Offshore Mooring Steel Wire Ropes(Extract)

1 Introduction

Steel wire rope constructions

Steel wire rope segments of mooring lines could be of various constructions as shown in Figure 1-1.

Other type of constructions may also be used if relevant experience can be documented.

The stranded rope constructions include a number of strands wound in the same rotational direction around a centre core to form the wire rope. The number of strands and wires in each strand (e.g. 6 ´ 19, 6 ´ 36, 6 ´ 61) are governed by required strength and bending fatigue considerations for the wire rope. This construction generates torque as tension increases.

The torque balanced spiral rope constructions (spiral strand, half locked and full locked coils) do not generate significant torque with tension changes. These constructions use layers of wires (or bundles of wires) wound in opposing directions to obtain the torque balanced characteristics. The half locked and full locked coil constructions consist of one or more layers of shaped wires over the basic spiral rope construction resulting in a design more resistant to the ingress of corrosion media. The shaped layer(s) of wires will also prevent any outer wire fracture from unwinding. These constructions will normally give higher load capacity related to nominal diameter due to the increased metallic area, compared to other constructions.

Figure 1-1

Steel wire rope constructions

2 Corrosion protection measures

A common design requirement is that wire rope segments in mooring lines are to be protected against corrosion attacks throughout the design life. The wire rope is therefore assumed to be fully protected such that its fatigue life approaches that in air. This is normally ensured by the following measures or combinations thereof:

— Sacrificial coating of wires.

— Application of a blocking compound on each layer of the strand during stranding. The compound should fill all crevices in the wire rope, strongly adhere to wire surfaces and have good lubricating properties.

— Surface sheathing of the wire rope by an extruded plastic jacket in order to prevent ingress of sea water and

flushing out of blocking compound.

The ends of each wire rope segment are normally to be terminated with sockets. Resin shall be used for pouring the sockets. For long term mooring the sockets should be provided with bend stiffeners (bend limiting devices). This is to protect the wire ropes from bending during installation operations when the bending radiuses are close to the minimum allowed value. To prevent water ingress in the socket a sealing system may be incorporated in the device.

Guidance for choice of wire rope construction

For long term mooring spiral ropes are normally used. These ropes maximise the available steel area and provide high strength to size ratio, high axial stiffness and limited rotation under load (i.e. torque balanced as mentioned above). Further, these constructions are considered to have high corrosion resistance since a lower proportion of steel wire area is exposed and the ingress of water to the centre of the rope is more difficult than with stranded ropes. Also, the closed and compact design of these ropes are very suitable to jacketing for added corrosion protection. Within this type of ropes half locked and full locked coil constructions have a higher wear

resistance than spiral strands because of the compact, near cylindrical surface. However, these are stiffer constructions and may, depending on the number of shaped wire layers, require more care during handling and installation. Locked coil ropes also require a larger bending radius than other constructions.

For mobile mooring stranded ropes are most commonly used. However, due to their flexibility, they may also be used in long term mooring systems as for example the upper short segment which is subject to winching damage, fairlead bending fatigue and splash zone corrosion. In these mooring systems stranded ropes may be considered as a “consumable” item which can be replaced every few years, whereas the lower segments are not intended to be replaced

Type of rope construction and extent of corrosion protection must be a case to case evaluation depending on factors like design life, level of bending stresses, environmental conditions, position of the wire rope segment in the mooring system and possibilities for replacement of the wire rope segment.

In DNV-OS-E301 Sec.4 Table I1 guidelines for choice of wire rope construction as a function of field design life and possibilities for replacement have been given. However, it should be emphasized that this table is a rough guidance and that there are no distinct limits of use for each construction.

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