Free bending characteristics of sheathed spiral strands under cyclic loading

Abstract

A theoretical insight is given into the non-linear free bending characteristics of axially preloaded and large diameter sheathed spiral strands experiencing high external hydrostatic pressure in, for example, deep water floating platform applications.

The theoretical model takes geometrical non-linearities and interwire friction fully into account. It is capable of estimating variations of strand plane-section bending stiffness, (EI)_eff, and hysteresis as a function of the imposed radius of curvature, magnitude of the external pressure, and level of mean axial load.

An oversight in a previously reported analytical model for following the reductions in strand flexural rigidity due to interlayer slippage has also been identified and subsequently corrected. In addition, the corrected model (which assumed small deflections) has been extended to cover geometrical non-linearities due to changes in lay angle and helix radius.

Numerical results are presented for a large diameter (102 mm OD) sheathed spiral strand experiencing various levels of mean axial load and external pressure. In particular, it is shown that under the plane-section bending assumption, the upper and lower bounds to (EI)_eff are, for all practical purposes, independent of the magnitude of mean axial load and external pressure. Moreover, the maximum possible values of radii of curvature associated with which plane-section bending is possible, have been identified in all cases. It is believed that even in the presence of interlayer slippage, the damping model (which is based on plane-section bending) offers reasonable and useful lower bounds to a strand's specific damping which should prove of value in certain applications involving cable dynamics.

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