This report investigates the influence of joint stiffness on the eigenfrequencies of offshore wind turbine supporting jacket structures. The joint is investigated individually, within the context of a jacket structure, and within the context of the entire offshore wind turbine structure. These analyses were specifically conducted for the case of a 10MW wind turbine jacket support structure. This report aims to present an analysis method that can represent the joints of jacket support structures for offshore wind turbines with high fidelity while also ensuring low computational cost during the analyses. The joint stiffness is captured through an implementation of joint submodelling by applying the Craig-Bampton reduction technique. Along with this, the report serves to provide a comparison of innovative wrapped composite joints and traditional welded joints when applied to a 10MW wind turbine. The wrapped composite joint was developed by Dr. Marko Pavlović at the Delft University of Technology, and it was investigated due to its superior fatigue performance compared to welded joint alternatives.
Two sets of analyses were conducted: the jacket analysed by itself, and the jacket analysed along with the tower, turbine, and piles (to be referred to as the ”offshore wind turbine”). Natural frequency analyses were conducted for the jacket and for the offshore wind turbine for the different joint types. It was found that for the jacket and for the entire offshore wind turbine, the models with welded joints had the lowest natural frequencies, the models with wrapped composite joints had the second lowest natural frequencies, and the models with rigid joints had the highest natural frequencies when compared to their respective counterparts.
In conclusion, both the application of the wrapped composite joints and the application of the analysis method of submodelling joints of the jacket are beneficial for solving issues related to the construction of jacket support structures for large wind turbines.

The offshore wind industry is developing larger turbines, often placed in deeper water. This requires larger foundations. The predominant foundation type is the steel monopile, typically consisting of two large diameter pipes, the monopile (MP) and the transition piece (TP). The Wedge Connection is a new type of connection between these components which is investigated and designed in this thesis. The core of the wedge connection is a dowel with an inclined plane on one end. A large number of these dowels is placed around the circumference in a flange fitted to the TP. Holes with an inclined plane matching the dowel are machined into an MP flange. During installation, the TP is placed on top of the MP and the holes and dowels are aligned. The dowels are displaced horizontally with a hydraulic system. The hydraulic pressure is converted into a vertical preload through the inclined plane.


In a comparison of connection concepts the strengths and weaknesses of the wedge connection with regards to other concepts are found. Two options are currently on the market, grouting and bolted flanges. Grouted connections have proven susceptible to long-term degradation, while bolted flanges have limited scalability and can be applied above the waterline only. The main benefits of the wedge connection are the option to apply it under water and the fact that no significant overlap is required. This makes the wedge connection a commercially attractive option.


The fundamental principle of the wedge connection is a vertical preload generated through an inclined plane. In a set of analytical equations the sensitivity to the wedge angle and friction coefficient is investigated. Based on these equations and the shear strength of the dowel, a preliminary design is drafted for a hypothetical foundation with 10 m diameter.


This design is modelled using ANSYS APDL. The connection is incrementally loaded up to the ultimate tensile load. A study is performed on the behaviour of the connection. The sensitivity of the connection to factors such as friction coefficient, input pressure and small geometry changes is considered.