Reducing the levelised cost of energy is crucial to accelerating the energy transition. To develop offshore wind solutions in greater water depths, a floating solution is required. The time-domain simulations of these Floating Offshore Wind Turbines (FOWTs) under wind-wave misalignment used in research and industry projects are computationally intensive and limits researchers and industry in their developments. To better understand the sensitivities of the fatigue loads of FOWTs to different parameters and environmental conditions, a computationally efficient method is needed. The aim of this research is to develop a frequency-domain method to quantify the effects of misaligned wind and waves on the response of a semi-submersible floating offshore wind turbine. Therefore, the following research question is defined: What is the effect of misaligned wind, windsea waves, and swells on the loads at the tower base of a semi-submersible type floating offshore wind turbine?

Several sensitivity studies are conducted to quantify the contribution of yaw-roll coupling effects and aerodynamic damping to the responses and loads. From these studies, it appears that the yaw-roll coupling can increase the response when excited at wind/wave directions in which the structure is asymmetric.
The magnitude of this effect is related to the wave peak period (and the resulting wavelength), the angle of misalignment with respect to the structure, and the apparent length of the structure. Also, the lack of aerodynamic damping in the direction of the rotor plane (side-side direction) leads to a noticeable
increase in the response, directly or through coupling effects. Finally, the frequency-domain method is compared with the time-domain simulations (BHawC-OrcaFlex) carried out by Siemens Gamesa. Although reasonable agreement is found for the load driving rigid body modes, significant differences in the tower bottom loads are found for the lowest and highest production wind speeds.

These results show that misaligned wind and waves can increase the response for headings where the structure is asymmetric due to coupling effects. Wind-wave misalignment leads to an increased response in the direction of the rotor plane due to the lack of aerodynamic damping. In general, the wind-wave misalignment can also have a mitigating effect on the maximum equivalent moment at the tower base, as the aerodynamic damping also reduces the response in the wave frequency range. Furthermore, the comparison shows the need to extend the frequency-domain method with the first tower bending modes and improvement of aerodynamic/mooring property estimation. Based on the findings and the conclusions, the recommendation is to investigate the floater specific sensitivities at an early stage of the design. Future research should focus on: the implementation of tower flexibility, improvement of the quasi-static estimation of mooring stiffness, frequency dependent aerodynamic properties, and implementation of second-order wave forcing.

In the simulation of floating wind turbines, a traditional rigid floater assumption becomes less valid while pursuing large size floating wind turbines with steel-efficient floaters. Up to date, hull flexibility still cannot be efficiently incorporated into aero-servo-elastic-hydro simulation tools, and the possible influence of hull flexibility has not yet been well-understood. Consequently, it is necessary to identify the significance of hull flexibility and the possible effect of it.

Recent researches have been investigating the influence of hull flexibility on substructural internal load, global responses and dynamics of the system. However, little has been done from a tower design perspective. Moreover, tower design for a floating foundation has also been seldom documented. To fill the knowledge gap, two research questions are defined: What is the difference in tower design with a floating foundation? and What is the effect of hull flexibility on tower design?

To answer the first research question, a FEM model with rigid hull is built based on four floating concepts designed for DTU 10MW wind turbine. The tower fore-aft bending natural frequencies are compared between fixed foundation and floating foundation. The second research question is answered by developing a FEM model with flexible hull based on a spar-buoy concept. The rigid hull model and the flexible hull model are compared by implementing structural analysis and fatigue damage estimation under waves load.

The result shows that the 1st tower bending natural frequency increases significantly(except for TLP) from a fixed foundation to a floating foundation, making it difficult to achieve a soft-stiff tower design. Furthermore, it is indicated that hull flexibility can decrease the 1st tower bending natural frequency, and the magnitude varies with different tower designs. A stiff-stiff tower decreases more while a soft-stiff decreases less. Lastly, the fatigue damage estimation implies that a soft-stiff design can be lack of fatigue strength to survive from waves load.

In conclusion, a soft-stiff tower design is difficult for large size floating wind turbine partly due to the increase in 1st tower bending natural frequency from fixed foundation to floating foundation, and partly because of strength requirement for fatigue load. As for a stiff-stiff tower design, without considering hull flexibility, there is a high uncertainty in the 1st tower bending natural frequency. As a result, for large size floating wind turbines, inclusion of hull flexibility is necessary for the tower design.