Nonlinear modelling of a point-absorber wave energy converter based on the weak-scatterer approximation

In this study, a nonlinear time-domain model based on the weak-scatterer approximation is developed to investigate the hydrodynamic performance of a point-absorber wave energy converter (WEC). The simplified WEC consists of a floating cylindrical buoy with a hemispherical bottom, a single tether and a power take-off (PTO) system. The efficiency and survivability of the WEC are analysed by the proposed numerical model in moderately steep regular waves and focused wave groups, respectively. The effects of wave steepness on the power absorption of the WEC and the PTO force are highlighted in the study of regular wave tests by comparison with a linear frequency-domain model, and the influence of the PTO damping coefficient on the performance is discussed. Then the survivability of the WEC in focused wave groups is analysed and validated against the published experimental data. It appears that in terms of the motion responses of the buoy and the mooring force, the present results have a satisfactory agreement with the experimental data. The time-frequency characteristics of the motion responses of the buoy are further analysed based on wavelet transforms.