Page 69 - Failure Analysis Case Studies II

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for potential dynamic resonances, but with very large structures and large masses these intuitions
can be misleading. The torsional stiffness of wire rope mooring lines, as compared with equivalent
solid bars, is so low that the period of natural torsion oscillation can be quite high, and there is
reason to believe from observed mooring line failures, that such motions can sometimes be excited
by the environmentally induced motions of floating structures. If the torsional amplitudes are large
enough, and vessel motions such as to allow very low tensions to occur, then the kind of torsional
instability described above can develop. Indeed, the destranding in the mooring rope illustrated in
Fig. 4 is thought to have been induced by just such behaviour.
This kind of dynamic torsional problem is initiated by the torque developed in the rope which
then causes rotation because of the different characteristics of the adjoining component. The use
of a torque balanced rope will effectively remove the rotational response and so overcome the
problem, but operating procedures that always maintain a minimum tension would also avoid the
rope being damaged.
It is also worth noting that the components in series which excite this kind of counter rotation
need not be rope and chain. Any torsionally mismatched pair will be susceptible, be they six strand
and torque balanced ropes, unbalanced ropes of different construction, or even two ropes of the
same construction but different diameter.

8. Problems encountered during installation

Mooring installation operations provide numerous opportunities for torsion related problems.
Apart from the tendency for a stranded rope to rotate under tension, one common mechanism for
inducing twist is dragging the rope along the seabed [8]. This can happen when installing a wire
rope mooring line deployed from a mobile unit. The problem only becomes apparent when the
tension in the rope is relaxed, and distortion is induced between fairlead and a deck mounted
winch. Reeling back onto the winch then causes miscoiling which can subsequently lead to severe
crushing damage.
The mechanisms which are the principal interest in this paper are those associated with raising
and lowering components, and specifically, operations involving six strand ‘work wires’. Numerous
operations which involve wire rope coupled to a chain that is lying on the seabed can induce twist
in the chain, which can, at a later time, be transferred into yet another component. If that
component is sensitive to imposed twist then the consequences can be very serious. An example of
such a deployment, based on the problems encountered during deployment of the P34 mooring
system [ 11, will be considered in detail and a hypothetical mechanism for the torsional damage to
the P34 spiral strand mooring lines analysed.
The operation to be analysed involves the installation of a mooring lined in 1000 m water depth.
The original intent was that the completed line should consist of a conventional drag embedment
anchor attached to somewhat in excess of 1000 m of chain, attached in turn to a spiral strand and
thus to a floating production, storage and offloading vessel (FPSO). The anchor and chain were
to be pre-installed with the free end of the chain lying on the seabed attached to a six strand wire
rope supported by a surface buoy. The particular stage of the operation being considered is the