Design Waves and extreme responses for an M4 floating, hinged wave energy converter

This paper presents a comprehensive analysis of measurements from a wave basin campaign investigating the use of Design Waves for the hinge response of the M4 wave energy converter (WEC). The experiments were carried out at a scale of 1:15 relative to a kW-scale ocean trial currently being built for deployment in King George Sound, off the coast of Albany, Western Australia. By averaging the largest body motion responses from long irregular wave realisations of extreme sea states, we determined the most probable extreme response — the NewResponse. The Design Wave was constructed by averaging the surface elevation time histories driving instances of the largest responses. Subsequently, the identified Design Wave was replicated in the wave basin. Our results show that the identified Design Wave is able to produce the hinge angle NewResponse of the M4 device with reasonable accuracy. The methodology applies to any linear system, and Design Waves of this type are expected to be applicable for a wide range of WEC motion responses. In addition to the experimental reconstruction of identified Design Wave signals, we analyse the effect of dunking – full submergence of the centre floats – and compare the maximum response and Design Wave signals for three severe sea states pertinent to the King George Sound location. In limiting-steepness severe sea states, the wave peak frequency is lower than the hinge motion natural frequency, and the NewResponse is largely independent of the sea state. However, the Design Waves are found to be somewhat sea state dependent. We relate both of these findings to the narrow-band nature of the hinge response at its natural frequency and the invariance of the spectral tail for fixed wave steepness.