Page 74 - Failure Analysis Case Studies II

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components having different torque/tension characteristics (e.g. chain and six strand ropes) is
likely to result in torsional oscillations as the tension fluctuates. This will introduce additional
fretting between the wires of the rope to compound fatigue. There is no reliable published fatigue
data to indicate how seriously this might affect endurance.

9.2. Torque balanced ropes

0 The choice of ropes with constructions having better torque balance characteristics, for pendant
ropes and work wires, would have undoubted success in reducing the introduction of turns into
chain, and subsequently spiral strand. However, these ropes are significantly more expensive
than the six strand constructions currently employed.
0 Ropes of such constructions are currently used routinely for diving bell hoisting and as single
fall ‘whiplines’ for cranes.
0 There is some precedent for using torque balanced ropes as work wires during installation, for
example in lowering the clump weights for the Lena guyed tower in the early 1980s.
0 Torsionally balanced ropes tend to have smaller outer wires than their six strand equivalents.
This makes them less robust and more vulnerable in aggressive mooring deployment operations.

9.3. Use of twist tolerant ropes

0 If installation procedures are likely to induce turns that can ultimately be transferred to com-
ponents with a low tolerance to twist, especially torque balanced wire rope, then one remedy is
the avoidance of such twist sensitive constructions for a mooring line.
0 Current developments in moorings for deep water include the use of polyester fibre ropes. Most
of the constructions selected to date for this application comprise a braided outer cover for a set
of essentially parallel sub-ropes which form the load bearing members. At present there is no
information available as to the torque/tension characteristics of these ropes, but given the low
level of twist in the sub-ropes, a reasonable level of tolerance to imposed rotation might be
expected. It is of interest to note that, necessity being the mother of invention and as a result of
good fortune, Petrobras were able to replace the damaged spiral strand by available polyester
fibre ropes.

10. Conclusions and recommendations

0 Wire ropes used as either mooring components or as work wires during installation can have a
tendency to twist under tension. This twist can be transferred from one component to another
(especially during installation operations) with potentially serious consequences as regards twist
sensitive components such as torque balanced wire rope, and even chain in extreme cases.
0 These mechanisms whereby turns can be generated are exacerbated by water depth, indeed the
capacity of chain to absorb some twist can overcome the problem completely in shallower water.
0 Quantitative models of the torque/tension characteristics of all components employed are neces-
sary to facilitate prediction of their torsional interactions. However, the first step in any such
prediction is to appreciate that such mechanisms occur at all.