Nonlinear Froude-Krylov force modelling for two heaving wave energy point absorbers

Most mathematical models used to assess the mo- tions of wave-energy converters are linear, which may lead to significant errors as these devices can have a strongly-nonlinear behaviour. This paper investigates the effects of nonlinearities in the computation of Froude-Krylov forces, which play a major role in the dynamics of the motion of heaving energy point- absorbers, with a focus on the influence of the device's geometry. Results show that Froude-Krylov forces nonlinearities could be negligible when the device is uncontrolled. In contrast, they become significant when control is applied to maximise motions, especially for a device whose immersed cross-sectional area varies over time: in such a case latching control parameters based on a linear model can prove to be inefficient. Furthermore, although the latching control can be adapted to the nonlinear behaviour of the device by tuning parameters accordingly, the amount of power production assessed through the linear models does not seem to be achievable. Index Terms—Wave energy, boundary element method, exci- tation force, Froude-Krylov force, nonlinear modelling