Multi-mode wave energy converters can generate power from multiple degrees-of-freedom, allowing them to harvest more energy than a converter operating in heave only. However, the control of these devices is complicated due to strong hydrodynamic coupling between the various degrees-of-freedom. This study attempts to provide a better understanding of the effects of pitch-surge coupling on the control and performance of a flat, cylindrical submerged wave energy converter. A linear Frequency Domain and a nonlinear Weak-Scatterer numerical model were both used to investigate the effects of hydrodynamic coupling on the performance of the device. Results showed that if pitching motions were not accounted for in the control strategy, pitchsurge coupling could significantly compromise the maximum power absorbed by the device. When nonlinear hydrodynamic coupling effects were included, the presence of pitch led to even worse performance than was predicted in the linear model, particularly at low wave frequencies. It was found that the nonlinear radiation forces, generated by the combined surge-pitch oscillations, were the cause for the difference between results.

INTRODUCTION

Ocean waves are an attractive source of renewable energy due to their high energy density and good variability properties (Ringwood et al., 2014). Among the different Wave Energy Converter (WEC) concepts proposed to date, Point Absorbers (PA) account for more than half of the existing designs (Mofor et al., 2014). Multi-mode PAs are a subset of this category, which are capable of harvesting power from more than one Degree-of-Freedom (DOF), such as heave, surge and pitch. In theory, this increases the amount of power that can be absorbed by up to three times when compared to a device operating in heave only (Falnes, 2002; Meng et al., 2019).

One of the challenges of designing multi-mode WECs is the strong hydrodynamic coupling between DOFs, particularly surge and pitch. In a study of a multi-mode, three-tethered WEC, a Multiple-Input-Multiple-Output (MIMO) controller was designed to optimise power absorption in the heave and surge DOFs, but the hydrodynamic coupling effects between surge and pitch were neglected (Sergiienko et al., 2019). As a result, it was found that up to 15% of the total power was lost through the pitch DOF. A better understanding of pitch-surge coupling effects in multi-mode systems is therefore required, to determine how these losses can be minimised.

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