Design and comparative analysis of mooring systems for a combined wind and wave energy system at intermediate water depth

Abstract

For intermediate water depths (typically ranging from 50 m to 80 m), designing steel catenary mooring systems for floating marine renewable energy (FMRE) platforms can be challenging due to the limited weight of suspended mooring lines. This can substantially increase mooring line tensions following large platform offsets. In contrast, mooring systems using synthetic fibre ropes offer the potential to prevent large platform offsets while reducing peak mooring line tensions. In this study, novel semi-taut mooring systems incorporating polyester ropes and steel chains are proposed for a combined wind and wave energy system – the semi-submersible flap torus combination (STFC) concept, deployed at a 50 m water depth. The STFC integrates a semi-submersible floating offshore wind turbine (FOWT), a torus wave energy converter (WEC) and three flap-type WECs. The dynamic responses of the STFC with different semi-taut mooring configurations under operational and survival environmental conditions are assessed in terms of key performance parameters such as the platform's motion responses and mooring line tensions. These performance parameters are compared against those of a chain-catenary mooring system. With the use of semi-taut mooring systems, significantly smaller mooring footprints as compared to the chain-catenary mooring systems can be achieved. Moreover, it is demonstrated that the semi-taut mooring systems are effective in reducing the maximum tension of the mooring lines. A basic cost analysis further indicates that semi-taut mooring systems offer substantial cost advantages over chain-catenary moorings in intermediate water depths.