Materials,Manufacturing and Testing Requirements for Offshore Mooring Steel Wire Ropes
Authors: Lohne Per William 
Published: October 2008

The trend to produce oil at constantly deeper water has led to the development of floating production solutions for the exploitation of the energy resources in these areas. It is a fact that steel wire ropes have been used and arc being proposed used, as line segments in the majority of the mooring systems of these units/ships. This paper specifics requirements for the materials, design, manufacture and testing of large diameter offshore mooring steel wire ropes and may serve as a technical reference document in contractual matters between the purchaser and the manufacturer. Typical applications covered are permanent moored floating production systems (IT%), offshore loading systems and mobile offshore units.

Introduction

Steel Wire Rope Constructions.

Steel wire rope segments of mooring lines could be of various constructions as shown in Figure 1. Other type of constructions e.g. eight strand, may also be used if relevant experience can be documented.

The six strand construction type includes a number of strands wound in the same rotational direction around a center core to form the wire rope, The number of strands and individual wires in each strand (e.g,6×41,6×49,6×6 1,6×91) are governed by required strength and bending fatigue considerations for the wire rope, This construction generates torque as tension increases.

The torque balanced strand type constructions (multistrand, spiral strand, half locked and full locked coils) arc attractive for use with permanently moored platforms/vessels since they do not generate significant torque with tension changes. These constructions use layers of individual 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 individual wires over the basic spiral strand construction resulting in a design more resistant to the ingress of corrosion media, The shaped layer(s) of individual 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.

Corrosion Protection Measures. A common design requirement is that wire rope segments in mooring lines arc 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 individual wires.Application of a blocking compound on each layer of the strand during stranding, The compound should till all crevices in the wire rope, strongly adhere to individual wire surfaces and have good lubricating properties.Cathodic protection by spinning zinc or other sacrificial anode alloy wires in one of the outer 3 layers of the wire rope during manufacture.Surface sheathing of the wire rope by an extruded plastic jacket in order to prevent ingress of seawater and flushing out of blocking compound.

The ends of each wire rope segment are normally terminated with sockets (cast or welded). Free bending at the sockets outlet can reduce the wire rope fatigue life.