Implied Reliability Levels in Different Load Models for Offshore Wind Turbines

Assuring uniform reliability levels across various system configurations is the intent of design standards based on the Load and Resistance Factor Design (LRFD) methodology. One such design standard for offshore wind turbines developed by the International Electrotechnical Commission was based on the European experience and may not necessarily represent conditions suited for U.S. waters where several offshore wind energy projects are being planned. It is, hence, of interest to investigate how uniform is the reliability of offshore wind turbines under various levels of wind and wave loads. We assess the reliability of bottom-supported offshore wind turbines in ultimate limit states associated with the fore-aft tower bending moment at the mudline. We compare reliability index estimates for different characteristic load definitions and assumed coefficients of variation for wind and wave loads, as well as for various hydrodynamic to aerodynamic load influences. Effectively, such variations serve to describe different sites and turbine designs. Since large-diameter monopile support structures are dominated by inertia forces, while jacket or tripod support structures with smaller diameter members are dominated by drag forces, we extend an available combined wind-wave load effect model for offshore wind turbines, to include both drag and inertia forces. We show that reasonably uniform reliability levels may be achieved for various combinations of wind and wave loads. Results suggest that drag-dominated wave load cases result in smaller and less uniform reliability estimates than is the case for inertia-dominated wave load cases.