Effects of floaters´ hydrodynamics on the performance of tightly moored wave energy converters

This study investigates the effect that several different floaters´ geometries have on the performance characteristics of tightly moored vertical axisymmetric wave energy converters (WECs). A cylindrical buoy with and without vertical and horizontal skirts mounted at its bottom, a cone, as well as two piston-like arrangements, consisting of an internal floater (cone or cylinder) and an exterior torus, have been examined and comparatively assessed. The WEC´s first-order hydrodynamic characteristics are evaluated using a linearised diffraction´radiation semi-analytical method. Axisymmetric eigenfunction expansions of the velocity potential are introduced into properly defined ring-shaped fluid regions surrounding the bodies and the potential solutions are matched at the boundaries of adjacent fluid regions by enforcing continuity of the hydrodynamic pressures and radial velocities. A dynamical model for the floaters´ performance in time domain is developed that properly accounts for the floater´s hydrodynamic behaviour, the coupling terms between the different modes of motion and of the power take-off mechanism. Numerical results are presented and discussed, in terms of the expected power production. The results show parametrically the effect that the varying hydrodynamic characteristics of each particular floater´s geometry have on the investigated WEC´s performance characteristics.