Numerical Study of a Three-float Wave Energy Converter - M4

Researchers at the University of Manchester have developed a moored, three-body line absorber M4 (shown in Fig.1 (a)) that can extract wave energy from various modes of relative motions (surge, heave and pitch) between the floating bodies. Experimental studies have shown that high crest capture widths of wave energy conversion can be achieved across a broad band of frequencies and the peak capture widths are greater than 25% of a wavelength in regular waves [1]. In the present project, numerical analyses for the wave energy converter M4 have been carried out using a frequency-domain potential-flow solver DIFFRACT [2] and a two-stage approach [3] has been used. In the first stage, the excitation forces, added mass and radiation damping of M4 are calculated. In the second stage, the motion equations are solved considering both hydrodynamic interactions and mechanical connections between the floating bodies of M4. Viscous effects have been included in the numerical study, and the power take-off system (PTO) is simplified as a linear rotational damper. Numerical results of relative rotations and moments at PTO have been compared with experimental measurements (e.g. Fig.1 (b) and (c)). Good agreements have been achieved and further investigations have been carried out for assessing the performance of the M4 device in multi-directional waves.