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.