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.