EFFECT OF HYDROSTATIC-PRESSURE ON STRAND BEHAVIOR

High density, supposedly impermeable, polythene sheaths have been offered by cable manufacturers for corrosion protection of large diameter spiral strands used in deep water floating production systems. Using previously reported orthotropic sheet theory, an insight is given into the effect of high external pressure on various sheathed cable characteristics, which include: 1 axial stiffness and hysteresis; torsional stiffness and hysteresis; free bending fatigue behaviour in the vicinity of end terminations. It is shown that such high pressures can significantly alter the pattern of interwire/interlayer contact forces. Recent published research has clearly demonstrated the central role such interwire contact pressures play as regards cables’ fatigue life under various modes of loading. The present theory shows that increases in contact forces in deep water sheathed cable applications lead to significant reductions in cable fatigue life. Moreover, such high external forces are shown to affect cables’ axial and torsional characteristics by suppressing the relative slippage of wires in the strand and increasing frictional forces between them. A higher cable stiffness will then follow. This then leads to significant variations in strands’ axial and/or torsional hysteresis which may have a considerable bearing on the overall dynamics of, for example, a guyed tower platform using sheathed strands. The significant influence of high external pressures on various cable characteristics have been demonstrated by numerical examples on a realistic 39 mm outside diameter (OD) multi-layered structural strand. Results are also presented for the case of zero external pressure relevant to unsealed mooring cables in deep water applications, in which case the absence of the sheath will permit water migration inside the strand and the cable behaviour is independent of the level of water depth. © 1990, Institution of Mechanical Engineers. All rights reserved.