ANCHORING SYSTEMS AND PROCEDURES FOR LARGE TANKERS

1. INTRODUCTION

Why is it thought that this booklet is needed?

It is because experienced seamen are losing anchors and/or cable, or experiencing windlass damage when anchoring VLCCs. This indicates that there is a need to consider the anchoring systems and the application of techniques to assist Masters and Owners in a better understanding of the factors involved.

Present anchoring arrangements for large vessels were developed by various bodies from a vast experience with anchors, equipment, and systems on smaller vessels.

In the early 1960's, before the advent of VLCCs, the International Association of Classification Societies (IACS) laid down ground rules governing the requirements for ship's anchoring equipment. Basically the requirements were that ships should be fitted with equipment capable of holding the ship at anchor in sheltered and semi-sheltered waters in winds of up to gale force strength. The rules only covered the broad parameters of the anchor system, chain diameter, and anchor weight for example. Many of the other components within the system were left to the discretion of the shipbuilder or shipowner.

The anchoring equipment on smaller vessels is generally acceptable, but equipment on larger vessels is subject to criticism and in some cases may even be regarded as being below an acceptable standard. Certainly anchors cannot be used on a large ship with the same degree of versatility as they are being used on small ships, where they are routinely used to assist in making certain maneouvres.

Development of equipment in this way led firstly, to unrecognised poor design on small ships becoming magnified and, therefore, noticed with the increase in ship size; and secondly to the insufficient upgrading of ships' equipment relative to an increase in actual ship size.

The table below illustrates this, and a comparison between a 25,000 Dwt. tanker and a 250,000 Dwt. tanker shows that whilst they vessel's weight has increased by about 1000 %, the cable strength has only been increased by slightly over 260%.

TABLE 1

WEIGHT					STUDLINK CHAIN			CHAIN BREAKING LOAD
SHIP DWT.	EQUIP. NO. (EN)	(kg) STOCKLESS	(kg) HHP	LENGTH (m)		DIAM. GR2(mm)	DIAM. GR3(mm)	GR2 TONNE	GR3 TONNE
25,000 50,000 100,000 250,000 500,000	2080-2330 2870-3040 4000-4200 6100-6500 9400-10000	6,450 8,700 12,300 18,800 29,900	4,837 6,525 9,225 14,100 22,425	605 632.5 687.5 742.5 770		70 84 97 120 152	62 73 87 107 132	263 368 477 694 1030	300 407 561 812 1165

Following a succession of accidents involving the anchoring system on large tankers, some concern has been expressed regarding the adequacy of the equipment fitted.

For instance, is it practical to have a direct relationship between deadweight and cable strength? Under what conditions of wind, wave and current is the anchor expected to hold the ship, and how much consideration has been given to dynamic forces? Furthermore, it must be realised that as the anchor gear is a system, the upgrading of one or more specific components to solve one problem may result in transferring failures to other parts of the system.

Most companies issue instructions to their Masters advising on the use and operation of the anchor equipment, but often these instructions are basic and do not contain sufficient information for the Master to make a reasoned judgement.

It would appear that information to Masters based on the following may be useful:-

a) The capability of the whole system should be known, including the details of the various parts of the system. These may be broadly divided into 4 parts:-

i) Type of anchor, proof load, weight and the expected holding power in relation to various sea bed types.

ii) Grade of anchor chain, size and proof load, and weight of the chain.

iii) Windlass capacity with respect to the amount of cable, together with anchor, that the windlass is capable of recovering in a vertical lift. The maximum rate of cable recovery for which the windlass is capable.

iv) Windlass brake performance criteria. Material specification and limiting speed for reducing the possibility of brake fade.

N. B. Regular maintenance and inspection of the windlass and anchor equipment are essential to ensure that the original performance specifications are maintained. (See Section 11).

b) Data relating to the forces generated by various weather and tidal conditions should be supplied to the Master to enable him to determine the effectiveness of the anchor equipment on his vessel under varying circumstances.

c) Data, or an approximate means of calculating this data, on the capability of the anchor gear to absorb the momentum of a moving ship.

The need to supply information such as that listed above, implies that the design and the understanding of anchoring systems and techniques could be improved. Nevertheless, because seamen have adapted their techniques in the light of experience, accidents with, and losses of anchoring equipment are now less common.

This booklet seeks to describe the development of anchoring equipment, together with the safe techniques which have been developed to make the best use of that equipment.

2. THE PROBLEM - A REVIEW OF INCIDENTS INVOLVING ANCHOR SYSTEM DAMAGE

2.1 Anchoring and mooring equipment is still achieving the results that experience has shown to be an acceptable compromise between cost and reliability. It is only when ships become large that the marginal performance, based on previously accepted criteria, is apparent.

Few significant anchoring problems arose until large vessels came into operation and the size of the anchoring equipment had been increased to absorb the greater forces.

TABLE 2

Summary of Anchor, Cable and Windlass Defects on Large Ships

No. of Ships At Risk	Ship Years	Anchor Defects	Cable Defects	Windlass Component Defects	Windlass Prime Mover Defects
474	2397	53	40	119	35

1) The ships included in Table 2 are:

a) 240m or more in length

b) Built 1965 to 1977 inclusive

c) In service 1969 to mid 1978 inclusive

2.1 Lloyds Register have over the years carried out a number of investigations into the incidence of anchor and chain failures, but the cause and the circumstances in which they have occurred, such as location, type of sea bed, weather conditions and operating procedures are generally not known in sufficient detail. Consequently, the causes of failure are difficult to identify. Additionally, equipment shortcomings are masked by the particular care being taken by ship's staff during anchoring operations.

2.2 The largest single cause of anchors being lost is due to the performance of the brake during the anchor drop. Whether it is due to the inefficiency of the brake, the method of application, or an excess of dynamic loading, requires further investigation.

It would appear that most brake failures occur due to their inability to absorb energy in the process of arresting the moving cable/vessel (dynamic load); rather than the inability to restrain movement of the anchored vessel (static load).

Though both restraints need to be met by the same equipment, the requirements are not identical.

TABLE 3 Causes of anchor losses

NO. REPORTED CAUSE

71 Anchors were lost. definitely or very probably due to windlass brakes failing to hold. In 69 cases all the cable ran out and in two cases the cable broke in way of the gipsy when the bitter end was reached in the chain locker.

5 Anchors were abandoned due to loss of motive power to the windlass at the time it was needed. (2 cables were cut, 3 were run right out and buoyed.)

2 Anchors were abandoned after windlass clutch failures, the cables being cut.

1 Anchor was abandoned after the windlass broke away from its seating, the cable being cut.

3 Anchors were abandoned for operational reasons (e.g. fouled on a pipeline). In each case the cable was cut.

2 Anchors were lost as a result of ship collisions.

10 Anchors were lost due to broken cables. In 3 cases links failed; in 7 cases details were not reported.

10 Anchors were lost due to failed anchor shackles or anchor shackle pins. In 4 cases parts of failed shackles were recovered with their pins missing, in one case the shackle body was recovered with its pin attached.

5 Anchors were lost due to fractured shanks.

3 Anchor heads vanished together with their crown pins.

1 Spare anchor left ashore without permission.

1 Spare anchor washed overboard when the weather deteriorated while it was being overhauled.

5 Anchors were lost without a cause being stated.

TABLE 4

Circumstances of apparent and definite windlass brake failures resulting in anchors being lost

1 Anchor suddenly vanished down the hawsepipe while the ship was proceeding at 14 to 15 knots in deep water. It would appear that neither the brakes nor the cable stopper had been properly secured.

1 Anchor was dropped due to a misunderstood order while the ship was proceeding astern under power at about 41/4 knots.

2 Anchors lost when windlass brakes were stripped down for overhaul without cable stoppers being properly closed.

7 Losses were reported as being due to heavy weather after the ship had been at anchor for some time.

28 Losses were reported as occurring while the anchor was being dropped.

19 Losses were attributed to "mooring", without further explanation.

3 Losses were attributed to grounding, without further explanation.

10 Losses were left unexplained in the reports.

If "mooring" can be assumed to be the same as "while the anchor was being dropped", then two thirds of all brake failures occurred during this operation. If the unexplained losses also occured during such operations the incidence rises to 80 per cent.

TABLE 5

Distribution of anchor defects

ITEM CRACKED BENT CORRODED WORN LOST

Shackle 1 2 1

Shackle pin

Shank 2 2 2 1

Crown pin 3 1

Crown bearings 1

Crown 13 1

Flukes 4 5 5

More than See Table

one component 2 2 2 3 3

2.3 Of all the items under review, chain cable appears to be the most consistently reliable. The incidence of failure and defect have not increased with ship size, in fact, rather the reverse. At present on large ships, an unusually good cable life seems to result from slipping windlass brakes, and by anchors that do not hold their design loads.

2.4 Nearly 3 % of lost anchors appear to have resulted from cable stoppers accidentally releasing the cable when they should have been closed. It would seem that normal ship vibrations are sometimes sufficient to permit guillotine bars to work upwards until they finally become ineffective and dangerous.

2.5 An increase in damage to anchors from heavy weather while they are housed in the hawse pipe has also been evident in recent years.

Wave impacts on the anchor crown drive the anchor further inboard causing very high bending moments on the shank and flukes, and the constant jarring causes pins to loosen.

It seems that it is not uncommon for designers to try to prevent the shank rattling in the hawsepipe by designing the hawse pipe to give the anchor three point support from two flukes and the shank, the shank being drawn into the ship in a manner inducing a wedging action in the hawsepipe.

Several cases have been reported where tugs had to be hired to free the anchors from their housed position. In each case the ship's own winches, hauling a wire round a sheave on the quayside were unable to generate sufficient tension to pull those anchors free.

2.6 The windlass components, bed plate and securing system must be capable of withstanding the stress and vibration imposed. Incidents have occurred when the clutch could not be engaged, due to the slewing of a bed plate, necessitating the slipping of anchor and cable.

2.7 Shanks of high holding power (HHP) anchors can sometimes bend sideways when a ship swings to wind and tide, and tries to drag the anchor round with it. The worst affected designs seem to be those with large bolsters built into the crown, e.g. A.C.14.

2.8 Although equipment failures are far too common, a review of the accident statistics suggests that correct operating procedures and maintenance checks could eliminate some of the causes for anchors being lost.

3. ACCEPTED DESIGN STANDARDS

The requirements for anchor weight and chain size, together with the method of calculating the Equipment Number (EN) are among the items which have been unified by the International Association of Classification Societies (IACS), which encompasses all major societies. Before the agreement of the IACS working party, the societies had used different methods to calculate the EN but the resulting values varied very little from one society to another. It was also felt that the unified requirements for anchor weights should not be markedly different from those specified under the societies' old rules. In fact, anchor weights were unified without much trouble as soon as it was realised that the requirements of all the classification societies already coincided very closely with the formula:-

EN x 3 = anchor weight (in kg)

Lloyds use an Equipment Number calculated as follows:-

EN = D^2/3 + 2BH + A/10 (metric)

where D = moulded displacement in tonnes to the summer load line

B = Greatest moulded breadth in metres.

H = Freeboard amidships, in metres, plus the sum of the heights at the centreline, in metres, of each tier of houses having a breadth greater than B/4.

A = Area, in square metres, in profile view of the hull, and of superstructure and houses above the summar load waterline.

The equivalent formula for imperial measure, using feet and Long Tons is:-

EN = 1.012 D^2/3 + 0. 1 86 BH + 0. 00929 A

Table 6 lists typical anchoring equipment required to be carried by tankers of various sizes.

3.1 A very important component that is not covered by the classification society rules is the windlass.

Many of today's windiasses are combined with mooring winches and their siting may be influenced by the needs of the mooring line leads. This can produce a less than ideal lead as far as the hawse pipe is concerned and thus reduces the effective pulling power of the windlass.

As far as the windlass is concerned IACS only require that it be fitted, efficiently bedded and secured to the deck. IACS do offer guidelines as to the required performance criteria of the windlass, but the final performance and power is left to the discretion of the owner or shipbuilder. (See Section 7).

TABLE 6

Tanker Anchoring Equipment

Tanker deadweight Tonnes 50,802 101,605 152,407 203,209 254,012 304,814

Tanker displacement Tonnes 66,408 127,805 188,389 247,213 303,842 362,012

Water Length Metres 217.6 262.7 298.4 320.3 335.9 347.5

Beam, B Metres 30.2 39.6 45.4 50.6 53.6 54.9

Loaded Draught, T Metres 12.80 15.24 17.07 18.47 20.12 22.86

Lateral Proj. area, A Metres² 2044 2341 2388 2434 2481 2527

Engine Power Megawatts 10.43 16.45 20.63 23.73 25.55 26.40

Propeller Static Thrust Tonnes 112 176.5 221.3 254.6 274.1 283.2

Lloyd's Equipment No. 2906 4224 5489 6414 7187 8198

Rule stockless anchor Wt Tonnes 8.70 12.90 16.10 18.80 21.50 24.50

Rule H.H.P. anchor Wt Tonnes 6.53 9.675 12.075 14.10 16.125 18.375

Maximum holding pull for

both stockless anchors Tonnes 60.9 90.3 112.7 131.6 150.5 171.5

Maximum holding pull for

both H.H.P. anchors Tonnes 130.5 193.5 241.5 282.0 322.5 367.5

4. THE FORCES TO BE WITHSTOOD

A survey, carried out by OCIMF showed that the majority of VLCC anchorage areas are in the range of water depths from 20-30 fathoms. Effective anchoring requires a length of chain outside the hawse pipe equal to 6-10 times the water depth. Since the length of chain required by IACS rules, and therefore carried on ships, is 210 fathoms, the maximum depth for effective anchoring is limited to 35 fathoms, and at this depth the cable would be payed out to the bitter end.

Once the vessel is at anchor, today's large tanker has equipment that may be regarded as being capable of holding the ship safely on reasonable holding ground within certain current and wave limitations. When the depth to draft ratio is less than 2, the current and wave forces can be very significant.

4.1 Table 7 shows that using IACS design criteria which consider only wind forces, but not current, the anchor equipment is sufficient to hold ships safely at anchor in gale force conditions. Notice that as the depth to draught ratio decreases, and also, as the current is introduced, so the standard stockless anchor would start to drag.

TABLE 7

Thrusts on Tankers due to 25m/sec (50 knots) wind and 2.5 m/sec (5 knots) Current in Sheltered Anchorage Free of Waves

Tanker Dead- Weight	Depth to Draught Ratio	All	Forces Acting	Head-On	Two A. S. S. Anchors, Holding Pull
Tonnes		Wind Tonnes	Current Tonnes	Total Tonnes	Tonnes
50,802	3.0 2.0 1.4 1.2 1.1	20.05 // // // //	18.79 30.56 47.54 56.58 63.37	38.84 50.61 67.59 76.63 83.42	60.9
101,605	3.0 2.0 1.4 1.2 1.1	22.96 // // // //	27.00 43.92 68.32 81.32 91.07	49.96 66.88 91.28 104.28 114.03	90.3
152,407	3.0 2.0 1.4 1.2 1.1	23.42 // // // //	34.33 55.85 86.87 103.44 115.86	57.75 79.27 110.29 126.86 139.28	112.7
203,209	3.0 2.0 1.4 1.2 1.1	23.87 // // // //	39.90 64.90 100.95 120.78 134.60	63.77 88.77 124.82 144.05 158.47	131.6
254,012	3.0 2.0 1.4 1.2 1.1	24.33 // // // //	45.56 74.10 115.26 137.22 153,67	69.89 98.43 139.59 161.55 178.00	150.5
304,914	3.0 2.0 1.4 1.2 1.1	24.78 // // // //	53.56 87.11 135.51 161.33 180.69	78.34 111.89 160.29 186.11 205.47	171.5

Therefore, to remain safely at anchor, a high holding power anchor, reckoned to be about 2 1/2 to 3 times as efficient as a standard stockless anchor, would have to be used.

An alternative method of illustrating this relationship is shown in table 7A.

NOTES

1 . Figures in brackets are cable scope, based on loaded tanker configuration.

2. Anchor cable force shown is the highest value for loaded or ballasted tanker.

3. 'A' and 'B' include 5 knot current.

4. 'A'includes drift force due to 20ft bow wave based on May 1981 draft of "API Recommended Practice for the Analysis of Mooring Systems for Floating Drilling Units".

5. 'C' includes neither current nor wave forces.

TABLE 8

All Forces Acting Head-On Wind Head-On, Current 10 Starboard,

Tanker Depth to Waves  Port

Dead- Draught

Weight Ratio Wind Current Waves Total Current Waves Total

FL FL FL FL= FT FL FL FL

Tonnes Tonnes Tonnes Tonnes Tonnes Tonnes  Tonnes Tonnes Tonnes

50,802 1.4 30.60 18.14 64.125 112.87 63.02 2.077 64.08 63.02 157.07

1.2 30.60 21.59 64.125 116.32 75.02 2.473 64.07 75.02 169.69

1.1 30.60 24.18 64.125 118.91 84.02 2.770 64.05 84.02 178.67

101,605 1.4 35.05 26.07 84.20 145.32 90.57 2.487 84.13 90.57 209.75

1.2 35.05 31.03 84.20 150.82 107.82 2.961 84.09 107.82 226.96

1.1 135.05 34.75 84.20 154.00 120.76 3.316 84.06 120.76 289.87

152,407 1.4 35.75 33.15 96.51 165.41 115.21 2.787 96.40 115.21 247.36

1.2 35.75 39.47 96.51 171.73 137.15 3.318 96.35 137.15 269.25

1.1 35.75 44.21 96.51 176.47 151.61 3.717 96.31 153.61 285.67

203,209 1.4 36.44 38.52 107.52 182.48 133.85 3.025 107.37 133.85 277.66

1.2 36.44 45.86 107.52 189.82 159.34 3.601 107.31 159.34 303.09

1.1 36.44 51.36 107.52 195.32 178.46 4.034 107.26 178.46 322.16

254,012 1.4 37.14 43.98 114.00 195.12 152.84 3.297 113.81 152.84 303.82

1.2 37.14 52.36 114.00 203.50 181.95 3.926 113.73 181.95 332.82

1.1 37.14 58.64 114.00 209.78 203.78 4.397 113.66 203.78 354.58

1.4 37.83 51.71 116.59 206.13 179.7 3.744 116.34 179.67 333.84

1.2 37.83 61.56 116.59 215.98 213.89 4.458 116.24 213.89 367.90

1.1 37.83 68.95 116.59 223.37 239.56 4.994 116.15 239.56 393.54

Thrusts on Tankers Due to 30.9 m/sec wind (60 knot), 6.1 m Waves, 204 m wavelength and

1.54 m/sec (3 knot) Current at Depth to Draught Ratio of 1.4, 1.2 and 1.1

Notes: -

 is the angle of the waves to the tanker's port bow to balance transverse forces due to currents acting at 10 on the starboard bow.

LONG THRUST = FL TRANSV. THRUST = FT

4.2 When wave effect is introduced, sufficient force can be generated which would cause even high holding power anchors to drag. A typical locale for this would be the English Channel during winter months.

4.3 Some points which come out of the data/tables that are possibly not common knowledge to the shipmaster are:

1) The IACS design and strength requirements for anchor equipment are based on wind forces only and make no allowance for other forces.

2) Forces generated by currents and waves can be of far greater magnitude than those created by the wind.

3) Forces generated by currents increase significantly with a reduction of underkeel clearance.

4) The relatively large increase in size and weight of anchor gear used for large ships result in a relatively low increase in strength and holding power.

5) IACS do not have rules for all components in an anchoring system.

4.4 The limitations of large ships' anchoring equipment becomes more critical when the ship is actually in the process of anchoring. Calculations and tests have shown that vessels of 250,000 dwt and over can suffer equipment failure if attempting to anchor whilst moving at speeds as low as half a knot over the ground.

This is because even though anchoring equipment has been increased in strength and size in compliance with IACS regulations, the capability of this gear to absorb the kinetic energy, or momentum, of large ships has decreased considerably.

5. ANCHORS

In the offshore industry the philosophy of anchoring is that the cable should break before the anchor drags. An anchor intended for permanent moorings should bite and develop its full holding power exactly where it is dropped, and since it is lowered under the control of a mooring, or anchor handling vessel, it needs only one fluke and no working parts. After use, it is of course, also recovered by an anchor handling vessel.

For these permanent moorings some very efficient anchors have been developed. Names such as Danforth, Bruce and Delta Flipper are familiar.

5.1 However, the safety of a ship at anchor depends to a very large extent on the degree of efficiency and characteristics of the anchor itself. The philosophy is that the anchor should drag before the cable parts. The design must be such that it must take a grip on any type of holding ground and be robust enough to take the heavy loads imposed upon it, not only riding to anchor, but also the dynamic loads imposed during anchoring and when swinging. It must stow easily, neatly, and firmly in the hawse pipe.

To permit the bower anchor to stow easily and neatly, it is not fitted with a stock. The head of the anchor is designed to pivot approximately 30°/45° either side of the shank. This not only helps the anchor to bite into the sea bed when anchoring, but also to fit snugly into the hawse pipe. It is, therefore, important when re-anchoring that the anchor be sighted (i.e. visually checked) and cleaned before use, to allow unrestricted movement of the flukes and therefore maximum holding power. The omission of the stock causes a loss of stability, and hence when dragging, the modern anchor tends to turn over.

5.2 The holding power of an ordinary standard stockless anchor is about three times its weight. As ship sizes have increased, the weight of anchors has increased, but not proportionally and efficiencies, therefore, have fallen until the holding power of ordinary standard stockless anchors on a VLCC may be regarded as marginal.

5.3 High holding power (HHP) anchors have become common in the last ten years and their extra efficiency more than compensates for the effect of absolute size.

The AC 14, probably the most popular HHP anchor was developed shortly after the second world war, and was shown to be about 2 1/2 to 3 times as efficient as the standard stockless anchor. During the development it was established that:

i) Fluke area, not the weight, is the most important factor in the holding power of an anchor in most types of sea bed, and:-

ii) that a suitable minimum scope of chain is critical if the anchor is to bury itself properly.

Some HHP anchors have proved to be up to 4 times as efficient as stockless anchors, but the 25% reduction in weight is the maximum allowed by the classification societies.

This reduction in weight must be limited because the weight of an anchor can sometimes be critical if a ship has to drop anchor in poor holding ground, such as soft mud or slab rock. Also, an anchor must be strong enough to withstand being dropped on solid rock or pulled against obstructions such as boulders on the sea bed and this strength usually requires additional weight.

5.4 Another point worth considering is that anchors must be designed to resist snatch loads which will be in excess of the steady loads which would cause them to drag. This increased holding ability, or resistance to snatch loads, is probably associated with suction effects set up behind the flukes if soil and water cannot flow into the area as the anchor tries to move.

5.5 The nature of the bottom has probably more influence on the holding power of anchors than any other single factor.

In sand, anchors will normally penetrate easily. During the short moment of tripping, when the anchor rises on its fluke points, it cannot fall on its side. As sand has a high coefficient of friction, the anchor must be capable of the easiest penetration.

Mud is a reasonably good holding ground, because it allows the anchor to penetrate completely and will cover the flukes. However holding power will generally be half that of sand.

Soft clay, combined with mud layers, is a variable holding ground. It seems to be good at first but when disturbed during penetration, the holding power can be immediately reduced to 30%, particularly where the clay is of an unconsolidated type.

Rock is a very bad holding ground. The holding power will generally be one third of the weight of the anchor. On a very hard bed such as rock, the anchor drags, standing up so that the flukes are on their points. As the pull increases, either the anchor will penetrate, or it will "walk" along until it comes to a soft patch in which it can embed itself. Alternatively, the flukes may catch on an obstruction or the crown become wedged between rocks.

5.6 The following test results compare the efficiency of a Standard Stockless Anchor (S.S.) against an A.C. 14 anchor under varying sea bed conditions.

Holding Power

Type of Anchor Shingle/Sand Rock with Layer Blue Clay

of Mud and Sand

A. C. 14 8 x weight 2.4 x weight 10 x weight

S. S. 3.5 x weight 1.8 x weight 3 to 4 x weight

6. CABLES

The anchor cable plays a vital role in the ability of a ship to ride safely at anchor. The scope of cable, or the ratio of the length of cable to the depth of water, affects the angle "A" at which the cable will pull on the anchor and thus affects the holding power of the anchor.

The figure shows that with only 5 degrees of cable scope angle (A) at the main anchor shackle the anchor loses nearly a quarter of its pull, and if the inclination is increased to 15 °, more than half the holding power is lost.

6.1 Over the years the recommended scope of cable has increased. Early seamanship books advised using a scope of 3, later editions increased this to 6, and even this scope may be insufficient.

The dramatic change in advised scopes corresponded to a change in cable material from iron to forged steel, the latter having a proof test 40% greater than the former and in consequence being much lighter. It is, however, recognised that the most suitable scope for cable should be judged on the basis of holding ground, wind, current and underkeel clearance. However, in crowded anchorages the optimum requirements may not be met.

6.2 Chain cable may be of wrought iron, mild steel, or special quality steel. Wrought iron and certain less efficient grades of mild steel may not be used with HHP anchors and presumably to keep some weight in the cable the Rules state that the extra special quality steel, U3, may only be used for chain 40mm or more in diameter.

The form and proportion of links and, shackles are in accordance with quoted International Standards.

Typical weights for anchor chain are shown in the table.

On a 250,000 dwt vessel, fitted with 130mm diameter cable, one shackle (15 fathoms) (27.5 metres) of that cable will weigh 10, 000 kg in air, or 8,500 kg in water.

6.3 Sufficient cable scope is an important factor in absorbing shock loads, thus reducing their impact on the ships windlass system.

An extended scope of cable can also protect the anchor as the vessel swings to tide or wind.

7. WINDLASSES

Classification societies and the International Standards Organisation (ISO) have in the past few years taken a much greater interest in the arrangements for handling large anchors and IACS have agreed windlass performance criteria having different requirements depending on the grade of chain used.

7.1 Lloyds took a different view. It was felt that the requirements set out by IACS were too complicated. Because windlasses are more or less standard items and the manufacturer does not usually know what grade of chain they will be used to handle, Lloyds based its Rules on one set of performance values. These are equivalent to those recommended by IACS for U2 chain, which apply in all cases. Other societies too, have yet to include the IACS recommendations in their rules.

7.2 Lloyds requirements are for "a windlass of sufficient power, suitable for the size of chain cable to be fitted and efficiently bedded and secured to the deck". The deck is to be strengthened to the Surveyor's satisfaction.

The performance criteria quoted determine if a windlass has sufficient power and is suitable. They relate to:

- a steady pull over 30 minutes

- a peak pull for 2 minutes

- a heaving speed

- a brake proof load

- a design which will not collapse under a load equivalent to the breaking load of the cable.

7.3 On a windlass fitted with two cable lifters the criteria do not require both anchors to be raised or lowered simultaneously.

7.4 An equation used for calculating windlass pulling power is:

Continuous pulling power "Z" (kg) = Constant x (chain dia mm)²

The constant is based on the chain grade; for grade 2 cable it is 4.25, and for grade 3 it is 4.75.

For example, in the following conditions:-

Wind Beaufort scale force 6

Current 3 kts.

Anchorage depth 100 metres

Lifting rate 9 metres/min.

On a vessel of 250,000 dwt., fitted with 120mm grade 2 cable we find that:

"Z" = 4.25 x 1202 kg

= 61200 kg

= 61.2 tonne

Assuming that the vessel is fitted with an AC14 anchor (weight 16 tonne) and that the cable weighs 0.6008 tonne per fathom then it is feasible that this vessel, allowing a 5% safety margin could lift just over 4 shackles of cable and the anchor in a vertical lift.

This performance capability would appear to be satisfactory and is in line with the windlass performance traditionally quoted for windlasses for small ships in seamanship books. However, it draws attention to the fact that there are physical limitations to anchoring in deep water.

7.5 As outlined in Section 2, windlass brake failure from whatever cause, is the reason for many lost anchors. Modern VLCC brake design, in a number of ways tries to overcome the inter-related factors of:

a) brake application

b) brake fade

c) maximum speed of payout.

7.6 In VLCCs the force required in the brake band necessitates such a large number of turns for the required mechanical advantage that the use of a handwheel has become, in some cases impractical.

When the brake was released manually, the anchor and chain would accelerate so quickly that the limiting speed would be passed before the operator could wind the handwheel back on far enough to regain control on the brake. This limiting speed is defined as the speed at which heat generated in the brake lining would cause fading of the brake. When this occurred the braking force was reduced so much that the anchor and chain could not be stopped.

To improve the response of the brake controls, hydraulic assistance is provided. As the brake is released and the speed increases, hydraulic pressure is generated, which re-applies the brake, so preventing the gypsy from exceeding a pre set rotation rate. To stop the windlass, it is necessary to re-apply the brake.

Another device utilises a system where the brake is held on by a spring. To release the brake, hydraulic pressure is used to oppose the spring and release the brake. As the speed increases, the control system reduces the hydraulic pressure, re-applying the brake, and so controlling the speed of release. This latter device is considered fail safe.

Equally important is the quality of the brake band and the proper functioning of the hinges and linkage to ensure:

i) minimal friction of moving parts, and

ii) maximum brake surface contact.

It may also be necessary to adjust the pivoting point on the brake band to compensate for reduced diameter due to brake weardown.

Windlass design has been subject to considerable advances in recent years, although the improvements have not yet shown up in accident statistics. The improvements are as a result of work on offshore platform mooring winches, the injection of money and experience coming to builders and designers from large tanker owners, and new ISO Standards and classification society rules.

8. STOPPER, LEADS AND STOWAGE

The arrangement of anchoring and mooring equipment on the forecastle is a compromise between the requirements of numerous inter-relating factors. For example, the angle at which the windlass is set will be influenced by the leads for associated mooring ropes, or the hawse pipe requirements which will in turn be affected by hull form and even the type of bulbous bow.

Furthermore, the windlass position must allow for cable stoppers between it and the hawsepipe and for proper securing arrangements for the anchor when in the stowed position. The windlass position must also be over the chain locker which will be positioned according to the internal structure of the forebody,

Classification society rules are imprecise. For example, hawsepipes are only required to be of "ample thickness and of a suitable size and form to house the anchors efficiently, preventing, as much as practicable, the slackening of the cable or movements of the anchor being caused by wave action", and an assumption is made that they will cause friction losses of 30%.

Mention is also made of the need to reinforce "those parts of bulbous bows liable to be damaged by anchors or cables". Also, substantial chafing lips are to be provided at shell and deck, or alternatively roller fairleads of suitable design may be fitted.

8.2 For the Master, the pertinent phrase is that referring to housing the anchors, where it states that the hawse pipe or anchor pocket must be of "a suitable size and form to house the anchors efficiently, preventing, as much as practicable, the slackening of the cable, or movements of the cable being caused by wave action".

The best method of securing the anchor, is undoubtedly by a purpose designed chain stopper, which even if the brake were to slacken and other securing arrangements fail, will prevent the anchor and cable running out. Such chain stoppers should be fitted with a means to prevent accidental vertical displacement of the securing bar or pawl.

In the early days of a ship's life, such a cable stopper, if properly positioned, will prevent movement of the anchor in the housed position. Inevitably though, wear on links, shackles, and on the anchor produces slackness in the system, hence the need for a secondary securing system.

If the anchor can move in the hawse pipe, especially in the loaded condition, the anchor flukes may pierce the bow. Prolonged violent slamming can fracture the hawse pipe, damage the anchor, loosen pins and fittings and even cause the cable to run out.

8.3 To prevent this, a secondary securing arrangement comprising of chains and bottlescrews is provided. Unless sufficient thought is given by designers and builders to this secondary securing arrangement it can constitute a safety hazard for personnel and create problems in heavy weather. At sea, it often requires retightening.

8.4 The chain locker should be both large and deep enough to provide an easy direct lead for the cable into the spurring pipe. Access doors should be watertight to prevent flooding of the forward spaces.

The modern requirement is to have a vertical cylindrical chain locker of such diameter that it prevents cable falling over and jamming in the locker. The bitter end is often secured externally where it is more accessible.

An efficient means of excluding the entry of water into the chain locker via the spurring pipe often receives little consideration from designers. Flooding of chain lockers is not uncommon after prolonged heavy weather. Unless the inspection doors to the chain lockers are maintained watertight, flooding of the forward storage area is also a possibility.

Eductors for draining chain lockers and forward spaces should be tested periodically and kept operational.

9. ANCHORING

It is not the intention of this booklet to provide anything other than guidance with regard to anchoring a large tanker. Masters, with knowledge of their ship, with their experience and with common sense, should be able to determine the most suitable method of anchoring their ship under given circumstances.

Nevertheless, the advent of large tankers has brought about methods of anchoring which require basic ship handling and seamanship. The techniques used may or may not be covered by seamanship books. Certainly, seamanship books have not, so far, related these techniques to the anchoring of large tankers. Therefore it is thought well worthwhile for this booklet to restate them in this context, even though some of the guidance may duplicate advice offered elsewhere.

9.1 Preparation for Anchoring

A certificated/licensed deck officer must supervise letting go or weighing the anchors, and should assign only experienced crew members to anchor work.

Before anchoring, the Master must satisfy himself that the berth is suitable for the ship, clear of other vessels and that the conditions provide a safe haven. Since conditions are subject to change and the movement of the vessel likely to alter the attitude of the anchor on the sea bed, there is a necessity for continued surveillance.

9.2 Prior to the anchoring party going forward, the officer in charge should be aware of the:-

- approximate anchoring position

- method of approach

- which anchor

- depth of water

- method of anchoring (see 9.4)

- final amount of cable.

The anchoring party should not only be on stand by forward, but have all preparations made - anchors cleared, steam and/or hydraulics on, anchor light/shape prepared etc. - before the vessel enters an anchorage area.

Remember that on a large tanker

THE APPROACH TO AN ANCHORAGE IS A SLOW, TIME CONSUMING OPERATION WHICH MUST NOT BE RUSHED.

Unless a vessel is to anchor in a position designated by an outside authority, the Master should try and identify a suitable anchorage position before entering the anchorage area. In a crowded anchorage, the plotting on the chart of the positions of ships already at anchor will usually enable a suitable anchorage position to be identified. When an anchorage position has been selected by the Master, pilot, or port authority, a planned approach can be made to the chosen position. Often, the best direction of approach to the anchorage can be determined by noting the direction in which other vessels of similar type, size and draft are heading. By approaching the anchorage on the same heading, manoeuvring in a confined area can be minimised.

9.3 Before letting go the anchor, the anchored position and length of nearby vessels should be borne in mind.

The anchoring speed over the ground should be as near to zero as possible.

9.4 Method of Anchoring

There are basically two methods of anchoring large ships. Both have merits, the success of either method depending largely on the windlass ability to control the rate of cable flow, and the capability of the anchor system to absorb the kinetic energy.

Which-ever method of anchoring is used, the vessel must be stopped over the ground before anchoring.

There are several methods of determining if the vessel is actually stopped over the ground, but many are inaccurate or impracticable. The most reliable method would appear to be information obtained from a dual axis doppler log, but even doppler logs are sometimes unreliable when the engines are operated astern.

The traditional method of estimating speed through the water by means of "eye" is still practiced on some large ships, but this method does not take into account tidal and current forces. Judgement based on visual transits and/or radar ranges of landmarks or adjacent ships at anchor are more reliable.

9.5 Commonly Used Anchoring Procedures

Method I

Stop the ship over the ground, walk out the anchor and cable until the anchor is about half a shackle off the bottom. Hold the cable on the brake, take the windlass out of gear and drop the anchor. Control the speed of cable flow by the brake.

Disadvantages

1) If the cable is paid out too fast, through improper application of the brake, this can result in the anchor and cable piling upon the bottom and lead to poor holding.

2) If the brake fails, (brake fade) then the cable will run out to the bitter end with consequent damage.

Comments

With smaller ships, the piling of cable on the bottom was avoided by stretching the cable as it was paid out. Additionally, after a couple of shackles had been paid out, enough for the anchor to take a hold, the ship was allowed to swing round to the prevailing forces (wind or current) before paying out further. If necessary the main engines were used to indicate or check the motion over the ground as required.

This can work well on VLCCS. The one difficulty is that of seeing the lead of the cable, and watching it 'grow'. In a loaded ship especially one with a flush deck, the hawsepipe is very low and the cable attitude is difficult to ascertain. The degree of engine assistance required is also difficult to estimate.

One distinct advantage of using this method of anchoring is that the brake will render before critical stresses are reached.

Method 2

Stop the ship over the ground and anchor the ship by means of walking the anchor and cable out under power until the complete length of cable required is paid out on the seabed.

Cautions

This method produces a controlled cable flow, but an accurate estimation of the vessel's movement over the ground is essential to avoid major damage to the vessel's windlass. In no circumstances must the windlass be allowed to operate at a rate in excess of the manufacturer's recommendation.

Comments

In no circumstances must the weight on the cable be such as to cause the windlass to "free wheel"; full power must be available and used at such times. The anchor brake may also have to be used in extreme cases to control the speed. If the lead and weight on the cable is incorrectly judged, then the first indication of there being too much stress in the system will be windlass damage. The wear and tear on the clutch mechanism and windlass drive will require vigilance to prevent serious damage to the vessel's anchoring capability.

At Anchor

At least two schools of thought exist as to which is the best way to ride to the anchor. The alternatives are the cable stopper (or guillotine), or the windlass brake.

The cable stopper will certainly provide a secure fastening for the cable and will not slip. However, any requirement to pay out additional cable requires disengagement of the stopper.

On the other hand, a well maintained windlass brake acts as stress limiter and if the weight on the 'cable is excessive, the brake will render. Marking the cable so that it is visible from the bridge gives a good indication that, for instance, in increasing weather conditions it may well be time to heave up the anchor and proceed to sea. By waiting too long, weighing anchor in bad weather can become a hazardous operation for those on the forecastle head.

While at anchor, considerable dynamic stress on the anchor system may be induced due to the yawing of the ship under influence of wind and tide. This yawing motion can be moderated by lowering a second anchor on to the sea bed.

In any case when at anchor in bad weather, stress on the cable can be eased by:

a) using the engines.

b) using the rudder to attempt to reduce yawing, especially in a current.

The best indicator of such stress is the behaviour of the anchor cable. Personnel should check this visually, looking for signs such as amount of change in the cable catenary or for unusual effects, such as shocks when the cable tightens.

10. EMERGENCY ANCHORING

Existing anchoring systems, used conventionally, would be most unlikely to arrest the drift of a large ship unless its speed over the ground was less than half a knot. As kinetic energy is a function of ships mass and velocity, there is a lesser energy absorption problem on vessels of smaller size. Strength of components being a critical factor, it is apparent from the data in Table 1 that vessels under 50,000 Dwt enjoy an advantage over larger tonnage.

10.1 However, bearing in mind that every available means must be used to prevent a ship from going aground, the use of anchors in such an emergency should be attempted. The objective is to bring the vessel to a halt off a lee shore or danger and some windlass damage is acceptable in the interests of the vessel, cargo and crew. In the absence of research data, the following guidance is offered. It must, however, be realised that the governing factors in all circumstances will be:

i) the size of the vessel

ii) the speed over the ground

iii) the steepness of the seabed and proximity of shoals

iv) the nature of the seabed and anticipated holding power of the anchors

v) the nature of the wind and sea

vi) the condition of the ship propulsion and steering system

vii) the condition of the anchoring equipment

viii) the availability (and power) of tugs.

The possible permutations within the stated conditions make it impossible to provide specific instructions for each set of circumstances.

10.2 In emergency situations anchors should be made ready for use at the earliest opportunity. Deteriorating conditions may preclude or delay this action later, for example heavy weather may make the forecastle head inaccessible.

10.3 In water too deep for the anchor to reach bottom, lowering the anchor or anchors to about 60 fathoms will reduce downweather progress. The anchor and cable may have the effect of a drogue and should help to keep the ship's head to the weather. It should be noted that recovering 60 fathoms of chain and anchor should be possible as this amount is within the design basis for windlasses.

10.4 Once the ship is in a water depth where the anchor can find the bottom, use of anchors to arrest the ship should be attempted. If the bottom is sand or mud, it may be possible that the ship's movement can be slowed down or even arrested by slowly lowering the anchor until it begins dragging along the sea bed. For larger vessels, at first the scope should be short and later it should be gradually increased as the ship's speed decreases. This action should bring the ship's head into the weather and slow her speed over the ground. The chance of success when using anchors on a rocky bottom is much lower, but nevertheless it should be attempted if this is the only alternative available.

10.5 In one case, a VLCC drifted some eight miles in I I hours and it seems probable that she was moving at a speed of at least 1 1/2 knots when she attempted to anchor. It appears that the anchor snagged on a reef, causing both anchor and windlass to break. Considering that the kinetic energy of the ship was possibly nine times the capacity of the anchor system this is not surprising. If the anchor and windlass had not failed it seems almost certain that the chain would have broken. However, it can be argued that the windlass partial failure, came close to saving the ship, since it probably absorbed most of the energy of the moving ship and must have considerably slowed its drift.

11. MAINTENANCE

Routine maintenance and inspection of all items of anchoring equipment is essential.

Correct maintenance will ensure that equipment continues to operate to its performance specification. Methodical visual inspections will detect defects before they become hazardous to equipment and personnel. Some ideas and areas for maintenance and inspection are suggested below.

1. At Sea

Routine greasing should be carried out at least as often as:

a) After leaving port.

b) After heavy weather.

c) Before anchoring/entering port.

It is important that old or dead grease is removed, especially from internal grease channels. This can be done by pumping gasoil or kerosene through the grease points.

Anchor securing arrangements should be checked frequently.

Routine greasing/maintenance must include a methodical inspection and check of items such as operating controls, safety guards and condition of spurring pipe watertight arrangements.

A more thorough inspection should be made periodically. Such an inspection should include, where practicable, brake linings and linkages, tightness of bearing keep nuts, cotter pins in place and a check that there is no excessive play in bearings.

Spare shackles and equipment should be kept clean and free.

11.2 Anchoring Operations

Vigilance during anchoring operations can pay dividends in the avoidance of incidents.

Obvious points to observe during inspections, and any deficiency to be rectified as soon and as far as possible are shackles (see 11.3), cable markings, missing or loose studs, bent or damaged anchor flukes.

It is important to see that the anchor and cable are washed clean of mud.

Mud in the chain locker not only chokes drain suctions but can be dangerous for those on the focsle when next letting go the anchor as the dry mud is thrown up with the cable.

11.3 Shackle Pins

Occasions have arisen when anchors have been lost because the spile pin securing the main pin of the "D" shackle has come free allowing the main pin to move.

At regular intervals the "D" and swivel shackles should be inspected to verify that the spile pins are still in position, thereby ensuring the security of the main pin. If a spile pin is missing it must be replaced, certainly before the anchor is used again.

As a preventative measure the main shackle pin is sometimes spot welded to the "D" as shown in the sketch.

Welding in this fashion permits the "D" to flex over the main pin without danger of the main pin coming out.

11.4 Brakes

Band brakes can.often be adjusted to take up the slack in the brake system by increments. This adjustment, or taking up the slack is, in one particular windlass, made, firstly, by inserting distance pieces, and for a larger adjustment, by reversing a connecting link.

Caution

Brake linings, apparently identical to those produced by well known manufacturers are being imitated by less scrupulous suppliers. They may not perform to specification.

Ensure that replacement brake linings are obtained from a reputable source.

1. Dry Dock

The following procedures are suggested:

a) Cables should be ranged on the dock bottom, anchors inspected, all shackles opened, examined and rejoined. The outboard length of cable should be moved to the inboard length.

b) Cable markings checked and renewed.

c) Bitter end connection overhauled.

d) Spare anchor inspected and overhauled.

e) Chain locker cleaned and coated.

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