Mooring design is a key challenge for floating wave energy systems and is arguably the most vulnerable component. Here we present experimental outputs assessing mooring loads on the M4 multi-float wave energy converter (WEC) moored to a single-point buoy with elastic cables, designed to reduce snap loads. Results are presented for a wide range of irregular sea states, varying significant wave height, wave period and directional spreading. Increasing directional spreading is found to reduce the maximum and standard deviations of the leading mooring line loads by up to 30-40% compared to unidirectional conditions.
Wave energy has the potential to significantly contribute to electricity generation, yet this extensive resource remains largely unexploited due to significant design challenges for wave energy converters (WECs). One of the key challenges is associated with the moorings which need to provide station-keeping without restricting motion – required for energy generation – excessively, and must withstand large unsteady loading. Snap loads in mooring lines in large waves are a particular challenge.
In this paper we assess the M4 multi-float WEC which is a hinged attenuator-type wave energy device designed to have similar capacity to offshore wind turbines. Previous experimental data exists for the M4 in unidirectional wave conditions. In Stansby and Carpintero Moreno (2020a) inelastic cables were used, whilst in Stansby et al. (2022b) a single elastic cable was used between a mooring buoy and the bed, with an inelastic cable installed between the mooring buoy and the M4 device. An improved mooring configuration using three solely elastic cables to a single buoy in pre-tension was investigated first, following modelling that suggested peak snap would be considerably reduced (Stansby and Carpintero Moreno 2020b). This was the case and has been further improved with a mooring buoy and an additional submerged buoy (see Figures 1 and 3). The idea is that with submergence the buoy will experience smaller forces and transmit smaller forces to the sea bed and thus to the anchors. This design was also expected to reduce snap loading throughout the mooring system.
