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.

Gates are an essential part of navigational locks, as they are responsible for creating a water barrier and can be repositioned to allow passage for maritime transit. Most lock gates are made of steel but a recent project in Sweden challenged the status quo by constructing the first stainless steel lock gate. The main question of this thesis aims to investigate the possibilities for the application of stainless steel in Dutch mitre gates and how optimisation through implementing a global arch shape can contribute to the material and weld reduction required to do this. To quantitatively assess the hypothesis of this thesis, Lock Veere was chosen as a case study. The finite element method was selected to simulate the operational conditions of a lock gate.
The first part of this thesis aims to define what needs to be taken into account when designing arch-shaped stainless steel mitre gates. The gate can be positioned in three different setups, which each provide different boundary conditions. Eight different load combinations were defined that could possibly be critical based on size, asymmetrical positioning or repetitive loading. In addition to the operational conditions, the requirements and limits of stainless steel in lock gates were considered.
The second part of this thesis investigates the feasibility and optimisation of the global shape. The effectiveness of shape is based on the loading system of a structure. For a lock, this is the water level difference that results in a uniform load over the width of the construction. Arches are relatively more resistant to this load, which allows for material reduction. Mitre gates have another dominant load situation; a point load with asymmetric support during an obstacle situation. Plates alone are not resistant to this load, so a frame around the plate is introduced. Based on the results found in this thesis, both components seem feasible and allow for further material reduction.
The third part of this thesis aims to prove the effectiveness of a complete gate design by simulating the construction with the operational conditions selected in the first part. The aim of the design was to be as simple as possible in order to meet the demands imposed by stainless steel. The final design allowed for 20\% weight reduction and 30\% weld length reduction compared to the traditional design.
Based on the results that were found, an answer to the main question posed by this thesis can be formulated. For the operational conditions found in Dutch mitre gates, an arch shape can be applied in combination with a surrounding frame. This reduces the weight, number of welds and weld length. For these reasons, manufacturing is expected to be less intensive and maintenance is more accessible due to component reduction. Global shape was thus able to contribute to the optimisation of Dutch mitre gate design. This optimisation through global shape adheres to the requirements for stainless steel. However, this result opens up interesting possibilities for further research on using global shape in combination with other materials as well.

The use of welding is widely adopted to assemble structural components in the construction industry for many years. To ensure safety of these welded components, many fatigue tests have been conducted on many different shapes and configurations of welded connections to precisely assess the fatigue
life. However, testing real-size structures and specimens is very limited due to it’s high cost and in-applicability. Thus, these full scale specimens are cut down into small scale specimens to allow applicability for testing. Different characteristics are exhibited between the full and the small scale specimens, as there are major difference in residual stresses induced by welding and cutting, which may give non conservative predictions for fatigue life.
In this thesis, the objective is to forecast the evolution of the residual stress field originated by welding of a full scale and small scale specimens of a cruciform joint at the weld toe after breaking it down into smaller specimens using a cutting process by performing a numerical analysis using Abaqus
finite element analysis software. To achieve this goal, first, a thermo-mechanical welding simulation was performed to obtain a welding
residual stress field on a 910 mm long cruciform joint, which is done in two main parts, starting with the thermal model in which a temperature field is analysed. The temperature field from the thermal model
is used as an input for the mechanical model in which the residual stress field is produced due to the temperature change and restriction of movement of material due to the shrinkage and expansion. Secondly, the 910 mm long full-scale cruciform joint was cut into five shorter specimens of 500, 210,
120, 75, and 20 mm. The welding residual stress (WRS) levels at the weld toe for each specimen was recorded and showed large stress losses and relaxations as the specimen gets shorter in length. a major longitudinal stress loss of 97% and 77% loss of maximum principle stresses when cutting down
the 910 to a 20 mm long specimen, making it almost free of WRS, but only a 5-6% loss of longitudinal and Max. principal stresses when going from 910 to 500 mm. Thirdly, after generating multiple welded specimens with different lengths, a tension load of 186.2 is set in the x-direction of the attachment plate of the cruciform joint, and the stress level at the weld toe was analyzed due to the applied load and the WRS. A 40% increase of the stress occur due to the
applied load, but a very slight decrease in longitudinal stresses for the 910, 500 and 210 mm due to depicting a plate-like behaviour in contraction due to poisson’s effect. Finally, the same specimens were analysed under the 186.2 MPa load but without including the WRS. Different shapes of stress distributions were found, and differences in stresses when comparing
the models with and without WRS in the models. The difference in longitudinal z-direction reached up to 282 MPa, while only 77 MPa in the transverse direction. The maximum principle stresses insured the importance of including WRS when performing fatigue assessment as it showed the fatigue failure to occur in the weld root with a crack to happen at the middle part. The specimens that exclude WRS would start cracking at the edges of the weld root, but in the central part of the weld seam when including WRS. The model that included WRS showed similar fracture location at the weld root as the fractured specimen performed in tests at TNO’s laboratories.
The next steps in this research is the modification of a modelling methods. The results can be improved and smoothed by modelling using the effective notch method were a radius is introduced at the weld toe and the root to eliminate the stress singularities. The welding simulation can be improved
to get better results by modelling the full cruciform joint without symmetry conditions, and include a weld order for all four welds with proper cooling time in between each weld.