The present paper reports a numerical study concerning a device for wave energy conversion which uses the relative vertical displacement between two bodies to absorb wave energy. Basically the device under analysis is a floating body with an internal oscillating water column (OWC), in which the wave energy is extracted through the relative displacement between the structure and the internal free surface. The paper presents in detail the methodology applied to define, from the hydrodynamic point of view, the device geometry. The numerical code used is a three-dimensional radiation-diffraction panel model based on the classic linear water wave theory and potential flow.

To proceed with the wave energy converter (WEC) evaluation the equations of motion (of each body), in the frequency domain, are expressed as functions of the complex amplitude of the displacements, which can be determined from the hydrodynamic coefficients (added mass and damping) and the complex amplitude of the excitation forces. From the relative stroke and the power take off (PTO) characterization the device mean power absorption is computed as well as the capture width. In the study it was assumed a turbine-generator for PTO equipment, as in a common fixed OWC system, being described through one term proportional to the relative velocity and another proportional to the relative displacement (which embody a spring effect related with the air compressibility). Both terms are controlled by the turbine characteristic. If the relative stroke is lower than a prescribed maximum, the (optimal) mechanical damping coefficient is computed from maximum power absorption. If larger, the mechanical damping coefficient is set to limit the stroke to the prescribed maximum value. To have a better understanding of the device power absorption level a dimensionless power absorption parameter is used (which relates the mean power absorption with the maximum attained with an axisymmetric heave motion buoy).