The impact of pitch-surge coupling on the performance of a submerged cylindrical wave energy converter

Abstract

Multi-mode wave energy converters are able to generate power from multiple degrees-of-freedom, typically delivering increased power absorption from ocean waves compared to devices operating in heave or surge only. However, the control of such systems is complex due to strong coupling between different degrees-of-freedom. This study aims to understand the effect of both linear and nonlinear pitch-surge hydrodynamic coupling on the performance of a submerged cylindrical wave energy converter. Results showed that when nonlinear coupling effects were considered, pitch had a much larger effect on device performance than what was predicted from a linear model. The maximum power that could be absorbed by the device at lower frequencies was significantly reduced as pitch amplitude increased. In terms of control, from linear theory, for any given pitch amplitude and phase, only the surge amplitude and phase was required to tune the device for maximum power absorption from the wave. However, when a nonlinear model was used, results showed that proper tuning of the pitch phase was also required to achieve maximum power absorption. As the pitch amplitude increased, nonlinear hydrodynamic effects caused by the combined pitch-surge motions became more significant, which in turn affected power estimates. Care should therefore be taken, since linear hydrodynamic models could potentially provide misleading predictions regarding the performance and control of multi-mode wave energy converters.