Supply vessels/anchor handling vessels

Drilling rigs are generally moored with 8 to 12 anchors. These are laid in a mooring pattern. Originally normal tugs were used for these operations, but very soon, there was a call for specialised vessels.

For anchor handling vesselss, it is very important to be able to work quickly and effectively. Much depends on the expertise of the captain and crew. The equipment and its design are also extremely important. Engine power has to be sufficient to handle chain and/or wire and anchors at the water depth concerned. The newest generation of AHVs has bollard pulls far in excess of 200 t.

Care should be given to the rated maximum bollard

94pull which in reality might be less, depending on the use of other power consuming equipment such as

bow (and sometimes) stern thrusters, winches, etc.

The winch often causes confusion. An AHV owner demonstrates maximum pulling capacity at the bare drum during the maiden trip, but a contractor requires high winch output when the drum is 70 to 100% wound with wire under working conditions. It is also possible that an owner limits the pressure of the hydraulic system below factory limits, to reduce winch wear and repair costs.

The dynamic capacity of the winch brake is particularly important when a long heavy chain must be deployed. Hydraulically and electrically braked drums are more efficient than band brakes.

For handling chain, many supply vessels have chain lockers below decks and a wildcat above the chain locker.

To ensure easy handling of chain and wire, simple, well-constructed tools are necessary. An experienced crew will also make the handling easier.

These specialised anchor handling vessels (AHVs) now have:

•A large deck space.

•Powerful winches, with auxiliary winches to reel extra wires.

•Large chain lockers, for storage of the chain.

•Large wire storage capacity.

•An adapted seaworthy design and very manoeuvrable with bow and stern thrusters. Some even with a dynamic positioning system.

•Space for drilling mud and fuel tanks for supply to drilling rigs.

•Small auxiliary cranes.

•One or two sets of towing pins and shark jaws.

•A stern roller that sometimes consists of two individually rotating drums.

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