Experimental and numerical investigation of WEC-type floating breakwaters: a single-pontoon oscillating buoy and a dual-pontoon oscillating water column

Wave energy converters(WECs) integrated into floating breakwaters or, more generally, their designs also used as wave absorbers can provide a space- and cost-sharing solution to enhance the efficiency of coastal protection. In this study, the hydrodynamic performances of an oscillating buoy(OB)-type single-pontoon floating breakwater(SPFB) and an oscillating water column(OWC)-type dual-pontoon floating breakwaters(DPFB) are evaluated, respectively, and are comprehensively compared with each other. A series of physical experiments were conducted to investigate the effects of wave parameters, geometrical dimensions, gap distances and PTO damping coefficients. Numerical simulations based on the fully nonlinear potential flow theory cross-check the measured data, and help to further understand the contribution of higher-order waves. It is found that both the wave attenuation and energy conversion of the OWC-type DPFB with a non-uniform draft (i.e. shallow front-pontoon draft and deep back-pontoon draft) are better than those of the OB-type SPFB with the same displacement. Meanwhile, for an OWC-type DPFB, a larger ratio of the chamber width and the total pontoon width has a beneficial effect on the wave attenuation, but has an adverse effect on the maximum energy conversion efficiency. Under the premise of the same pontoon draft, the maximum conversion efficiency of the OWC for the optimal opening ratio is higher than that of the OB for the optimal PTO damping, but the effective frequency bandwidth is almost identical between two devices. Strong wave nonlinearity can generate more energy dissipation induced by the viscous effect and the vortices, resulting into the reduction of energy extraction.