Models for Offshore Wind Turbine Foundations and Their Influence on Long-Term Loads

In order to predict long-term loads on offshore wind turbines while evaluating ultimate limit states for design, one usually needs to carry out time-domain turbine response simulations. The accuracy of such predictions depends on proper modeling of the inflow turbulence and wave processes as well as of the turbine, the support structure, and the foundation. In order to represent realistic turbine response dynamics for monopile-supported wind turbines common in shallow waters, it is important that the soil-foundation-structure system is described by engineering models routinely used in practice. For the foundation, alternative models may be used that account for soil properties and nonlinear pile behavior under lateral load. The objective of this study is to investigate the effect of alternative monopile foundation models of shallow-water offshore wind turbines on extreme loads associated with 20-year return periods needed during design. Fixed-base, apparent fixity, coupled springs, and distributed springs foundation models are compared. 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 speed and wave height combinations are employed as we study comparative response statistics, power spectra, and probability distributions of extreme loads for the fixed-base and the different flexible foundation models. A discussion on the varying dynamics, on short-term response statistics, and on extrapolated long-term loads from limited simulation is presented. Sea states involving wind speeds close to the turbine's rated wind speed are found to control 20-year loads, and the flexible foundation models are found to experience higher extreme loads than the fixed-based case. Overall, the three flexible foundation models appear to yield fairly similar long-term loads computed based on an Inverse FORM (First-Order Reliability Method) approach for the selected turbine and soil profile used in this study.