This paper presents the numerical modelling of two point absorber wave energy converters (WECs) with and without a moonpool under focused wave conditions. The numerical model applies the overset mesh technique in order for the mesh to conform with the large-amplitude WEC motion induced by the focused wave groups. The incident wave group is first examined by a mesh convergence test and by comparing with the experimental data. The simulations are then carried out with the presence of the WEC. In total, three wave conditions are considered, each with the same wave period but with different wave heights. Nonlinear effects on the WEC motion are clearly exhibited when the wave steepness increases. The accuracy of the numerical results is carefully assessed against experimental data. Furthermore, the effects of the moonpool on the dynamics of the WEC are also discussed, where the WEC motion is compared for the case with and without a moonpool under the same wave conditions.
In recent years, the possibility of harnessing energy from ocean wave resources has gained great interest, where different design concepts of wave energy converters (WECs) have been proposed, such as oscillating water columns, bottom-hinged pitching devices, floating pitching devices, overtopping devices, and point absorbers. Point absorbers are one of the simplest WECs. Their characteristic length is generally smaller than the typical wavelength at the peak wave frequency. Meanwhile, they are typically subjected to large-amplitude motions close to resonance. In such a condition, a highly nonlinear wave-structure interaction is expected, where local wave breaking and overtopping may occur. Moreover, the damping coefficient of the WEC can be composed of not only the radiation wave damping but also the power takeoff (PTO) damping, as well as the viscous damping. The viscous damping force itself can be important in many cases, which are due to vortex shedding and shear stress force; see, for example, Gu et al. (2018), Palm et al. (2018), Wei et al. (2015), and Giorgi and Ringwood (2017). Such characteristics make the wave-structure interaction process highly complex and distinct from the traditional large-volume offshore structures.