Page 61 - Failure Analysis Case Studies II

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or used for raising and lowering, wire rope commonly acts in series with other components. These
different components operate in static and dynamic equilibrium with each other, transmitting
mechanical loading along the line between surface and seabed. The different classes and sizes of
components inevitably have different mechanical characteristics, and while such properties as
strength, stiffness, mass, and even differences in fatigue are well understood, the responses to twist
and applied torsion, though widely appreciated as a potential source of problems, are generally
not well understood. And it is in the torsional response of the different components that some of the
most significant differences in behaviour can be observed. The interplay between these differences is
important in determining the overall behaviour of a system, but not only as regards the behaviour
of the system which has been conceived by the mooring designer. The historical sequence of
operations, starting from the installation of the first component, can effectively alter the con-
figuration, and thus the response, of the final system.
Because of the relative sizes and weights of the different components used, water depth becomes
especially important when considering torsional behaviour during deployment operations. These
problems have parallels in the field of mine hoisting where the torsional response of wire rope has
long been recognised as an important consideration [2-4]. The considerations given to the mine
hoisting problem, especially in the context of developments for ultra deep shafts (3000-4000 m) in
South African gold mines [5-71, has been a valuable source of information for understanding the
mooring system and work wire handling problems.

3. The selection of rope construction

There is a significant range of different wire rope constructions supplied by rope manufacturers
for offshore mooring use, each with different combinations of attributes. When selecting wire rope
for different applications a number of different considerations will apply and the final selection
will inevitably represent a compromise. The essential characteristic of a rope is that it has high
axial strength and stiffness, in relation to its weight, combined with low flexural stiffness. This
combination is achieved in a wire rope by using a large number of steel wires, each of which is
continuous throughout the rope length, which when loaded axially in parallel provide the tensile
strength and stiffness, but when deformed in bending have low combined bending stiffness provided
their bending deformation is decoupled.
To facilitate handling it is necessary to ensure that the rope has some integrity as a structure,
rather than being merely a set of parallel wires; this is achieved by twisting the wires together.
Whilst this gives the rope coherence, it also creates lateral forces within the rope which increase
with the axial tension. Consequently, each wire is gripped between its immediate neighbours so
that when there is a local fracture of an individual wire, the clamping forces generate frictional
shears. As a result within a short distance of the break, the broken wire is carrying its full share of
the rope load. This clamping can also increase bending stiffness raising wire stresses associated
with operation over pulleys and drums. To minimise this effect and so maintain bending flexibility,
every wire in the rope must have some freedom to slide along its path within the rope construction,
effectively to cancel out the differences in bending strain implied by simple beam bending con-
siderations (this is the essential difference between a rope and a solid bar).
The above bending requirements would be satisfied by a simple twisted bundle of wires; however,