Non-linear behaviour of an idealised submerged WEC with Coulomb damping

Abstract—Wave energy converters (WECs) with Coulomb-type damping are relatively common as this behaviour is typical of hydraulic power take-off (PTO) systems. This study investigates motion response of a mass-spring system under combined linear and Coulomb damping, as a simplified representation of a WEC with a Coulomb damping force in the PTO. Motivation for this work stems from observed heave motions, of a submerged model-scale WEC with Coulomb PTO damping, which in some cases exhibited large offsets/drifts from the mean position. For submerged devices any restoring force must be engineered, as the
hydrostatic restoring force is zero. Given a choice of restoring force and Coulomb coefficients, we are interested in how to make the optimum choice to minimise displacements in large waves. In this paper a simple model is used to investigate this question. In the range of parameters considered, the extreme
displacements are minimised by adopting negligible Coulomb damping force (relative to the exciting forces) and a weak detuned spring coefficient. The low-frequency drifting behaviour is seen to be important for both mean and extreme behaviour of the oscillator.