Comparative analysis of numerically simulated and experimentally measured motions and sectional forces and moments in a floating wind turbine hull structure subjected to combined wind and wave loads

Multi-body time-domain finite element models, which implement a recently developed numerical approach for determining forces and moments in floaters, are developed to simulate rigid-body motions and sectional forces and moments of a reference 5-MW braceless semi-submersible wind turbine in turbulent winds and irregular waves corresponding to below rated, at rated and above rated conditions. The simulated responses are compared with measurements of a 1:30 scaled model test using a real-time hybrid testing approach. In general, agreement between simulations and measurements are very good. Differences in spectral densities of the measurements and simulations have been quantified while the reasons for the differences have been thoroughly analyzed and discussed based on comparisons of measurements in different conditions and numerical parametrical study. Effects of non-linear wave excitation loads and drag forces on the rigid-body motions and sectional forces and moments are analyzed while dominant load components in fore-aft bending moments in five cross-sections in the hull of the reference model are identified. The interface between the pontoons and central column of the reference model is identified as the most critical part. Both low frequency and wave frequency load effect should be accounted for. Mean forces and moments from wind and waves result in a change in configuration of mean wetted body surface of the hull when compared to its configuration in calm water. This may result in a considerable change in resultant sectional forces and moments even though change in resultant of the hydro pressure forces on whole of the wetted body surface could be very limited. For the analyzed model, simulated fore-aft bending moments of the model in wind and waves could be obtained by superimposing the results for wind only condition, and wave only condition except that the corresponding averaged wind induced forces and moments should be applied on the numerical model. This simplification can significantly reduce number of cases of short-term analysis required in long-term analysis. However, applicability of the simplification should be analyzed case by case in particular for a blunt structure with relatively large volume of displaced water in waves with relatively small wave length. Analysis and discussions given in this paper are based on available measurements of the model test. Hydroelastisity and structural vibration of the columns and pontoons of the hull are not accounted for by the numerical and experimental models. Suggestions for design of future model tests are given in this paper.