Page 68 - Failure Analysis Case Studies II

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torque

moderate load

chain at negligible

~
twist

Fig. 6. Schematic representation of the proposed torsional behaviour of chain as a function of tensile load, compared
with a six strand wire rope.

6. Torsional characteristics of chain

A common practice is the use of mooring systems which combine wire rope and chain in series.
The torsional properties of chain are somewhat of a mystery other than a general acceptance that
relative rotation of the order of 3" per link is possible with only nominal torque resulting. Because
of the torque reaction which can be generated in rope, unless there is perfect torque isolation
between components, the response of chain is significant. However, because of the dearth of
specific information it is necessary to speculate, and approximate the behaviour of chain.
Under a given constant tensile loading chain can be expected to have a limited range of rotation
(the accumulated 3" per link) at more or less constant torque. This essentially frictional torque will
increase with tensile load. Once the limit of this rotation has been reached the chain will rapidly
become very stiff, but eventually a limit will be reached at which the chain will start to knot up,
and if the tensile load is constant this process might be expected to continue at a more or less
constant torque. At zero tension there is effectively no resistance to rotation and chain can absorb
rotation almost indefinitely. These characteristics are illustrated diagrammatically in Fig. 6.

7. Interactions in installed mooring systems

With six strand rope and chain in series, and assuming both ends are prevented from rotating
as tension increases, the rope will generate a torque which will be transmitted through the whole
line. Initially, the chain will take up its 3" per link, allowing some corresponding unlaying of the
rope, but as the chain stiffens, further transfer of rotation will meet increasing resistance and the
torque will increase more rapidly. In a quasi-static situation this should not cause any problems,
but the dynamic response may well be different. Most experienced engineers have a natural feel