The increase in demand for renewable energy has resulted in higher demand for wind energy. To meet this requirement, the wind turbine sizes are increasing rapidly, and this results in increasing load on the connections between MP-TP and the segments of the tower. This research focuses on traditional L-flange connection and novel C1 wedge connection, former the most widely used in the offshore industry and other being new to the offshore industry. The objective of this thesis is to evaluate the limits of the L-flange and C1 wedge connection. The design of the L-flange connection carried out on the basis of Petersen’s theory[1]. It is designed to have higher ULS resistance with steel mass as low as possible. This analytical designed is then compared to finite element analysis (FEA). Previous study of Cheng[2] is used to validate the setup and methodology for FEM in Ansys. For the same overturning moment, C1 wedge connection is designed using the design tool provided by C1 Connections. The design check for flanges is carried out. The design is then compared to finite element analysis (FEA). Fatigue limit state was verified for both the connections. Based on the study, the following conclusions are made. Firstly, L-flange connection has around 30% higher ULS resistance compared to design overturning moment of 609MNm whereas C1 wedge connection has 55% higher ULS resistance. Secondly, L-flange connection has higher meridional deformation at the same elevation in the shell as compared to C1 wedge connection. The gap opening at the interface of the flanges is studied and it is observed that C1 wedge connection opens after the loss of contact force at the interface which is generated more efficiently through the pretension of stud whereas L-flange connection being an eccentric connection starts opening without full loss of contact force at the interface and the secondary path of load transfer is activated. This results in the lower meridional rigidity provided by the L-flange connection as compared to the C1 wedge connection. Lastly, C1 wedge connection provided an opportunity for several optimizations to have the same ULS resistance as that of L-flange connection. The mass of steel reduced for these optimizations. The L-flange connection weighed 53.72 tonnes for both ULS and FLS criterion whereas C1 wedge connection weights 20.8 tonnes for ULS criterion and 28.8 tonnes for FLS criterion.

The global focus on climate change and the transition away from fossil fuels has highlighted the importance of renewable energy sources. Offshore wind turbines are being optimized and are therefore growing in size and power. This research focuses on bolted ring-flange connections, a connection type that plays a crucial role in the design of offshore wind turbines, as they transfer the external force between parts of a turbine. The objective of this thesis is to analyze how the increasing dimensions from current to future offshore wind turbines and geometric imperfections impact the reliability of analytical approaches for load transfer functions (LTFs) for these connections. Two components of this objective are considered: examining the influence of different dimensions of ring-flange connections and analyzing the impact of various gaps between flanges on LTFs for 'current generation' and 'next generation' turbines. Analytical calculations are compared to results obtained with finite element analyses, which are assumed to represent an actual connection. Based on the research findings, the following conclusions are made. Firstly, the widely used tri-linear approach by Schmidt/Neuper [18] for obtaining the LTF in bolted ring-flange connections is found to be unreliable for current and future turbine sizes. This method highly underestimates the forces in the bolts when initial gaps are present between the flanges. Calculations performed with this approach could lead to an overestimation of the turbine's lifetime compared to reality by multiple years, possibly causing more maintenance or early failure. Alternative approaches, such as a very new and not yet approved polynomial approach, show reliable results, providing accurate estimations of bolt forces for large connection diameters. Additionally, currently verified tolerances for gaps between flanges (1 mm over 30° and 2 mm over entire circumference) are outdated, and larger gap heights or smaller gap lengths are expected in practice, especially for future turbines. These gaps lead to a larger bolt force in practice, decreasing the fatigue resistance and lifetime of the structure. Even though very small gaps are expected to occur often, lower bolt forces are obtained compared to larger gaps, both with an expected height to length ratio of 𝑢𝑔𝑎𝑝/𝑙𝑔𝑎𝑝= 0.53 ∗ 10^−3. In analytical design calculations, it therefore is recommended to consider larger sized gaps with a gap length of approximately 1600 mm with its expected gap height.