AXIAL TORSIONAL COUPLING OF SHEATHED SPIRAL STRANDS IN DEEP-WATER APPLICATIONS

The coupled axial/torsional behaviour of sheathed multi-layered helica l strands (currently employed in deep water platform applications) is addressed. Based on a series of theoretical parametric studies on a nu mber of multi-layered spiral strand constructions covering a wide rang e of cable (and wire) diameters and lay angles, straightforward formul ations (aimed at practising engineers) have been developed for obtaini ng the upper (no-slip) and lower (full-slip) bounds to various strand stiffness coefficients. It is demonstrated that the stiffness coeffici ents are (for all practical purposes) largely independent of the level of cable mean axial strain and magnitude of water depth. In terms of cable construction, the lay angle is found to be of first-order import ance, with the other geometrical parameters playing a minor (second-or der) part with regard to the values of strand stiffness coefficients. The proposed formulations are all amenable to simple hand calculations using a pocket calculator. Numerical examples are given to facilitate the use of the newly developed routines. They should prove of value i n designing offshore structures for serviceability limit states in lon g-term applications: these include the determination of the natural fr equencies of the structures which will be higher for (the traditionall y ignored) no-slip behaviour of the cables, and the in-service detecti on of axial/torsional wave propagation due to individual wire fracture s under, say, fatigue loading conditions.