Offshore wind turbines are complex systems that exhibit intricate interactions among the loads, the environment, and the turbine structure. Therefore, wind turbine design significantly relies on the ability to understand this coupled system and accurately estimate the in-service design loads.

A review of the available literature indicates that most of the current understanding of the structural dynamics of offshore wind turbines stems from research on 5 MW turbines. However, as the wind sector progresses towards taller turbines with larger rotor diameters, the interaction between the rotor and support structure becomes more relevant. In addition, with blades more slender, flexible, and lighter than ever before, the complexity of the turbine dynamics significantly increases. Accentuated geometric nonlinear effects, combined with a more sophisticated blade geometry and anisotropic material behavior, contribute to the challenging structural response of modern blades. Hence, there is a need for further investigation into the influence of modern blades on the turbine response and resulting design loads.

In this context, the present study performs a sensitivity analysis on the IEA 15 MW Reference Wind Turbine to investigate which blade parameters have the greatest influence on fatigue loads during normal turbine operation under turbulence. The sensitivities of different blade parameters are ranked, a reflection on the type of relationship (i.e., either monotonic, quasi-monotonic, or non-monotonic) between each blade parameter and the model response is provided, as well as an indicator of the change caused in the magnitude of the loads. One of the goals of this research is to support blade manufacturers in improving their blade designs by tuning the parameters in new blade models to achieve lower fatigue loads at different positions of the turbine structure. Furthermore, this study aims to highlight parameters that should receive special attention during the manufacturing phase, as slight deviations in their values from the original blade design may considerably increase the fatigue loads.

In this work, aeroelastic simulations are performed with HAWC2, and fatigue loads are quantified using Damage Equivalent Loads, while the first-order Elementary Effects method is employed to assess the parameters’ individual effects on the turbine response. Lastly, two strategies are adopted to ensure that the causes of changes in the dynamic response of the turbine are mainly attributed to variations in blade parameters: turbulence files are generated only once per wind condition and reused in all simulations, and the controller is retuned with HAWCStab2 each time a blade parameter is altered.

The findings reveal a dominant influence of the shear centre on the flapwise moment at the blade root, as well as on the fore-aft moments at the tower-top and tower-bottom across different wind speeds. These moments are critical with respect to fatigue loads since they align with the wind direction and experience a high number of load cycles during turbine operation. This research also unveils that even slight differences in the shear centre position along the chord can have a significant impact on the fatigue loads. For the torsional moment at the blade root - a challenging load component in modern blades due to their increasing length, slenderness, and flexibility - the flapwise bending stiffness, followed by the edgewise bending stiffness, exhibit the highest importance levels at below and near rated wind speeds. In contrast, at above rated wind speeds, the importance of the edgewise swept increases significantly, surpassing the influence of both flapwise and edgewise bending stiffness. Lastly, the torsional stiffness typically plays an important role at near and especially above rated wind speeds in several load components at the tower.

Although the findings may vary based on the size, type, and control of the wind turbine evaluated, the results of this thesis are expected to contribute to the load analysis of other turbines.