Analysis of Wave Drift Forces on a Floating Wave Energy Converter

Abstract

Wave drift forces acting on floating wave energy converters (WEC) are often the most important loading component for the design of the mooring system. On one hand these forces may be, at least, one order of magnitude larger than wind and current forces, and on the other hand the floating structure and mooring system may respond dynamically to the slowly varying wave drift forces. The paper presents an analysis of the wave drift forces on an articulated floating wave energy converter. Particular attention is given to the effects of the wave energy extraction on the time history of the horizontal drift forces. The hydrodynamic calculations are carried out by a frequency domain Green function panel method, resulting on the transfer function of the WEC motions as well as the transfer function of the mean drift forces. The power takeoff system is represented by a simple linear model where the extracted power is related to the relative velocity in the articulation and the damper of the PTO. With the transfer function of the mean drift forces, the variance spectrum of these same forces is calculated for stationary irregular seastates, and finally time histories of the drift forces are produced for typical operational conditions.