Segmented and continuous beam models for the capture of ocean wave energy by Pelamis‐like devices.

Abstract

Pelamis‐like devices are elongated structures (floaters rather than sitters) aligned perpendicular to crests which flex or rotate longitudinally because of incoming waves. The Pelamis device is similar to Cockerell’s rafts and consists of a chain of cylinders connected so that there is a resistance to the changing of the pitching angles between adjacent cylinders. The flexing causes pistons to move, which, in turn, drive turbines that generate electricity. If the segments are sufficiently short and numerous, the device can be modeled as a continuous beam, with the conventional partial differential equation for the beam dynamics being modified by inclusion of a term proportional to the time derivative of the fourth spatial derivative of the vertical beam displacement. The effect of the incoming wave is approximated by a quasi‐static buoyancy term proportional to the difference of the local heights of the beam’s center line and that of the water height associated with the incoming wave. A theory is given for the prediction of the maximum power output that can be achieved for the continuous beam model and of how well different segmented models can approach the ideal limit. Performance is markedly better when the segment lengths are less than a quarter wavelength.