Page 73 - Failure Analysis Case Studies II

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stiffness. Obviously comparable reductions in chain diameter to reduce the rope tension would
have equivalent effects.
The operation analysed here is only one example of a process which can induce rotation, though
this is perhaps the category of operation most likely to induce the higher amounts of twist.
Inevitably when working in even greater water depth much higher twists will be involved.
It must also be pointed out that the torque/tension model used for these calculations is fairly
simple and has not been validated for ropes of the size used offshore. The analysis reported here
has also been performed using the even simpler two-term model of rope response [9] predicting a
35% lower rotation in the pendant rope, but otherwise a very similar pattern of behaviour.
The reported facts relating to the P34 mooring line losses are that the deployment procedure
was as described above, and that the spiral strand was found to be torsionally damaged beyond
recovery when connected to the FPSO. The mechanism described here has been deduced from an
analysis of the operation. There are no observations that can confirm, or otherwise, the relative
states of twisting in different components at each stage, but no other explanation has been
advanced.

9. Steps to avoid induced torsion

Apart from expensive solutions such as using two lines to different AHVs, or other devices
attached to the connection between components to prevent the transfer of turns, what else might
be done to avoid this kind of problem? There are essentially three categories of solution: one
involving rotating connectors (swivels), the second involving the use of torque balanced ropes, and
the third involving the selection of twist tolerant components. The merits of each are considered
below:

9.1. Rotating connectors

0 Conventional swivels are of little use here because they have high friction and therefore really
only operate at very low tension. This is ideal to release torque to facilitate handling of a rope
adjacent to a connector when restrained on the deck of an AHV.
0The use of special ‘low friction’ swivels in the case study above may have little benefit when
coupled between chain and pendant work wire. This is because of the combination of significant
tensile load and very low torque (associated with the low torsional stiffness of the unloaded
chain). Furthermore there is no validated quantitative data available for the relationship between
load transmitted and ‘break-out’ torque for these devices, which makes any analysis impossible.
0 Permanent installation of a ‘low friction’ swivel between the chain and spiral strand should have
the benefit of limiting the transmission of accumulated turns from chain to rope as the mooring
system is tensioned. However, such a policy would still run the risk of residual turns in the chain
forming knotted clumps with serious loss of strength and fatigue performance (this risk is of
course present with any option that does not prevent twisting of the chain in the first place, and
in fact one of the mooring chains in the P34 operation described above was broken at just such
a knot during retrieval).
0 The use of a ‘low friction’ swivel as a permanent connection in a mooring line which combines