Abstract
Many types of device have been proposed for generating useful electrical energy from the kinetic and potential energy of ocean waves. One class of device consists of a closely spaced array of floats whose vertical oscillation drives a power take off system (for example Manchester Bobber, Fred Olsen F O3 , Trident Energy, Wavestar). Typical dimensions are a radius a ∼ 5 m, a centre to centre separation s ∼ 4a to operate in wavefields with peak periods in the range 5 ≤ T ≤ 12 and water depth of 25m -50m.
For these systems, the response of each float is dependent on both the excitation force due to the diffracted wave-field and forcing due to waves radiated by the oscillation of the devices. It is widely known that these interactions cause both the response and power output of a float within an array to differ from the same device in isolation. High power interaction factors can be attained in regular waves providing that both the mass and mechanical damping on all floats are specified in terms of wave frequency. To account for hydrodynamic coupling between the floats, optimal tuning requires that the mechanical damping on each float is specified in terms of the velocty and acceleration of all floats in the array. In practice, this is non-trivial. A simpler system in which mechanical damping on each float is based only on its own motion in regular waves has been shown to produce slightly less power, but still greater power than if the devices were in isolation [Justino & Clement, 2003]. A similar approach has also been shown to increase power output in irregular waves [De Backer et al., 2009] when response is modelled by superposition. Mass variation is typically limited to a finite range and, for a buoyant device, a variation of mass would modify draft and hence radiation damping and added mass. It is therefore simpler to modify only the mechanical constraint on each float. A simpler system in which the masses of all the floats are fixed, and the mechanical damping values of each float are selected independently of the motion of the other floats was shown to increase power output of a 5 x 1 array in regular seas [Thomas et al., 2008]. Although these predictions of increased power output are promising they are reliant on the validity of linear theory for modelling shallow-draft float response in regular waves and do not describe irregular wave response. Response and power output of a comparable mechanical system has been studied in irregular waves by superposition [Cruz et al., 2009] but it is unclear whether steady-state regular wave responses can develop. After demonstrating increased power output by application of float-specific mechanical damping, the validity of linear analysis for predicting shallow-draft float response is addressed by i) comparison of predictions of undamped float response to experimental measurements of an array of heaving devices and ii) modification of the forcing spectrum due to 2nd order sum- and difference- frequency forces. Hydrodynamic analysis is conducted using WAMIT.