The demand for offshore wind turbines (OWT) is rising due to the increasing need for renewable energy. This growth is reflected not only in amounts of OWTs but also in their power rating capacity and size. Currently, the L-flange is the most commonly used connection to join the tower to the foundation. However, this connection type has trouble to handle the unprecedentedly high overturning moments that must be transferred between OWT segments. The C1 Wedge Connection (C1-WC) offers a solution by using multiple fasteners to connect a fork-shaped upper flange (UF) to a lower flange (LF), which are both welded to 100+ mm thick tubular steel sections.
The UF is currently manufactured using submerged arc welding to join its three components: the crown (CR), inner web (IW), and outer web (OW). This research investigates the feasibility of electron beam welding (EBW) as an alternative technique. EBW offers increased production speed by utilizing higher power density, allowing the weld to be made in a single pass. However, its full-scale (long-term) performance is unproven. This study explores its feasibility through small-scale experimental tests. The thesis also examines an alternative weld location in the C1-WC UF, comparing the current and alternative weld positions in terms of fatigue and ultimate limit state (ULS) by using finite element analyses.
The main research question is:
To what extent is the welding technique 'Electron Beam Welding' feasible for application in the C1 Wedge Connection™ upper flange manufacturing?
Numerical analyses offer insights into fatigue performance and the use of increased strength in the net cross-section. Although the alternative design, with the weld located in the gross cross-section, demonstrates acceptable fatigue performance under preloaded conditions, it does not outperform the base-case design in service life. Moreover, the increase in net cross-sectional yield strength of the alternative design remains unutilized in the ULS. This suggests that the UF geometry could be designed more efficiently: by reducing the thickness of the inner and outer webs with 19%, an UF mass reduction of 13%, 3.3 tonnes of steel, is achieved.
Experimental tests presented challenges, particularly regarding the precision required for EBW. A slight misalignment in the weld led to incomplete fusion at the weld’s root. Hardness testing revealed a significant increase in hardness in the heat-affected zone (HAZ) and fusion zone (FZ). Charpy impact tests failed to meet the minimum requirement of 31 Joules at -40°C, while tensile tests showed that failure occurred outside the weld. This raised concerns about the weld’s ductility.
Overall, EBW is not yet a feasible option for full-scale production of the C1-WC, mainly due to the needed stringent tolerance requirements and thereby in achieving a consistent welds along the full OWT circumference. Submerged arc welding remains the most reliable method for assembling the UF. Future research should focus on improving EBW’s robustness and reliability, particularly in understanding its fatigue performance and how to account for geometric imperfections.
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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. ... Read more

The field of wind energy has experienced significant expansion and development driven by the need to reduce reliance on fossil fuels. Offshore wind turbines have become increasingly popular leading to larger turbines with greater energy output to match the growing demand. The most common foundation type of offshore wind turbines is the monopile foundation, consisting of two large diameter steel tubulars, the monopile (MP) and the transition piece (TP).
As wind turbine generators (WTGs) increase in size, the transportation and installation of MP foundations becomes more complex and expensive, requiring specialized vessels with adequate lifting capacity. This research investigates a new connection – the underwater wedge connection – as a method of connecting an MP to a TP underwater. By having an underwater connection, the size of the MP can be reduced as it no longer has to exceed the waterline.
The wedge connection consists of a dowel with an inclined plane and two flanges. A number of dowels will be placed around the circumference of the connection, fitted onto flanges on both the MP and TP. As the dowels are pushed into position, the inclined plane creates a vertical preload between the two flanges. This allows a tensile load on a segment of the connection (caused by the bending moment in the foundation) to be transferred to the foundation via two load paths: reduction of the preload, and by loading the dowel itself in shear.
First an analysis on the current state of art and relevant design codes was carried out. This analysis highlighted the requirements the connection needed to satisfy, in order to serve as starting points to make decisions regarding the design. The preliminary design of the connection is made using analytical calculations. The structural integrity of the flanges and the dowel at the Ultimate Limit State (ULS) was verified. A 3D model of the connection was then created to perform numerical analyses using ANSYS Static Structural. The behaviour of the connection under ULS and Fatigue Limit State (FLS) loading is studied. The opening and failure point of the connection along with the sensitivity of various parameters are also investigated.
The results show that connection was able to effectively transfer both tension and compression loads. The connection has a mechanical advantage of 1.95, meaning that it achieves the same preload as the ULS load by applying only 51.3% of the ULS segment load during installation. The connection opens gradually and only opens at a load higher than the ULS load. Even after opening, it continues to transmit loads effectively, with ultimate failure governed by the yielding of the lower flange. Additionally, the connection exhibits good fatigue resistance, with a low fatigue damage level of 3.8%.
It is recommended to conduct experiments to validate the numerical model employed in this study. Further studies should also be carried out to investigate the impact of structural imperfections on the behaviour of the connection.
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