The taut wire position reference system

Taut wire systems have been in use since the earliest days of DP, indeed, the first DP vessel Eureka of 1961 had taut wire position reference. Generally taut wire systems are less popular and numerous than hitherto, but are still being fitted on occasion. As with the Artemis system, taut wire technology has been overtaken by alternatives such as

Chapter 6 83 Position referencing

DGPS and Fanbeam.

A taut wire system consists of a davit or A-frame located on deck, usually at the vessel side. A weight of up to 500kg is lowered to the sea floor. Once located, the wire is placed under continuous tension, paying in and out to compensate for vessel movements. The position of the vessel is thus defined by the vertical angle of the wire, and the length of wire deployed. Wire length is read into the DP system at the moment the weight lands and the system switches to 'tension' mode (known as 'mooring' in the Kongsberg/Bandak taut wire). Wire angle is determined from the angular offset of the Gimbal head at the end of the A-frame through which the wire is deployed.

These angular measurements are transmitted to the DP system as voltages from gimbal potentiometers.

The taut wire system is robust and simple to operate. Often the vessel will be fitted with two units;

one on each side of the ship. The accuracy of the data is dependent upon a number of factors, chiefly water depth. A typical taut wire system has a depth capability of 350m, with an accuracy of typically within 1-2m up to about 200m depth. At greater depths the accuracy starts to deteriorate, and is dependent upon sea state and strength of tide.

The taut wire system requires a bit of planning for successful operation. Maximum scope (horizontal range) is determined by wire angle, limited to 28° or 30°; horizontal range is thus related to water depth. This means that in very shallow water the horizontal range may be only a few metres. Deep-water capability is, of course, limited by the wire length.

DPOs must carefully check the sea floor configuration to ensure that they do not lower the weight onto sea floor hardware, or endanger the operation the vessel is supporting (divers do not like having a taut wire weight lowered onto their heads!). It is normally advisable for the taut wire in use to be that on the side of the vessel away from the operation, eg if a vessel is working port-side to a platform, then the starboard taut wire will be used. With the vessel manoeuvring in the approach stage of the operation, it might be necessary to re-spot or reset the taut wire one or more times. This involves deselecting the taut wire from the DP, lifting the weight a few metres, waiting for it to swing to the vertical, then redeploying it.

When the vessel is engaged in underwater operations (dive or ROV support, for example) it is vital that the divers or ROV pilots are informed of the presence of the taut wire(s) and of their positions.

Care must be taken in re-spotting the taut wire if divers or ROV are in the vicinity, and they must be informed of this.

In a conventional monohull vessel, with the taut wire located at the vessel side, there is a deployment limitation on the bilge keel. The 28° or 30° wire angle scope is obviously not available in this direction. On the DP system, a display view will show taut wire status and maximum operational areas.

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Strictly a mechanical system, the taut wire requires regular maintenance, and it is necessary to carry spare wires and weights; wires occasionally break and weights are occasionally lost! A recommended move is to crop the wire at intervals. If the vessel is working continually using taut wire, then the wire will become worn where it spools on the drum and gimbal head. Cropping the wire by 10m at approximately two-week intervals refreshes the area of wear. It is necessary to record the crop and the remaining wire length.

Other types of taut wire system may be encountered. A moonpool taut wire is available for vessels which need to work in areas of surface ice. This type of system works directly out of the vessel bottom, the unit being low in which the wire is led directly to the nearby platform and secured.

Both these types of unit are nowadays very rarely seen. A further alternative is the deadweight taut wire, in which the tension capability is provided by a suspended weight. This system has the benefit of not requiring mains power for operation, thus will continue to give a position during a blackout.

DP Operator's Handbook

Chapter 6 85 Position referencing

The advantages and disadvantages of a taut wire system as a position reference for DP can be listed as follows:

Advantages:

• Accurate

• Reliable

• Quickly and easily deployed

• Not dependent upon third parties Disadvantages:

• Vulnerable to fouling by ROV or other underwater operations

• Limited horizontal range of operation

• Limitations in very shallow and very deep water

• Mechanical in nature, regular maintenance required

• Need for spare weights and wires to be carried

DP OPERATOR'S HANDBOOK

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DP Operator's Handbook