Torsional interactions in multi-component moorings
Offshore moorings for oil and gas platforms often consist of several components in series, including lengths of wire rope, chain, and fibre rope that provide desirable properties at different depths and locations from seabed anchor to platform fairlead. While untwisted chain and most fibre ropes do not generate torque in response to axial load, wire ropes in offshore use are often not torsionally balanced, and when a complete mooring leg comes under tension the torque generated will cause turn not only in the wire rope but also in other components in the line. The response of mooring chain is strongly non-linear, that of fibre rope less so, as the torque generated in the wire rope(s) is propagated into the rest of the line. The paper explores these phenomena for some typical mooring elements. Practical examples show how the torque and turn generated from the interaction during installation with work wires as well as between the mooring components themselves can lead to serious problems. Finally the authors discuss how these problems might be avoided.