Modelling of seabed interaction in frequency domain analysis of mooring cables

Abstract

Mooring cables under wave loading interact dynamically with the seabed; this interaction is nonlinear and can be modelled in full only by performing lengthy time integration of the equations of motion. However, time domain integration is far too computationally expensive to be carried out for all load cases. A new method of modelling the interaction between a cable and the seabed in the frequency domain, but without considering frictional effects and impact, is therefore proposed. The section of cable interacting with the seabed is truncated and replaced with a system of coupled linear springs, with stiffnesses linearised from static catenary equations. These springs would model the behaviour of the truncated cable and hence the time-varying boundary condition at the touchdown. The entire cable-spring system is then analysed in the frequency domain with a centred finite difference scheme. The proposed method has shown to increase the accuracy of frequency domain analysis in certain cases with affordable computational overhead.