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.

Climate change, as a result from global warming, requires an energy transition: the reduction of greenhouse gas emissions from fossil fuels and a radical innovation of the global energy system to proceed apace. Offshore wind is an important source of clean, renewable energy, and it plays a key role in the transition. 80% of the worldwide offshore wind is to be produced on locations in deep waters; here floating foundations are required, that to date are far more expensive than their bottom-fixed counterparts. To reduce costs of floating wind energy, reliable, detailed predictions of the system’s loads and motion response are crucial. Floating offshore wind turbine structures are designed using ’aero-hydro-servo-elastic’ software codes that simulate the dynamic response of a floating offshore wind turbine system to the offshore environment. Predictive accuracy can be improved by comparing simulation results from a model of a known system against measurements taken from the real-world system, a so-called model validation. One promising state-of-the-art aero-hydro-servo-elastic software code is BHawC/OrcaFlex, developed by Siemens Gamesa Renewable Energy (SGRE). Due to its novelty, however, validation of the code has only been carried out to a limited extend, giving rise to uncertainty about the interpretation of simulation results. The purpose of this MSc. thesis project is to validate the performance of BHawC/OrcaFlex by comparing its simulated load and motion results to measurements on a real-world floating turbine from the Hywind Scotland floating offshore wind farm (Hywind). Measurement data and a description of the ’as-built’ system were made available by the wind farm owner Equinor ASA. In order to establish an achievable level of modelling accuracy and predictive value of BHawC/OrcaFlex, the code was verified against another aero-hydro-servo-elastic software code: OrcaFlex, by setting up a similar model of the Hywind system in both codes. Limited information is available on the performance of OrcaFlex in floating wind load and motion predictions. Therefore, it was in turn verified against a wide range of industry-standard aero-servo-hydro-elastic software codes, using a modeled system that closely resembled the Hywind turbine and load cases that step-by-step increased in complexity, to further isolate causes of discrepancies between the models. OrcaFlex predictions matched very well across all load cases. The main differences were attributed to differently modeled additional linear hydrodynamic damping, as the official damping prescription resulted in prediction errors. In the BHawC/OrcaFlex verification against OrcaFlex, both models were subjected to multiple load cases that step-by-step increased in complexity, to further isolate causes of discrepancies between the models. Simulation results from running both models appeared to be nearly identical, though some discrepancy was observed from due to the simplified aero-servo-elastic OrcaFlex code. The final validation of BHawC/OrcaFlex to full-scale Hywind measurements is performed at below-rated, rated and above cut-out wind speeds with multi-directional wave and current components. In general, BHawC/OrcaFlex motion frequency domain predictions appeared to correspond well to the actual Hywind measurements. Most phenomena in the low-frequency, wave-frequency and high-frequency region were captured by the simulations. However, large errors were observed in the mean surge, sway and bridle line tensions predictions. Most discrepancies were found originating from errors in the model set-up, e.g. lack of hydrodynamic damping, simplifications in the wave model or errors in the mooring system set-up. Tuning of the mooring system showed improvement of the results, but further improvements could be made. Several sensitivity studies were added on parameters, such as hydrodynamic drag, tower damping and mooring drag. This showed overprediction of the surge/sway and roll/pitch frequency responses can be mitigated by both additional linear and viscous hydrodynamic damping. The main recommendations for further research are to further analyse errors identified in the model set-up. In addition, some yet unexplained phenomena that are not captured by BHawC/OrcaFlex in the current model, are to be addressed. Finally, a the development of a standardized approach to relate model validation studies in the field of floating wind to cost improvements could further quantify the value of future comparison studies.

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.