VLCC - Anchoring.doc

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

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

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

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