Device Resonance Response in a Wave Energy Converter : Investigation of Surge Resonance in a Heaving Point Absorber

Abstract: Wave power possesses vast potential to become a considerable energy contributor. The main challenge for wave power to become competitive on the energy market is to minimize the delivered energy cost simultaneously as designing a wave energy converter (WEC) strong enough to survive extreme sea conditions. The Swedish company CorPower Ocean is developing a heaving WEC that converts the emerged relative heave motion to electricity. CorPower Ocean minimizes the delivered energy cost by letting the WEC resonate with the incoming wave providing maximization of the annual energy output. However, system characteristics of the WEC structure can produce resonance in other modes than heave which might affect the energy performance. This thesis targeted the device resonance response in CorPower Ocean’s WEC in the surge mode. More specifically, the thesis investigated, through simulation, when resonance in surge occurs and whether the impacts of the resonance should be mitigated. The results indicated that the WEC is resonant in surge when the incident wave has a period between 6.5-10.5 s and 13.5-19 s. Moreover, the WEC is resonant in surge when the oscillation period in surge is 15-17 s. The surge resonance period and its bandwidth increase for higher wave loads. The surge resonance is negatively affecting the heave motion and the WEC’s performance. It is not exactly known what in the structure that causes surge resonance.Two methods were used, results from one method showed tendencies that the mooring system was the instigator of the surge resonance, however, that could not be confirmed.