Foundation models for offshore wind turbines

The objective of this study is to investigate what effect the use of alternative models for monopile pile foundations for shallow-water offshore wind turbines has on extreme loads associated with long return periods that are needed during design. We employ a utility-scale 5MW offshore wind turbine model with a 90-meter hub height in stochastic simulations; the turbine is assumed to be sited in 20 meters of water. Selected 20-year wind-wave combinations are employed as we study comparative time histories, power spectra, response statistics, and probability distributions of extreme loads for fixed-base and flexible foundation models. Two alternative flexible foundation models are considered and longterm loads are studied in comparison with the use of a fixed base model. A discussion on the varying dynamics, on short-term response statistics, and on extrapolated long-term loads from limited simulation is presented. It is shown that root-mean-square (RMS) tower loads are higher for the flexible foundation models than for the fixed-based one. This is due to the smaller stiffnesses of these models. Extreme loads are also higher for the flexible foundation models though not to the same degree as the RMS loads. Seastates involving wind speeds close to the rated wind speed control long-term 20-year loads which can be as much as 15% higher for the flexible foundation models compared to the fixed-base one.