The first and higher harmonic components of the resonant fluid response in the gap between two identical fixed rectangular boxes are experimentally investigated in a wave basin. Gap response is excited by transient wave groups (being based on scaled versions of the autocorrelation function of sea-state spectra, representing NewWaves, the average shape of large waves in a sea state). Several different wave groups with different maximum surface elevations, spectral peak frequencies and bandwidths are used, while the bilge shape of the boxes and approach angle of the waves are also varied. Unlike a simple regular wave, it is complicated to separate the harmonic components for a transient wave group due to nonlinear wave–wave and wave–structure interactions. A four-phase combination methodology is used to separate the first four harmonic components, and this also allows higher harmonic components to be isolated with simple digital frequency filtering. Harmonic components up to 14th order in the incident wave amplitude have been extracted. It is shown that for an incident group with appropriate frequency content, the linear gap response may be substantially smaller than the second harmonic component, which is strongly driven via quadratic coupling of the linear terms from the incident wave and occurs in the gap resonant modes. Double frequency excitation may have important practical implications for offshore operations. Fourth and zeroth (long-wave) harmonics in the gap are further driven via quadratic coupling of the second harmonic itself. Linear damping coefficients for the first few modes of the gap resonant response are derived from measured time series using a numerical fit and shown to be higher than those from linear diffraction calculations.

This paper considers arrays of wave energy devices consisting of oscillating rigid bodies undergoing optimum displacements in regular waves. A previously published identity concerning the directional behaviour of the interaction factor, q, (which characterises an important effect of the array configuration in the context of point absorber theory) is shown to be valid under more general conditions. In fact, this identity is shown to be a consequence of a more fundamental result. Boundary element hydrodynamic calculations carried out on simple 3-member arrays are shown to be consistent with these theoretical proofs. Some aspects of the behaviour of the arrays are also investigated.

Ocean wave energy has been of interest since at least the mid-1970s and great advances in the understanding of the fundamental principles of wave energy extraction and converter modelling have been made using linear hydrodynamic analysis. This paper reviews efforts that have been made to use nonlinear hydrodynamics to analyse wave energy converter behaviour and performance. Both 'partially nonlinear' and fully nonlinear potential flow methods, as well as computational fluid dynamics methods solving the Navier-Stokes equations and smoothed particle hydrodynamics have been used for this purpose and are reviewed here. These more complex methods have been applied primarily to single devices, so array and mooring line interactions are not considered. While a number of studies have been performed in these areas and results are encouraging, further advances are required to give accurate prediction or reproduction of experimental results.

Wave energy converters and other offshore structures may exhibit instability, in which one mode of motion is excited parametrically by motion in another. Here, theoretical results for the transverse motion instability (large sway oscillations perpendicular to the incident wave direction) of a submerged wave energy converter buoy are compared to an extensive experimental dataset. The device is axi-symmetric (resembling a truncated vertical cylinder) and is taut-moored via a single tether. The system is approximately a damped elastic pendulum. Assuming linear hydrodynamics, but retaining nonlinear tether geometry, governing equations are derived in six degrees of freedom. The natural frequencies in surge/sway (the pendulum frequency), heave (the springing motion frequency) and pitch/roll are derived from the linearized equations. When terms of second order in the buoy motions are retained, the sway equation can be written as a Mathieu equation. Careful analysis of 80 regular wave tests reveals a good agreement with the predictions of sub-harmonic (period-doubling) sway instability using the Mathieu equation stability diagram. As wave energy converters operate in real seas, a large number of irregular wave runs is also analysed. The measurements broadly agree with a criterion (derived elsewhere) for determining the presence of the instability in irregular waves, which depends on the level of damping and the amount of parametric excitation at twice the natural frequency.

Circular arrays of fixed truncated vertical cylinders are known to support so called "sloshing near-trapped modes" when subjected to incident water waves of the appropriate frequency and direction. In this paper waves radiated from circular arrays of truncated cylinders oscillating independently in still water are considered. For a square array of four cylinders, peaks in the condition number of the radiation damping matrix are used to identify body motion modes of interest. Body motions which excite the same free surface motion local to the array as the sloshing neartrapped mode are associated with enhanced radiation from the array. In contrast, at lower frequencies oscillations with low wave radiation to infinity are found when the cylinders oscillate in phase, in either heave or surge. For an array with eight cylinders this almost wave-free behaviour is dramatically enhanced, and a very simple structure which appears to be "motion-trapping" may be constructed.