Numerical reproduction of extreme wave loads on a gravity wind turbine foundation

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Numerical reproduction of extreme wave loads on a gravity wind turbine foundation

Bredmose, H. / Skourup, J. / Hansen, E.A. / Christensen, E.D. / Pedersen, L.M. / Mitzlaff, A.

A fully nonlinear 3D Navier Stokes solver with VOF (Volume of Fluid) treatment of the free surface is used to reproduce two extreme laboratory wave impacts on a gravity wind turbine foundation. The wave climate is irregular waves with a current. Numerical results for inline force, overturning moment and run-up are compared to measurements. The extreme wave loads for the two events are associated with slamming onto the under side of a horizontal platform placed 9.1m above the still water level. For such impacts, the computed wave loads are strongly sensitive to the shape of the incoming waves. A comparison with a Morison-type estimation of the wave loads shows that this much simpler approach can reproduce the overall trend of the wave load history, but not the extreme moment. The two examples of wave impacts presented show that extreme wave loads can be computed by a Navier-Stokes solver combined with the VOF technique. The computations presented took about 16 hours each on a standard PC with a 3.4GHz Pentium 4 processor. Thus with application of parallel computers, computations of much longer time series is possible. Besides offering a 'numerical wave tank', computations of the type shown here allows examination of flow details - either in a post-processing manner as in this paper - or as a tool for designing laboratory setups on advanced structures.