Effects of wave spreading on performance of a wave energy converter

Ocean waves are irregular, nonlinear and directionally spread (short-crested). The irregularity of waves can be
considered by superposing components at different frequencies. In many frequency-domain analyses, second-order contributions can be calculated to take account of the nonlinearity in wave-structure interactions. The directional spreading of the waves is usually described by introducing a directional spectrum. Several directional spectra have been suggested by researchers [1]. In short-crested waves, the wave energy propagates in different directions around principal wave directions, which will affect the performance of Wave Energy Converters (WEC). Especially, less energy may be harnessed for some directionally sensitive WEC.
In present paper, we have examined the performance of an attenuator type WEC by calculating the relative rotations (pitch and yaw motions) between floating modules in uni-directional and multi-directional waves. The emphases have been put on operational sea states and only linear incoming waves have been considered. In the next section, the description of multi-directional waves in a numerical model will be introduced briefly. Then the motions of an attenuator type WEC and shear forces (in vertical and horizontal directions) acting on power take-off system (PTO) will be investigated. Different wave spreadings have been considered.