Extreme wave loading on offshore wave energy devices using CFD

Many different types of wave energy converters have been proposed in recent years. The two primary design considerations are the need to generate energy at competitive economic rates in average sea states and the need for the wave energy converters (WEC) to survive extreme wave conditions. Due to the complexity of most offshore wave energy devices and their motion response in different sea states, model scale tank tests are common practice for WEC design. Full scale tests are also necessary, but are expensive and only considered once the design has been optimised. Computational Fluid Dynamics (CFD) is now recognised as an important complement to traditional physical testing techniques in offshore engineering. Once properly calibrated and validated to the problem, CFD offers a high density of test data and results in a reasonable timescale to assist with design changes and improvements to the device. This paper c © Proceedings of the 8th European Wave and Tidal Energy Conference, Uppsala, Sweden, 2009 deals with the results of test cases leading towards simulation of the full dynamics of Pelamis and the Manchester Bobber. The test cases presented involve the interaction between waves and fixed horizontal cylinders and results are compared with experimental data to validate the CFD codes. Also results for fluid-structure interaction of an oscillating cone on the water surface are presented. The surface elevation and diffraction effects are discussed, as well as the forces on the structures due to the waves and motion respectively. Four different CFD codes are applied to simulate the test cases: Smooth Particle Hydrodynamics, a Cartesian Cut Cell method based on an artificial compressibility method with shock capturing for the interface, and two pressure-based NavierStokes codes, one using a Finite Volume and the other a control volume based Finite Element approach.

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