Design of a mooring system for an inertia tube wave energy collector

The dynamics of a point absorber wave energy collector (WEC) were investigated using the University of New Hampshire (UNH) developed finite element computer program Aqua-FE and tank testing. The WEC design considered here makes use of a buoy rigidly connected to a long, vertical inertia tube which is open at the top and bottom. A piston-rod assembly is enclosed and connected to the power take-off (PTO) mechanism. Due to inertia of water within the tube, relative motion between the piston and buoy-inertia tube structure occurs, and this drives the PTO. The Aqua-FE model was then used to design a slack mooring system sufficient for holding the WEC on station while minimizing interference with its energy absorption function. The Aqua-FE model was created and validated by comparison to wave tank measurements made using a 1/9.4 scale physical model in experiments conducted in the UNH 36.6 m long by 3.66 m wide by 2.44 m deep wave tank. The mathematical model was then applied to predict full scale response to seas representing extreme storms expected at UNH´s offshore test site south of the Isles of Shoals, NH. Predicted mooring loads were used to specify mooring system hardware. The Aqua-FE model for this system was evaluated using scale model results for free-release tests in heave (vertical displacement), pitch (angular motion), as well as heave in a series of single frequency waves. Wave periods, Froude-scaled to full size, spanned the range of periods observed at the UNH site.