Hydrodynamics of Multiple Floating Point-Absorber Wave Energy Systems With Inter-Body and Bottom Slack-Mooring Connections

Point absorbers constitute an important class of offshore wave energy converters. If employed in the extensive exploitation of the offshore wave energy resource, they should be deployed in arrays, the distance between elements in the array being possibly tens of meters. In such cases, it may be more convenient that the array is spread moored to the sea bottom through only some of its elements (possibly located in the periphery), while the other array elements are prevented from drifting and colliding with each other by connections to adjacent elements. This kind of mooring arrangement is addressed in the paper in a simplified way by considering an array of two buoys. Two opposed slack-mooring lines connect the floater pair to the bottom, while a third line, from whose mid-point hangs a weight, connects the two buoys pulling them towards each other. The centres of the buoys and the mooring lines are in a vertical plane parallel to the incoming wave direction, so that body and mooring motions are two-dimensional. The whole system — buoys, moorings and power take-off systems (PTOs) — is assumed linear, so that a frequency domain analysis may be employed. In the numerical simulations, two identical hemispherical buoys oscillate in heave and surge, acted upon by the waves, the mooring system and their PTOs. The PTO consists of a linear damper whose force is proportional to the heave velocity. Results from numerical simulations, with regular and irregular waves, are presented for the motions and power absorption of the converters, for different mooring and PTO parameters. Comparisons with the simpler cases of one single buoy in the moored and unmoored situations show significant differences.