Fatigue and extreme wave loads on bottom fixed offshore wind turbines. Effects from fully nonlinear wave forcing on the structural dynamics.

Since the world’s first offshore wind farm was built in the early 1990s in Denmark, the offshore wind industry has increased tremendously in Europe, and will increase even more the next years. Both the water depth and the size of the wind turbines have increased continually since the first offshore wind farms. As wind farms are being moved further offshore the wave loads become larger compared to the wind loads and therefore more important in the design of offshore wind turbines. Yet, the water depth is still only shallow or intermediate where the waves should be described by nonlinear irregular wave models. In today’s design, however, often only linear or second-order irregular wave theory is used to describe the stochastic process of the waves. The extreme waves are often described by the fully nonlinear stream function theory, which only is valid for regular waves on a flat bed. For this reason it is important to investigate the significance of nonlinearity for irregular waves both in the determination of the extreme loads where the irregular nonlinear waves can become more steep than waves from nonlinear regular wave theory and in the determination of fatigue loads where nonlinear waves will transfer energy to higher frequencies which can be close to the wind turbines eigenfrequency. In the present thesis the response of an offshore wind turbine placed on a monopile foundation is investigated when exposed to linear and fully nonlinear irregular waves. The focus of the investigations is the consequence of incorporation of full nonlinearity in the wave kinematics. In the main part of the thesis six wind and sea states with increasing wind speed and significant wave height are considered. The wave realizations are considered at four different water depths to investigate the effect of water depth on the wave nonlinearity. A fully nonlinear potential-flow model, Engsig-Karup et al. (2009), is used to calculated both the linear and fully nonlinear wave kinematics. The wave forces are calculated by Morison’s equation. The aeroelastic calculations are carried out in Flex5, Øye (1996), to study the dynamic effects of the wave nonlinearity. In first part of the thesis, the linear and nonlinear wave realizations are compared and the static wave forcing based on the two wave theories analysed. This analysis is followed by dynamic calculations where the effects of wave nonlinearity on the structural dynamics are investigated. Focus is on the sectional moments in the tower and monopile. The equivalent loads and accumulated equivalent load due to the six wind and sea states are further calculated and compared. The wind forcing and the aerodynamic damping are often dominating over the effects from the waves. The misalignment between the wind and wave directions is therefore also included in the analysis. In this way it is possible to investigate how the nonlinearity of the waves affects the structural dynamics and fatigue damage in situations where the effects of the wind are insignificant. Damping of the structural response is an important parameter, when the nonlinearity of the waves is investigated. Besides aerodynamic damping other damping effects also exist which affect the structural dynamics. The magnitude of the hydrodynamic damping is therefore also investigated in the thesis. To investigate the effects of the soil in the dynamic analyses, a soil model, Hededal & Klinkvort (2010) and Klinkvort (2012), is implemented in Flex5. With this model it is possible both to investigate the structural response and damping due to the soil and compared it against classical p-y curves combined with a constant damping factor. The potential flow solver is further compared with a CFD-solver, where the detailed flow around the monopile when exposed to waves and the corresponding pressure acting on the cylinder are calculated. The structural response due to the forces from the CFD-solver is compared against the structural response due to the forces based on the potential-flow solver and Morison’s equation. Finally a small study of the effect of including wave directionality in the dynamic analysis is performed. All the analyses in this thesis contribute to the understanding of how important the wave nonlinearity is in the design of offshore wind turbines.