Effects of dynamic axial stiffness of elastic moorings for a wave energy converter

Abstract

This work studies the effects of the dynamic axial stiffness of elastic moorings on the dynamic behaviour of a point absorber wave energy converter. Following two mooring analysis procedures, coupled dynamic analysis of a taut-moored WEC with three legs is performed using the FEM program DeepC in three irregular wave conditions. Two synthetic fibre rope materials are investigated, i.e. a normally stiff polyester and a wire-lay 3-strand nylon rope. The results of WEC motions and mooring tensions obtained from a quasi-static stiffness model and the dynamic stiffness model are compared and discussed. The former analysis applies the non-linear stiffness working curves of the ropes in the simulations, while the latter utilizes the dynamic stiffness expression with an iterative process following a practical mooring analysis procedure. For the nylon rope, the influence of the load amplitude on the dynamic stiffness and the WEC response is presented and analysed. It was found that the quasi-static stiffness model tends to underestimate the maximum mooring tensions, leading to 30%–40% lower results compared to the one accounting for the dynamic stiffness effects. For the studied WEC system, the nylon rope shows advantages over polyester, because of the lower mooring tensions and higher WEC motions.