Finite Element Analysis of an Offshore Wind Turbine Monopile

Large diameter monopile foundations for offshore wind turbines are subject to large horizontal loads and overturning moments. These foundations have diameters ranging from four to seven meters with the potential to become larger with the development of larger wind turbines. They are often analyzed by means of the p-y method to characterize the pile-soil interaction. While the method is theoretically rigorous, the input p-y curves, for example those recommended by the American Petroleum Institute (API), are based upon very limited field data. This paper describes some analyses of a large diameter monopile in dense sand. The pile characteristics are representative of the foundations for a 3 to 5 MW wind turbine. The soil characteristics are representative of dense sandy soils that may be encountered at wind farm sites in the southern North Sea and offshore the Northeast United States. The pile was modeled by the p-y method and also by 3D finite element analysis (FEA). The paper describes the details of the FEA model development, important in minimizing numerical inaccuracies in 3D analyses. Equivalent p-y curves, back-calculated from the 3D FEA, may be useful for p-y analyses, which are simpler than 3D FEA. The results suggest that the API method over predicts soil resistance and under predicts pile deflection for large diameter monopiles subjected to lateral load and in stiff soils.