than had been anticipated although significantly
more expensive than platform based. Vessel based options were particularly
attractive for tracked vehicle options as there is a good record of deploying
tracked vehicles from vessels for trenching, whereas their weight makes their
deployment more difficult for platform deployment
4.1.3.1.Equipment Development
The HS system was designed around existing proven dredging and subsea
technology as appropriate to the individual components.
At the time of contract award a number of the HS system components had to be
purchased and assembled for the trial (see reference 18). The main components
to be procured included the Warman dredging pump and discharge hoses. In
addition the pump drive motor, ROV interface / dredge skid and the subsea hose
connectors had to be designed in detail and manufactured, however these were
all based on existing in-house HS experience and designs. The Warman pump
procurement was on the critical path and the motor whilst based on a standard HS
ROV subsea motor was significantly larger power output than the existing design.
The dump valve assembly was designed and manufactured from specifically for
the trial using HS experience and good subsea practice.
The PSL equipment was selected in part due to the fact that the equipment all
existed or had already been ordered on a speculative basis for this or other
potential contracts. The Pump umbilical winch and suction hose were standard
PSL units of which they had a number of identical units used for subsea cuttings
transfer, whilst the pump / motor unit and variable speed drive (VSD) were already
on order. BP witnessed all the equipment undergoing a FAT.
The equipment supplied by HS and PSL for the Blyth trials included:-
Halliburton Subsea
i.
Workclass ROV.
ii.
ROV umbilical & winch c/w winch hydraulic pack & hoses.
iii.
ROV control van c/w deck umbilical.
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iv.
ROV under-slung dredge skid c/w 6” suction hose connection system,
dredge arm, and optional agitator head.
v.
Generator for HS and PSL equipment c/w with distribution panel.
vi.
Dredge pump (Warman) and motor (HS) c/w dump valve system.
vii.
Discharge hose (4”) c/w connection system to dump valve.
viii.
Echoscope 1600B 3D real time sonar c/w tilt mechanism.
PSL
i.
VSD control van c/w power / deck cables & console (ROV shack).
ii.
Umbilical and winch c/w hydraulic pack.
iii.
Dredge pump (DiscFlo) and motor c/w PSL suction hose connector.
iv.
6” suction hose 40m long (PSL connector at one end, bare flange at other).
Dredge ROV at Blyth
More detailed descriptions of the HS and PSL equipment are included in their
respective reports.
4.1.4. Onshore Testing / Trials
BP investigated a number of onshore and near shore trial site options. They
included options to use various harbour locations and also sea lochs. The Blyth
Dry Dock Facility operated by Euro-Seas Engineering Solutions & Testing Ltd
(EEST) was selected as it provided the best overall test site solution. Principally it
allowed the trial to be conducted in a totally controlled environment both on the
quayside and underwater. Other attributes of Blyth included:
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•
Main facility in the UK for shallow water subsea trials, and has a track
record of successful trials including stack up tests, ROV systems and
tracked vehicles. EEST the Blyth operators principal service is to provide a
commercial facility for subsea trials.
•
Series of large dry docks the seabed properties of which can be accurately
controlled.
•
The dry dock can be filled and emptied as required, flows and different
conditions can be simulated.
•
There is a large amount of space, and the site has all required utilities.
•
Support services such as cranes, scaffolding, and fabrication are available
on site or close by from sub-contractors that have a track record with
EEST.
•
Good office facilities are available including viewing facilities, catering,
changing facilities, communications etc.
•
Blyth is a private location and access is restricted to our participants and
observers.
The identified downside was the likelihood of restricted visibility in the flooded dry
dock.
The initial plan was to undertake the onshore trials during June 2000 followed by
an offshore trial in August 2000. Due to the time taken to complete the selection
and award process the onshore trial date was unrealistic by the time the contracts
were awarded.
The onshore trials were eventually undertaken during August 2000, following
preparation of the trial site which commenced in July 2000.
4.1.4.1.Aims of the Onshore Trials
The trials at Blyth were intended to be a representation of some of the
parameters of drill cuttings recovery at NW Hutton. In particular the trials were
aimed at gathering information on pump performance and estimating recovery and
dilution rates. The trial was intended to -
•
Demonstrate the ability of the Warman (HS) and Discflo (PSL) pumps to
deliver cuttings to a head equivalent to the delivery head height at NW
Hutton.
•
Measure the recovery and dilution rates of a series of sample cuttings of
differing consistency (varying components and shear strengths).
•
Demonstrate the ability of the ROV to hook-up and detach the suction and
discharge hoses to and from the pump unit.
•
Demonstrate the ability of the ROV to manoeuvre and position the PSL
suction hose, to ‘dock’ onto the hose, and to automatically disconnect from
the hose in the event of an ROV shut down.
•
Demonstrate the ability of the ROV to operate the dredge arm to sweep the
dredge head through the pile.
•
Demonstrate the operation of an agitator head to break up cuttings at the
nozzle intake.
•
Demonstrate the operation of the dump valve assembly.
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•
Demonstrate the ability of the ROV to deal with or avoid different hard and
soft debris.
4.1.4.2.Onshore Trial Set-up
The schematic of the trial facilities show how the equipment was configured at
Blyth for testing the subsea dredging equipment. The EEST scope of supply for
the trials included the following: -
•
Provision of Dock facilities filled with water for ROV dredging activities.
•
Design and provide a 60 metre tower based on a mobile crane to simulate
the head height from the sea surface to NW Hutton pipe deck level, with 4”
dia. hose with flanges or couplings to match contractors hose.
•
Provision and operation of the separation system to separate solids from
water.
•
Provision of sample areas in dock three (two sets of three, 6 x 4m – filled to
BP requirements).
•
Provision of personnel and management, to engineer, prepare and operate
the facilities and to monitor the trials (for separation performance, solids
and water content, video record, flow meter and weight measurement).
•
Equipment to monitor performance of equipment on test.
•
Provision of ad-hoc services including cranes
1
7
13
2
8
14
3
9
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
Target
Dredge head
ROV
Suction hose
Pump
Discharge hose (including 7)
140m removable section
Riser bend
Riser pipe
Riser head/ syphon preventer, 63m above dockside
Downcomer
Receiver pipe
Sampling pipe
Discharge hose
Discharge head
Skip
Lagoon
Dockside
Dock 3 (Flooded)
Dock 2 (Dry)
Schematic Flow Diagram (NTS).
15
4
10
16
5
11
17
6
12
North West Hutton
Drill Cuttings Recovery Trial
EEST
Euro-Seas Engineering
Solutions & Testing Ltd
Figure 1 – Trial System Layout
Procurement of equipment and construction of the trial facilities commenced in
early July 2000 and Dock 3 was flooded on 16th August 2000. Figure 1 shows a
simplified schematic of the test set-up. Full details are contained in reference 9.
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In order to test the dredging system performance a total of seven different sample
seabed areas were created in the dry-dock. At the time of the onshore trial there
was a significant level of uncertainty over the properties of drill cuttings in general
and significantly at NW Hutton. The sample areas were made with a
correspondingly conservative range of properties and included debris to simulate
what may be dredged offshore in actual drill cuttings. The samples were made
from either sand or clay and were contained within temporary formwork.
In order to monitor dredging performance the flow rates were measured and the
‘output’ from each trial was fed to a settling lagoon to enable both solid and liquid
quantities to be estimated. During each test, samples were taken from the
complete dredge flow and these were then analysed to give spot reading of solid:
liquid ratio. All other relevant parameters were recorded and logged.
4.1.4.3.Onshore Trial Results
Full Details of the trial results are contained in reference 9 (EEST), reference 10
(HS) and reference 19 (PSL).
Overall the trials at Blyth proved to be a success demonstrating that the system is
able to work.
‘Success Criteria’ for the Blyth trial with the equipments performance is included in
Appendix 4. On most criteria the system was judged to be a success and for most
parameters that ‘failed’ or could not be tested, they were identified as being
readily solved or would not be an issue during the offshore trial.
However there was a significant system limitation in that the results for both
pumps showed very low average solids to water ratios. The instantaneous ratios
reached a peak of 1:26 volume to volume solids to water (1:13 mass : mass solids
to water ratio) with most results being much lower. The overall ratios were poor
due to the difficulty of achieving steady dredging performance in a short duration
trial. The best result was a recovery of a total of 4 tonnes of wet sand during a
total pumped volume of 65m
3
.
Date
Dredge
Head
Sample
Number
Sample
Contents
Maximum ratio
of wet solids to
water (wet
mass: mass)
Maximum ratio of
wet solids to water
(wet vol: vol)
PSL Pump
20.08.00 Caged
Suction
Simulat-
ed
Seabed
Existing
dock
Not measured
21.08.00 Caged
Suction
2
Sand
Not measured
22.08.00 Rotating 4
Soft Clay
1:16
1:32
22.08.00 Rotating 6
Medium
Clay
1:189
1:373
22.08.00 Rotating 7
Hard
1:171
1:337
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Clay
Warman Pump
23.08.00 Rotating 3
Soft clay
1:135
1:267
24.08.00 Rotating 5
Medium
Clay
1:200
1:395
24.08.00 Rotating 1
Sand
1:13
1:26
24.08.00 Rotating Simulat
ed
Seabed
Existing
dock
1:36
1:72
It is difficult to directly compare the results of trials using separate pumps as the
dredging technique altered during the trials as a consequence of experience with
the earlier trials and the equipment configuration also varied (principally change of
dredge heads).
Results for both pumps showed very low average solids ratios. However, there
are some reasons for the low solids ratios other than the suction head: –
•
During the first trial, using the PSL pump on a sand sample, the suction
hose became plugged with sand. This is probably because the caged
nozzle on the dredge head was pushed deep into the sand pile, whilst the
pump was running at a slower speed than was necessary to keep the sand
in suspension in the 6” suction hose (flow rates were measured in the 4”
discharge line). When the sand trial was repeated with the Warman pump,
the agitator head was fitted, and was not pushed so deeply into the mound,
At the same time, due to the lessons learnt in the first trial, the flow rate
(and therefore the velocity) was increased to prevent settling.
•
The agitator head was designed to break up the very hard surfaces of
some of the samples (up to 120kPa). However, the design of the rotating
cutter was such that gravel became jammed between the cutter and the
back plate very easily, stopping the head from turning. Without the cutter
rotating the only action on the hard surface was from sweeping the dredge
arm from left to right, which was highly inefficient.
•
The cutter head was designed to break up the clay adjacent to the suction
intake. Thus there was no direct flow of water through the cutter head,
which became easily clogged up with clay (irrespective of whether the clay
was hard or soft).
•
The agitator head grill was designed to prevent the intake of particles larger
than 30mm. However, it became partially or completely blocked with gravel
very quickly, and frequent back flushing using the 60m head of water
proved necessary.
There are several factors differentiating a trial at Blyth with an actual recovery
operation at NW Hutton, and these are significant when analysing the results from
Blyth. These were:
•
Very restricted visibility at Blyth that does not clear readily.
•
Lack of method to fully simulate differential losses in 145m water colum n.
•
Difference between sample materials dredged at Blyth and ‘real ‘NW
Hutton cuttings. The discrete constituents of samples i.e. granular sample
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(sand) and cohesive samples (compacted boulder clay) in reality are likely
to provide a limited test for evaluation of the ability to recover drill cuttings.
Not withstanding these limitations a trial took place that demonstrated the ability of
the systems to lift a mixture of solids and water to the required delivery head at
acceptable flow rates. The trial highlighted the deficiencies in the design of certain
components (as it was intended to), and proved a useful exercise in
understanding some of the operational difficulties that might be involved in
offshore locations.
4.1.5. Post Onshore Trial Development
Following the onshore trials it was agreed that a detailed analysis of the systems
performance was required using independent experts to establish if higher solid to
water ratios (better than 1:10) could be achieved by the equipment tested at Blyth.
Dredging Research Limited (of the UK) and Delft Hydraulics (Netherlands)
provided this expertise.
DRL provided practical expertise on how the system could best be improved in a
qualitative manner and also undertook some basic calculations on dredging
parameters. They also used their knowledge from the JIP task 5b to better define
the expected nature of the drill cuttings at NW Hutton.
Their opinion was that following some system modification the system would be
able to operate within the required solids to water range of 1:10 or better.
Delft Hydraulics were approached initially as an option to undertake some highly
controlled testing of the dredge heads in their flume tank. However they
suggested that their numerical models would enable the system to be evaluated
initially without physical testing. Their work commenced with testing and studying
the properties of drill cuttings and then using this data and HS’s estimated
reaction forces and torques for the ROV system to numerically model the dredging
operation.
The Delft study concluded that the ROV based dredging system could achieve
solid to water ratios of 1:10 or better on the materials expected using a Breebot
crown type cutter. The estimated reaction forces that the ROV can produce would
be more than required for a crown type cutter. The cohesive nature of the cuttings
makes the use of static suction dredging inapplicable due to the small pit size that
would be formed; suction dredging would require a moving action.
A more detailed summary of DRL and Delft’s work is included in Appendix 4, and
Delft Hydraulics full report is reference 8.
4.1.5.1.Equipment changes
Following the equipment review with DRL, discussions with the JIP partners and
completing the Delft Hydraulics desk study the majority of the equipment upgrade
recommendations were implemented.
Post Blyth Trial Equipment Modifications implemented:
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•
Vacuum gauge mounted on dredge skid.
•
Semi automated dredge arm sweeping system.
•
Subsea pump discharge pressure gauge.
•
Improvements to subsea hose ROV operable connectors.
•
A crown cutter dredge head was designed and built (by Delft Hydraulics).
•
Angled suction nozzles were designed and built.
•
Suction hose size was reduced from 6" to 4".
•
Addition of buoyancy modules added to suction hose.
•
An ROV suction hose connection eject mechanism was designed to allow
release in 'dead sub' situations.
A proposal to vary the length of the dredging arm during dredging operations to
reduce the ROV movements during dredging was not implemented due to the
complexity and cost of the proposal for potentially limited gains.
4.1.5.2.Factory Acceptance Test
On completion of the equipment review with DRL and discussions with the JIP the
decision was made to defer the offshore trial to 2001 due to the level of work
required on the system and the o nset of poorer autumn/winter weather offshore.
The requirement for additional large scale onshore testing was discussed in detail
and a number of options for trial sites were reviewed including the use of a
Norwegian Fjord and returning to Blyth. Options for partial system tests were
included, including the use of the Delft Hydraulics flume tank for crown cutter
testing.
The decision was made to go directly to offshore trials following the equipment
upgrades with testing limited to a wet Factory Acceptance Test at HS of the
upgraded system elements. The decision was made following a thorough review
of the options, cost benefits, likely outcome, and uncertainty over visibility and
ability to manufacture representative samples.
A significant shortcoming identified was the limited information that could be
obtained from further onshore / near shore testing (at significant cost). Tank
testing at HS was able to test the various components function with the exception
of the dredge head performance (at reasonable cost). The key element that could
only be tested with large scale testing was the overall dredging performance. Due
to the wide variability in drill cuttings properties from the DRL/Delft work it would
be very difficult to accurately simulate the cuttings in a test facility and there would
always be doubt on the results until the system was tested on NW Hutton cuttings.
The factory acceptance tests were undertaken at HS test tanks over the period of
2
nd
to 4
th
May and 24
th
May. All system components that had been modified were
fully tested and adjusted until they operated smoothly. Components tested
included:
•
Suction hose buoyancy
•
Pump to Discharge hose ROV mateable connector
•
ROV dredge skid to Suction hose connector
•
Suction hose to pump inlet connector
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•
Dredge arm operation
By the end of the FAT all elements were working to BP’s satisfaction.
Full details of the tests are contained in the HS report on the FAT (reference 11).
4.1.6. Offshore Trial System
The subsea dredging system comprised of:-
HS
i.
Workclass ROV.
ii.
ROV umbilical & winch c/w winch hydraulic pack & hoses.
iii.
ROV control van c/w deck umbilical.
iv.
ROV workshop and ROV deck hydraulics pack
vi.
ROV under-slung dredge skid c/w suction hose connection system, dredge
arm, with nozzle(s) and cutter head.
vii.
Cabling from VSD container to ROV control van and winch HPU.
viii.
Dredge pump (Warman) and motor (HS) c/w dump valve system.
ix.
Delivery hose connection system to mount on to PSL and HS pumps.
x.
Discharge hose (4”) c/w connection system to dump valve.
xi.
4” Suction hose - 45m long.
xii.
Tuggers, tugger frames, guide wire, chutes, intermediate skids and clump
weight (DMA)
NW Hutton Skid deck (above) showing hose deployment
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HS ROV system on Production Deck Laydown area (above)
xiii.
EchoScope 1600B 3D real time acoustic camera c/w tilt mechanism.
xiv.
ROVtech Eyeball ROV for environmental monitoring.
Eyeball ROV (above)
Echoscope sensor (above)
PSL
i.
VSD control van c/w power and control cables.
ii.
Umbilical and wi nch c/w hydraulic pack.
iii.
Dredge pump (DiscFlo) and motor.
iv.
Pump inlet connector (receptacle and hose adapter)
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PSL Equipment on skid deck (above)
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4.2. Environmental Monitoring
4.2.1. Initial Equipment Selection
In order to ensure that the most appropriate environmental monitoring equipment
was selected for the offshore trial, the types of equipment required was not
prescribed at the outset. Instead, external parties were invited to identify the most
appropriate equipment that would meet the Task 6 objectives. The objectives for
the environmental monitoring were to:
•
Monitor the resuspension, transport and deposition of drill cuttings from the
trial lifting operation at NW Hutton
•
Assess the impact on the surrounding seabed
•
Estimate the recovery timescale arising from any adverse impacts.
To ensure that the environmental monitoring provided as much information as
possible, a number of gaps in scientific knowledge were also identified for review
by external parties during the tender process. These identified gaps inc luded
contaminant uptake (and subsequent biological effects, for example, detoxifying
enzyme induction) in fish living on/around the pile. Throughout the selection
process it was also the intent to ensure close co-operation with SERAD who were
also planning similar operations to monitor resuspension events resulting from the
impact of fishing trawls with a cuttings pile. The intent was to incorporate any
experiences from the SERAD operations into the Task 6 environmental monitoring
programme, wherever practical.
4.2.1.1.Background / Introduction
The received tenders were reviewed by a team of BP, JIP Sponsors and DNV
personnel. Following the ITT and competitive tendering exercise, CEFAS were
awarded the environmental monitoring workscope.
The proposed CEFAS environmental monitoring strategy is outlined in the
schematic below. In addition to their preferred environmental monitoring
approach the CEFAS tender included a number of optional additions. This
strategy was used as the baseline from which any changes to the workscope
were monitored.
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Objective
Field Condition
to be measured
Method
Background conditions
Response of pile to natural
conditions
Dispersion of cuttings material
Hydrodynamics during trials
Tidal currents and waves -
long-term Minilander
(Task 1A)
Suspended sediment
concentration - long-term
Minilander (Task 1B)
Waves/Currents 4 short-term
Minilanders (Task 3A)
ROV - OBS mast (Task 3B)
ROV - ADCP sections (Task
3C)
Suspended sediment
concentrations 4 short-term
Minilanders (Task 3D)
Calibration data - CTD, water
samples, suspended sediment
profiles (Task 3E)
Bed state around platform
Camera record (Task 1D)
Temporal evolution of plume
Spatial evolution of plume
Sea bed mapping (Task 1C)
Particle size profiles - LISST
(Task 3I)
CTD profiles + water samples
- lab studies (Task 3H)
Settling velocity of plume
particles
ROV mounted water supplier
(50 samples)
SSC (Task 3F)
PSA (Task 3G)
Fate of plume sediments
Footprint of redistributed
cuttings material
Sea bed mapping (Task 4A)
ROV TV survey (Task 4B)
Sediment tracer experiment
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Objective
Field Condition
to be measured
Method
Real-time Telemetry of Observed Data
Figure 1 - Project Strategy - revised 12 October 2000.
Background conditions
Plume source
Volume and elevation of
discharge
Concentration and
composition of pile material
Side scan plus swathe bathy
(Task 2B)
Side scan plus swathe bathy
(Task 2A)
Geotechnical properties from
sediment cores (Task 2C)
Thickness of deposition
Coring (Task 4C)
Fate of plume sediments
Sea bed chemistry and
benthos
Effects and recovery
timescale
Repeat coring programme
(Task 5A)
Options in the original scheme of work which have been removed
Optional additions to the present scheme of work
Impacts on food chain
Biological uptake of
contaminants
Analysis of meiofaunal and
macrofaunal samples
Longevity of impacts
Return of water column to
background state
Extended deployment of long-
term Minilander for 14 days
after trials
Observe deployment site and
investigate logistics
Early reconnaissance at NWH
J Rees + 1 to undertake
weekend visit to NWH
Catch of commercial fish
Establish presence of any
hydrocarbons in the water
column
Distinguish between
hydrocarbons from produced
water plume from platform and
that from lifting trials
Deploy tuned flourometer on
ROV
Assess visual impacts
Measure surface water sheen
Garret screens
Sea bed chemistry and
benthos
Coring (Task 1E)
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4.2.2. Equipment Development
Following the original contract award, the environmental monitoring system
developed significantly as the proposed equipments benefits and limitations
became better understood and as the whole trial system for NW Hutton became
better defined. One of the main focal points for the review and further clarification
of the environmental monitoring workscope was a Workshop held on 12
th
March
2001 attended by representatives from the DTI, SERAD, CEFAS, DNV and
UKOOA JIP Sponsors. The objectives of the workshop were to:
•
Confirm the selected technology given the anticipated size of the plume
and the resultant layer of disturbed cuttings pile material on the seabed
•
Look into any increased flexibility in the scope of work, given the costs
associated with a high calibre vessel; and
•
Explore other potential sources of disturbance monitoring.
Details of the Workshop can be found in Appendix 4. Based on these discussions
the following 3 potential options were identified for the trial:
•
A platform-based monitoring scheme;
•
A vessel-based monitoring scheme; or
•
A hybrid scheme which combines platform-based operations and the use of
a vessel-of-opportunity.
These options were distributed to the JIP sponsors and voted upon to identify the
JIP’s preferred way forward. In the subsequent months, further minor revisions
were made to the scope of work (and associated costs) such that all parties were
satisfied with the level and degree of monitoring identified within the Task 6
budget. The finalised environmental monitoring workscope was reviewed and
approved by the independent Scientific Review Group (SRG).
Following these reviews the main changes to the scope of work were as follows:
•
a move away from a vessel based monitoring system, with minilanders
deployed at the edge of the 500m zone, to a system with the equipment
platform deployed and located close to the dredging operation;
•
the deletion of studies investigating the uptake of hydrocarbons, metals,
and into commercial fish species (cod, haddock) and detoxifying enzyme
induction; and
•
the inclusion of a limited fluorescent tracer experiment.
4.2.3. Final System Description
The final environmental monitoring package comprised a series of seabed landers
(Minilanders) which were used to monitor:
•
The long -term (background) suspended sediment, tide and wave climate
•
The suspended sediment concentration in an approximate 40 - 100m
radius around the platform
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•
Daily (aqualander) water samples which were analysed for concentrations
of total hydrocarbons, barium, endocrine disruptors (APEs) and sediment
load (for calibration of sediment monitors)
A Minilander Being Deployed Offshore
Other equipment utilised to monitor the environmental impacts included:
•
An eyeball ROV in addition to the workclass (dredging) ROV which was
used to provide a flexible, visual means of identifying any generated
sediment plumes.
•
The use of a fluorescent tracer to mimic the sediment backflush event
which was anticipated to represent a ‘worst case’ sediment resuspension
event.
Samples of recovered material (water/solids) were also taken at the surface
(SWACO Unit and platform mud pits) by CEFAS personnel for an oil-water
partitioning experiment. Further details regarding the environmental monitoring
package can be found in reference 7 (CEFAS contract Report on NW Huton Drill
Cuttings Recovery Project – Environmental Monitoring, August 2001).
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4.3. Topsides Cuttings Processing and Disposal System
The Topsides processing and disposal system was designed with the following criteria:
•
To determine an option for disposal of recovered material that will support the
recovery operation. (This does not mean the option chosen would necessarily be
suitable for a large scale recovery operation)
•
The final selection must provide an environmentally acceptable option, within the
constraints of legislation, recovery operations, platform operations and budget.
•
It is important there are no undue environmental impact / implications.
•
The solution should match disposal rates to recove ry rates to support the recovery
trial. The design criteria maximum rate for the recovery trial is 20 m
3
/hr solids and
180 m
3
/hr liquids. The planned maximum recovery volume is 900 m
3
of cuttings
and upto 10,000 m
3
of associated water.
•
Needs to be cost effective for trial purposes.
4.3.1. Background and Introduction
The initial part of the selection process was to determine the possible options for
disposal of the recovered material. A number of options were reviewed. These
included overboard discharge of clean fluids, onshore cuttings and bulk disposal
and insitu reinjection of recovered materials. The options are summarised on the
flowchart in section 15.7 and the various perceived advantages and
disadvantages discussed in the attached table.
It was evident the most feasible option which supported the above objectives was
to utilise a system which would slurrify the recovered material and then inject it to
a deep subsurface horizon.
The initial step to confirm feasibility was to determine the ability to inject materials
into the sub-surface strata. A study was commissioned from BP rock mechanics
group in UTG - Sunbury. The results and recommendations are in report,
WT02_Study Report EOS-NWHD-BPA-WEL-REP-002-1. This confirmed the
feasibility and provided indicative injection horizons, fracture generation
predictions and likely injection rates and pressures.
Preparing this form of study is a normal initial stage when considering a cuttings
reinjection system. CRI is used widely in the Industry as a means of disposal of
drill cuttings. However, where it is used, the wells are specifically designed and
constructed to accept a CRI stream and the injection occurs at medium depth
formations. In the specific case of NW Hutton, there were no wells originally
designed for CRI, therefore all injection had to take place into the reservoir. Since
the reservoir is sandstone, it was not clear, at the design stage, whether fracturing
or matrix injection would occur.
A major challenge to the Project was to find a well that was suitable for injection of
the recovered material. Initially 5 wells were identified on the basis they were
water injector or oil producer wells which had little or no further utility.
The mechanical condition of the wells was reviewed, which eliminated a number
as being unsuitable. Other considerations included the location of wells in the
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reservoir and possible loss of production. Eventually one well was identified and,
with the acceptance of the NW Hutton Partners, designated for use.
A significant lesson from selecting the well was that, if a full-scale operation were
ever contemplated, any injection wells would require significant modification
(workover) to enhance their suitability.
4.3.2. Initial Equipment Selection
A study was carried out by Wood Group / JP Kenny into disposal options,
FT06/FS08/2 Study Report, EOS-NWHD-WDG-FAC-REP-006 Rev.0. This study
concluded a CRI system could provide a suitable disposal method. This report
was produced based on full cuttings mound removal. It did not consider the
special circumstances of a limited trial. Two options were proposed, either to use
a stand-alone temporary system or to modify areas in MM2 on the platform to
carry out the process.
Tenders were invited from two CRI companies, MI SWACO and Apollo Services.
These companies were selected on the basis they were providing CRI systems,
directly or indirectly, for BP at the time of the tender.
SWACO proposed five temporary systems and Apollo proposed modification of
the existing platform mud modules. Each te nder provided indicative costings. In
addition to the direct costs of the tenderers, budget installation or modification
costs were obtained from Wood Group.
The standalone options had a much lower impact on NW Hutton, i.e. it did not
require significant modifications and therefore had lower installation costs.
However, its operating costs were slightly higher. Given the limited nature of the
trial, the standalone option was selected as it provided the best overall evaluation.
4.3.3. Equipment Development
Havi ng been awarded the contract, SWACO were able to appoint an engineer as
single point of contact into the Project; and this engineer became an integral part
of the Project team. This occurred early in the project. This greatly facilitated the
development of the original five proposals from SWACO to the system that was
eventually utilised. As part of the Project management requirements the operating
procedures for the system and all modifications were subject to risk and peer
review as required.
The key deve lopments made, were:
•
Inclusion of MI mud engineering personnel to the offshore team.
•
Enclosure of the equipment to provide a reasonable working environment.
•
Operational integration with the Deutag personnel to utilise the platform
storage capacity for holding slurry, and use of platform mud pumps for
injection.
•
Integration of the AFOS bulk bentonite system.
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•
Development of a “fit for purpose” system to handle excess large cuttings in
skips. This was done by a combination of mechanical design of the
equipment and agreement of operating procedures and protocols.
4.3.4. Final System Description
For the purposes of the Project, the start of the Cuttings Processing and Disposal
System began at the upper flange of the delivery hose from the Halliburton unit.
The SWACO CRI unit was installed as a stand -alone system on the North West
Hutton pipe deck.
Swaco Processing Unit on Pipe Deck
The initial function of this system was to separate any large or metallic debris from
the drill cuttings mound material delivered to the surface by a submersible pump
from the seabed.
The four Shale Shakers in this system were located on two levels, designed to
meet a height restraint imposed by the maximum submersible pump output. From
the outcome of a peer review by BP the top deck had two circular motion Shakers
facing the West side of the platform, and these were dressed with coarse mesh
screens. These two Shakers acted as scalpers to remove any large materials
such as metal and marine shells. Two ditch magnets were located in the Shaker
header tank to remove as much steel as possible. All solids discharged from these
upper Shakers were gravity feed into cutting containers (skips).
The combined liquid/solid underflow was gravity fed onto a second set of two
SWACO Linear motion Classifying Shakers located on the intermediate deck.
They faced the East side of the platform; the function of these two Shakers was to
classify all cuttings. They were dressed with 40 mesh screens (to provide a d90
cut point of less than 300 microns). All oversized solids discharged from these two
Shakers were gravity fed into the Slurrification Unit coarse tank to be ground down
to sub 300-micron size and reclassified.
The liquid underflow was gravity fed into the fines tank and then via a weir
overflow from the fines tank into the existing platform mud gutter returns line.
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This flowed into the mud process system within the existing platform mud handling
system and tanks were used for storage and chemical treatment of the slurrified
material. A stand-alone AFOS bulk system was used to provide viscosifying
materials to the system.
The injection process was handled in a batch mode. When a batch was ready, it
was transferred to the high-pressure mud pumps, which injected the slurry, via
temporary high pressure piping into the injection well.
The complete system required the co-ordination of several groups of personnel.
Considerable effort was made in pre-planning the management system for the
process, which proved very beneficial.
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5.
OFFSHORE TRIALS
5.1. Equipment Mobilisation and Installation
The final schedule for the offshore trials is incorporated in Appendix 2.
The Flow diagram showing the overall equipment schematic is included in reference 1
together with equipment layout drawings for the topside and subsea equipment.
A simplified view of the equipment location:
Installation of generator’s, electric cabling, gas detection, structural steel, piping, hoses
and other support services was undertaken between April and June 2001. The main
equipment was mobilised to NW Hutton in mid June 2001. The installation of the subsea
equipment, cuttings processing and injection equipment and environmental monitoring
equipment took place during the latter part of June 2001.
The installation went smoothly but took longer than expected for a number of reasons.
There was significant problems mobilisng personnel due to extensive bad weather (fog),
however the principal reason / root cause was under estimating the difficulty of
mobilising such a large amount of temporary equipment onto a platform that does not
routinely see this level of activity.
5.1.1. Subsea Dredging System
The installation of the workclass ROV system, the PSL equipment for pump
deployment and the installation of the winches for installing the subsea hoses
went well. Non-productive time was higher than expected as there were a number
of relatively small installation issues that slowed progress. A significant period of
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fog and poor weather had a relatively minor impact on the overall installation but
did affect equipment arrival offshore and the ability to respond to any last minute
requirements. Details of the Halliburton and PSL equipment installation is
contained in references 13,14,15 and 20.
5.1.2. Topsides Cuttings Processing and Disposal System
The SWACO processing unit was straightforward to mobilise and install on NW
Hutton which can be attributed the unit being fully assembled onshore. The areas
of difficulty were the large number of interfaces and in practice a number of these
did not go to plan although they were all resolved without delaying the project
offshore. The installation procedures are described in reference 23 and the
lessons learned from the installation are described in more detail in section 5.4
and the SWACO report reference 21.
5.1.3. Environmental Monitoring
Installation of the winches required to deploy the environmental monitoring
equipment went smoothly. Approximately 14 days before the trial was intended to
commence, the Long-Term Minilander was deployed to establish the background
suspended sediment climate.
Prior to the dredging activity, the remaining four minilanders and aqualander were
deployed to their appropriate positions. The long-term minilander, the (platform)
west side minilanders and aqualander were deployed to the seabed using the
platform pedestal crane and auxiliary winch. The minilander orientation was
verified, and adjusted if necessary, using the eyeball ROV. The (platform) east
side minilanders were deployed to the seabed using the pedestal crane and ROV
friendly hook and ‘flown’ out to the intended 100m and 120m positions by the
workclass ROV.
The offshore assembly of the CEFAS equipment was straight forward and the
deployment of the minilanders to the seabed was comparatively simple with no
significant problems. The o verall time to deploy the equipment was longer than
expected due to the limited resources offshore and the need to prioritise other
activities above the CEFAS equipment deployment. The CEFAS equipment
deployment was completed prior to commencement of dredging.
Minor difficulties encountered included the east side minilanders deployment and
the spooling of the winch rope when recovering minilanders. The east side
minilanders which were deployed to the seabed by crane were on the limit of the
ROV’s weight capability. When the ROV picked them up to carry them to the
required location it only just managed to lift the first minilander, it did however
manage to move it out to the correct area. The second minilander to be moved by
the ROV had additional buoyancy added to reduce its weight in water.
Recovering the minilanders involved spooling the soft line back onto the winch
drum.
The installation of the OBS sensor onto the eyeball ROV was straightforward
offshore, however the eyeball ROV was unable to readily accommodate the
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ADCP. Note: Once dredging operations started it was agreed that, given the size
and location of the plume relative to the eyeball ROV, the utilisation of the ADCP
on the eyeball ROV was not practical. Hence the ROV mounted ADCP was not
used during the trial.
The drawing below shows the as deployed locations of the CEFAS equipment and
the dredging area.
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5.2. Equipment Operation
The organisation of personnel offshore, their interfaces and the overall operation was
undertaken in accordance with the BP Bridging and Interface Document (reference 5)
and the BP Operational Procedures Document (reference 4).
5.2.1. Subsea Dredging System
The susbea equipment was operted in accordance with the BP overall procedure
and the detailed HS and PSL procedures (references 4,16 & 20)
Testing of the subsea equipment commenced on 8
th
July 2001 with a subsea test
of the Warman pump on water. The pump was stopped, restarted, and operated
at various rates up to 110m3/hr. The operation was monitored by the eyeball
ROV. On successful completion of this test the Warman pump was checked by
the eyeball ROV and it was discovered that the dump valve accumulator had
discharged, this required the pump to be recovered to surface. The exact cause
of the problem was not found so a second manual valve was fitted to the dump
valve to allow the ROV to manually shut off the dump valve.
The initial dredging trial commenced on 11 July after deployment of pump and
connection of hoses went very smoothly in the morning. The initial dredge run
used the Breebot crown cutter and the Warman pump and was at low flows whilst
SWACO and Deutag balanced their systems. Solids content was low as the
velocities were too low to carry larger particles – average flow of 30m3/hr.
Operations were stopped after a total of 55 minutes to allow review of how the
operation was progressing. The dredging was re-started at full flow rate (average
of 75m3/hr), and it was noticed that the pumping performance was variable which
was traced to intermittent opening of the dump valve (review of the subsea
pressure signals indicated it was opening every 20 seconds during the last 4
minutes of pumping). The dredging ROV then stopped dredging to close the
manual dump valve.
The second dredging trial on 12 July utilised the Warman pump and a simple
caged suction nozzle as the free flowing nature of the cuttings found during the
previous trial meant that this would be more successful. This trial dredged for
approximately 2.5 hours with a number of stops to backflush, change technique,
or relocate the suction nozzle. The amount of solids fluctuated and was initially
low as the pilots experimented to find the best method. The best performance
with the nozzle was to push it in directly down into the cuttings and thrust forward
with the ROV creating a collapsing hole. During the operation wire rope was
picked up and was successfully backflushed to clear the nozzle. A minor problem
with the eyeball ROV problem meant it was not able to monitor the whole of this
days trial.
Dredging on the 13 July continued using the same setup and technique of
submerging the dredging nozzle into cuttings. The eyeball ROV obtained some
excellent video footage of the operations as visibility was good and these are
included on the ‘video highlights’. During backflushes for clearing restrictions
visibility for both ROV’s was lost, however it quickly returned. Returns to surface
visibly contained more solids than the previous dredging operations.
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The final dredging trial involved changing pumps to test the PSL Discflo pump.
The suction nozzle was again used. Due to the different size of the Discflo pump
it pumped at a higher average flow rate than the Warman pump. Overall it’s
performance appeared similar to the final test with the Warman p ump.
Problems with injection of dredged water into well A39 limited the overall duration
of the trials to the volume of the NW Hutton mud module 2 pit capacity. As a
result the dredging operations finished on 14 July.
Overall the operation of the subsea equipment worked well and it is described in
more details in the relevant contractors reports.
5.2.2. Topsides Cuttings Processing and Disposal System
The SWACO unit was fully commissioned and tested on water on 10 July 2001.
The system was operated in accordance with SWACO’s procedures (reference
23).
During the first dredging operations on 11 July the cuttings processing system ran
well, easily handling the cuttings. Most cuttings were very fine and passed
through the 40 mesh screens (sub 300 mircron). During the test with full flow of
the SWACO discharge pump it was too high for pit room to accept so the flow was
throttled back and the subsea pump reduced it’s flow. The inlet trough within the
mud module was then reconfigured by changing the sluice arrangement to
prevent any fluid overflow or splashing at high flows.
Throughout the dredging operations samples were routinely taken at the inlet to
the SWACO unit for accurate analysis of water:solids ratio. Additional samples
were taken of the solids from the header box and metal in the dredged stream
was collected by ditch magnets in the header box. The inlet samples were initially
taken from the header box itself however due to the header box filling with solids it
was considered that more representative samples could be taken from the tee
piece in the riser upstream of the header box.
The SWACO team monitored for VOC’s at the SWACO header box/shakers and
the potential presence of H2S was monitored by a combination of pocket and
portable H2S detectors on the SWACO unit and in MM2 around the mud pits. The
platform Safety Advsier undertook regular checks for Low Specific Activity scale.
The Mud Engineer measured the properties of the recovered material entering the
mud pits.
The output from the SWACO CRI system was transferred to MM2 mud handling
system. After treatment it was pumped, with the high-pressure mud pumps, via
temporary HP lines, to the injection well, A39.
After the temporary high-pressure lines had been rigged and pressure tested, a
test i njection was carried out. A 300 bbl pre-flush was injected, followed by a 20
bbl high viscosity bentonite pill, which was displaced with 299 bbls of over-flush
(dredged seawater and platform supplied seawater). When the initial high
viscosity pill reached the formation the re-injection rate was limited to 0.5 bbls /
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min by the A annulus pressure reaching its maximum allowable pressure
(MAASP).
The inability to inject solids whilst dissappointing had been anticipated and
contingencies put in place to allow the trial to continue. The main impact was to
reduce the duration of the lifting operation to that required to fill the mud pits. This
reinforces the earlier comments relating to the importance and possible expense
associated with preparing disposal wells.
During dredging operations, the recovered seawater was routed through the Sand
traps and treatment pits to allow maximum solids separation by gravity i.e. the
agitators were not run in the treatment pit and these pits were allowed to gravity
overflow into the next pit wherever this arrangement was possible. The dredged
water in the pits contained 2.6% - 3.6% sub 300 microns solids (% solids was
estimated by fluid density). An initial GC – MS (Gas Chromatography Mass
Spectrometry) showed <1 ppm of oil.
It was not practical to remove the solids from the pits using the SWACO vacuum
system as originally planned. The recovered solids that were left in the bottom of
the pits were removed with high viscosity pills circulated into each pit in turn, with
the slurry being backloaded into skips for onshore disposal.
Dredging returns passing over the SWACO shakers
5.2.3. Environmental Monitoring
The CEFAS monitoring equipment recorded data and was not operated as such.
A known limitation with the monitoring equipment was that the data was not
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available real-time, hence any problems with the equipment could only be
identified retrospectively (see Section 5.3.5). During the trial CEFAS personnel
made a photographic record of the trial and had a member of their team working
with the monitoring ROV.
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5.3. Results and Discussion
5.3.1. Safety
The onshore and offshore trials were completed safely.
All offshore project work was undertaken within the NW Hutton safe system of
work and all activities were subjected to risk assesments. Each day at the shift
start each team held a tool box talk to discuss the days activities. Job specific tool
box talks were held prior to commencing each new activity. On a number of
occasions during the equipment mobilisation and installation immediately prior to
critical activities, operations were halted to for ‘Time Outs For Safety’ to ensure
that the activity was undertaken safely by ensuring all involved personnel fully
understood all aspects of the activity in question.
Prior to commencement of the operations evacuation exercises were undertaken
to ensure that the personnel could be safely and quickly evacuated in the event of
an unsafe condition occurring during the trials (caused by either the trial itself or
by other platform activity). The exercises went smoothly and only minor changes
were made following the exercises.
Throughout the trials the recovered material and equipment was monitored for
LSA Scale, H2S and VOC’s. No LSA scale was recorded offshore. A low level of
H2S was measured with a peak reading of 5ppm (with average levels much lower)
a level which is well within safe occupational exposure limits. Maximum VOC
levels were 5ppm.
5.3.2. Overall
Summary of Statistics
Dredge ROV in water
71:38 hrs
Eyeball ROV in water
64:49 hrs
Dredging cuttings
06:54 hrs
Max % wet cuttings (from samples)
17%
Average % wet cuttings (from mud pit
returns and skip volume)
Approx 4%
Volume of liquid recovered
353 m3 solids + water plus 25m3
water during pump test (389m3
measured from pit returns)
Volume of cuttings pile removed
14m3
Volume of dredge liquid injected
253m3
Skip volume accumulated
Approx 1m3
Peak H2S level recorded
5ppm (on shale shakers)
Low Specific Activity Scale
None detected during offshore
monitoring
Notes
Amount of cuttings pile removed is calculated from the specific gravity of the mud
pit returns plus estimate of dry solids leaving shaker and assumes a 40% water
content in the in-situ cuttings.
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Energy useage for all the susbea equipment was approximately 200Kw.
Summary of Each Dredging Test
Date
8 July
11 July
12 July
13 July
14 July
Dredge
Head
N.A.
Breebot
crown
cutter
Caged
suction
Caged
suction
Caged
suction
Pump
Warman
Warman
Warman
Warman
PSL Discflo
Test
Duration
55minutes
46minutes
2hrs38min
1hr50mins
45minutes
Total
Volume
pumped
25m3
23m3
132m3
132m3
71m3
Maximum
wet solids by
analysis
N.A.
N.A.
6% by vol
(1:15 by
vol)
14% by vol
(1:7 by vol)
12.5% by
vol
(1:7 by vol)
Maximum
wet solids by
observation
N.A.
N.A.
8%
17%
16%
Maximum
wet cuttings
in mud pits
(pit slurry –
excludes
cutting to skips)
N.A.
3.6%
3.6%
3.6%
3.1%
Notes:
1. Test Duration includes minor stops for backflushing etc.
2. Total Volume pumped is as recorded by magnetic flow meter including both
solids and liquid phases
3. Maximum wet solids % is derived from the samples taken at the SWACO
unit header box / inlet, see reference 25 for details
4. Maximum wet solids by observation is the solids % observed when the
sample settles out in a calibrated measuring cylinder. see reference 25. It
should be noted that the variation between the results by observation and
(much more complex) analysis was relatively small typically 1 or 2 percent.
5. Maximum wet cuttings in mud pits is calculated from specific gravity (SG) of
pit slurry – it excludes cutting to skips and uses assumed SG and water
content of drill cuttings. The % quoted is an average typically over a 10m3
volume of recovered material.
6. No samples were taken on 11 July for analysis as the shaker area was
being continuously monitored for H2S and LSA.
Full details of the sample analysis is contained in reference 25. In addition to
measuring the water to solids ratio SWACO undertook a number of other tests:
retort tests to identify oil water and solid content of the samples taken (see section
5.3.5 for discussion); particle size distributions and measurement of the dry solids
density.
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5.3.3. Subsea Dredging System
Operation of the dredging ROV is described in detail in the HS report reference 18
and can be seen visually on the ‘highlights video tape’ (reference 6). Operation of
the PSL Discflo pump is described in detail in reference 21.
Equipment performance data was recorded for the subsea dredging equipment
during each trial and was logged on a PC at approximately 1 second intervals. An
interpretation of the dredging operation can be made from this data when it is
graphically presented. A selection of approximately 10 minute snapshots of the
various dredging trials are presented below. The complete data recorded on the
PSL datalogger is contained in reference 22.
5.3.3.1.Dredging Trial 11
th
July
The initial 11
th
July dredging trial used the Warman dredging pump and the
Breebot crown cutter (which was made specifically for the offshore trials by Delft
Hydraulics following their study work). This was the first trial and the sytem was
being optimised and the operators were ‘learning’ how the system behaved. The
drill cuttings were very soft and appeared to be non-cohesive which made them
relatively easy to dredge. The properties were at the low end of the expected
range, however previous drill cutings cores at NW Hutton had not been taken this
close to the jacket. Cores close to the jacket base or inside the jacket may have
resulted in a different initial equipment setup and would be useful at other
locations where cuttings removal is being planned.
Graph 1 –
Note all Graphs are Contained in a Separate Document
‘Offshore Trial Dredging System; Operational Graphs’ Reference 2.
Graph 1 shows a 10 minute snapshot of this first trial. From this graph it is clear
that the dredging operation had continual stop – starts and was therefore not in a
steady state. At the start of the graph there is pressure but no flow indicating that
there is a blockage in the hose. The pump was shutdown to backflush the line
and re-started at 15:46:30, initially there was no flow until pump pressure builds
and flow increases. At 15:51:30 the pump is stopped and the slow pressure
decrease is indicative of some form of restriction in the hoses which slows down
the backflush of the 70m head of water. The eyeball ROV observed the plume
during this backflush. The pump restarted at 15:53:00 and following this there
were pressure dips at approximately 20 second intervals. These pressure dips
were identified as a fault with the dump valve intermittently opening as pressure
on it increased causing partial opening.
The rotating action of the crown cutter created some disturbance as it swept
through the cuttings locally reducing visibility.
During this trial no samples were taken from the inlet to the SWACO cuttings
processing unit as the emphasis was to monitor for any evidence of H2S or LSA
scale. No LSA scale was detected and levels of H2S recordd were less than
1ppm.
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Flow rates during the first trial were low with the pump motor operating at a low
frequency (~43Hz) and this may be partially responsible for the rapidly occurring
blockages and some of the secondary disturbance by the crown cutter.
5.3.3.2.Dredging Trial 12
th
July
The 12
th
July dredging utilised an angled suction nozzle. The crown cutter was
specifically designed to allow dredging of cohesive (clay type) materials and
previous surveys had indicated that this type of material may be present. The first
trial showed material to be relatively free flowing and non-cohesive which is ideal
for suction dredging. Several suction nozzles had been fabricated and the ‘bent’
suction nozzle was fitted as the DRL review had indicated this arrangement to be
preferrable.
Graph 2
– see reference 2
Graph 2 shows the dredging parameters whilst CEFAS took their first partition
experiment samples from the SWACO unit header box. At this time (15:15:00) the
pump was operating at a fairly low flow rate similar to the pump rate on Graph 1,
however it is clear that the parameters recorded were much steadier. The
reasons for this are not definitive but are probably a combination of pilot
experience, closure of manual dump valve and the change of dredge head to
suction nozzle. Towards the end of this snap shot the motor frequency (pump
flow rate) is being increased to bring it up to the design rates (80 to 100 m3/hr).
At a number of occasions on the graphs the flow rate was very steady (e.g.15:17
to 15:18 on Graph 2). This steady flow was typically caused by the dredge head
either being clear of the cuttings pile and pumping only water or the dredge head
being choked by cuttings which will cause the Warman pump’s vacuum protection
valve to open.
Further review of the ROV videos and pump data could allow an estimate of the
amount of time that the system pumped water rather than dredged solids. This
data could then be used to calculate the overall percentage of solids during actual
solids recovery which will be higher than the 4% wet cuttings calculated for the
trials as a whole. Clearly in order to operate the system efficiently for a full mound
recovery this non productive dredging time would need to be minimised.
5.3.3.3.Dredging Trial 13
th
July
Graph 3 illustrates the 13
th
July dredging again utilising an angled suction nozzle
and the Warman dredging pump. The pump was operating at the design rates for
the NW Hutton trials of 80 to 100m3/hr, and the regular oscillations of the flow rate
indicate that unstable flow was occurring due to the dredge head entraining solids.
The graph is marked to indicate the time of CEFAS’s second partition experiment
samples being taken. Note that there is a time lag of over 1 minute between
solids entering the suction nozzle and reaching the SWACO header box. Sample
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29 was taken at 14:15:00 with a 7.5% wet cuttings conte nt by volume from the
subsequent onshore analysis.
An unexplained pressure drop occurred at 14:16:30 from 9bar to 8bar, the flow
rate also dropped but the pump motor frequency/speed remained constant. This
may have been caused by some form of inlet (suction system) restriction. At
14:17:30 the fluctuation in the flow rate reduced which is indicative that the system
was pumping water rather than dredging solids, probably to allow a possible
restriction to be cleared. In this case dredging was then stopped (at 14:21) to
allow a backflush to clear the system.
Graph 3
– see reference 2
Graph 4 shows the trial continuing on 13
th
July. A typical pump shutdown to
backflush a blockage occurred at 14:55:05. The pressure decreased over a 1
minute period probably due to accumulated material in the hoses. A sample was
taken at the SWACO unit just before this shutdown with a 3.6% soilds content.
Graph 4
– see reference 2
The Dredging trials were all undertaken just outside of the jacket footprint adjacent
to legs B2 and B3. Some of the dredging operations were undertaken very close
to the jacket members. The results do not indicate any loss of performance when
the dredging was close to a member, although the operators found it simpler to
operate clear of members. As a result the ROV relocated between dredging
operations. On completion of the run shown in Graph 4 (at 15:00) the ROV was
very close to a member, so the suction hose was disconnected and re-positioned.
During disconnection some secondary disturbance always occurred from the
residual sediment lying in the hose that was disturbed. Graph 5 shows the final
dredging run on the 13
th
July with the ROV relocated away from the jacket
members slightly. During this run samples were taken with wet cuttings levels
upto 16% by observation, subsequent onshore analysis by filtration ammended
this result to 14% wet cuttings by volume. Two samples were taken during the
time frame of Graph 5 one with the 16% peak (14% by analysis) the other with
5%, however there is no apparent reason for this significant variation so it is
assumed to be caused by the variable nature of the dredging operation.
Graph 5
– see reference 2
5.3.3.4.Dredging Trial 14
th
July
The PSL Discflo pump is significantly different to the Warman pump a nd is suited
to pumping viscous slurries. The Warman pump was recovered and the PSL
pump deployed for the final trial on the 14
th
July to establish if there was any
significant difference in dredging performance.
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Graph 6 shows the pump operation shortly after the commencement of the trial
(16:29:00) when the motor frequency was being adjusted by the operators to
establish the preferred flow rate. The Discflo pump has a higher capacity than the
Warman pump and was typically operated with an average flow rate around
110m3/hr. Even though a sample taken at 16:52 was fairly early in the trial a
result of 7:1 water to wet cuttings by volume was obtained. The samples for the
3
rd
CEFAS partition experiment were also taken at this time.
Graph 6
– see reference 2
Comparison of the PSL pump graphs with the Warman pump graphs (with suction
nozzle) shows little difference in the general patterns. One difference is that the
PSL pump shows higher variations in the discharge pressure during steady
dredging operations.
Throughout the trials the operators were varying their techniques to optimise
performance. Initially the dredge head was swept from side to side as this was
identified as the optimum technique for the Breebot cutter to takes slices of
cuttings. Once the dredge head was changed to the suction nozzle the sweeping
technique was clearly not efficient and the best techniuqe appeared to be to thrust
the dredge head forward and down by thrusting the ROV slightly. The limited
duration of the trial meant that it was not possible to quantify these differences.
Another change in technique was for clearing nozzle blockages. Once the system
time lag and affect of the SWACO inlet header box were understood there was
less need to backflush due to the elimination of false indications of low solids.
Initially low (visible) solids on the shakers was taken as indicative of low solids
being dredged; when the dredge head was buried this implied there must be a
blockage. To clear the blockage the dredge head was lifted and pump was
stopped to backflush naturally, the pump was restarted and this was repeated until
normal pressure – flow was achieved. Backflushing created the largest visible
plumes during the trials so the operators worked to minimise backflushing or any
other pump stoppages without first pumping water to clear the solids.
Later in the trials the technique was improved to reduce the amount backflushed
by only partially reducing the motor speed till flow reduced to zero. By observing
the raised suction nozzle with the eyeball ROV the debris could be seen falling
away and motor speed increased early to minimise any reverse flow. Graph 7
shows two backflush operations in this manner with considerably reduced
backflow and hence much reduced secondary disturbance. The ability to
continuously vary motor and pump speed was extremely valuable throughout the
trial and this ability to vary speed of the pump to clear the nozzle with a minimum
backflow is one example of this.
Graph 7
– see reference 2
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Throughout the trials the amount of stop start operations were significant. Graph
8 shows the full dredging operations for 13
th
July where there were 5 stop start
operations with the Warman pump whereas on the 11
th
July there were 5 in a
much shorter duration of dredging.
Graph 8
– see reference 2
The frequency of stop start operations tended to reduce as the operating
experience increased. Clearly in order to maximise the systems performance the
amount of time not productively dredging would have to be minimised. Whilst the
overall ratio of water to wet solids was only approximately 25:1 (4%) the highest
sample achieved was 6:1 (14%) by volume. It is not straight forward to deduce
what the effect of minimising non-productive dredging time would have been.
However from the trial results it is reasonable to ‘estimate’ that the average water
to solid ratio during stable dredging operations on NW Hutton would have been in
the range of 20:1 to 10:1 by volume. With more time available to dredge the
system used would probably have been able to achieve an average of
approximately 10:1 by volume.
Key Observations from the offshore dredging operations:
1. Ratio of Water to (wet) Solids by volume varied from 6:1 to almost entirely
water. The overall average was approximately 25:1 including starting up,
stopping and clearing blockages. Steady dredging operations during the
trial were probably in the range of 10:1 to 20:1
2. Virtually no plume from suction nozzle.
3. Suction nozzle seemed to perform well when sunk deep into cuttings and
used to form a collapsing hole.
4. Breebot crown cutter was tested first and it was known to be less suited to
soft material, however because it was tested first it’s performance was
probably less than optimal and no comparisons can be made between the
dredge heads.
5. The cuttings dredged were at the ‘low’ end of the expected properties,
however coring had not been undertaken adjacent to the jacket base. This
is recommended prior to dredging operations at other sites.
6. Trial duration was not long e nough to properly differentiate between pump
performance or benefits of one technique over another.
7. Drill cuttings found to be very soft in the locations dredged. They were of
low cohesiveness but did not flow as such. It is likely that there would be
some increase in the properties with depth and these may favour a crown
cutter (Breebot).
8. Value of driving the motors/pumps with a variable speed drive was
demonstrated by clearing the suction nozzle using the excellent pump
control that the VSD provided.
9. Dredging performance was much improved over Blyth probably due to a
mixture of system improvements and increased visibility. This
improvement confirms the decision not to undertake further complete
system testing onshore before the offshore trials.
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10. Dredging operations contained significant number of stop start operations
and time when the system was pumping only water, all of which reduce the
systems overall dredging performance.
11. Eyeball ROV observation of dredging operations was invaluable to give an
all round view of operations and it could always locate up tide of the
dredging.
12. Maximum dredging depth achieved was in excess of 0.5 metres.
5.3.4. Topsides Cuttings Processing and Disposal System
The topsides processing system worked extremely well. A number of detail
lessons on design modifications are recorded in the SWACO final report. Most
significantly, the system was well able to handle the volumes of solids and liquids
delivered by the subsea unit.
The recovered material presented little problem for the CRI unit to handle. The
dredged solids were discrete and did not cause blinding of the shaker screens.
The material was very easy to slurrify to the required standards for injection.
The recovered material did not present any conditions that would have created an
unsafe situation. There was no LSA indication; there was only a short-term peak
of H2S to 5ppm, otherwise levels were low; and low levels of VOCs (max. 20
ppm) recorded.
The system was quite extensive and required co-ordination of a number of groups
of personnel.
Reinjection of slurry as planned was not available which resulted in all solids
being retained at surface. All recovered seawater was disposed of by reinjection.
Storage of the recovered slurry in the mud pits indicated visually and qualitatively
that gravity segregation of solids and liquids (in this trial at least) is relatively rapid
and efficient.
5.3.5. Environmental Monitoring
On the whole, the majority of the environmental monitoring equipment functioned
satisfactorily upon deployment.
The following problems were identified:
•
The OBS (Optical BackScatter Sensor) and Conductivity sensor on the
East Side Minilander 2 were embedded into the seabed for the duration of
the trial;
•
A degree of biofouling and subsequent loss of sensitivty of the ESM2
suspended sediment logger on the Long Term Minilander was detected
after approximately 14 days; and
•
The fluorescent tracer intended to mimic the release of cuttings
material/water during a backflush event came out of suspension in the
transport containers and consolidated, rendering it unfit for its intended
purpose.
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The following section summarises the observed background environmental
conditions and incremental effects resulting from the dredging activity.
5.3.5.1.Environmental Monitoring – Confirmation of Assumptions
One of the main assumptions that affected the positioning of the environmental
monitoring package was the reported NW Hutton tidal data. It was assumed that
the tidal conditions near the seabed at NW Hutton would be similar to that
measured at the surface i.e. virtually circular. This assumption was based upon
BP surface current data. Using this data, the minilanders were positioned on the
seabed such that the sideways looking ADCPs, effectively ‘boxed in’ in any plume
and ensured that any plume would be detected regardless of the tide/current
conditions. The drawing in Section 5.1.3 shows that this was largely achieved.
The post monitoring analysis of the tidal / current data indicated that, rather than
being circular, during the trial, the water movements showed only two complete
circuits with an occasional flow reversal. Furthermore, during the dredging period,
the residual current flow at the seabed was dominated by a weak southerly and
south-easterly current of approximately 0.1 knots (4.6 cms
-1
). These water
movement and current data confirm that minilanders 4 and 5 and the aqualander
were in appropriate positions, relative to the dredging site, to observe and sample
any plume(s) generated by dredging or backflushing.
5.3.5.2.Environmental Monitoring - Results
The main results of the environmental monitoring for the Task 6 project have been
divided into:
•
Sediment resuspension (Background and during dredging activities); and
•
Chemical (contaminant) analysis of seabed water/trapped resuspended
sediments (from Aqualander / Minilanders) and recovered materials
(water/solids) at the platform (SWACO Unit and Mud Pits).
Sediment Resuspension
A number of sediment resuspension events were observed during the trial. The
sources of this disturbance appeared to be both naturally occurring and
anthropogenic (man-made) events.
A number of the resuspension events occurred at times when no dredging was
ongoing. These events were fairly small and typically raised the background
suspended sediment concentrations approximately 5 fold (approximately 1
mg/litre background to 5 mg/litre). The most likely explanations for these events
are natural sources of disturbance such as tidal and wave activity.
ROV dredging and positioning (thrusters) resulted in the localised generation of
sediment plumes.
The environmental monitoring results confirmed the observations from the video
footage and can be summarised as follows:
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•
The plume from dredging is small and detectable (approximately 15
mg/litre) close (4m) to the dredging site. This increase in sediment load is
similar to that observed by natural events (for example, wave and tidal
resuspension of seabed material);
•
No plume from ROV dredging was detected at any of the minilander
positions which were positioned fur ther away, 43-120m from the dredge
site.
•
Back flushing represents the most significant source of suspended
sediment resulting from the trial. During a backflush event, the initial
suspended sediment concentrations in close proximity to the source (4m),
reaches a high of approximately 240 mg/litre for a short time (1-2 min)
before decreasing;
•
During the trial approximately 20 back flushing events occurred, the
majority of which were not detected by the minilanders at the 88m position
•
Two small resuspension events (of approximately 2mg/litre suspended
material) were tentatively identified at a point 88m from the disturbance
event. These observations were made on the 13
th
July, at 14:30 and 15:20
GMT and are within the measured background range for natural sediment
resuspension events
•
A larger resuspension event was identified on the 13
th
July between 17.20
to 23:00 GMT which does not correspond to any dredging period. During
this observed event, the sediment concentration temporarily (for
approximately 6 h) increased by 10mg/l. Dredging has been tentatively
discounted as the source of this disturbance.
Contaminant Analyses
A number of samples of water, suspended sediment and recovered cuttings/water
mixture were taken during the trial to determine their background levels and to
identify any secondary pollution resulting from the dredging operation and its
associated activities e.g. backflushing events. The range of contaminant analyses
included total hydrocarbon, barium, endocrine disruptors (Alkyl Phe noxy
Ethoxylates, APEs), and LSA (Low-Specific Activity Scale) radioactive analyses.
Contaminant analyses (total hydrocarbons, barium and endocrine disruptors -
Alkyl Phenoxy Ethoxylates, APEs) of the seabed water samples (taken by the
Aqualander) confirm that:
•
The measured contaminants (pre-trial) are at normal background levels for
the North Sea;
•
At the 88m position, no oil plumes resulting from dredging or backflushing
were detected. Similarly, the concentrations of barium or APE (endocrine
disruptors) in the surrounding seawater were not signficantly increased by
dredging/backflushing;
•
The barium and APE concentrations within the seawater separated from
the recovered drill cuttings were higher than background levels;
•
The LSA analyses indicated that the Radium-226 or Actinium -228 in the
recovered solids are close to background levels, hence the material does
not pose a radiation hazard;
•
No surface oil sheen was observed that could be attributed to the dredging
operation and associated activities.
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Recovered Materials At Surface
The partition experiment monitored the behaviour of the recovered oil as whole
samples of dredged fluids were recovered to the surface and allowed to separate
with time.
The oil content in the recovered water and solids (following gravity separation of
the solids) ranged between approximately 5-70 mg/l (water fraction) and 10-92
mg/l (solid fraction). There was no evidence of an enhanced oil/solid separation
with time. The cuttings pile heterogeneity and instantaneous variations in
recovered solids and water meant that it was not valid to draw conclusions on the
mass balance for the recovered oil within the water and solid fractions.
Initial results from the SWACO analyses of recovered solids at surface indicated
that the oil on recovered solids at surface (after processing) ranged between 4 -
9.5% oil on cuttings (dry weight). Previous core samples from the NW Hutton pile,
in close proximity to the dredged area have been analysed and are known to
contain up to 7% (oil on cuttings wet weight). Assuming an average water content
of 50% (range, 15-70%), the oil on recovered solids is of the same order of
magnitude with the known oil content within the NW Hutton pile content.
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5.4. Lessons Learned
This section contains details of the key lessons learned and observations from the
project. More detailed information is contained in the individual reports listed in the
references (Section 9.0). A key part of documenting the lessons learned and
observations were brainstorming and other sessions held during the Offshore Trial
Review Meeting on 21 August 2001 (reference 3).
1.
Category
Observation
Notes
Learning /
Recommendation
2.
Safety
Safe Completion of the
project
Differing company cultures meant
that significant effort was required.
Emphasis on safety
throughout paid off
3.
Good use of STOP and
TOFS
STOP used throughout offshore
phase. Time Out For Safety used
effectively to ensure work continued
safely before activities became
unsafe
Benefit of effective use
of platform standard
safety tools across
project team.
4.
H2S Detectors – personal
detectors excellent, fixed
type poor
Fixed detectors had erroneous
alarms, these were traced to
incorrect battery charging.
5.
Performing Authority
overload
Additional workload of trial
overloaded key platform personnel
including performing authorities and
SAEP
Require additional
resource or identify
different ways of
working
6.
Additional project specific
safety adviser avoided
overloading platform SEA
Enabled walk the talk of safety
culture for project and was able to
run RA’s and join toolbox talks
Found to be essential
7.
Working with SIMOPS
(priority conflicts, adjoining
operations)
Equipment layout meant significant
risk of dropped objects onto lower
work sites without careful
management
Careful management
avoided any incidents
8.
PA on SWACO equipment
not heard
Noise from nearby generators was
louder than the SWACO unit itself
Require local flashing
lights to indicate
alarms
9.
Safety action overload
Not enough time left to do the job
10. Project
Manage-
ment
JIP Task 6 Steering
Committee was a success
Benefited from a small
group of interested,
informed partners
11.
Project Cost -Substantially
underestimated what could
be achieved with budget
Early studies indicated several
systems could have been tested for
the final cost of testing a single
system. Budget prices were very
different to subsequent tendered
costs
Tenders are required
for reliable cost
estimates.
Underestimated
overall system
complexity
12.
Loss of continuity during
project delays resulted in
changes not properly
documented
Changes of personnel inevitably
occurred over the 18 month project
duration. (e.g. error in elec.
schematic not picked up, correct no
of H2S monitors not in workpack)
13. Interfaces
Developed an effective
team with all parties
working well together
offshore
All parties helped others as
required. E.g. SWACO helping
Deutag during quiet periods.
Clear benefit of
building a team
onshore and having
entire team focussed
on a common
objective offshore.
14.
Generally good direct
communication across
interfaces. However PSL
Interfaces with remote team
members (PSL, CEFAS) are harder
to manage by their very nature
Particular attention is
required with remote
interfaces which is
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interfaces. However PSL
felt ‘left out’ at times from
project team as unable to
regularly attend weekly
meetings
to manage by their very nature
interfaces which is
harder with language
barriers. Simplify
interfaces where
possible
15.
HS - PSL Engineering
(equipment) interfaces were
difficult
Communications & distance made it
difficult to get interface details right
and for each to know what the other
was doing
See above, different
equipment standards
and approaches made
the interfaces more
complex.
16.
Offshore interface location
did not always appear
logical in constructed
system; interfaces around
flow meter and injection
riser were overly complex
(3 parties involved where it
could have been 2)
Project interface locations stayed
constant throughout project with
little change. Design changes after
equipment purchased complicated
matters (two wells to one and
introduction of flow meter spool into
riser hoses
Simplify, minimise
interfaces where
possible.
Balance issues of
changing interfaces
as design changes
against complexity this
adds where equipment
owned by others.
17.
WGE undertook some
activities differently to
Deutag’s expectations
Deutag not offshore continuously so
WGE could not always discuss
activities that interfaced before
commencing them
18. Design
and build
of
equipment
No ROV TMS due to Mod 2
Production laydown having
a very restricted load
capacity
A TMS would increase ROV
excursion distance and increase
launch / recovery weather window.
good weather during trials meant
that the operations were not
restricted
For a short duration
trial you can survive
without a TMS. A TMS
and heavy weather
deployment system
would be essential for
a full cuttings recovery
to minimise downtime.
19.
Good decision-making
following Blyth trial
Changes to subsea equipment
improved it’s performance – subsea
hose connector mods, dredge head
re-design
Onshore trials were
worthwhile
20.
Generators oversized, no
switchover/backup supply
Generators sized for all equipment
operating - a very conservative
scenario for maximum load.
21.
No SWACO MDU / cabling
issues
BP’s requirements for electrical
isolation / switching for temporary
equipment were not clear / made
available to SWACO.
The SWACO / WG
interface agreed
during design was not
optimal. Clear
definition of
requirements and
responsibilities is
essential.
22. Equip -
ment
Mobilis -
ation
OIS requirements not clear
esp. electrical. PSL HV
lead had to be full
permanent spec for
temporary cable.
This was identified as a potential
area of difficulty and the project
team met OIS before inspections
commenced. Several changes
required to PSL equipment even
though 3
rd
party DNV certified.
This resulted in last minute
equipment changes to cables and
the wrong size gland was
subsequently supplied for HV cable.
Better definition of BP
/ OIS electrical
requirements should
be obtained to avoid
last minute equipment
changes.
23.
Equipment arrived late due
to lack of vessel capacity &
cancelled sailing due to bad
Project requirements had been
alerted to BP logistics weeks in
advance. Not clear if project
Project shipping
logistic requirements
were not fully
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weather
contractors communicated with
correct ASCO contact.
understood by
logistics personnel.
Ensure particular
attention is given by
project personnel to
ensure logistics
personnel (BP &
ASCO) fully
understand projects
requirements.
24.
Accuracy of weights of
equipment
Weights and dimensions load sheet
good but could have been more
accurate, it used design weight
which were often light.
Get real / actual
equipment weights as
early as possible.
Particularly for heavy,
wide, high loads
25. Offshore
Installa-
tion
Length of the productive
working day
A significant amount of non
productive time was experienced
each day waiting on permits and
due to withdrawal of permits
Project programmes
need to factor in
sufficient non
productive time to
allow for permit
system.
26.
Marrying up equipment -
substitute equipment not
identical
Had to re-drill holes for PSL winch
as substitute identical winch
appeared to be slightly different
dimensionally. Could have been in
tolerance but cumulative tolerances
or deck deflections could cause
misfit.
Substitute identical
equipment may not fit.
Trial fit exact piece of
equipment where
possible or review
impact of misfit if not
possible. Use slotted
holes where possible
to increase tolerances.
27.
Not all minor installation
requirements were fully
identified
Details of minor cables not
identified in contractors procedures
– they require proper installation on
trays
Ensure all minor
installation
requirements are fully
identified
28.
Hose deployment – Worked
but requires good
communications.
High friction loads and some
snagging of unions occurred during
hose recovery. Optimisation of
chute design and inclusion of rollers
would reduce friction loads.
Chute shape not ideal.
Scale cross section
drwg. Would have
highlighted this.
29.
Complex Crane Lifting Ops.
Required through out
mobilisation
A number of cross hauled lifts
inside minimum crane radius
Detailed procedures,
onshore assessments
and offshore RA’s
were essential to the
safe & efficient
completion of lifting
ops. Consider extra
winches for equipment
cross hauling
30.
No onshore involvement of
offshore construction
personnel
Was planned, requested (for FAT’s
RA’s) but did not occur. Offshore
trips by onshore team with photos
etc. was substituted
Ensure onshore
involvement of
offshore construction
personnel
31.
Electrical isolations and de-
isolations required platform
personnel to perform them,
until equipment handed
over.
Senior Authorised Electrical Person
was single point for isolations
caused delays until equip released
SAEP was
overloaded. Review
workload of SAEP -
reduce requirements
for project workload
32.
Project received excellent
platform support during
Developed close working
relationship prior to offshore trial.
Value of aligned goals
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operational phase.
Aligned goals
33.
Single point accountability
(Offshore Const. Supv.) for
installation of all project
equipment was invaluable
Dedicated WG & BP onshore
support was also very useful.
Ensured smooth
installation of
equipment and
minimised conflicts
with platform ops
34.
QA of equipment (X-over &
winch wire length)
Several items of equipment arrived
offshore out of spec.
Double / triple check
critical equipment
35.
Over side working
restrictions slowed down
operations.
Was not clear what constitutes over
side working. Limited standby
vessel availability due to fog or
Hutton requirements acerbated the
issue
Project to better
understand what
constitutes over side
working and plan
accordingly.
36.
Deck layout worked well –
large amount of equipment
offshore. Required
significant effort to manage
available deck space
Mobilised over 150 lifts of
equipment onto NWH. Project
equipment occupied all available
space. Double stacking not
permitted
Large amount of
equipment required,
time and effort to
avoid choking the
platform with
equipment. Develop a
layout plan and get
deck crew involved
early
37. Offshore
Trials –
Subsea
Recovery
ROV right vehicle, cuttings
would have been too soft
for tracked vehicle
Reaction loads, movement of the
dredge arm tended to move vehicle
- could spud the ROV.
38.
Sonar and Echoscope
Survey system worked well
Valuable to system
performance
39.
PSL equipment (VSD,
winches, pump) all worked
well
Direct deployment over side by PSL
winch A frame was not an option at
NW Hutton but crane deployment of
pump went smoothly
40.
Trial too short to
differentiate bet ween
pumps. Dredging
performance was not
optimal
Well problems limited duration to
available mud pit capacity. Trial too
short to identify optimal dredging
technique
Not an issue for full
recovery.
Consider accessing
other dredging
expertise
41.
Suction hose buoyancy
good but not optimal
Suction hoses were very
manoeuvrable
Big improvement over
Blyth trial. Value of
onshore tests.
42.
Vacuum relief & dump not
required for discflo pump
PSL do not consider them
necessary due to Discflo pumps
unique design
43.
Lack of working vacuum
gauge on dredge arm
Vacuum gauge would help in
control of dredging operation by
allowing operator to know how
much suction was being applied.
Consider vacuum
gauge on pump inlet
as alternative /
additions
44.
5 inch bolt found inside
Discflo pump casing
No damage was caused confirming
pumps debris resistance
45.
Hose guide wire
configuration limited pump
deployment options (had to
deploy pump after hose).
Lack of space limited options for
sitting equipment. More space
would have allowed operationally
better layout
Equipment space
used for subsea
equipment was
insufficient for optimal
layout for full removal
operation
46.
Suction hose –buoyancy
and design both ends for
lifting
Hose was only designed to be lifted
by one end. Recovery would have
been simpler if it could have lifted
Difficulty of recovering
long hoses after they
have been used and
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by either end.
possible twisted.
47.
Cuttings at NWH relatively
benign,
System designed assuming worst
case for cuttings
Expectation they were
hazardous added
costs.
48.
Guide wire anchor block
was lost, and Warman
pump required 10te pull to
lift 3te pump skid
Equipment sinking – cuttings load
bearing capacity
Design for soft nature
of seabed
49.
Platform crane deployment
of equipment to seabed
good
Problems identified in RA such as
rotation / twisting of loads subsea
did not occur. Only just enough
crane headroom to deploy 30m
lengths of hose from pipe deck
Careful attention to
equipment design and
deployment procedure
paid off
50.
Deployment of subsea
equipment went very well
Blyth trials and
persevering with
FAT’s paid off
51.
Need video link between
ROV and SWACO – video
link with eyeball very
valuable
Would allow ROV pilots to monitor
dredging returns and SWACO to
see ROV ops.
52.
Eyeball ROV proved to be
invaluable, it undertook
some work tasks while work
ROV was on breakdown,
and it was able to visually
monitor dredging ops from
the position of best visibility.
The eyeball ROV video picture
displayed in the dredge ROV
control van was essential for good
control of dredging ops as the
dredge ROV’s cameras are looking
from behind and do not get a good
picture, they are also unable to
move out of the plum in the same
way as the eyeball can.
53.
Eyeball ROV could not fly in
plume and could not readily
carry ADCP due to size
Once operations commenced it was
clear it would be impractical to fly
ADCP into a small plume given
plume size and location near jacket
so did not work at getting ADCP
onto eyeball
Flying an eyeball into
the plume may only be
feasible for large
plumes with
deployment of ROV
above plume.
54.
Radio comms. worked well
At times a second channel would
have simplified comms. when
independent simultaneous ops were
occurring
An additional channel
would have allowed to
cranes working
simultaneously while
injecting into A39
55.
Flow meter and subsea
pressure gauge plus other
PSL instruments and data
logger were essential for
dredging.
Vacuum gauge did not work which
would probably have been useful.
Magnetic flow meter was excellent
choice after problems with several
types at Blyth
56. Offshore
Trials –
Topside
Process-
ing /
Disposal
Change management for
reduction from 2 wells to 1
well for injection versus
equipment requirements
The knock on effects in terms of
equipment design were not fully
investigated when the number of
injection wells was reduced –
interfaces could have been
simplified with hindsight.
Review all impacts for
all changes.
57.
SWACO inlet trough filled
with solids before solids
overflowed onto shakers,
the solids that filled the inlet
trough were lost down the
riser when pumping
Inlet trough was a standard design
used for mud’s carrying cuttings
Need different inlet
trough design to allow
for a water – cuttings
slurry .
Re-Design inlet trough
to ensure no backflow
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stopped or back flushed
/ loss of cuttings
58.
Planned for lots of
scenarios, actual scenario
different.
The variety of plans meant the
system could handle the actual
scenario.
Essential to plan for
uncertainty.
59.
X-over arrangement at well
complicated and part
supplied wrongly fabricated.
X-over complex because of need to
make use of existing HP hoses to
minimise costs
For key components
that cannot be easily
identified, check
fabrication is correct
by trial fit up.
60.
Lack of equipment
redundancy & spares
Trial nature meant could not justify
more than minimum of spares /
redundancy
Ensure adequate
spares for full cuttings
recovery.
61.
AFOS Bentonite transfer
system did not work well
The use of individual sacks rather
than 1te bags would have been
simpler
62.
Cutting’s properties better
than expected
Designed for worst case
Worst case could be
‘eased’ but deeper into
pile cuttings may have
‘worse’ properties
63.
Coping with flow into MM2
initially caused splashing
until flow re-routed.
Need different arrangement for
higher flows
64.
Mud Engineer for pit
management worked well -
Mud pits rigged as sand traps to
separate the solids and water. Mud
engineer also measured recovered
cuttings parameters
Ensure proper control
of the mud pits,.
65.
Poor injectivity into well A39
Lack of injectivity may have been
because we had not fractured the
well initially although an injectivity
test was carried out prior to the trial.
66. Environ-
mental
Contamination not a big
issue – equipment washed
off by sea as lifted to
surface
67.
Flying the eyeball through
the plume to monitor it’s
size was not practical inside
the jacket.
To monitor a very large plume it
may be feasible for a vessel
deployed ROV or an ROV deployed
above the plume
68.
Reeving of soft line onto
lander winches was slow
An automatic spooling
device would have
speeded up the
operation
69.
Sampling arrangements at
inlet to SWACO unit
Need to ensure representative
samples are taken, to ensure this
changed sampling from header box
to T on riser hose
Re-design required
with close attention to
sample point.
70.
Icebox for refrigerated
samples worked well
Cheaper and easier to source than
powered cooled container
Use again!
71.
Mini-landers – not ROV
friendly.
Eye for ROV operable hook was too
low. Note: Minilanders not normally
deployed by ROV
Can manage with
current design but not
ideal
72.
Tracer experiment – tracer
went hard.
Incorrect solids to water ratio
caused settling
Check all chemicals
several days before
use
73.
Difficult to transfer Garret
screen equipment to
standby boat.
In practice weather during trial was
not suitable for this test except on
the final Saturday run (would
require 1m or less swell)
Equipment suitable for
transfer to NW Hutton
standby boat must be
suitable to go in FRC
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6.
ANSWERS TO JIP NEED TO KNOW QUESTIONS
The task 6 CTR included a list of questions to be answered by task 6
1.
What is the typical rate at which cuttings can be removed from the sea-bed?
From the trial results, the dredging system utilised could recover cuttings from the
seabed at an average rate of 10m3/Hr and potentially this system may be able to
achieve higher rates (see 2 below for discussion on associated water). Significantly
higher rates are possible if larger system size is used, however in many
circumstances it may be difficult to install or utilise significantly larger systems due
to platform limitations.
2.
What is the range of composition of the slurry as it arrives at the surface?
The recovered slurry had a water to solids of 6:1 or more with significant fluctuation
in water content. During the trials the water percentage in the recovered slurry was
high, the short trial duration limited the ability to achieve the systems optimum
performance, however steady state ratios were in the range of 10:1 to 20:1 water to
solids. However it is considered that a system similar to that used at NW Hutton
could achieve an overall ration of 10:1 water to solids for a full recovery operation.
3.
What determines the ratio of solids to water and how can it be minimised?
This is determined by equipment design and dredge technique & experience plus
the properties of the cuttings. With regard to equipment design and selection the
dredge head, pump and dredge vehicle are all important. The improvements
between Blyth and NW Hutton trials indicate that the improved dredging
instrumentation assist with obtaining and maintaining good system operation.
4.
What is the degree of secondary pollution (i.e. water column impact and area /
thickness of cuttings re-settlement) due to re-suspension caused by the
dredge -head and the ROV / crawler? What is the environmental impact of
secondary pollution?
Dredging by suction produces little secondary pollution or plume; a larger localised
plume is produced by cutter suction dredging (using the Breebot crown cutter). The
dredging ROV created a localised plume by its movement during dredging. The
largest plume was created by backflushing the hoses to clear blockages. Little
secondary pollution was discernable at a distance of 100 metres from the dredging
operations, and no effects were seen on the sea surface.
5.
How much would removal to surface cost?
The cost of removing a typical 25,000m3 pile is discussed in section 7. The budget
estimate depends on the system selected but could range from over £5 million to
£7.5 million for recovery of the cuttings from seabed to surface. As discussed in
section 7, these estimates include allowances for a number of items that are highly
variable and likely to be platform specific (e.g. structural modifications). These
estimates do not include any cost to cover storage, transport and disposal of the
recovered solids and water (covered by task 7 of the JIP).
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6.
What is the energy consumption for removal (Operation)?
The subsea system requires approximately 200kW to power the ROV and the
dredge pump, this would equate to approximately 500Mw hours for lifting the
cuttings pile to an elevation of +70m.
7.
What is the condition of the seabed under a cuttings pile?
The trial did not dredge through the cuttings down to the seabed so no information
was gained on this.
8.
How do you know when removing seabed rather than cuttings i.e. can you
see at dredge head or do you need to sample slurry at surface?
The trial did not test this, however it is envisaged that there will be a change of
consistency once the seabed is reached, and bathymetry data would also provide
information on the depth through the pile that may be supplemented with coring
results. In practice to remove all the cuttings it may be necessary to remove some
of the seabed beneath the cuttings. The pre-installation bathymetry is unlikely to be
exact and there may have been seabed settlement that may complicate identifying
the interface.
9.
Can the ROV / crawler operate / be controlled reliably within the jacket
footprint?
From the trial it is clear that the jacket structure does not provide any restrictions on
access of dredging vehicles except in the vicinity of the conductors. A large tracked
system would also be able to enter the jacket footprint; however crossing the jacket
members once the cuttings have been cleared would require close consideration.
10. What are typical downtimes for weather and system reliability (i.e. debris
blockages etc.)?
ROV’s can achieve 90-95% reliability for 24 hour ops. once they have passed
through the initial settling in period. Provided sufficient spares are carried offshore
a dredging system should be able to achieve 90% reliability. This may not be
achieved if the system suffers damage from large amounts of debris or if the
cuttings are abrasive. Weather downtime is very specific to the method of
launching the equipment and the routing of hoses and umbilicals through the
splashzone. An initial estimate of around 20% weather downtime is considered
reasonable for year round operations.
11. What is the best way to clear new debris as it is uncovered by pile removal?
Working around debris as it is exposed is possible given good visibility, as found at
NW Hutton. Once debris is excavated it would be moved into a basket for recovery
to surface. Potentially this activity could be undertaken by a second ROV that is
also used for monitoring the dredging and environmental monitoring.
12. Is it possible to remove a large 'conical' pile without major slumping, if so
how?
Dredging from the side would cause slumping if properties were similar to NW
Hutton, however it is probably possible for an ROV dredging system by working
from top down depending on its properties. Tracked vehicles may not be suitable
for working at the top of the pile dependent on the pile characteristics.
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7.
APPLICATION OF SUBSEA SYSTEM TO FULL SCALE
REMOVAL
Assuming removal of a typical 25 000m3 pile from 150m of water, topsides similar to NW
Hutton.
This section only considers the Subsea Dredging Sys tem. Processing and disposal of the
cuttings and associated water is considered by Drill Cuttings JIP, Phase 2 Task 7.
7.1. Existing System
The system used for NW Hutton could be used for a full removal of a typical pile with the
jacket, topsides and platform services available. The existing system would not be
optimal but given the trial results and assuming some minor changes an ROV based
system is considered to be viable and an estimate of the time to remove a pile can be
made:-
Assumptions
•
25000m3 cuttings pile
•
Dredge rate of 100m3/hr at 1:10 solids ratio average
•
Soft non cohesive cuttings as seen at NW Hutton trial
•
System Operational 8 hours per shift
•
Disposal not considered but will lift 225000m3 of water that will require disposal
•
Learning curve, weather and downtime due to disposal (waiting on wells or boats)
not factored in.
Recovery Duration
Flow Rate
12 hour Dredging 313 days
100m3/hr
24 hour dredging
156 days
100m3/hr
For 12 hour operations the entire recovery would take in excess of 12 months allowing
for equipment mobilisation, installation, reduced productivity during the initial stages and
weather downtime of the subsea equipment or disposal system. Increasing mechanical
properties of the cuttings with depth and debris are also likely to increase the duration.
A key factor on the system used is the rate of recovery required to match the dredging
system to the disposal system for the lifted cuttings/water. A significantly higher
dredging rate could be achieved with comparatively little change to the system.
Assuming rates of 150m3/hr durations reduce significantly.
Recovery Duration
Flow Rate
12 hour Dredging 208 days
150m3/hr
24 hour Dredging 104 days
150m3/hr
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Building the various factors into the estimates it would be reasonable at these rates to
remove an entire pile in about 12 months with single shift dredging.
With the existing system used on NW Hutton weather downtime would become a
significant issue during the winter months and a different ROV launch system would be
required and potentially a different method of deploying the riser hoses would also be
required. A typical ROV crane deployed system can operate in 3m significant waves
which would have weather downtime approaching 50% in winter. Changes can be
made that would potentially allow the system to operate in conditions up to sea state 6.
Significant structural modifications would be required to allow operation in adverse
weather as ‘heavy weather deployment systems’ have high dynamic reaction loads.
The above durations do not factor in the practicalities of removing all the drill cuttings and
they assume that a constant recovery rate can be achieved. In practice as the cuttings
pile gets smaller the overall productivity is likely to reduce:-
•
The cuttings may have higher shear strength and more cohesive properties with
depth.
•
Increased debris is possible with depth from the early part of the platform life
•
Interference of jacket horizontals near the seabed.
•
Difficulty of identifying cuttings seabed interface.
In practice to recover all of the cuttings a law of diminishing returns is likely to apply and
recovery of the last 5% to 10% may be very difficult. An area of particular difficulty being
accessing the cuttings on the template and around the conductors.
7.2. Optimum System
Whilst there is no single optimal system a ‘projection’ of the optimal system can be
described based on the work to date.
7.2.1. System Capacity
The system capacity is heavily dependent on the requirements of the
cuttings/water disposal/processing system, and the dredge system could be
designed to fit the requirements of the receiving system within the limits of the
deployment system.
Whilst a system capacity of 100m3/hr (total flow) was about right for the trial and
the ROV system employed, a capacity of 150 – 200m3/hr may provide better
economics and performance for a full removal. This capacity would probably not
require a significant increase of system size or weight.
7.2.2. Dredging System
From the work at NW Hutton where the surface cuttings were very soft a ‘flying’
ROV based dredging system was ideal as it would be difficult to manoeuvre a
tracked vehicle across the cuttings surface. If the cuttings have increased shear
strength with depth (or at other locations) then a tracked vehicle may be suitable
for dredging the deeper cuttings as it may be able to apply higher reaction forces.
Alternatively a system to ‘spud’ the ROV would allow it to have higher reaction
forces.
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Dedicated equipment is required, a ROV etc. built for cuttings removal would be
smaller and simpler with full integration of dredging and ROV controls requiring
less personnel, it may have variable buoyancy, spudding system, integrated
pump. Automation of the dredging systems would be required to reduce
personnel.
Whilst a tracked vehicle has some benefits in that it potentially could react against
higher dredging forces this would not be the case on soft cuttings, where it may
sink and the tracks may have difficulty providing the traction. The optimum
system may be a flying ROV system that also has removable tracks that are
mateable subsea.
Existing tracked vehicles are all heavy and this poses problems for deployment.
They are all designed to be deployed from DP vessels by an A, frame. Adopting
them for platform deployment is possible but will require significant modifications
to some platforms.
A significant proportion of the plume generated during dredging came from
interaction of the ROV with the seabed and it’s thrusters. It is possible that the
tracks of a tracked vehicle will cause more significant disturbance.
The dredging system has to be able to handle debris and from the short duration
of the trial little experience has been gained in this area. However the view is that
the system will dredge around most larger debris and the problems are likely to be
caused by rope and wire that can be sucked through screens. A debris basket for
recovery of debris would be required.
The monitoring ROV proved to be of great use as it was able to move to view the
dredging operations from the best location. It also managed to disconnect hoses
and connect ROV operable hooks. It is considered that a second monitoring
ROV would be essential during the early phase of full recovery when the system is
going through the ‘learning curve’. A larger support ROV that can undertake more
tasks such as debris recovery would reduce the duration of the operation and
improve system performance; it is the recommended option for a second ROV.
A further development of the second ROV would be to equip it for dredging
operations either as a backup or as a second full dredging spread. This option
may be appropriate in the event that the prime dredge vehicle was tracked and
the ROV could provide support (monitoring and debris removal) and dredge areas
where the tracked vehicle could not access.
The existing combination of suction nozzles and crown cutter suction head would
cover a wide range of dredging conditions from soft material as found at NW
Hutton through to more cohesive materials tha t some cuttings core samples have
indicated.
7.2.3. Deployment System
In the event that the platform topsides are still in-situ the first choice of method of
equipment deployment is likely to be by a combination dedicated ROV crane / ‘A’
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frame and platform crane for equipment that is not routinely recovered. The
subsea equipment will require to access the entire cuttings pile which may extend
over 50m from the jacket. In order to achieve this a platform deployed system is
likely to require multiple deployment locations.
Weather downtime of the entire dredging operation is principally dependent on
deployment system and it’s weather limitations for launch and recovery of the
dredge spread. Standard ROV crane deployment systems are fairly limited from
a platform and significant weather downtime would result in winter. However a
guide wire deployed heavy weather launch system allows launch and recovery in
up to sea state 6 (4 to 6 metres significant wave height and 28 to 47 knots wind)
and would give year round weather downtime of approximately 20%. Riser hose
deployment is not routinely required during the dredging operations, however
once deployed it needs to be left in place to minimise downtime. A guide wire
system may not be adequate for protecting the hose from wear in adverse
weather and some form of conduit may be required, this might be an unused
conductor or ‘J’ tube.
Weather downtime for a vessel based operation is likely to be significant if the
platform is in place due to restriction on working alongside. If the platform has
already been removed then moon pool launched ROV’s are able to work from
vessels in up to sea state 6 and would limit downtime to similar levels to above.
Whilst a vessel deployed system has obvious cost implications and safety risks
due to the duration alongside, it may be a realistic option under certain
circumstances (this is obviously very dependent on the cuttings disposal
method):-
•
May be unaffected by platform operations that impact platform based
solution.
•
Does not require platform deck space or beds.
•
Vessels are proven for deploying large tracked vehicles.
•
Vessel can move to closest point around platform (but may have problems
reaching jacket centre).
•
Equipment mobilisation is undertaken in port.
•
May be able to facilitate larger system using higher flow for shorter
duration.
•
Ideal for a campaign moving from platform to platform as it only mobilises
once.
7.2.4. Environmental Monitoring
Environmental monitoring of a long -term recovery operation would be essential to
determine the overall impacts of the operation. It is envisaged that the monitoring
would be based on the system proven at NW Hutton and that the background
levels of resuspension (prior to the dredging operation commencing) would be
determined. The suite of sensors would be similar to those used in the trials.
The subsea equipment would be specified to monitor both secondary disturbance
and the localised impact of the dredging operations. The core of the system to
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monitor the operation would be a series of 4 or 5 minilanders located around 50
metres from the various dredging areas. To maximise the flexibility of the
environmental monitoring package, it is recommended that real-time telemetry is
used to relay data to the platform for rapid interpretation. This will allow the
monitoring package and sensors to be repositioned as required to ensure the
collation of high quality results.
Real-time telemetry would also aid the detection of biofouling or other potential
problems with the monitoring spread, thereby e nhancing the system’s efficiency.
To allow detailed examination of specific dredging activities the monitoring would
be augmented by the second ROV carrying real time monitoring equipment (such
ADCP of OBS) which could ‘fly’ into any open water plumes to obtain real-time
resuspension data.
Assuming that the observed sediment resuspension events from dredging and
associated activities were not larger than anticipated, it is feasible that as the
operation progressed, the environmental monitoring could be reduced so that
specific dredging events were targeted.
It is not envisaged that the potential effects of sediment resuspension on food
chains would be required in any future dredging operation.
7.2.5. Manpower
Based on optimising the system and integrating the operation of the subsea
dredge system with the ROV a significantly reduced manpower could be used for
a full recovery operation. Potential numbers for the subsea team offshore are
listed below. These numbers would also vary depending on the cuttings disposal
method and platform requirements.
Supervision
2
Subsea
4 – 6 (12 Hour)
Platform support
2
(12 Hour)
TOTAL
10
Say 18 person team for 24 hour cutting removal ops.
7.3. Technical Implications for other pile types
The system described here is principally for the case where there is a platform to deploy
the subsea dredging system from. In the event that there is no platform then a vessel
deployed system would be the only option for a dredging system. For a vessel based
option (with no in-situ jacket) it is possible that grabs or other systems not considered
suitable for cuttings recovery with a jacket in place would be realistic options.
In the event that the cutting properties are found to be very different from those
encountered at NW Hutton then a different type of dredging system to that used at NW
Hutton may be most appropriate.
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7.4. Schedule
Using the existing HS / PSL system a cuttings removal operation could commence within
a period of several months dependent on the difficulty to mobilise the equipment onto the
platform. In reality the need to work in poor weather conditions through the winter is
likely to significantly increase this due to the envisaged difficulty of installing suitable
systems for ROV deployment and hose protection in bad weather on most platforms.
An increased capacity optimum system is likely to involve a larger and heavier
equipment spread on the platform and would extend the schedule.
In the case of NW Hutton only a small (500kg) increase in spread weight for the ROV
system would require a complete redesign of the installation. In the event that
deployment equipment was required for severe weather then a major rethink of the
equipment layout would be required which could result in significant structural
modifications.
A realistic schedule would be 6 months or longer from project sanction to commencing
equipment mobilisation.
7.5. Financial
Based on the rates tendered for the offshore trial, factored to take into account the
experience to date a first pass high level budget estimate can be prepared for the
recovery of a full cuttings pile to surface. There are a large number of uncertainties and
assumptions required which are outlined below:
•
25,000 m
3
cuttings pile.
•
Dredge rate at 1:10 solids ratio average.
•
Soft non-cohesive cuttings as seen at NW Hutton trial.
•
System Operational 8 hours per 12 hour shift (included downtime for ‘waiting on
platform’).
•
Weather downtime assumed to be 20%, (based on ROV heavy weather launch
system).
•
Support monitoring / debris removal ROV included in costs.
•
Onshore support costs including engineering estimated at 10% of sub total based
on typical BP subsea projects.
•
Disposal not considered in this section (addressed by JIP task 7) but assume will
lift 250,000 m
3
of water.
•
Learning curve and downtime due to disposal (waiting on wells or boats)
not factored in.
•
Environmental monitoring not included. Monitoring costs are typically in the
range of 5 to 10% of the total project cost for a dredging operation (where
monitoring is required) and would probably be significanty more for a full cuttings
pile removal.
•
An allowance for minor structural modifcations is included. The cost of significant
structural modifications for subsea deployment systems are not included, these
are highly platform specific, but could be very costly in the case of deploying a
tracked vehicle from some platforms.
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•
Cost of relocating equipment around platform to access all parts of cuttings pile is
not included, platform specific but could be significant for larger footprint piles.
•
Excludes offshore support costs: personnel, catering, logistics, power
generation. In the event that production has ceased then the cuttings recovery
operation may have to bear the full cost of continued platform operations.
The cost of processing, storing and disposing of the recovered cuttings and
associated water is not included in these estimates as it is covered by the JIP task
7.
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Option 1
Option 2
Option 3
Option 4
Option 5
Dredge Rate
100m3/hr
100m3/hr
150m3/hr
150m3/hr
200m3/hr
Working Day
12 hour
dredging
24 hour
dredging
12 hour dredging 24 hour dredging 24 hour dredging
Deployment
Platform
Platform
Platform
Platform
Vessel
Vehicle
ROV
ROV
ROV
ROV
ROV
Operational Duration
(days)
313
156
208
104
78
62
31
42
21
16
Downtime (days)
weather only
weather only
weather only
weather only
weather only
Platform Modifications
(£k)
200
200
250
250
n.a.
System Preparation
(£k)
400
400
500
500
500
System Mobilisation/
demob (£k)
100
100
125
125
125
System Installation (£k)
100
100
125
125
175
System Operation (£k)
2,441
1,794
1,726
1,248
2,730
Weather downtime (£k)
488
359
345
250
546
Onshore Support (£k)
244
179
173
125
273
Offshore support costs
Not Included
except 2
dedicated platform
personnel
Not Included
except 2
dedicated platform
personnel
Not Included except
2 dedicated platform
personnel
Not Included except
2 dedicated platform
personnel
Not applicable
Budget Total
Before
Contingencies
£3,974k
£3,132k
£3,244k
£2,622k
£4,349k
Personnel
6 Subsea
3 Support ROV
2 Supv.
2 Platform
12 Subsea
6 Support ROV
2 Supv.
4 Platform
6 Subsea
3 Support ROV
2 Supv.
2 Platform
12 Subsea
6 Support ROV
2 Supv.
4 Platform
No platform in-situ
total £4.6k/day
total £8.4k/day
total £4.6k/day
Total £8.4k/day
Complete system
including vessel and
support ROV
£35k/day
Dredge Spread
£2.5k/day
Dredge Spread
£2.5k/day
Dredge Spread
£3k/day
Dredge Spread
£3k/day
Support ROV
£0.7k/day
Support ROV
£0.7k/day
Support ROV
£0.7k/day
Support ROV
£0.7k/day
Notes
Allow 25% for
increased size on
fixed costs and
dredge spread
Allow 25% for
increased size on
fixed costs and
dredge spread
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From the table above higher production rates and 24 hour working will tend to reduce
costs, however the picture for a specific location will be somewhat more complex. 24
Hour working and higher production rates both increase the cost risk as they are more
optimised solutions. Higher production rates increase day rates and potentially may
have a significant impact on the setup costs where the size/weight results in a
requirement for significant topside modifications. Production rates have to be matched to
the disposal system and this may require lower production rates in the case of re-
injection or higher rates if material is being transferred to a waiting vessel for ship to
shore. 24 Hour working reduces the available time for repairs, increased downtime cost
whilst waiting on spare, weather etc. and requires more personnel offshore.
Vessels are generally more expensive than a platform based solution, however the
difference is not as high as might be expected. Where there is a requirement for
significant topside modifications due to a lack of structural capacity or deck space a
vessel option may give the lowest cost dredging option. However the high day rate will
result in it being susceptible to overruns on the duration whether for technical reasons or
weather etc.
As a result of the accuracy of the assumptions and the uncosted items a significant
allowance should be added to these estimates. The table below incorporates an
estimate for the uncosted items listed above (significant modifications, environmental
monitoring, equipment relocation, platform support costs and general contingency).
Overall the cost of these items combined could be in the region of £1 to £2 million. With
the larger allowance probably required for the higher risk platform options (larger system
and 24 hour working). The vessel option has fewer uncertainties in some respect as the
installation of the equipment on a vessel is well proven.
There will be uncertainty in the equipment performance until a significant amount of
cuttings recovery has been undertaken. An allowance has been included in the table
below to cover 50 additional operational days to cover unforseen problems such as the
system not delivering a sufficient production rate of solids during initial learning curve,
unexpected cuttings properties, system breakdown and debris handling. A consistent 50
days has been applied for the options as the duration of these problems is not
necessarily linked to system capacity/performance.
Finally an estimate of budget accuracy has been made for each option.
With the allowances included the cost of the platform based options is very similar at just
over £5million with costs per m3 from £206/m3 to £214/m3. The vessel based option
comes in significantly more at just over £7m (£288/m3) due to it’s sensitivity to over runs
on duration.
The lowest cost solution will be highly dependent on the details of the specific location,
and the lowest cost solution for the entire recovery and disposal may not utilise the
lowest cost dredging solution. Therefore it is recommended that a budget estimate for
removal of a large mound to surface (excluding disposal) is in the range of £5million to
£7.5million or £200/m3 to 300/m3 for cuttings lifted.
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Option 1
Option 2
Option 3
Option 4
Option 5
Dredge Rate
100m3/hr
100m3/hr
150m3/hr
150m3/hr
200m3/hr
Working Day
12 hour
dredging
24 hour
dredging
12 hour
dredging
24 hour
dredging
24 ho ur
dredging
Deployment
Platform
Platform
Platform
Platform
Vessel
Vehicle
ROV
ROV
ROV
ROV
ROV
Operational Duration
(days)
313
156
208
104
78
Budget Total Before
Contingencies
£3,974k
£3,132k
£3,244k
£2,622k
£4,349k
Estimate to cover
uncosted items
£1000k
£1500k
£1500k
£2000k
£1000k
Allowance for 50
additional operational
days
£389k
£575k
£415k
£600k
£1750k
Total Budget Estimate
£5363k
£5207
£5159
£5222k
£7099
Aproximate Cost, £ / m3
214
208
206
208
£288
Budget accuracy
+ 20%/-10% + 20%/-10%
+ 25%/-10%
+ 25%/-10%
+ 25%/-10%
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8.
CONCLUSIONS
8.1. Overall
Recovery of contaminated drill cuttings from the seabed to the surface appears to be
operationally feasible and indications are that secondary pollution is relatively low based
on the equipment utilised in the trial operation. A system similar to that trialled could
theoretically be used for recovery of a drill cuttings mound to surface. (note that this
does not imply that a suitable onward disposal plan is available)
The levels of oil recorded in the dredged material suggest that previous estimates of
contamination were representative of the trial site selected. The oil in general remains
bound to the solid drill cuttings material with relatively small amounts migrating into the
dredged water.
The practical implementation of cuttings recovery on an operational platform would
appear to be applicable to a limited number of installations capable of supporting the
operation. The operation would be challenging with a significant duration, significant cost
implications and a significant impact on routine platform operations.
The direct cost of the dredging operation could be in the region of £5 to £7.5 million for a
25,000m3 cuttings pile or £200/m3 to £300/m3 to lift to the material to the surface. This
equates to over £4000 / m3 of oil removed assuming 5% contamination. Note that this
does not include transport or disposal costs, which will be significant.
Equipment mobilisation was difficult, time consuming and had a significant impact on the
platform. Mobilisation and operation of the complete system required about 40 additional
personnel on NW Hutton (including personnel for cuttings processing and disposal).
The mobilisation and trial operation was completed safely however the significant
number of additional personnel inevitably increases the overall risk profile of the
installation.
A significant number of ‘lessons learned’ have been collated that should be reviewed
before any similar operations are considered in future.
Detailed knowledge of the properties of a cuttings pile is essential to establish the correct
dredging /topsides processing equipment and technique.
8.2. Subsea Dredging System
•
The trials in general have confirmed the results of previous study work and no
significant surprises have been found.
•
A credible cuttings recovery system has been built and tested that utilises existing
subsea and dredging technology.
•
Suction dredging causes little secondary disturbance based on this trial. The
cutter suction dredging caused localised secondary disturbance.
•
The trial duration was short and there is significant scope for the equipment to
operate at higher operational efficiency, however the level of improvement is
difficult to estimate. During the trials the dredging technique varied and appeared
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to improve, the duration did not allow proper comparison between techniques or
equipment.
•
All the principal dredging components functioned well. The selected ROV based
system was an effective solution for operating and dredging close to jacket
members.
•
Peak water to solids ratios of 6:1 (by volume) were achieved during the trials and
whilst average water to solids levels were between 3% and 4% including starting
up, shutting down etc., it is considered that a long term average of around 1:10
could be achieved with the system.
•
It is apparent that the overall water to solids ratio will be increased by
inefficiencies created by debris, difficult operational sites, start up and shutdown
etc. It is essential that this is considered in reviewing options fo r, and impacts of,
cuttings recovery.
•
Following the Blyth trial, improvements were made to the system that contributed
to a significant improvement in dredging performance offshore.
•
Good visibility at NW Hutton made a big impact on improving dredging
performance compared to the Blyth trial.
•
The NW Hutton drill cuttings were found to be very soft with low cohesion
throughout the recovery trial. Suction dredging worked well on the soft cuttings it
was tested on and was the correct dredge head selection for these properties.
•
In order to correctly select dredging equipment for a particular cuttings pile, it is
important to ensure as much as possible is known about the particular cuttings
pile by coring and other survey means.
8.3. Topsides Cuttings Processing and Disposal System
•
The large volume of recovered water during a full scale recovery operation of
potentially 10:1 water to solids ratio or higher would be operationally difficult to
handle and process.
•
There are likely to be significant environmental issues associated with handling
and disposal of the solids and the large volumes of associated seawater.
•
The system used was suitable for the Project purposes.
•
If a large-scale operation were contemplated, the system would need to be re-
evaluated.
•
The selection and preparation of the injection wells is critical to the success of the
process. In a large-scale operation, considerable effort would be required to
prepare (workover) the wells.
•
It appears the levels of LSA and H
2
S in the recovered material were negligible, but
contingency procedures must be in place.
•
Detailed pre-project planning of the system management procedure is very
beneficial.
•
Solid / liquid gravity separation appears to be relatively rapid and efficient given
adequate storage and residence time.
8.4. Environmental Monitoring
•
The environmental monitoring results confirm the ROV visual results which
included low levels of secondary disturbance.
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•
Contaminant analysis confirmed that the background levels of barium and total
hydrocarbons were not significantly increased in the Aqualander seawater
samples. Levels of APEs and barium were higher in the recovered water at the
platform topsides. LSA levels were low and similar to background levels. There
was no detectable oil in the plumes generated during the trial.
•
The environmental monitoring and associated results have advanced
understanding of the potential for drill cuttings recovery operations to cause
secondary pollution.
•
The level of environmental monitoring for full cuttings recovery would require
careful evaluation to balance the level of coverage with technical difficulty,
particularly with regard to long-term impacts.
•
The oil in the cuttings pile appears to remain principally attached to the cuttings
with relatively little migrating into the lift water during dredging. The low level of oil
in the associated dredged water may be associated with the high ratio of
recovered water to solids.
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9.
REFERENCES
9.1. BP
1. Offshore Trial Drawings.
2. Offshore Trial Dredging System; Operational Graphs.
3. Drill Cuttings Trial Review Meeting Minutes, 21 August 2001.
4. Offshore Trial: Operational Procedures Document, Rev 2 28.06.01.
5. Offshore Trial Bridging and Interface Document, Rev 3 25.06.01
6. Video Highlights of Offshore Trial.
9.2. CEFAS
7. CEFAS Contract Report on NW Hutton Drill Cuttings Recovery Project – Environmental
Monitoring, August 2001.
9.3. Delft Hydraulics
8. Research ROV Cutter Suction Dredge, by Lourens Aanen, Walther van Kesteren, Dick
Mastbergen, WL Delft Hydraulics, February 2001.
9.4. Euro-Seas Engineering Solutions & Testing Ltd (Blyth Dry Docks)
9. Report On Onshore Trial Results – August 2000, Rev 0 28.12.00, Document No.: ES/WT/005.
9.5. Halliburton Subsea
10. Phase 1 Trials Report – Blyth, Rev B 13.12.00, Document No.: AB-T-JR-00927.
11. Wet FAT (Tank Trials) Report , Rev A 11.06.01, Document No.: AB-T-JR-01003.
12. HSE Management Plan, Rev 1 29.03.01, Document No.: AB-T-MD-01243.
13. Hose and Deployment Procedure, Rev C 01.07.01, Document No.: AB-T-PR-01683.
14. Dredging Pump & Umbilical Deployment & Recovery, Rev B 23.05.01, Document No.: AB-T-
PR-01684.
15. Installation and Load Test Procedure, Rev B 30.05.01, Document No.: AB-T-PR-01685.
16. Operating Procedures for Project Phase 2 Offshore Cuttings Recovery Trial, Rev A
Document No. AB-T-PR-01686.
17. Operating Procedures for Project Phase 1 – Blyth Cuttings Recovery Trial, Rev A Document
No. AB-T-PR-01687
18. Phase 2 Offshore Trials Report, Rev C, Document No. AB-T-JR-01016.
9.6. PSL
19. Report on Blyth Test PSL Pump and VSD, Rev 02, Date: 19.10.00, by PSL Services AS.
20. Project Handbook Trial Pumping of Drill Cuttings, NW Hutton - Phase 1 & 2, Document No
241.1.127 Rev. 04, by PSL Pipeline Process Excavation.
21. Report on NWH Offshore Trials PSL Pump and VSD. Proj No 241.1.161 Rev 02.
22. PSL Pump Datalogger Output, Excel Spreadsheet.
9.7. SWACO
23. Operating Procedures, Rev 02 07.06.01, Document No. 0342.
24. Operational Report for the Drill Cuttings Mound Processing Offshore Trial onboard North
West Hutton. Report No. 0378.
25. North West Hutton Sample Analysis. Procedures and Results
.
9.8. Wood Group Engineering / JP Kenny
26. Drill Cuttings Mound Recovery to Topsides Facilities, Rev A 29.12.99, Document No.
06.1781.01.U.3.002 (JPK No.)/ 37E034F0003/SS/RE/0003 (WGE No.).
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10. ABBREVIATIONS
ADCP
Acoustic Doppler Current Profiler
APE
Alkyl Phenol Ethoxylates
CTR
Cost Time Resource
CEFAS
The Centre for Environment, Fisheries & Aquaculture Science
DMA
Dead Man Anchor
DNV
Det Norske Veritas
DRL
Dredging Research Ltd
EEST
Euro-Seas Engineering Solutions & Testing Ltd
FAT
Factory Acceptance Test
HPU
Hydraulic Power Unit
HS / HSS
Halliburton Subsea (Services)
j.b.
Junction Box
JIP
Joint Industry Project
JPK
JP Kenny
LSA
Low Specific Activity (Scale)
MDU
Mains Distribution Unit
OBM
Oil Based Mud
P.A
Public Address
PSL
Formerly known as AGR Services AS
ROV
Remote Operated Vehicle
SAEP
Senior Authorised Electrical Person
SIMOPS
Simultaneous Operations
SME
Small Medium Enterprise
SRG
Scientific Review Group
TMS
Tether Management System
TOFS
Time Out for Safety
VOC
Volatile Organic Compound
VSD
Variable Speed Drive
WGE
Wood Group Engineering Ltd
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11. APPENDICES
12. APPENDIX 1 – OFFSHORE TRIAL DRAWINGS
The Following Drawings are included in Reference 1 ‘Offshore Trial Drawings’
12.1. NW Hutton – Topside Production / Drilling Deck Layout SSA-0322-D-
0001-00
12.2. WGE Flow Diagram for Offshore Trials SK-37E-34G003-010 rev D
12.3. SWACO Rig Plan Layout Cuttings Mound Processing and Disposal
System - Drawing ARQ 1143 012
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13. APPENDIX 2 – SCHEDULE
13.1. Key Project Date s
25/04/00
JIP agreed to tender evaluation of subsea system
with option for second pump.
July 00
Commence preparation of Blyth site for trials
20/08/00
Commence trials at Blyth
24/08/00
Complete trials at Blyth
August 00
Trial fit of support steelwo rk for Subsea equipment
offshore on NW Hutton.
August 00
SWACO Equipment FAT
4/09/00
JIP Agrees to postpone offshore trials to 2001
13/12/00
Delft Hydraulics Study Kick Off Meeting
17/01/01
Delft present study results
06/02/01
Present Delft results to JIP and recommend that go
straight to offshore trial with only onshore FAT.
06/03/01
JIP agree to offshore commencing on 1 July 2001
May 01
HS Equipment FAT (2
nd
to 4
th
and 24
th
May)
11/6/01
Long term minilander deployed
15/6/01
Final components of subsea system at BP supply
base
16/6/01
Main equipment packages start arriving on NWH
2/7/01
Dredge ROV test Dive
8/7/01
Test run Warman pump – deliver sea water to
SWACO unit
11/7/01
Commence Dredging trials
14/7/01
Complete Dredging Trials
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13.2. Offshore Trial Schedule
ID
Task Name
3
Ship Equipment Offshore
4
Install HS and PSL equipment on NWH
5
Commission HS and PSL equipment on NWH
6
Dredge ROV Test Dive
7
Deploy subse hoses, pump etc.
8
Perform offshore trial
9
Recover Subsea Equipment
10
Demob subsea equipment
11
Topsides Processing
12
System Installation
13
Carry out installation
14
Process system commissioning
15
System Operation
16
Perform offshore trial
17
System shutdown, clean, decomm.
18
Perform teardown
19
Environmental
20
Install Longterm Minilander
21
Deploy and test environmental equipment
22
Deploy Aqualanders and monitor ops with Eyeball ROV
23
Wood Group - Preparation for Phase 2
24
Prepare utilities & install temp piping on NWH
25
Install steelwork for HS system
26
Assist with equipment hookup
27
System Commissioning
28
Remove Temporary Equipment
02/07
Perform offshore trial
Perform offshore trial
M T W T F S S M T W T F S S M T W T F S S M T W T F S S M T WT F S S M T W T F S S M T W T F S S M T W T F S S M T W T F S S M T
04 Jun '01
11 Jun '01
18 Jun '01
25 Jun '01
02 Jul '01
09 Jul '01
16 Jul '01
23 Jul '01
30 Jul '01
06 Aug '01
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14. APPENDIX 3 – PROJECT ADMIN
14.1. JIP Task 6 CTR – ‘Agreed Scope of Work’
Title
Pilot Lifting Operation
Task 6
Objective
1. Verify and define removal systems and technology currently available.
2. Perform system test to confirm operability.
3. Implement an offshore test of equipment to determine the operating
parameters, feasibility and cost of cuttings (OBM) retrieval.
4. Understand the short and longer term environmental impacts of pile cuttings
disturbance.
Method
Using North West Hutton as a suitable location for the pilot programme the
following will be conducted (N. W. Hutton has been selected due to its availability
and the expected timing of full decommissioning):
1. Engage contractor market to determine availability and costs of cuttings removal
systems
2. Develop equipment and conduct a “wet” trial of up to four systems and select
the two most efficient (and effective) for trial offshore.
3. Implement an offshore trial of max. two systems at N. W. Hutton to test recovery
efficiency of cuttings in and around jacket footings and debris. Obtain all
necessary data and samples. 2 X 24 hour recovery duration trials are being
targeted which will remove up to a maximum of 900m
3
(plus recovered sea-
water) of cuttings (Approx. 3.6% of the total).
4. Dispose of the material generated in an environmentally responsible manner.
(Base case is to re-inject the cuttings and all associated fluid).
5. Implement a thorough environmental monitoring programme to assess the
impacts of disturbance (An option could be to have the environmental
monitoring listed as a separate item to separate the hardware issues from the
environmental issues).
6. Conduct a thorough evaluation of all results for application to full-scale removal.
Timescale
9 months
Inputs/
outputs
Prior BP study on hardware potential (Note BP considered the ability to safely
execute such an offshore trial in summer 2000 as ‘go’/ ‘no go’ criteria for being
included on the bid list. Phase I knowledge, info from area (2) (3) and
comprehensive reporting requirements on:
1. Details of the current availability and effectiveness of cuttings lifting equipment
2. Operating parameters obtained during the “wet” trial
3. Information on equipment performance offshore and comparison
4. Cuttings pile behaviour under removal conditions
5. Assessment of the environmental impact of disturbance (useable in any
comparative assessments)
6. Details of the disposal operation
7. Data for correlating plume (component) dispersion modelling
8. Accurate assessment of the technical and financial implications of complete
recovery
9. Removal at the edge of the pile, to ascertain ‘when to stop’ and how quickly the
seabed recovers
Other issues
1. Interface between JIP and BP as operator, particularly w.r.t. Gateways for
equipment selection and decisions to award, selection for offshore trial and
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readiness to go offshore
2. Slurry/water samples for onshore trials/tests (7) & interfaces with area (7)
3. Acceptability of data format for modelling (4)
4. SRG input to monitoring programme
5. Other types and sizes of piles
6. Site restoration (possibly for scope of areas 2 or 3)
7. Marine Laboratory tests on single well site (May/June)
Checklist of
questions to
be answered
1. What is the typical rate at which cuttings can be removed from the seabed?
2. What is the range of composition of the slurry as it arrives at the surface?
3. What determines the ratio of solids to water and how can it be minimised?
4. What is the degree of secondary pollution (i.e. water column impact and area/
thickness of cuttings re-settlement) due to re-suspension caused by the dredge-
head and the ROV/ crawler? What is the environmental impact of secondary
pollution?
5. How much would removal to surface cost?
6. What is the energy consumption for removal?
7. What is the condition of the seabed under a cuttings pile?
8. How do you know when removing seabed rather than cuttings i.e. can you see
at dredge head or do you need to sample slurry at surface?
9. Can the ROV/ crawler operate/ be controlled reliably within the jacket footprint?
10. What are typical downtimes for weather and system reliability (i.e. debris
blockages etc.)?
11. What is the best way to clear new debris as it is uncovered by pile removal?
12. Is it possible to remove a large 'conical' pile without major slumping, if so how?
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14.2. Project Organisation
The organisation offshore during the trial is shown below. The onshore project organisations
are shown in Reference No. ?? – Offshore Trial Bridging & Interface Document, Rev 3
25.06.01. section 2.
Offshore Operational Phase
Drill Cuttings
Project Manager
OTL
Maint. Supv.
Swaco
Technical Advisor
MI
Mud Engineer
SWACO
Cuttings Processing
AFOS
Bulk Chemicals
Deutag
Drilling / Pumping Equipment
Wellserv
Wellbay equipment
Team Leader
Cuttings Disposal
WG Personnel
Trades inc. piping, rigging
E &I
(Platform Support
WG Supv co-ords
project requirements)
WG Construction Sup.
Echoscope
Sonar system
Halliburton Subsea
Subsea dredge system
HS (ROVTECH)
Eyeball ROV
CEFAS
Env. Monitoring
PSL / AGR
Pump & VSD
Team Leader
Subsea
BP Offshore Project Mgr
NWH OIM
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15. APPENDIX 4 – SUPPORTING INFORMATION
15.1. The Key Elements of the Tender Scope of Work
1.
DESCRIPTION OF THE MOUND
The mound is estimated to have a volume of 25,000 cubic metres and is known to envelop the whole of
the bottom bracing level of the jacket structure, the subsea drilling template and a second level of central
jacket framing which surrounds the template. The bottoms of the jacket pile sleeve clusters and the
bottoms of several risers and 'J' tubes are also known to be covered.
The mound is some 6 metres deep at the central area under the platform and runs out about 6 metres to
the north and about 40 metres from the south, east and west sides of the jacket base. The seabed
elevation is -144 LAT metres.
The drill cuttings mound can be described as a pile of drill cuttings from drilling operations containing scrap
debris (such as, scaffold poles, clips, brackets, grating, cable trays, wire rope slings, hand tools, piping and
pipe fittings, nuts and bolts, steel plates and sections, heavy hoses, protection sheeting etc.) with some
sand from the process separators and possibly some layers of hardened cement.
Supplementary information regarding the physical and aspects of the cuttings mound and a set of
drawings of the mound and its location relative to the NW Hutton structure, are included in Section IX.
2.
REMOVAL SYSTEM
It is envisaged the selected system will have the best combination of the following attributes:-
•
The system will perform the task safely without harm to personnel, equipment and facilities.
•
The system will perform the task with a minimum of environmental disturbance.
•
The system has been optimised for deployment from the platform defined location.
•
The system can manoeuvre to both outer and internal areas of the mound. Can access and
manoeuvre within the bottom plan framing of the structure and appurtenances. Or, can direct
dredge and debris tools to all necessary locations of the mound to perform removal.
•
The system can perform the removal dredging and pumping of the mound materials and manage
the removal of random and mixed debris.
•
The system can perform the task efficiently and continuously, interfacing with topsides treatment
and re-injection process.
•
The system is commercially economic.
3.
THE WORK
The cuttings and other materials are to be recovered from the seabed at -144 meters LAT by a diverless
system with minimum secondary disturbance and delivered to the +57 metre platform level drilling mud
gutter where the cuttings will be received for secondary processing, by others, before re-injection, by
others, into redundant wells.
Debris materials are to be recovered with minimal secondary disturbance to the platform topsides.
Shipment to shore and disposal will be undertaken by others.
More information is required on the following areas that will be evaluated from the Phase 1 and Phase 2
trials:-
•
Dredge head/s
•
Pump/s
•
Debris handling
•
Vehicle/s
•
Removal and recovery system integration
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•
Removal and recovery system, treatment, re-injection scheme integration. (this will include others)
The recovery, treatment and re-injection scheme is required to operate in a co-ordinated and combined
manner.
The work will be divided into phases:
3.1
Phase 1 - Onshore Trial
The results of this work will provide further physical knowledge, performance measurements and
compatibility information for the various system components, before taking them to the offshore
environment. The data provided will be subject to evaluation and comparative assessment to select the
optimum systems.
The aim of Phase 1 is to test available dredging equipment, in order to assess performance and
compatibility between dredge heads and pumps in a simulated seabed with associated debris. This trial
should also demonstrate the lifting of mud and debris to the equivalent height of the NW Hutton mud
handling deck above sea level.
In order to prove a suitable system the CONTRACTOR should simulate the complete package proposed
for the (Phase 2) works and demonstrate the capability of the overall system, vehicle (ROV or tracked) to
manoeuvre, showing reach, dexterity, and capability to access all areas of NW HUTTON.
Provide capability to monitor entire operation in zero visibility.
The CONTRACTOR will provide detailed procedures prior to mobilisation covering but not limited to:
•
HSE plan
•
Mobilisation and Execution
•
Organisation chart, interfaces and roles and responsibilities
•
Communications
•
Budgets, cost control, reporting and forecasting
Objectives
•
Complete Operation safely, remotely and diverless
•
Produce minimum plume
•
Dredge head to remove up to a maximum of 20 m3/hr of solids at an optimum ratio 3:1 water to
solids
•
Dredge head and hoses to handle debris up to 8” diameter and handle/recover from blockages
•
Dredge head to be capable of operating 50 metres from pump or vehicle
•
Pump to lift cuttings and Debris from -144 metres LAT to head height of +57 metres
•
Capability to remove and handle large debris for recovery at a later date
•
Reliability during operation
•
Monitoring operation in zero visibility
•
Compatibility / Operability of equipment
•
Manoeuvrability of vehicle with dredge head in operation
The preferred location for the trial is a (COMPANY supplied) dry dock with tower crane. The mound will be
simulated by suitable material in the dock and water filled, with the crane providing means of deployment
and hydrostatic head.
The duration of the Phase 1 Trial should not exceed 5 days at the trial site.
3.2
Phase 2 - Offshore Trial
The results of this work will provide further physical knowledge, performance measurements and
compatibility information for the system in the offshore environment and working in the full integrated
scheme. The data provided will be subject to evaluation and comparative assessment to select the
optimum systems.
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Based on the lessons learnt in phase 1 the CONTRACTOR will prepare detailed procedures for completing
the phase two works from the NW Hutton platform. This should include and not be limited to the following:
•
HSEMS interface document and HSE plan
•
Simultaneous operations
•
Details of any platform modification if required
•
Equipment layout, lay-down areas, weights, sizes, and power / service requirements.
•
Details of proposed dredging areas to demonstrate feasibility of completing phase three
•
Equipment reliability expectations.
•
Complete RISK Assessment for the entire operation.
•
Emergency recovery procedures of all equipment.
•
Plume containment and monitoring.
•
Detailed schedule
•
Request list of Client supplied items or services.
Objectives
•
Safe Operation
•
Minimise Environmental Disturbance
•
Installation of all equipment within budget and schedule
•
Achieve recovery targets (to be agreed between the COMPANY and the CONTRACTOR before
Phase 2 mobilisation)
•
Remove cuttings and any Debris
•
Access and manoeuvrability to all parts of the mound
•
Delivery to surface in required quantities ( water / cuttings ) ratio.
•
Confirm the feasibility of complete mound removal within environmental guidelines
The scope of the trial at NW Hutton will be limited. The duration is planned to be two to three operational
days for each system. Limited material will be recovered to pilot an integrated process. The integrated
process includes, the Contractors recovery system, the topsides treatment and re-injection by other
contractors. The area for the trial will generally be the east face area as shown on mound drawing 06-
1781-SK-V -0-015 Rev 02 Plan on Horizontal framing - Phase 2 area. (Section IX Documents and
Drawings).
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15.2. Success Criteria for Phase 1 Trials at Blyth
Amended to indicate trial outcomes following post trial review meetings on 29 August
Where applicable each pump will be considered separately
SUCCESS
Grey Zone
FAILURE
Comments
1. Trial Safety
No Accidents or incidents
Any accidents
One minor near miss with
EEST operation
2. Dredge
Equipment
Safety
No accidents or incidents
caused by or associated with
the equipment
Avoidable incidents due to
unfamiliarity of equipment
Any accidents caused by or
associated with the equipment
3. Performance
The system performance is
fully quantified (within the
limitations of the trial site) so
that what it can / can’t do is
clearly understood
System performance irregular Do not understand
equipment’s capabilities at
end of trial
Do not understand the
parameters that affect the
solids ratio
4. Head
Pump produces simulated
Blyth trial head (approximating
to -144m to +63m) at design
flows (50 to 100 m3/hr)
Pumps achieve simulated
Blyth trial
head with less than
50 m³/hr
Pumps fail to reach
simulated Blyth trial head
with any flow rate
Both pumps produce full
head with capacity in
reserve (PSL to confirm
this)
5. Seabed
Types
Dredge systems (i.e. both
pumps) can excavate all
seabed types tested at Blyth
(with cutter head)
Either dredge system has
significant limitations in it’s
ability to excavate soils other
than the softest
Both dredge systems can only
excavate softest seabed tested
at Blyth (with cutter head)
Difficulty excavating clay
material. Intake appears to
block, 1
st
sample always
showed best result. Clays
offshore may have
significantly different
properties - likely to be non-
reactive
6. Debris
Dredge systems can lift or
leave in situ all debris types
tested at Blyth without blocking
(with or without cutter head)
Dredge systems have trouble
handling some types of
debris.
Dredge systems have trouble
handling all types of debris.
Soft rags and boot caused
problem to rotating head.
7. ROV
ROV operates without difficulty
and can handle the dredging
ROV has periodic difficulties
that may be overcome with
ROV fails to hold station
while dredging and has
Needs to be trimmed
correctly
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and can handle the dredging
system
that may be overcome with
practise
while dredging and has
difficulty manipulating
dredge head
correctly
8. ROV
ROV can manoeuvre with
dredge system connected
ROV can manoeuvre with
difficulty...
ROV cannot manoeuvre with
dredge hose connected
ROV can handle system
once correctly trimmed.
ROV had difficulty flying
when dredge skid was
blocked (probably linked to
a hose snag) - emergency
recovery procedure
required
9. Ratio /
Control
Good water to solids ratio -
better than 6:1, flow rate and
ratio can be controlled within
20% of desired
Water to solids ratio in range
6:1 to 9:1, flow rate and ratio
can be controlled within 50%
of desired
Water to solids ratio too high to
be effective >10:1, cannot
control flow rate or ratio.
Best test result of 20:1 on
sand (from lagoon results).
Flow rate could be
controlled. Ratio Needs to
be much better.
10.Ratio /
Control
Good control of water /solids
ratio, can maintain a steady
ratio
Significant fluctuation in
solids water ratio.
Solids water ratio varies widely
and cannot be controlled
Could not control ratio
11.Deployment
Whole system can be
deployed quickly / easily with
little anticipated difficulty for
offshore trial
Awkward deployment that
could lead to reduced
operating opportunity at NW
HUTTON.
Difficult to deploy system at
Blyth or significant difficulty
envisaged for deploying
equipment offshore
Not Tested at Blyth due to
visibility
12.Deployment
ROV can connect/disconnect
hoses with little difficulty
ROV can only
connect/disconnect hoses
with difficulty - a significant
duration would be used
offshore each day on this
activity (2 hours or more)
Unable to make hose
connections subsea.
Not fully tested due to
visibility.
ROV - Camlock connector
would only release with
difficulty.
13.Blockages
System can be operated so it
does not get blocked
System suffers blockages but
dump valve works and
system can restart dredging
without equipment recovery
System gets blocked and
requires equipment recovery
unblock
Dump valve functionality
was successfully tested,
system had difficulty
recovering from a fully
blocked (sand) suction hose
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- this may have been
partly/totally due to a kinked
hose.
14.Plume
No significant plumes
produced in any configuration
(motor or suction head)
Plume size is minimised in
certain configurations, while
others results in significant
plumes
Significant plume produced
in all set-ups and
configurations (HOLD
dimensions of significant
plume)
15. Start - Stop
System can start up, stop and
restart readily, back flush
system not required, creating
only small plumes even when
back flush is required
System can start up, stop
and restart readily even after
emergency stop, back flush
system is required; plume is
created (Hold max. Size)
System has trouble starting or
restarting, significant plume
created
16.Reliability
System proves highly reliable
with full availability each day
for last (2) days of Blyth trials
and no major equipment
damage or wear.
Reasonable reliability
achieved by end of trials, up
to ½ hours lost per day, only
limited equipment damage or
wear
Significant amounts of
equipment unavailability /
breakdown throughout trials,
reducing system productive
time be 3 or 4 hours per day,
significant wear found at end
of trial.
System proved reliable,
only issue was that a bolt
came loose on Warman
bearing housing - not
applicable to offshore as
this is not the bearing that
will be used.
17.Motors
No problems with electric
motors
Some motor problems that
may not be just teething.
Electric motors fail to run
pumps under load or
overheat
Performed exactly to design
18.Plume Size
Small plume less than 4
metres from dredge head / not
visible on surface
Plume 4 to 8 meter from
nozzle / just visible on
surface
Plume fills dock and is
clearly visible on surface.
(This may be the only
observable plume size due to
dock visibility limitations)
None from dredge head
19.Visibility
ROV will operate and dredge
in zero visibility
Sufficient visibility returns
quickly on interrupting
dredging that only slightly
reduces overall system
efficiency.
Zero visibility prevents
operator using dredge head
20.Visibility
Echoscope monitoring system
works and is aid to dredging
Echoscope system works but
does not assist with dredging
Echoscope system does not
work or not suitable for
Echoscope system proved
useful as pilot aid and for
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works and is aid to dredging
operation
does not assist with dredging
operation
work or not suitable for
offshore trials
useful as pilot aid and for
seabed mapping.
21.Modifica -
tions
Minor modifications only
required before mobilisation to
NW HUTTON
Modifications required but no
impact on offshore schedule
Major modifications required
before offshore mobilisation
requiring additional time.
Schedule delay required if
further onshore testing of
mods to cutter head are
required
22.Time
Trial completed within planned
schedule
Trial starts late / overruns by
up to 2 days
Late start / overrun by 3 or
more days
23.Budget
Trial Completed within budget
Budget over run less than
10%
Budget over greater than 10%
24.
All engineering calculations as
expected
Operation of systems needs
a rethink.
Operation of systems is not as
expected by design calcs.
25.Trial Site
Trial site performs to design
and does not impact / hold up
trials
Minor problems with trial site
and only minor hold ups with
equipment trials
Trial site does not perform as
expected and has significant
impact on equipment trials
26.Trial Site
Trial site creates reasonable
simulation of NW HUTTON
trial
Simulation accuracy not
known.
Trial site is perceived to have
significant limitations
compared to NW HUTTON
(that weren’t appreciated at
commencement of trial)
Within known limitations of
the visibility, water depth
and simulated cuttings
using clay.
Bold = Critical Factors - Project review within BP and with Steering Committee before proceeding to offshore
Other Issues Identified from Trials:
•
Flow meters: doppler meter ineffective at solids levels tested, Panametric meter worked and available in intrinsically safe version
for offshore
•
Control System: worked well, motor variable speed drive allowed good control of flow rate
•
Team work: HS, PSL and Omnitech all worked as a team. Having all equipment in ROV control cabin was invaluable.
Equipment Actions Identified
Importance
Timescale
1. Change suction hose size, to 5” or 4”
High Priority/ Nice to have
3 weeks
2. Hose Buoyancy and possible ROV buoyancy
Essential
2 weeks
3. Modify Suction Head. Shroud, depth stop, flow path
Essential
4 weeks
4. ROV connectors (CAMLOCK)
Essential
3 weeks
5. Pump - Hose Connectors (inlet PSL connector and outlet)
Essential
2 - 3 weeks
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6. Gratings on Pump Frame
Essential
1 weeks
7. Pressure Transmitter
Nice to have
2 weeks
8.
Wet FAT to prove modifications (HS test tank)
Essential
additional 1 week
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15.3. Post Blyth Trial Review of System with Dredging Research Ltd.
Following the Blyth trials the results were reviewed with DRL and HS to identify what
worked and did not work and how to best improve the system.
DRL were well placed to provide this assistance as they are a specialist dredging
consultancy tha t were working on other JIP tasks (5b) and had also worked on Task 6.1
of the JIP Phase 1.
The general consensus from all parties was that some modifications should be made to
the system, followed by a second onshore ‘proving’ trial to demonstrate that these
modifications contributed to an improved system performance. (The options for further
onshore testing are discussed more fully in section 4.1). DRL believed that with these
modifications the system could achieve solids to water ratios in the acceptable range.
Areas of the dredging system that were discussed as potentially requiring changes to
improve the overall system performance included:-
Dredge Head – Caged Suction Nozzle
The caged nozzle worked acceptably on the first trial on sand until the hose became
blocked. However, the nozzle had been pushed deep into the sand mound and this may
have contributed to the suction hose becoming blocked.
The actual cuttings pile is likely to present a combination of granular materials mixed with
clays and othe r components, and is likely to have low cohesive properties, then it is likely
that a plain caged nozzle would be used for much of the dredging. However, the open
surface area of the nozzle was large when compared with the area of the suction intake,
and some modification to the nozzle should be considered to maintain high inlet
velocities.
Dredge Head - Cutter
The agitator head requires modifications so that the flow of water and materials
continually passes through the head into the suction line, rathe r than the cutter head
being mounted adjacent to the intake. In this way the likelihood of the head becoming
clogged with cohesive materials is reduced, as there is a continual washing effect from
the water passing through the head.
The cutter design sho uld be modified to a crown - cutter type, designed to continually
remove slices of material from the surface of the pile and deposit them into the path of
water flowing into the mouth of the suction intake.
The drive to rotate the head should be set to between 30 and 40 rpm, and the coupling
must be designed to prevent hard objects from becoming jammed in the rotation
mechanism and causing the head to stall.
Suction Hose / Hose Management
The suction hose bore was too large and must be reduced in order to maintain sufficient
velocity throughout the system to prevent settling.
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The suction hose should be fitted with buoyancy so that it is almost neutral when flooded.
The ROV connection end of the hose must be neutral when flooded to assist the ROV
manipulator in engaging the hose into the skid receptacle.
Suction Hose Connection (ROV skid)
The suction hose connection must be modified so that there is a greater degree of
compliance between the male and female connectors when engaging and disengaging.
This will be achieved by machining a new male connector with a reduced diameter
behind the cam groove, so that there is less horizontal contact between the bodies when
mated. At the same time the female cams will be machined so that they are retracted
within the female body when the cam arms are in the disengage position.
The new male will incorporate a ring collar. A second set of hydraulic cylinders, enabled
from the disengage function and the failsafe accumulator circuit; will act on the male ring
providing a forced separation of the connector halves.
Suction Hose Connection (Pump inlet)
The PSL connection will be modified (by PSL) to incorporate a face seal to reduce the
possibility of water intake at this point.
Discharge Hose Connection (pump outlet)
The discharge hose connection at the dump valve spool and hose lower pump piece will
be modified to incorporate an open-framed bucket and cone arrangement. This will be
used firstly to guide the hose end (male) into the pump outlet (female). Once mated the
guide will also hold the connectors together while the ROV throws the cam arms into the
‘lock’ position.
Control of Dredge Head
A contributory factor to the low solids ratios throughout the trial was the low visibility, and
the lack of feedback on the position of the dredge head to the operator of the dredge
arm.
Maximum efficiency of the system is dependent primarily on positioning of the dredge
head (whether the nozzle or the crown cutter is fitted). The possibility of maximising
solids collection by feedback of information to the dredge arm operator is to be
investigated, and the following devices / systems will be considered.
Vacuum Gauge
Fitting of a vacuum gauge, at (or close to) the dredge head, would allow the operator to
determine whether the head is fully immersed in the pile. This will allow the operator to
raise the head when necessary to reduce the risk of plugging.
Echoscope
The Echoscope sensor proved extremely useful in navigating to the sample areas in zero
visibility, and may prove to be capable of accurately calculating volumes of recovered
material. However, it had only limited success in monitoring the arm during dredging.
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Dredge Head Guide
A simple alternative solution to maintaining the head at the correct position in the mound
(i.e. partially immersed) may be to incorporate some form of plate into the dredge head.
The plate would prevent the cutter head from being completely immersed in the mound,
ensuring through flow of water past the head and thus helping to prevent the head from
clogging up.
To further the system development DRL used their experience from Task 5b to develop a
revised range of soil types that would encompass the expected NW Hutton drill cutting
properties. The Task 5b results used drill cuttings core samples taken specifically for the
JIP and of which a carefully controlled geo-technical analysis was undertaken under
DRL’s guidance to best identify the drill cuttings key dredgibility parameters. 16.4
contains the DRL guidance notes on selecting soil types for further testing and was used
by DRL as part of their work.
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15.4. Delft Hydraulics Study of Dredge Head Performance and ROV
Compatibility
Delft Hydraulics are the worlds leading dredging research institute with an extensive
track record of developing and testing dredge heads utilising their test facilities. The
experience of these tests has been built into numerical models that allow them to predict
the performance of a dredge head under prescribed conditions. Whilst Delft Hydraulics
has the capability for undertaking dredge head trials in their existing facilities they
recommended that they undertake a desk study prior to any dredge head trials. The
options for further onshore testing are discussed in section 4.1.
Delft Hydraulics has the capability for testing a cutter suction dredge head in their flume
tank facility. The purpose of a testing programme would be to confirm that the proposed
dredge head can achieve the required dredging performance (primarily solids to water
ratio) under conditions that simulate an ROV.
The desk study’s scope was to review the available data (cutting properties, ROV and
dredge system specifications) against the required dredging performance (solids ratio,
flow rates, no plume etc.). The Delft numerical models were then used to estimate the
forces and torques required to meet the dredging performance for both a simple suction
nozzle and a cutter suction head. To allow for the limitations of the input data and
numerical models the work included a sensitivity analysis to look at the impact of varying
key parameters. Limitations of the numerical modelling include: variations in cuttings
properties, accuracy of ROV force estimates/ROV limitations, differences between ROV
and large scale dredging operations, and the accuracy of the model itself.
The study commenced with an investigation of the soil parameters of several drill cutting
piles. The rheological parameters of a soil sample originating from Beryl A were studied
in more detail. Based on the results of the soil parameters an estimation was made of
the forces and torques on a Breebot crown cutter used to remove the cutting piles. The
forces during suction dredging were also estimated.
The water-soil ratios for several operation conditions of the ROV combined with the
Breebot cutter were then calculated.
Results and Conclusions
The Delft Hydraulics work estimated that the ROV dredging system when equipped with
a crown cutter (Breebot) is able to dredge the cuttings at ratios better than 10:1. Delfts
key findings and recommendations (Section 9.4 Reference No. 14 – Research ROV
Cutter Suction Dredge, by Lourens Aanen, Walther van Kesteren, Dick Mastbergen, WL
Delft Hydraulics, February 2001 for full report):-
The geo-technical information supplied by BP/DRL shows a large variation in material
properties of drill cuttings from several piles, especially in grain size distributions. All
distributions clearly show an artificial mixture of different original distributions in the silt,
sand and gravel range, which is typical for rock cutting debris. From only a few locations
plasticity limits were determined. They reveal however, together with the grain size
distributions, that the mineral composition of the clay fraction can vary tremendously from
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location to location and probably also within a pile. The mineral composition of the clay
fraction is determining the cohesive behaviour.
Cutting forces and torques were calculated for the 1:8 scale serrated edge cutter
“Breebot”, which has also been tested in the past in the dredging flume. Given the
“cohesive” behaviour of the drill cuttings, the calculations were performed for clay with an
undrained shear strength of 10 kPa. The calculated forces and torques are almost linear
with the undrained shear strength. The resulting maximum cutting forces on the ROV are
less then 0.12 kN and therefore much lower than the design level of 2 kN. The maximum
torque is less then 0.022 kNm, which is less than 6% of the available torque of 0.35 kNm.
In the calculations ductile failure is assumed. In reality, it is to be expected that cracking
might occur, especially when the sand/gravel content exceeds 60%. In that case, cutting
forces and torques will be reduced. However dynamic fluctuations can be amplified
depending on the natural frequencies and damping characteristics of the ROV. Because
the ROV is force-controlled, amplification is expected. This could be reduced by
acceleration control of the ROV. It is recommended to perform an experimental
simulation in order to improve the dynamic response of the ROV.
The applicability of a stationary suction pipe for hydraulic excavation of the drill cuttings
is limited because of the cohesive behaviour. Outside the erosion pit, the maximum flow
area is limited by the undrained shear strength. Give n an undrained shear strength of the
Beryl sample of 4 kPa and a residual (remoulded) strength of 0.5 kPa, the radius around
the pit that will flow towards the pit is limited to respectively 0.04 m and 0.27 m at the
design cutting depth of 0.175 m. Therefore hydraulic excavation requires hauling or
sweeping action. The results of hydraulic excavation are strongly bed dependent.
It is unknown how the Breebot cutter will operate in the drill cuttings and therefore it is
concluded that the practical feasible water-soil ratio will be somewhere between 4 and
7.4. When, due to operational conditions, the sweep velocity is badly controlled and is
dropping below 0.1 m/s the water soil ratio will exceed the design ratio of 10.
The application of a small cutter of 300 mm in excavation drill cuttings requires
downscaling of practical experience in cutter dredging. One of the major lessons learned
from cutter research in the past performed for the Dutch Dredging Research Association
is that scaling effects will be present due to the conflicting scaling laws for
hydrodynamics (Froude controlled), cutting processes (strain rate controlled) and mixing
process (residence time controlled).
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15.5. DRL Guidance Notes on Test Materials
Delft Hydraulics Cutting Tests – Guidance notes on test materials
DRL - 22
ND
N
OVEMBER
2000
The tests which are currently in progress for Task 5B of Phase II suggest that our
understanding of the geo-technical properties of cuttings is very limited. In essence, it
appears that the permeability of the materials is very low but it is not clear whether this is
due to the well-graded character (i.e. a wide range of particle sizes) of the materials or
due to the unusual composition of the pore fluids (i.e. a high hydrocarbon content,
properties of chemical additives). The chemical effects of additives on pore fluid / fine
particle interaction may also be important.
The 3
rd
-party geo-technical data that are available are often suspect (i.e. unreliable in
normal soil mechanics terms). This applies particularly to measurements of density and
water content. The density tests yield wildly disparate values. Low value may be due to
the unconsolidated nature of the materials (probably true in many cases) but are often
likely to reflect sample disturbance. Some values are very high (i.e. higher than typical
densities of rocks) and these may be due to mud additives. Water content is expressed
in terms of the (unit) dry weight of solids and, therefore, may be expected to yield ‘odd’
results if the materials contain varying proportions of high-density mud additives. Finally,
shear strength data must be interpreted with caution because most tests were
undertaken on material which is not appropriate for vane tests (the method used in the
vast majority of cases).
For all of these reasons, we think that the most prudent approach is to specify the test
materials only in terms of what we know and understand and to avoid specifying detailed
properties which we do not fully understand. At present, we know only the following:
The particle size distribution of the cuttings varies widely, both from pile to pile and within
individual piles but the vast majority of tested samples can be described as well-graded;
The cuttings have a low strength – ignoring the reservations e xpressed above
concerning interpretation of shear strengths in different types of material, we know that
all shear strength tests have yielded values of less than 40 kPa;
The evidence suggests that the materials are generally of low to medium plasticity
(although two tests have shown high plasticity).
The test materials should therefore comprise well-graded, low-strength soils of low to
medium plasticity.
Two soils should be used, one representing the finer types of cuttings (which we shall
assume also are the most plastic) and one representing the coarser types of cuttings
(which we shall assume also are the least plastic). Because it is extremely difficult to
fabricate well-graded soils which include a significant clay fraction, it is recommended
that natural soils be selected which have (approximately) the required particle size
distribution and plasticity.
The condition in which they are tested can, at this stage, only be defined in terms of vane
shear strength because of the difficulties of interpreting densities and water contents.
Drill Cuttings JIP Task 6 Final Report
Revision: 2 Final
Page: 97 of 103
Date: Jan 2002
We take the pragmatic approach of ignoring the fact that vane shear strengths in soils
containing sand are suspect and simply assume that a sandy test soil with a low vane
shear strength is likely to be broadly similar to a sandy cuttings samples of low vane
shear strength. The important aspect of test soil selection is to identify a soil with
(approximately) the correct grading and plasticity.
Sandy soil
This soil should comprise predominantly sand with a high silt content and a very low or
negligible clay content. Figure 1 below shows the preferred grading envelope. The <425
micron fraction should be of low plasticity. For guidance, we are looking for soil with
plasticity characteristics lying in the shaded area shown in Figure 2.
100
90
80
70
60
50
40
30
20
10
0 .001
0.01
0.1
1.0
10
100
0
Sa nd
Gravel
Silt
Clay
Fine
Fine
Fine
Medium
Medium
Med ium
Coarse
Coa rse
Coa rse
Cobb les
Pa rticle size, mm
P
e
rc
e
n
ta
g
e
p
a
ss
in
g
.
Figure 1. Grading envelope for sandy test soil.
0
10
20
30
40
50
60
0
10
20
30
40
50
60
70
80
90
100
Liquid Limit
P
la
s
tic
it
y
In
d
e
x
A
li
ne
.
Figure 2. Plasticity envelope for sandy soil
Drill Cuttings JIP Task 6 Final Report
Revision: 2 Final
Page: 98 of 103
Date: Jan 2002
The soil should be placed in the test tank at a water content which yields a vane shear
strength in the range 35-40 kPa. It will therefore be necessary to undertake a series of
simple tests to determine the required (sea) water content. These can be carried out by
varying the water content of thoroughly remoulded soil and testing to determine the
shear strength. When placing the prepared soil in the tank, it will need to be tamped
gently to eliminate air voids. We recommend that the tank then be filled with seawater
and left for at least 24 hours before undertaking the cutting test. About 6 vane tests
should be carried out in the soil immediately before the test commences.
Supporting test data should include particle size distribution, Atterberg limits and water
content. We suggest that at least 3 particle distribution tests should be undertaken and
at least 6 Atterberg limit / water content determinations on samples taken from different
parts of the placed test soil.
Clayey Soil
This soil should comprise predominantly silt with some clay and sand. The preferred
particle size distribution envelope is shown in Figure 3. The soil should have plasticity
characteristics lying in the shaded area shown in Figure 4.
100
90
80
70
60
50
40
30
20
10
0 .001
0.01
0.1
1.0
10
100
0
Sa nd
Gravel
Silt
Clay
Fine
Fine
Fine
Medium
Medium
Med ium
Coarse
Coa rse
Coa rse
Cobb les
Pa rticle size, mm
P
e
rc
e
n
ta
g
e
p
a
ss
in
g
.
Figure 3. Grading envelope for clayey test soil.
Drill Cuttings JIP Task 6 Final Report
Revision: 2 Final
Page: 99 of 103
Date: Jan 2002
0
10
20
30
40
50
60
0
10
20
30
40
50
60
70
80
90
100
Liquid Limit
P
la
s
tic
it
y
In
d
e
x
A
li
ne
.
Figure 4. Plasticity envelope for clayey soil
Soil preparation should be carried out as for the sandy soils, i.e. try to place the soil so
that it has a shear strength of 35-40 kPa. If funds permit, it would be interesting to test a
clayey soil with a shear strength of about 5 kPa. The test sample preparation method
and supporting tests should be the same as those suggested for the sandy soil.
Drill Cuttings JIP Task 6 Final Report
Revision: 2 Final
Page: 100 of 103
Date: Jan 2002
15.6. Environmental Monitoring Workshop Minutes
Priorities agreed at Meeting 12 March 2001
During the meeting at DNV with members of the JIP and stakeholders a prioritised
environmental monitoring programme was agreed for monitoring the NW Hutton trial lift
of cuttings.
Priority
(highest first, CEFAS task nos. in brackets)
Priority 1
•
Long-term Minilander (T1A+T1B+T1D)
•
Short-term Minilanders (T3A+T3D) – investigate the use of Horizontal ADCP’s to
measure suspended sediment, these may be mounted on additional landers
deployed from platform and positioned by dredging ROV.
•
Water Samples (T3H, T3I + T3E)
•
Tracer (new task – simulating backflush event, with grab sampling to assess
tracer distribution)
•
ROV TV Survey (T4B) by platform eyeball ROV if no vessel based ROV
•
Samples of dredged water taken at topside discharge (analyse for heavy metals &
hydrocarbons)
•
Surface oil sheen, by Garret screen deployed from FRC
Priority 2)
•
OBS mast on ROV (T3B)
•
ADCP on ROV (T3C)
•
ROV Water sampler (T3F+T3G)
•
Vessel based eyeball ROV
Priority 3)
•
Seabed Mapping (T1C/ T4A+T1A)
Review of CEFAS’s outstanding options
Impact on food chain - out
Pro-active project management - In
Longevity of impacts – decide during trial
Monitor produced water/benthic plume with Fluorometer – out
Surface Sheen – Garret screens – In – Priority one, (deploy by FRC from Standby boat /
platform)
In summary, the proposed programme virtually stands but with a few modifications.
Drill Cuttings JIP Task 6 Final Report
Revision: 2 Final
Page: 101 of 103
Date: Jan 2002
15.7. Flow Chart of Disposal Options
Flow Chart of Disposal Options For NW Hutton Trial.
Is the material
slurrified
?
Can it be reused?
Convert to spud mud
Ship to shore
Y
Y
N
A
Can it be Recycled?
Separated out?
Flocculate and
O/W separation
Solids (skip & ship)
Clean water (discharge)
Oily Waste (ship)
Y
N
C
B
D
Pump to Boat
Can we inject elsewhere?
Treatment on-shore
Ship to secondary injection
N
Y
F
E
Can material be
directly injected?
Prepare for
insitu injection
Inject to available
well(s)
G
N
Y
Drill Cuttings JIP Task 6 Final Report
Revision: 2 Final
Page: 102 of 103
Date: Jan 2002
Discussion of Flow Chart of Disposal Options.
Outcome Process Route
Advantages
Disadvantages
Unknown Issues
A
Slurrified material
taken into the mud
tanks and viscosified
& treated.
The resulting product
is pumped to a
supply boat and
transferred to shore
for storage.
The material is then
shipped to rigs as
premixed spud mud.
Reuse.
None of the liquid phase requires
additional disposal.
Trial limited by mud tank capacity, 2000
bbls plus resident time for treatment
Supply boat capacity for WBM is typically
2200 bbls. This could be relieved by
using larger supply boat; Far Swan has
3600 bbls storage and could take on
deck storage.
Query on supply boat availability in July.
Process will not support a continuous
trial.
Compatibility of final
product with
requirements for spud
mud, especially <3% oil
content (dilute?)
Can the total produced
volume be stored
onshore?
How many wells will
require premix spud mud
(time to reuse all the
mud?)
B+C+D
Slurrified material
taken into the mud
tanks then pumped to
a clean up unit.
Substantially reduces the volume
of material to be disposed.
Utilises existing technology; there
is a proposal on file from AFOS
for a system.
Particle removal is 99.5% solids
over 2 micron.
Target Hydrocarbon content is 20
ppm (0.0002%)
Dealing with disposal of different
streams is more cost effective.
It will support a continuous trial.
Equipment rental is £1568 per day.
Personnel costs are £2940 per day plus
consumables and installation.
Solids recovered in outcome B will
probably be oil contaminated by outcome
D. Requires onshore processing.
Will the discharge have any effect on the
environmental monitoring?
Outcome C will require
DTI acceptance of limited
hydrocarbon discharge.
Drill Cuttings JIP Task 6 Final Report
Revision: 2 Final
Page: 103 of 103
Date: Jan 2002
Discussion of Flow Chart of Disposal Options.
Outcome Process Route
Advantages
Disadvantages
Unknown Issues
E
Material is taken into
the mud tanks.
Treated as necessary
then shipped to
onshore disposal
site.
No on-site discharges
All material is being disposed onshore at
high cost.
All material is being dealt with by the
same process.
Trial limited by mud tank capacity, 2000
bbls plus resident time for treatment
Supply boat capacity for WBM is typically
2200 bbls. This could be relieved by
using larger supply boat; Far Swan has
3600 bbls storage and could take on
deck storage.
Process will not support a continuous
trial.
Onshore disposal sites
not fully identified for
liquid waste.
F
Material is taken into
the mud tanks.
Treated as necessary
then shipped to
offshore disposal
site.
Recovered large
solids could also be
transferred.
Enables full offshore disposal of
recovered material to substrata.
Trial limited by mud tank capacity, 2000
bbls plus resident time for treatment
Supply boat capacity for WBM is typically
2200 bbls. This could be relieved by
using larger supply boat; Far Swan has
3600 bbls storage and could take on
deck storage.
Query on supply boat availability in July.
Process will not support a continuous
trial.
Requires DTI acceptance
of the proposal.
G
Slurrified material
taken into the mud
tanks and viscosified
& treated.
Material then injected
into an available well
to a deep subsurface
horizon.
No discharge from the
Installation.
Utilises known technology.
Does not require significant trans-
shipment.
The injection well will not be suitable in
future for production.
Are wells suitable for
injection?
Will it be possible to
maintain injectivity at an
appropriate rate?
Dti accepptance of
method required.
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