Elastic Response of Floating Offshore Wind Turbines to Waves, Current and Wind

This thesis is concerned with the development of a hydro-aero-elastic coupling approach to determine the dynamic motion of Floating Offshore Wind Turbines (FOWTs) to combined wave, current and wind loads. In this study, it is aimed at obtaining a numerical approach that can be applied for dynamic analysis of both single- and multi-unit FOWTs. The numerical coupling approach is developed in frequency domain and the hydrodynamic and aerodynamic loads are obtained by use of a Green function method and steady blade-element momentum method, respectively. The numerical approach is coupled with finite-element method to determine the elastic motion of the FOWTs. An analytical model is developed to linearise the aerodynamic forces and moments on operating rotors of FOWTs. The aerodynamic load is presented with a harmonic function with the wave frequency, so that it can be linearly added to the equation of motion of the floating structure in frequency domain. The mode-shapes of the FOWTs are obtained by use of finite-element method and a reduced-basis approach. Next, the calculated mode-shapes are added as generalised modes to the equations of motion of the FOWTs to determine their flexible-body responses due to the environmental loads. Finally, assuming that the current speed is small, the wave-current interaction with FOWTs is studied by use of boundary integral method with a Green function for small current speeds. HYDRAN-XR, is a potential flow solver that is integrated with finite-element method for hydroelasticity analysis of floating structures. In this study, HYDRANXR is further enhanced to obtain the wave-current-induced motion of floating structures and aerodynamic loads on single- and multi-unit FOWTs. Rigid-body and elastic responses of several floating structures and a single-unit FOWT to various environmental loads, namely (i) wave loads only, (ii) combined waves and current, (iii) aerodynamic loads on a rotor and finally (iv) combined waves and wind are determined by HYDRAN-XR and compared with available laboratory measurements and numerical data in literature. Next, rigid-body and elastic motion of three single-unit FOWTs, namely SPAR, barge and semisubmersible FOWTs to aligned and misaligned wave, current and wind loads are determined and compared. To investigate the effect of current on the dynamic motion of the FOWTs, wave-induced hydrodynamic forces and moments on the structures are calculated and compared with those when current is present. Furthermore, the responses of the three FOWTs for several current speeds and misalignments of incoming waves with wind and current are obtained and compared. Overall, it is observed that the wave-current interaction affects the wave-induced motions of the SPAR FOWT more than other two FOWTs. Finally, a multi-unit FOWT, the wind-tracing FOWT is introduced (subject of interest to the industry sponsor of the project). The layout of its mooring system is identified in a parametric study. The rigid and flexible-body motion of the wind-tracing FOWT to various environmental loading are determined and discussed. Discussion is provided on the importance of including the hydrodynamic loads due to wave and current interaction and the elasticity of the entire structure. It is shown that the motions of the wind-tracing FOWT are dominated by the wave-induced hydrodynamic loads and the hydroelastic motion of the substructure is significant.