Fatigue Analysis of Floating Offshore Wind Turbine Tower in Composite Material

Abstract

The expansion in the offshore wind industry is growing rapidly due the substantial amount of energy it can generate. Floating Offshore Wind Turbines (FOWT) have high potential in the energy market for sites with deep water depths. However, these FOWT’s platforms are subjected to high dynamic coupling loads which can lead to fatigue in their substructures. The superior properties of FRP materials including better corrosion-resistance, strengthto- weight ratio and fatigue performance make them better candidates compared to steel. As the size of structures increases, the advantages of steel diminish. However, the lack of design guidelines and rules along with technological limitations highlight a research gap for the use of FRP materials in the offshore renewable industry. Unlike other aspects of FOWTs such as foundations, the tower linking the turbine and foundation has not been a primary focus of innovation. The FibreGY project aims to enable the use FRP materials in the marine renewable sector particularly in large offshore wind and tidal platforms. In this thesis, the fatigue analysis of a semi-submersible twin-turbine FOWT tower made with composite materials will be performed. Bureau Veritas (BV) software HydroStar will be used for the FOWT hydrodynamic response. The newly in-house software OPERA developed within the research department of BV Marine and Offshore will be used for the FOWT global response under environmental loads. The stresses within the FRP tower will be obtained by using the analytical model developed in this thesis in a Python script. BV fatigue guidelines will be used to assess the fatigue life of the FRP tower. Design optimization of the FRP tower will be carried out using the developed Python tools for the fatigue life and BV ComposeIT software for the static analysis. FEMAP software will be used for FOWT modelling, static analysis, buckling and modal analysis. The results show that the fatigue damage of the FRP tower using carbon fibre and epoxy resin is very low and sufficient for 20 years design life. The design optimization of the FRP tower shows a reduction of nearly 66% in weight compared to the original total weight with the reference laminates. The optimized FRP tower is able to withstand static loads and fatigue damage while respecting the deflection and buckling criteria.