Offshore Wind Turbine Substructures at Intermediate Sea Depths Preliminary design of a floating substructure and comparison with a fixed substructure

Abstract

The demand for electricity is increasing and offshore wind farms have great potential to bridge this void. This thesis explores the potential of having floating offshore wind turbine substructures at sea depths between 30 and 60 m because of the limited studies in this area. The study will provide a comparative analysis of a fixed lattice-based substructure and a floating semi-submersible substructure for a 5 and a 15 MW wind turbine. Furthermore, this thesis will address the various factors that need to be considered in the selection of the solutions and will cover a preliminary design of the two substructures. The preliminary design is based on simplified methods that can be used in the early stages. Focus lies on the analysis of global response and structural resistance as well as a brief frequency observation. Due to the complexity of the floating solution, this thesis aims to summarize and clarify the main theory behind them. In comparison to the floating substructure, the design method for the jacket substructure is more straightforward and well-documented. The main purpose of the preliminary design of the jacket substructure is to compare the material consumption and how it changes with the intermediate sea depths. The results from the design iteration provide a deeper understanding of the overall behavior of the substructure and validate the applied simplified methods. Furthermore, the results show the need for design changes for varying depths for floating and fixed substructures. A floating substructure of the type semi-submersible is possible at intermediate sea depths. However, according to the preliminary design of this particular semi-submersible, it was less material efficient than the fixed jacket substructure.