An empirical model to predict elongation of polyamide mooring lines

Abstract

Polyamide mooring lines are being proposed for station-keeping of floating wind turbines in shallow water. However, their mechanical behaviour is quite different to that of the polyester fibre ropes used for deep water offshore oil & gas platform moorings. This paper presents an empirical model which can predict the specific strain response of a polyamide 6 mooring line under stochastic loading. Particular attention is given to the non-linear load-strain behaviour, hysteresis, and the influence of loading history. The identification of the model, using 12 stochastic tests performed on wet 3-strand polyamide sub-rope samples, is described. This model reproduces the specific features of the response of fully wetted PA ropes well, with reasonable accuracy and, for the extreme (maximum and minimum) values it provides a much better prediction than the linear “dynamic stiffness” model. This new model can be integrated in the time domain processing of a suitable mooring analysis software.