Design of a Mooring System With Synthetic Ropes for the FLOW Wave Energy Converter

Abstract

Martifer Energia is developing a concept of a wave energy converter (WEC) to be used at near shore locations with water depths starting at around 40m. It is a floating device composed of two bodies connected by a one degree of freedom articulation. The energy is extracted at the articulation by a power takeoff system actuated by the relative motion between the bodies. One of the important components of many WECs is the mooring system, since usually the cost is large compared global cost of the device. In this case the WEC will be moored by a spread mooring that allows the device to weathervane with the environmental loads. The paper presents one design solution for the mooring system investigated during the development stage of the concept. It is composed of four hybrid lines, each one with a segment of nylon rope connected to the floating device and a part of chain that contacts with the sea bottom and ends at the anchor. The wave frequency hydrodynamics are first calculated with a frequency domain boundary element method. The nonlinear cable dynamics problem, which is coupled to the slow drift motions of the floater, is solved in the time domain by a finite difference method. The design considers the climatology of the future area of operation of the prototype. Since the loads on the lines depend on the characteristics of the lines themselves, the design solution is obtained iteratively. Appropriate safety factors are considered. The result is the number of mooring lines, their angular separation, length and diameter of each line component.