Essentials in Coupled Dynamics of Floating Offshore Wind Turbines

Abstract

At the moment, offshore wind is an indispensable source of energy. Compared to onshore wind, wind speeds offshore are considerably higher and more constant. Furthermore, offshore wind turbines reduce noise and visual impacts. In contrast to fixed bottom foundations, floating wind can be economically feasible in global offshore waters that exceed a depth of 60 meters. However, floating wind has its challenges. Research has shown that the interaction between floater motions and wind turbine controller can be significant, especially above rated wind speed. The interaction between the hydrodynamics and aerodynamics above rated wind speed is the main reason why fully coupled aero-hydro-servo-elastic simulations are needed to determine the motions and mooring loads of the system. The downside of fully coupled simulations is, next to the need of comprehensive technical background, the high computational time which makes these simulations unsuitable for concept design studies. The purpose of the research is to developed a fast and simplified model that can be used for design iterations in concept design stages of Floating Offshore Wind Turbines (FOWTs). The simplified model must be able to predict the general motions and mooring loads with acceptable accuracy. Moreover, the simplified model should be based upon available wind turbine input data in an early design phase. A sensitivity analysis is carried out to determine the contribution of various physical phenomena, present in fully coupled aero-hydro-servo-elastic simulations, on the motions and mooring loads of the GustoMSC Tri-Floater with a 5MW NREL wind turbine located at its center. The fully coupled simulations are performed with aNySIM-Phatas. By this analysis, the main physical contributors to the motions and mooring loads of the GustoMSC Tri-Floater are selected to include in the simplified model. It is confirmed that the interaction between the aerodynamics and hydrodynamics is significant, especially for platform pitch motions in the above rated conditions. Furthermore, the (controlled) aerodynamic thrust calculated from Blade Element Momentum (BEM) theory, as well as the wind drag loads are found to be the main driving forces on the system. The simplified model is constructed by coupling an external Python code to the multi-body time-domain software aNySIM. Whereas aNySIM is responsible for the hydrodynamic and mooring calculations, Python is used for the simplified aerodynamic calculations. The effect of the wind turbine controller on the aerodynamic thrust is included by the use of the steady state thrust curve, aerodynamic damping ratios and filter on the incoming turbulent wind. The simplified model is compared with fully coupled aNySIM-Phatas simulations. Compared to fully coupled simulations, obtained results suggest that in a free floating condition the differences in the mean are found to be within 5% and 10% for platform surge and platform pitch, respectively. The mean and maximum differences in the forward fairlead tension were found to be within 2% and 5%, respectively. Moreover, the results show that the simplified model is 5 times faster and much easier to construct compared to the fully coupled simulations. The results of this research are encouraging as the simplified model can serve as a tool to predict the general motions and mooring loads with acceptable accuracy. Also, as Python is not coupled to aNySIM before to the best of the authors knowledge, this coupling opens doors to other simplified modelling strategies.