html 11 1








Routines when cargo handling





Routines
when cargo handling
11.1  

Legislation and rules


All
transportation of liquefied gas is controlled by international and national
legislation. The international legislation is set up by IMO, which is the
UNÅ‚s maritime division. All new laws and legislations are renewed and
updated when the old rules are no longer appropriate for itłs original
intent. As we gain more experience and knowledge about accidents, the laws and
rules will be revised to fit what has been learned.
The
various cargoes that is allowed for transport on gas carriers and the types of
gas carriers that may carry the them is updated as we gain better knowledge
about the products. Over time we can observe the various types of cargoes to
see if there are any changes regarding health hazards or environmental
pollution. Some cargoes have changed TLV from 400 ppm to 1 ppm. The reason for
this is the experience we have got with the cargo, and it is very seldom that
new products came into the market with 100% guaranty that they are safe in all
manners.
What
kinds of cargoes we can carry and which types of gas carriers that may carry
the various cargoes depend on the toxicity of the cargo. The toxicity ratings
of the various cargoes will always change over time as we gain more
information about their potential health hazards. One example is VCM “Vinyl
Chloride" that had a TLV of 50 ppm in the early 1980s, but in the 1990s the
TLV had been changed to 2 ppm.
What
types of cargoes a gas carrier may transport, depends on what precautions have
been taken in the vesselłs design and construction, to prevent cargoes from
pollute the environment.
In
order to ratify IMO rules and legislationłs, a given number of the IMO
member states must to abide by them. All flag and port states may have there
own set of rules and legislation that differ from the IMO rules. The IMO has
set up a minimum of standards that all gas carriers must be constructed and
classed according to.
 


11.1.1  
International rules
There
is three international conventions to protect the environment and those are
SOLAS (1974) with protocol of 1978 and appendixes to 1991, MARPOL 73/78, and
STCW 78/95.
SOLAS
«Safety of Life at Sea contain rules and legislationÅ‚s on safety
certificates such as: Safety Construction Certificate, Safety Radio
Certificate, and Safety Equipment Certificate. These three certificates cover
the safety of crew, ship and safety equipment. Over time when as we gain more
knowledge, the rules and regulations will be updated to avoid similar
accidents from happening again in the future.
 


 


11.1.2  
Certificates and documents we
are required to have onboard according to SOLAS 74/92 appendix 3:
 





Intact Stability Booklet


SOLAS 1974 Regulation II-1/22




Minimum safe Manning document


SOLAS 1974/89 Regulation V/13b




Cargo Ship Safety Construction
Certificate


SOLAS 1974 Regulation I/2 as amended by
GMDSS amdts.




Cargo Ship Safety Equipment Certificate


SOLAS 1974 Regulation I/2 as amended by
GMDSS amdts.




Cargo Ship Safety Radio Certificate


SOLAS 1974 Regulation I/2 as amended by
GMDSS amdts.




Document of compliance with the special
requirements for ships carrying dangerous goods


SOLAS 1974 regulation II-2/54.3




Dangerous goods manifest or stowage plan


SOLAS 1974 regulation VII/5(3)



 


MARPOL
«International Conference on Marine Pollution contains rules and
regulations that are designed to protect the environment from pollution on a
short and long term basis. MARPOL specifies what we are allowed to pump or
throw overboard, and also what equipment we must have onboard to prevent
pollution of the environment.


 


MARPOL
defines clean water as water with less than 15 ppm of contamination.
 
11.1.3   
Certificates and documents related to MARPOL 73/78 required to be onboard
according to SOLAS 92 appendix 3:






International
Oil Pollution Prevention Certificate


MARPOL
73/78 annex 1 regulation 5




Oil
Record Book


MARPOL
73/78 annex 1 regulation 20




Dangerous
goods manifest or stowage plan


MARPOL
73/78 annex III regulation 4



 


STCW
78/95 «SeafarersÅ‚
Training, Certification and Watch keeping contains rules and regulations on
qualification certificates of officers and rating onboard vessels. STCW 78/95
was implemented 1st February 1997 and is based on two parts. Part A
contains the minimum standard requirements for qualification of all personal
on various types of ships. Part B contains the minimum requirements of what
all personal need to know in order to receive a certificate for their various
ranks on all types of ships.
11.1.4   
Certificates and documents related to STCW 78/95, we are required to have
onboard according to SOLAS 92 appendix 3






Certificate
for masters, officers or ratings


STCW
78/95 article VI



 


IMO
«IMO Gas Code contains rules and regulations that are meant to make the
transport of liquefied gases as safe as possible for persons onboard as well
as the environment. There are three gas codes issued by IMO.
·     
Code for existing ships carrying Liquefied gases in bulk.
·     
Code for construction and equipment of ships carrying liquefied gases
in bulk.
·     
IGC code International code for construction and equipment of ships
carrying liquefied gases in bulk.
 


The
gas codes contain requirements for the construction of gas carriers. This
includes requirements on stability, leakage after collision/ grounding and
cargo compartments. In the gas code we also find a list of products that are
classified as liquefied gases, as well as the requirements covering how those
cargoes are to be transported. The gas code also covers toxic cargo and what
types of safety devices are required to carry those cargoes.
The
vessels Certificate of Fitness states which cargoes the vessel is permitted to
carry.
We
are required to always have onboard the personal safety protection equipment
for the type cargo we are carrying, as specified for in the vessels
Certificate of Fitness. Especially important are instruments for atmospheric
measurements and personal safety equipment.
 


11.1.5   
Local rules
Some
port states have local rules that are stricter than the IMO rules and are
designed to protect local waters. All countries try to protect own territorial
waters against pollution. Local rules may be stricter on clean water, shore
line pressure, navigation after sunset etc.
There
are three countries that have especially strict rules and have a large amounts
of import and export of liquefied gases and those are: USA, Italy and Japan.
In
the USA the US Coast Guard issue a Certificate of Compliance on all gas
carriers that are to load or discharge in US waters. According to US rules and
regulations, all gas carriers that visit US waters are to be inspected by the
US Coast Guard in accordance to the vessels Certificate of Compliance. Gas
carriers that fail the US Coast Guard inspection will not be permitted to
birth before the vessel has fulfilled the standards set by the Certificate of
Compliance.
One
main difference between IMO and USCG is that in US they have a higher standard
of security regarding strength on pressure vessels. That means that most of
the gas carriers have one safety relief valve setting according to IMO and a
lower one according to USCG.
Gas
carriers that are built according to IMO gas code have no problem in attaining
a Certificate of Compliance. The owner of the vessel must send the USCG
diagrams of the mid ship section, GA plan, fire and safety plan. Those
drawings and plans must be sent to the USCG in long before the vessels first
arrival US waters, if possible when the vessel is new.
Italy
issues all vessels that carry gas in Italian waters an Italian safety
certificate called “RINA". The vessels Class Company can issue the RINA
certificate if it is approved by RINA. The RINA certificate is renewed and
surveyed together with the vessels IMO Certificate of Fitness.
Japan
has their own rules and certificates on gas carriers. Those rules cover for
the most part measurements of vessels. That means there are other net and
gross weights on the vessel in Japan than on the IMO load line certificate.
Japanese authorities will survey all gas carriers that handle cargo in Japan.
If the documentation on the vessel is found unsatisfactory by the authorities,
they can require that the vessel must go in dock for new measurements.
In
Norway it is the Norwegian Maritime Directorate that makes all the local
rules, those rules are applicable for all vessels in NOR or NIS. The Norwegian
Maritime Directorate uses the Class Companies to issue certificates and to
conduct surveys on those vessels.
Then
we have some local rules around the world that does not allow gas carriers to
navigate after dark. Some ports have regulations concerning the wind speeds.
The ship/ shore checklist must state if there are any restrictions on wind
speed or the height of waves. Information about the terminals is found in the
“Guide to port entry" or from the agents, terminals or your own company.
 


11.1.6  

Publications
We
required to always have onboard the latest edition of publications related to
cargo, cargo handling, SPM, ship to ship transfer etc.
The
most important publications we must have onboard are: SOLAS 74/92 latest
edition, MARPOL 73/78 latest edition, STCW 78/95 latest edition, the latest
IMO gas code for the type of gas carrier we are on, ICS Tanker safety guide
liquefied gas, and ICS Ship to ship transfer guide for liquefied gases.
Vessels that are registered in either NOR or NIS must have the latest edition
of the Norwegian Maritime Directorate rules. In addition we must have the
guidelines and publications from SIGTTO, ICS, OCIMF and USCG. In the Exxon
vessel inspection guide we can find references to all publications they
require us to have onboard.
 


11.2   
Loading
routines


Before
we can commence loading we need a confirmed loading order from the owner. In
the loading order we will find the quantity to be loaded, at which temperature
we will receive the cargo, and what temperature we are to discharge the cargo.
Further we find information about the load port and discharge port. It may
also state what size there is on the terminal lines and flanges.
The
quantity to be loaded is given in metric tons. There are three different ways
to state the quantity, either:
 





4000
mt +- 5% MOLOO


Then
we can load from 3800 mt to 4200 mt on owners option




4000
mt +- 5% MOLCO


Then
we can load from 3800 mt to 4200 mt on charterer option




4000
mt


Then
we have to load 4000 mt



 


MOLOO
means that it is the owner, represented by the captain that states the
quantity to be loaded. MOLCO means that it is the charter that states the
quantity.
The
cargo loading temperature is given either fully refrigerated, ambient or on a
given temperature. Ambient means the temperature is equal to the air
temperature if the shore tank is located on the surface. It also states what
temperature the cargo is to be discharged at.
The
maximum allowed filling limit when loading is 98% and it is the safety relief
valve setting and the cargo temperature that give the filling limit. To find
the filling limit we can either use the operation manual for the vessel or the
cargo density table. When using the density table, we can calculate the
temperature of the cargo from the absolute pressure, in this case 4 bar.
When
we have the basic information on the cargo, we must start planning the
loading. We then calculate the filling limit in each cargo tank and then plan
the loading rate. The loading rate is determined by three factors: cargo
temperature, the ambient temperature, and whether we do or do not have vapour
return.
When
we are loading and we need to run the cargo plant, there are various ways to
run the plant. It depends on the temperatures and the flexibility of the
plant.
If
we are set up to load two different cargoes e.g. ethylene and propane, then we
must separate the cargoes from each other. We call that segregation of cargo
tanks, cargo cooling plants and lines. Taking out small pipes on the cargo
lines does segregation, those pipe parts are called “spool pieces". The
spool pieces are taken out of the lines and the main line is blanked of flange
covers. We also have to segregate the cargo cooling plant e.g. two plants are
used for ethylene and one is used for the propane. It is stated in the
Certificate of Fitness how the lines and compressors can be segregated. If we
have 4 cargo tanks there could be a possible segregation with cargo tank 1+2
and 3+4.
When
we are loading a partial cargo we must try to use the manifold that is linked
directly to the tank that we are loading. If we are going to load on tanks 2,3
and 4, then we use the manifold for tanks 3+4. All spool pieces are marked
according to the diagram. Many of the spool pieces have the same diameter but
have a different length, so try to keep your spool pieces orderly. Normally
the spool pieces are mounted on the cargo lines, so there should not be any
problem keeping them organised.
Before
we can commence loading we have to cool down the tank shell as mush as
possible, the optimal is less than 10oC above cargo loading
temperature. The resulting temperature of the tank shell depends on how much
time is used, the amount of cargo remaining on board, and the arrangement of
the lines to the cargo tanks. We need thermometers on the outside of the tank
shell in order to achieve the proper temperature. If we are going to load
fully refrigerated propane, we must try to get the temperature on the outside
on the cargo tank shell below
35oC before we start loading. When
the cargo tank shell is chilled down before arrival, the time used for loading
will be reduced.
Before
we arrive port the cargo tank shell must be chilled down, and cargo lines and
spool pieces must be readied. Only then will we be able to reduce the time
used in port.
When
loading, the liquid is either pumped or pressured onboard from the shore tank.
If we are loading by pressure, the vesselłs cargo tank pressure must be
lower than the shore tanks pressure. This way of loading is mostly used on
fully pressurised vessels. When we are loading by pumps we must follow the
cargo tanks pressure to hold it below the safety valve set point. After
completion of the loading, we need to free the loading hose/ arm of liquid by
use of hot gas. The hot gas is produced by the vesselłs cargo compressor or
from the shore tank or terminalłs compressor. On some terminals they use
nitrogen to free the hose/ arm of liquid. When using this method we must keep
a close watch so we do not get so much nitrogen into the cargo tanks.
Before
we commence loading cargo it must be issued a checklist, load planning,
loading log, ship/ shore checklist and a time sheet. The vesselłs plans for
loading have to be discussed and agreed to by the terminal loading master and
safety officer. Read carefully the checklists and pay special attention to any
notes about maximum pressure or minimum temperatures on the loading hose/ arm.
All deviations from the planning have to be noted in the deck logbook and
discussed with the loading master/ safety officer.
It
is very important that we are familiar with both the vessels and the terminals
emergency routines, so that we all know what to do if there is a cargo leak or
uncontrolled venting.
When
we transfer cargo from other vessels, we must follow our company quality
manual and the ICS Ship to Ship Transfer Guide.
 


11.2.1  

Loading without vapour return
When
loading without vapour return, only the liquid line is connected to the
terminal. On some terminals we also have to connected a vapour line, but it is
only for emergency use, and goes directly to flare. The cargo liquid is pumped
or pressured to vesselłs cargo tanks through the liquid lines.
To
avoid high pressure in the vesselłs cargo tanks we need to pressure control
the cargo in the tanks. To control the pressure we use the cargo cooling
plant. During the entire loading process we must check the tank pressure, and
we must do our utmost to avoid uncontrolled venting. Uncontrolled venting
happen when the cargo tanks pressures rise to the set point of the safety
relief valves and they open. The cargo tank vapour will then be led to the
vessels vent mast. To avoid uncontrolled venting we have to reduce the loading
rate or stop loading if we can not increase the cooling capacity.
 
11.2.2
  
Loading without vapour return but use of cargo cooling plant
When
commencing loading, we always start with a slow rate to check that there is
not any leakage on the terminal lines/ arms/ hoses or the vessel lines/
valves. When we are sure that there is no leakagełs and the cargo tank shell
is close to the same temperature as the cargo, we can then increase the
loading rate slowly to the agreed maximum rate. While we are increasing the
loading rate we must watch the cargo tank pressure carefully.

In
order to avoid having too much pressure in the cargo tanks when loading, we
can either reduce the loading rate or stop loading, if the cargo cooling plant
is on maximum capacity. If we can increase the cooling plant capacity, we have
to do it before reducing the loading rate. It is important for the vessel to
load at the rate that is stated in the charter party or is agreed to by the
loading master. If we have to reduce the loading rate due to foul gas, we will
then have to clarify it with the loading master and it must be noted in the
deck logbook. When we reduce the loading rate the cargo temperature from shore
will increase.
On
all types of gas carriers, it is important to check the cargo tank pressure
all the time while loading. We have to do our outmost to avoid uncontrolled
venting.
11.2.3   
Loading with vapour return
The
safest and fastest way to load is when we have vapour return, and that can be
done on all types of gas carriers. When we are loading with vapour return the
liquid hose/ arm is connected to the vesselłs liquid manifold and the vapour
hose/ arm connected to the vesselłs vapour manifold. The cargo liquid is
pumped or pressured onboard through the vesselłs liquid lines and to the
cargo tanks that are to be loaded. The cargo tanks excess pressure is
evacuated through the vesselłs vapour lines to shore.
Before
we commence evacuating any vapour from the vessel we must be sure that the
vapour is returned to the shore tank and not to flare. If the vapour is
evacuated to flare, the vessel will be charged for the amount of vapour that
is burned. We must be aware that if we evacuate more vapour than is agreed to,
the terminal can develop problems with the shore tank pressure.
 


11.2.4    
Loading with cargo
cooling plant and vapour return
In addition to the vapour return, we use the
vesselłs cargo cooling plant. If we have an indirect cargo cooling plant, we
cool down the cargo tank shell or the vapour phase in the cargo tank. If we
have a direct cargo cooling plant we condense the vapour in the cargo
condenser and the condensate is pressured to the cargo tank.

While
we are loading we must try to keep the cargo tank pressure as low as possible,
in doing so the vapour return will help us great deal. When vapour return
exists, we can increase or reduce the amount of vapour to shore by throttling
the vapour manifold valve. How mush vapour we can send to shore must be agreed
upon with the loading master before commencement of loading. It is the
terminalłs capacity to receive vapour that determines the rate of vapour the
vessel can send to shore while loading.
Before
we open the vapour manifold valve to send vapour to shore we must 
be sure that the vapour goes back to the shore tank and not to flare.
If the vapour is sent to flare, the vessel will be charged for the amount that
is burned in the flare.
On
fully pressurised gas carriers we must not send to shore so much vapour that
the cargo is chilled down to less than
10oC. As an example, if
we load propane and the cargo tank pressure is taken down to near 0 bar, the
cargo temperature will be about -42oC.
After
the loading is completed the terminal loading hose/ arm has to be freed of
liquid. To evacuate the liquid from the loading hose/ arm we either use the
vessel cargo compressors and blow hot vapour, or the terminal uses nitrogen
and blows onboard. If the terminal is using nitrogen, we must minimise the
amount of nitrogen to the cargo tanks. Try to blow the line into one cargo
tank only. If we get to much nitrogen in the cargo tanks, we will develop too
high condenser pressure and our cargo compressors may stop, and in the worst
case we may have an uncontrolled venting.
When
loading fully refrigerated or semi-refrigerated gas carriers we must try to
evacuate as much vapour to shore as possible in order to get the lowest
possible cargo tank pressure. When loading with a high rate, the shore tank
liquid level and the cargo temperature will be reduced and the vapour phase
increased. When the liquid level and the cargo temperature is reduced, the
terminal can take more vapour from the vessel. The maximum loading rate
depends on the cargo temperature, temperature of the cargo tank
shell
before commence loading, cargo cooling plant capacity, size of the loading
lines and the ambient temperature.
 
11.3   
Routines
when cooling or heating cargo at sea


Cargo
procedures while the vessel is at sea depend on what has been stated in the
charter party. Some cargoes are to be discharged fully refrigerated, while
others at a given temperature. In addition to the charter party, the vessels
cargo handling equipment performance is important, especially the capacity of
the cargo cooling plant, capacity of the cargo heater, insulation on the cargo
tanks and the length of the sea voyage.
 


11.3.1  

Cooling of the cargo while at sea
On
fully refrigerated gas carriers we do not have any choice, we must maintain a
low pressure and temperature of the cargo. How to run the cargo cooling plants
at sea depends on the plant itself, type of cargo, cargo tank insulation, the
ambient temperature and the length of the sea voyage. The fewer cargo plants
and hours we are running the plants, the lower fuel consumption do we have,
and maintenance cost will be reduced.
As
long as the cargo plants are running, we have to watch that we donłt get
vacuum in the cargo tanks. We must try to maintain a positive cargo tank
pressure e.g. 0,01 bar or higher, this is to avoid air leaks into the cargo
tank.  Some cargoes like ethylene and butadiene, will be contaminated
if the content of oxygen is to high. To avoid a vacuum in the cargo tank there
may be a pressure switch on the suction side of the cargo compressor or on the
cargo tank connected to the cargo compressor, that stops the compressor when
there is to low suction pressure.
On
semi-refrigerated gas carriers we have a few more options how we may handle
cargo at sea, than with a fully refrigerated gas carrier. We may either
maintain the cargo temperature or cool down the cargo, depending on the
charter party.
The
number of cargo cooling plants we have to use depends on the cargo, the
ambient temperature, capacity of the cooling plants, length of the sea voyage
and the insulation of the cargo tanks. We must try to cool down the cargo to
the discharge temperature as soon as possible and then maintain the
temperature the rest of the voyage. We will then be able to discharge the
cargo earlier if we are sent to another port of call. As long as the cargo is
onboard it may be sold to another customer, so we must be prepared to
discharge the cargo earlier than planned. We must always keep in mind that
cooling down the cargo demands a lot of energy. Read the charter party/
loading order carefully and run the cargo cooling plants as economically as
possible.
Always
check the weather forecast and air temperatures for your voyage. The seawater
temperature has a major influence on your cooling capacity. Higher seawater
temperature results in reduced cooling capacity. Pressure is defined as the
movement of molecules, and when a vessel is pitching or rolling the molecules
will move faster and the result is higher cargo tank pressure. Try to cool
down the cargo as mush as possible before you enter into bad weather.
When
the cargo cooling plants is running we must fill out the cargo-cooling log. In
the cargo-cooling log we must record the various pressures, temperatures and
ampere for each of the cargo cooling plants. We must also record when we have
changed condensate and to which tank we have pumped the condensate. This to
avoid overfilling of cargo tanks and interrupt the cargo cooling process.
On
fully pressurised gas carriers we can only control the cargo tank pressure if
there is cargo compressor onboard, if not, we have to vent the vapour to the
atmosphere. The cargo tank steel of fully pressurised gas carriers are
normally designed for minimum temperature of
10C°.
That means we normally control the cargo temperature and pressure and do not
lower it.

11.3.2  

Heating of cargo on the voyage
The
charter party or loading order must state if we have to heat the cargo on the
sea voyage before discharging. Depending of the vesselłs cargo equipment,
there are normally three different ways to heat the cargo. We can pump the
cargo through the cargo heater to another cargo tank. We may use the cargo
compressor to blow hot vapour down into the liquid in the cargo tank. We can
also use indirect cargo plant and pump e.g. ethanol in coils either outside of
the cargo tank shell or in the vapour phase inside the tank.
On
fully refrigerated gas carriers there are no options for heating the cargo at
sea. If the vessel is equipped with cargo heater we may heat the cargo when
discharging.
On
semi-pressurised gas carriers we may heat the cargo during the sea voyage if
we have the cargo equipment for it. The most effective way to heat the cargo
is to use the cargo pumps and the cargo heater. Then we pump the cargo from
one tank through the cargo heater and then to another cargo tank. If we have
submerged pumps this is normally not a problem. If we use deep well pumps, we
must keep in mind of the shaft bearings and the shaft itself when the vessel
is pitching or rolling. As we pump liquid from one cargo tank to another we
must check the sounding on the cargo tank we are pumping to. Check the
sounding in the other cargo tanks also, and record the cargo heating log
temperatures and soundings on all tanks. When we check all the tanks while we
are heating the cargo we can avoid overfilling any of the cargo tanks.



11.4   
Routines
when discharging


Before
we may commence discharging any cargo, the captain must get a written
permission from the owner to discharge the cargo.
Certain
information is needed before arriving at discharge port, such as the discharge
temperature given in charter party, backpressure on the terminal, and size of
the terminal lines and flanges. The captain can receive this information
either from the agents or directly from the terminal.
When
all the information is received we must make a discharging plan. The
discharging plan must contain which cargo tanks are to be discharged, and the
sequence and rate of the discharging. If the cargo has to be heated and we
need to use a booster pump, and agreement must be made with the terminal as to
the minimum/ maximum cargo temperature and maximum backpressure. We must also
prepare the correct reducer to fit the terminal flanges.
The
cargo pumps we plan to use have to be checked according to the
manufacturerłs specification ref. operation manual. Check closely the oil
and mechanical seals. 
The
vessels discharge plan must be discussed with the terminal before commence
discharging. When the discharge plan is agreed to, fill out the ship/ shore
safety checklist. The discharge plan and all checklists must be signed and
followed by the vessel and the terminal. All deviations from the discharge
plan or checklist must be noted in the deck logbook and cleared with the
terminal.
There
are different methods for discharging a gas carrier, and they depend on the
cargo equipment onboard and the facilities at the terminal. There are two main
methods used in discharging a gas carrier, either by pumps or by pressure. If
shore backpressure is very high we must use a booster pump in addition to the
ordinary cargo pumps or cargo tank pressure.
When
we use cargo pumps to discharge the cargo we must first check that the pump is
free by turning the pump shaft by hand. When we can turn the shaft by hand we
may then start the pump.
When
the terminal and the vessel are ready to commence discharging, we start the
process by using only one pump. The delivery valve on the pump is only
slightly open as well as the one back to the cargo tank. When the line is
chilled down we then close the valve down to the cargo tank and open the
pressure valve a little bit more. Check that there is no leakage on the
vesselłs
lines and valves and as well as the terminal lines.
When
both the vessel and terminal lines are chilled down we can increase the
discharge rate according to the discharge plan and on the terminal advice. Do
not increase the discharge rate more than what the terminal has asked for, in
the worst case the terminal will stop discharging and the vessel is charged
for the delay.
While
discharging, it is always the terminal that sets the rate and required the
cargo temperature. If the terminal asks the vessel to reduce the rate then we
must comply. To reduce the rate we either throttle the pressure valve on a
pump or stop a pump. If we throttle the pressure valve, we then generate
pressure in that cargo tank. To avoid increased cargo tank pressure it is best
to stop the pump. If the vessel refuses to reduce the rate the terminal can
stop the discharging and then hold the vessel responsible for the delay or
damage caused by the high discharge rate.
If
the shore tank pressure is to high the terminal must flare the excess
pressure, or they can arrange a vapour return to the vessel. Reasons for the
high shore tank pressure may be either a discharge rate that is high, to high
temperature of the cargo, or the cargo compressors on shore may have to small
capacity.
If
the vessel must use the cargo cooling plant while discharging, permission to
do so must be stated in the charter party or agreed to by the receiver so that
the vessel will be compensated for the extra fuel consumption. Always while
discharging we must record in the deck logbook if there have been any
deviations from the discharge plan. Onboard we must do the utmost to avoid any
claims from the terminal.
 


11.4.1  

Discharging by cargo tank over pressure
When
we must discharge the cargo tank by pressure, the tank that receives our cargo
must have a lower pressure than our own. This way of discharging is the
simplest, but when the liquid level decreases, the cargo tank pressure also
decrease. That means we have to compensate for the lost pressure, and that is
done either by hot gas from the vessel or shore, or by nitrogen from shore.
This way of discharging can be done by semi and fully pressurised gas
carriers.
 


11.4.2  

Discharging with centrifugal pumps
The
most common pumps on gas carriers are centrifugal pumps, either of deep well
or submerged type. Most gas carriers have one pump in each cargo tank and they
are also normally equipped with one or more booster pumps on deck. When
discharging fully refrigerated and semi-pressurised gas carriers we normally
only use the pumps in the cargo tanks. If we have to discharge against high
backpressure then we also need to use the booster pump either onboard or at
the terminal.
While
we are running the cargo pumps we have to check that the pump is running the
correct direction and the liquid level is reduced in the cargo tank. Record in
the discharge log pump pressure and ampere and discharge volume every hour. On
new cargo pumps there is an anti rotating device to protect the pump from
running the wrong way. Most of the pumps also have a non-return valve on the
pressure side to avoid leaks if the pump has or are stopped. Check the oil
level and the mechanical seal when the pump is running.
The
booster pumps must be used if the backpressure is higher than maximum pressure
for the pumps in the cargo tank. Booster pumps can be set up either in a
series or parallel, depending on backpressure and the rate agreed upon. We
must never increase the pump pressure higher than the pressure limit on the
terminal discharging hose/ arm.
When
transferring cargo from one vessel to another we must follow the company
quality manual and the ICS Ship to Ship Transfer Guide. While transferring
cargo either ship to ship or ship to shore there must be a good communication
between the two parties. All changes from the discharge plan or checklists
must be reported in the deck logbook.
 


11.4.3   
Discharging through the cargo heater
Sometimes
we may have to discharge the cargo with a higher temperature than we have in
our cargo tanks. In those cases we use our cargo heater while discharging.
When using the cargo heater we must first check that the heating medium is
circulating. Seawater or oil is used as a heating medium, however some
terminals do not accept water as heating medium. When we have checked that the
heating medium is circulating we may then pump cargo through the heater.
Normally we can by-pass the heater with some cargo, but watch the temperature
of the liquid manifold. Never pump cargo to shore with lower temperature than
the minimum temperature given in the checklist. In the worst case the
shoreline may be damaged.
If
we are using water as heating medium we must keep the seawater temperature
from dropping below 5oC as it goes out of the heater. If the water
is freezing, the cargo heater may be damaged and start leaking.
The
cargo temperature on the vesselłs liquid manifold depends on the amount of
cargo that is pumped through the cargo heater. The manifold temperature is
adjusted by bypassing the cargo heater. Less cargo through the heater will
result in lower temperatures and higher flow “discharge rate". If the
water temperature is 5oC or less we must not use the cargo heater
unless we have facilities to heat the water before we pump it to the cargo
heater.
Some
terminals do not allow the use of cargo heaters that utilise water as heating
medium; they require oil as medium. If we must discharge at a terminal that
does not accept our heater type, we then must try to heat the cargo at sea.
11.5   
Routines
during change of cargo


All
gas freeing operations requires a statement in the charter party or direct
orders from the company operation. Gas freeing is very expensive whether we
gas free a single tank or the whole vessel, 
and we must try to avoid those unnecessary costs.
Normally
we have to gas free all the cargo tanks when we are changing cargo and there
must be visual inspection made of the cargo tanks. Other reasons for gas
freeing cargo tanks are for example times when we have to repair anything in
the tank or when we have to go in dry-dock. The tanks are gas free when they
are free of flammable, toxic or neutral gases and the tank atmosphere is pure
air.
When
we have to gas free cargo tanks, we must discharge as much liquid as possible
so that we can reduce the time used to liquid free the tank. To gas free cargo
tanks we use the vesselłs cargo compressors, inert gas generator and the
cargo fan.
When
we have received a confirmation from the charterer or owner that we are to
change cargo, we must set up a plan for the cargo change. The time we need to
gas free the vessel depends on the cargo ROB and what type the next cargo is.
Some
products we may carry, such as propane and butane can be loaded on each vapour
phase, we only need to be liquid free. Other products such as ethylene and
butadiene require that we are gas free before gassing up with the cargo to be
loaded.
There
is two temperatures we have to keep in mind when gas freeing. Those are the
temperature of the cargo tanks when start the gas freeing process, and the
expected air temperature when we commence blowing air. The reason that those
two temperatures are important is that we have to heat the tank shell the same
amount of degrees as the difference between the two temperatures. As an
example if we start with
99oC in the cargo tank and the ambient
temperature is 19oC, we must heat the cargo tank shell 118oC.
It is always the heating process that takes the longest time when gas freeing.
11.5.1  

Gas freeing
The
first we have to do when gas freeing is to liquid free the cargo tanks. To
liquid free the tanks we can either blow hot gas down in the pump sump or
pressurise the tank and empty the tank through the empty blow line. It depends
on the type of gas carrier we are on and the pipes in the cargo tanks. When we
blow hot gas, we blow it either through the condensate line, empty blow line
or the pump. If we blow through the pump we must check that the pump does not
start to rotate. When the hot gas is blown down in the pump sump the liquid
will be boiled off. As long as we have liquid the cargo tank pressure will
increase as we blow hot gas. When there is no more liquid left in the cargo
tank, pressure will be stabilised and only the tank shell temperature will
increase. We must try to have as high pressure in the tank as possible when we
liquid free the tank. On fully refrigerated gas carriers we must watch the
cargo tank pressure at all times to avoid an uncontrolled venting.
Continue
to blow hot gas down in the cargo tank until we have about 5oC
above the seawater temperature. Read the temperature on both sides of the
cargo tank shell. Keep in mind that the cargo tank shell can have a thickness
of 20 mm or more.
We
then stop the cargo compressors when we have reached the planned cargo tank
temperature. Then we have to vent off the cargo tanks pressure, either through
the vapour manifold into the water or to the vessels vent mast. If the vessel
is equipped with purge tanks or we have a gas recovery plant we can use the
cargo compressors and condensate the overpressure to the purge tank. The purge
tank is a small tank “pressure vessel" located either on deck or in a hold
space. When we have reached atmospheric pressure in the cargo tank we can
commence to inert or purge the cargo tanks with nitrogen.
If
we are using inert gas the oxygen content by volume has to be less than 5%,
that is an IMO requirement. The inert gases have to be as dry and warm as
possible before we send it to the cargo tank. If the vessel is equipped with a
heater on the inert gas line we must use it to make use of the density
difference between the inert gas and the cargo vapour. When we have
differencełs in the density it is easier to achieve a good displacement
purging. The differences in density tell us if we must blow the inert gas
through the cargo tank vapour or liquid line.
We
must try to hold as low cargo tank pressure as possible while we are inerting
to avoid turbulence in the cargo tank. Start the inerting with as low rate as
possible, that way we will get the most effective purging. Displacements
purging means the cargo atmosphere is pressed out by the inert gas. Just after
we have commenced the inerting we have to measure the cargo tank atmosphere
for HC vapour in the part of the cargo tank we have blown inn the inert gas.
During the whole inert operation we have to measure the cargo tank atmosphere
for HC vapour content until we reach the planned content less or equal to LEL
or the limit stated in the company QA manual.
While
we are inerting, the oxygen content will not be higher than the oxygen content
we have set on the inert gas generator e.g. 2% by volume. The HC content will
be reduced as long as we are inerting and we do not stop the inerting before
we have reached the LEL for the actual cargo. As an example the LEL on propane
is 2% by volume, so we have to inert until we reach 1,5% by volume. When we
have less HC content than LEL on the actual cargo we can commence 
ventilating the cargo tank with air.
 


11.5.2   
Example on displacement purging

In
the IMO regulations we must use a safety factor of 2 as the margin for error
on measurement and instruments. That means that when we draw flammability
diagram, the line for critical mixture with 2% oxygen by volume gives 4% HC
content by volume. We then have to inert until we read 2% HC by volume before
we commence ventilating with air. It is important that we also inert all
liquid lines, condensers and cargo compressors before we stop the inerting, to
have a neutral atmosphere in them.
Equipment
we can use when venting cargo tanks with air are cargo compressors, cargo fan,
inert gas blower, booster compressor or portable cargo fans. The kinds of
equipment we use depend on the vesselłs cargo equipment. If the vessel is
equipped with a vent heater we must use it to get as warm and dry air as
possible. While we are venting with air we have to measure the oxygen content
and also check that the HC content is reduced to 0% by volume. Before we stop
the air ventilation we have to measure the cargo tank atmosphere for 0% by
volume of CO
- CO2 (Carbon monoxide - Carbon dioxide) and the oxygen
content must be 21% by volume.
CO
Carbon monoxide


CO
is a very toxic gas and extreme caution should be taken prior to entering a
tank that has been previously inerted and ventilated.
The
HC content in the cargo tanks can not be higher then when we started venting
with air, unless we have forgotten to purge any lines or other cargo
equipmentłs.
The
humidity and temperature of the air we use for venting gives us the necessary
temperature of the cargo tank shell. If the cargo tank shell has a temperature
that is much lower than the air, the air will condense on the steel and we
will get water in the tank.

For example at an ambient temperature on 25oC and 75% relative
humidity and  an average cargo
tank shell  temperature of 5oC,
will produce 10,75 g/m3 of water 
condensation.



Temp
Water


4°C
6.30 g/m3


5°C
6.75 g/m3


6°C
7.22 g/m3


24°C
22.20 g/m3


25°C
23.50 g/m3


26°C
24.80 g/m3



At
25oC the maximum content of water is 23,5 g/m3 and 75%
humidity gives us then 17,625 g/m3. At 5oC the maximum
water content can be 6,75 g/m3 that give 17,625 g/m3

6,75 g/m3 = 10,87 g/m3 water. If we are venting with
10000 m3/h, 108,7 kg of water will be released an hour. We then
understand how important it is to heat the cargo tank shell to same
temperature as the ambient temperature and that we have to use dry and heated
inert gas and air. If we get water into the tank while venting we have to use
either an ejector or rags to dry up the water.
If
we had heated the cargo tank shell to 30oC before venting with air
at 25oC the relative humidity in the tank atmosphere will be
approximately 57%. At 30oC the maximum content of water is 31 g/m3
and we had 17,625 g/m3 that gives us 17,625 g/m3 divided
by 31 g/m3 = 56,8% and we do not develop any problems with water.
While
we have a visual inspection of cargo tanks we must use that time to sweep and
clean up dust and check that there is not any foreign substances on the tank
top. Check that all bolts and nuts on the pump and cargo tank lines are tight.
Also check that the pump is in the correct position, if a pump is damaged it
is very costly to repair and gas free the tank.
11.5.3  

Gassing up cargo tanks
To
gas up a cargo tank means that we change the cargo tank atmosphere from air to
either a neutral, toxic or cargo atmosphere. There are different methods and
ways to gas up cargo tanks and they are dependent on the specifications from
the charterer and what type of cargo to be loaded. If the vessel is set up to
load a cargo that requires low dew-point and a low content of oxygen, we must
then use nitrogen to purge out the humidity and oxygen from the cargo tank. If
there isnłt any required limit to the humidity and oxygen content can be
more than 1% by volume, we can then use inert gas to purge out the humidity
and oxygen from the cargo tank. Before loading ammonia we can gas up the cargo
tanks directly with ammonia vapour if the terminal and charterer agree on
that, otherwise we have to use nitrogen.
Normally
we purge with nitrogen alongside a terminal or jetty, it can also be done at
anchorage with a barge. Some new gas carriers have their own nitrogen plant
onboard and can purge while they are at sea. Before the nitrogen is blown down
into the cargo tank we must try to heat it as much as possible, up to 60oC
or more.
Nitrogen
vapour expands when it is heated and the warmer we have the nitrogen, the
lower the consumption and time used is. To heat nitrogen onboard we can use
either the superheater feed by steam or an electrical heater.
While
we are purging with nitrogen we must have as low tank pressure as possible,
less than 0,02 bar. When we start purging we must start with a low rate to
avoid turbulence in the tank. Normal method is displacement purging and uses
the different density to push out the old atmosphere from the cargo tank. Just
after we have commenced purging we must measure the oxygen content in the part
of the tank we blow in the nitrogen to see the reduced content of oxygen. Also
check the oxygen content in the middle of the tank just after commence purging
to be sure that it isnłt turbulence in the atmosphere.
The
purging rate must be discussed and cleared with the loading master before we
commence purging. Always start with a low rate and increase to maximum when we
are sure that there is no turbulence in the atmosphere. We have to calculate
for each purging what is the most economical, either the lay time or the use
of nitrogen. There is a big differences in harbour fees and nitrogen prices
around the world, so what was the cheapest in one port could be the most
expensive in other ports. Before we have completed purging we have to purge
through all lines and all cargo equipment to be sure that we do not have any
air left in the cargo systems.
When
the surveyor has, according to the specifications given from the shipper or
charterer approved the cargo tanks and cargo equipment, we can then commence
purging with cargo vapour. Some gases as ethylene and butadiene require less
than 0,1% oxygen by volume.
For
liquefied gases as methane, ethylene and ethane most shippers require a dew
point of less than
45oC. The charter party always specifies the
maximum content of oxygen and the maximum dew point.
11.5.4  

Examples on parallel purging

Purging
with cargo vapour is mainly done at a terminal or at anchorage, it depends on
where we are in the world and what type of cargo we are purging with. The
method that is most friendly to the environment is to conduct the operation in
a place where it is possible to condensate the cargo and nitrogen vapour. The
most common method is to load some cargo from shore and then do the operation
out on the road. At some terminals we are allowed to do the operation
alongside and send the cargo vapour to the terminal flare.
We
must always use the difference in density while we are purging; the lightest
vapour to be purged must go into the top of the cargo tank. Then we take the
vapour to be purged out through the bottom liquid line in the cargo tank.
11.5.5  
Example
on nitrogen serial purging


Which
method we should use, either serial or parallel depends on the experience and
the lines onboard. On one vessel it can be easiest to purge parallel, another
vessel get the best results from using serial purging. When we are gassing up
we have to avoid opening any lines after the cargo tanks are completed gassed
up. That means on most vessels we must gas up using the parallel method.
The
more flexible the vessel is built, the easier it is to purge and gas up the
vessel.

11.6   
Formulas to be used
when changing atmosphere in an tank
There
are some formulas that we can use to calculate the consumption of nitrogen or
inert for changing atmosphere in tanks and the time to be used for the same
purpose. The formula is a bit different, if we use inert versus nitrogen.
11.6.1   
Using nitrogen
When
we use nitrogen, the oxygen content is 0%. That means we should purge 21%
oxygen from the air to a given maximum content of oxygen in the tank e.g.
0,2%.
 
11.6.2   
Numbers of volume changed is ln (original O2 content/ desired O2
content)



 ln


Natural
logarithm




Original
O2 content


The
original content of O2 in the tank that we should purge




Desired
O2 content


The
specified O2 content given in the charter party




Numbers
of volume changed


The
number of times the specified tank capacity needs to be completely
purged of nitrogen



If
we have one tank at 1000 m3 capacity and the O2 content,
according to the charter party,
should
be less than 0,2%, we start with air in the tank. The calculation will be as
follows:






Number
of volume changed =


ln
( 20,8% / 0,18%)




 




ln
( 115,56)




Number
of volume changed =


4,75



The
desired O2 content has been set to 0,18%, to be less than 0,2%.


The
nitrogen consumption will then be 1000m3 * 4,75 which equals 4750m3
nitrogen. This is the minimum required nitrogen. When ordering nitrogen add
10% to the minimum needed (5225m3) nitrogen.


11.6.3  

Using inert
When
using inert we also use the formula with ln “natural logarithm" but we
must calculate the O2 content in the inert gas also.
 


11.6.4   
Numbers of volume changed is ln (original O2 content/ desired O2
content)



 ln


Natural
logarithm




Original
O2 content


The
original content of O2 in the tank that we should purge




Desired
O2 content


The
specified O2 content given in the charter party




O2
content in the inert


The
O2 content that we set the inert gas generator to give, never
above 5%




Numbers
of volume changed


The
number of times the specified tank capacity needs to be completely
purged of nitrogen



Take
an example with the same tank at 1000m3. The charter party states
maximum 2% O2 and the O2 content on the inert is set to
0,5%. We start with air in the tank.



Number
of volume changed =


ln
( 20,8% - 0,5% / 2% - 0,5%)




 




ln
( 13,53)




Number
of volume changed =


2,61



We
have to subtract the inert gas O2 content from the original and
desired O2 content.
The
total consumption of inert will be 1000m3 * 2,61 = 2610m3.
11.6.5   
Use with allowed vacuum
On
vessels that have facility to have some vacuum on their tanks they can use
their compressors to create the minimum allowed pressure in their tanks. 
If your vessel can have 30% vacuum in the tanks, it means a 0,7 bar
absolute pressure. 
When
you have 30% vacuum you have already quit 30% of the oxygen, which means you
have 20,8/100*70 = 14,56% oxygen left.  When
we come alongside we pressurise the tanks with nitrogen to 1 bar absolute.
Then we can continue the purge normally.
To
pressurise the tanks to 1 bar absolute we need 30% of the total capacity of
our tanks.  If we should purge one
tank on 1000m3, we need 300m3 of nitrogen.
 









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