Slow-drift of a floating wind turbine: An assessment of frequency-domain methods based on model tests

The second-order hydrodynamics of a semisubmersible offshore wind turbine is investigated in this paper by analyzing and proposing a set of different options for estimating the slow-drift motions during its design. A case study consisting of a three-floater semisubmersible designed to support a 1.5Mw turbine is considered. An experimental campaign focused on characterizing second-order surge response was carried out and its most salient results are documented in the paper. The campaign was conducted in two different facilities and comprised decay tests, regular, bichromatic and irregular waves. Wind has not been considered in this phase of the research. Numerical modeling with frequency domain solver WAMIT has been carried out. Due to location depth and mooring length restrictions, the natural periods of horizontal excursions are smaller than those of well studied DeepCwind platform. This may change the importance of the different second-order components, something investigated in present research by comparing simplified and full Quadratic Transfer Functions (QTF) computations. Results obtained with experiments and simulations are compared, focusing on the mean and slow-drift motions and forces. It is shown that the Newman approximation underestimates the second-order response in some cases while the white noise model retains the main physics involved, a novel result which may change the paradigm for mooring design of these artifacts in the near future.