3 rd International Conference on Ocean Energy, 6 October, Bilbao 1 A time domain analysis of arrays of floating point-absorber wave energy converters including the effect of nonlinear mooring forces Pedro C. Vicente 1 , António F. de O. Falcão 1 and Paulo A.P. Justino 2 1 IDMEC, Instituto Superior Técnico, Technical University of Lisbon, 1049-001 Lisboa, Portugal E-mail: pedro.cabral.vicente@ist.utl.pt; antonio.falcao@ist.utl.pt 2 Laboratório Nacional de Energia e Geologia, 1649-038 Lisboa, Portugal E-mail: paulo.justino@ineti.pt Abstract The extensive exploitation of the offshore wave energy resource may require the deployment of dense arrays of point absorbers, the distance between elements being possibly tens of meters. In such cases, it may be more convenient and economical that only elements in the periphery of the array are directly slack-moored to the sea bottom, while the other elements are prevented from drifting and colliding by connections to adjacent elements. Previous work was done in a base configuration of three floating point absorbers located at the grid points of an equilateral triangular, with a solid weight located at the centre of the triangle, which was extended to more complex equilateral triangular grid arrays. The study was based on frequency domain analysis which requires, not only the power take-off system (PTO) to be linear, but also linear mooring forces, which is quite unrealistic as a model of slack moorings. In the present paper those restrictions are removed by using a time-domain, rather than a frequency domain, analysis, which allows nonlinear mooring forces to be considered. The mooring cables are approximately modelled as catenary lines in a quasi-static analysis. The results show very different behaviour for the horizontal and vertical motions of the floating converters, namely the possibility of occurrence of low-frequency horizontal oscillations of large amplitude. Even in the case of incident regular waves, such horizontal motions were found to be non-periodic, a behaviour that is typical of nonlinear systems. Keywords: Arrays, Catenary, Moorings, Point absorbers, Wave energy. 1. Introduction Free floating devices are a large class of wave energy converters (WECs) for deployment offshore, typically in water depths between 40 and 100m. As in the case of floating oil and gas platforms, such devices are subject to drift forces due to waves, currents and wind, and so they have to be kept on station by moorings (early contributions to the mooring design of wave energy converters can be found in [1,2]). Although similarities can be found with such applications, the mooring design will have some important differences, one of them associated to the fact that, in the case of a wave energy converter, the mooring connections may significantly modify its energy absorption properties by interacting with its oscillations [3]. Among the wide variety of floating wave energy devices, point absorbers have been object of special development effort since the late 1970s. They are oscillating bodies whose horizontal dimensions are small in comparison with the representative wavelength. Examples of devices are the IPS buoy [4], Aquabuoy [5], Wavebob [6] and PowerBuoy [7]. Their rated power ranges typically from tens to hundreds of kW. The extensive exploitation of the offshore wave energy resource may require the deployment of dense arrays of absorbers, the distance between elements in the array being possibly tens of meters [8]. However, little attention seems to have been devoted in the published literature to the mooring design of free- floating point absorbers in dense arrays. This may be explained by the present stage of development of the technology (focusing on single prototypes) and/or by the restricted availability of such information. In such cases, it may be more convenient that only elements in the periphery of the array are directly slack- moored to the sea bottom, while the other elements of the array are prevented from drifting and colliding by connections to adjacent elements.