Hydrodynamic characterization in a wave flume of a small scale wave energy device moored with piles inducing friction

Accurate identification of hydrodynamic characteristics for wave energy converters is a prerequisite for designing effective control strategies and power-optimization models. The identification could, in principle, be based on physical model experiments. However, in practice, mechanical imperfections, particularly Coulomb friction at pile-hinge connections, introduce a non-sinusoidal force component that contaminates the measured signals and yields spurious values of added mass and radiation damping coefficients. Those coefficients are the cornerstone for a power optimization study for the next stages. This paper presents a method to identify and quantify the dissipation contribution due to mechanical limitations by performing forced-oscillation tests on a 1:20-scale rectangular-shaped floating breakwater wave energy converter with a moonpool, tested in the wave flume lab of the University of Padova at different frequencies and amplitudes. Dry condition tests, carried out with and without the friction due to the pile constraints, characterize the mechanical dissipation and spurious dry stiffness independently of any hydrodynamic effects. Tests in water (wet tests) for both heave and pitch degrees of freedom are used to identify hydrodynamic characteristics contaminated by the square-like component due to friction. Overall, the procedure enables the acquisition of corrected force signals to better identify hydrodynamic characteristics (added mass, radiation damping, and frequency response function).