Second-order wave-diffraction force on a floating buoy with a caisson underneath

A wave power device comprising a floating circular buoy on the water surface and a submerged cylinder on the sea floor has been proposed in previous studies. This paper considers the wave interaction with a stationary wave power device of this kind and investigates the second-order hydrodynamic interaction between the buoy and the caisson. To achieve this, the first-order wave diffraction problem was first solved based on the eigenfunction expansion method. Then a semianalytical solution for the second-order double-frequency wave force applied on the buoy was developed based on the indirect method. After conducting convergence tests, the results from the present method were compared with those from the direct method and the higher-order boundary-element method (HOBEM) and good agreements were obtained. Then calculations were performed for different sizes of the caisson. The contribution of each constituent component to the whole second-order double-frequency wave force was investigated and the effects of the caisson size on the hydrodynamic loads experienced by the buoy are discussed.