Page 62 - Failure Analysis Case Studies II

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(a) spiral strand (b) six strand (c) multi-strand
Fig. 1. Typical rope constructions.

such a structure would, if freely suspended, simply untwist. It would also be very vulnerable to
damage of the outer wires, which if broken at one location, would just fall away and serve no
further contribution to the load bearing function of the rope. The resolution of these problems
provides the basic motivation for the design of different rope constructions. Wire rope can be
constructed to be more or less ‘torque balanced’ (or non-spin, or spin resistant) to minimise any
axial torque generated by tensile load, and any tendency to rotate when suspended. However such
constructions tend to have other disadvantages, so before proceeding to a detailed consideration
of torsional behaviour of wire ropes, it is relevant to consider the relative merits of the three generic
classes of rope construction.

3.1. Spiral strand

Geometrically, perhaps the simplest of rope constructions, spiral strand consists of concentric
helical layers of wire (Fig. l(a)). The outer layers of a spiral strand, which constitute the bulk of
the cross section, generally have wires of the same diameter, opposite helical senses, and the same,
or similar, helix angle (but consequently different helical pitch, or lay length) with a core often
geometrically similar to that of conventional stranded ropes (of mixed wire sizes but common
pitch and sense). The characteristics of spiral strand, as relevant to offshore mooring applications,
can be summarised as follows:
0 spiral strands offer high strength and stiffness for a given diameter and wire grade;
0 spiral strands can provide a high degree of torque balance;
0 wires tend to be of larger diameter than in stranded ropes of comparable diameter-this benefits
corrosion resistance but can limit the tensile strength of the wire employed (especially if shaped
wires are used as in mining or ropeway applications);
0 the outer rope surface is essentially cylindrical which facilitates sheathing in polymer to provide
long term corrosion protection;
0 exposed outer wires are vulnerable to damage and this construction is unsuitable for operation
on and off multi-layer winch drums at more than low tension (unless having ‘full lock’ outer
wires, and especially if sheathed);