Foundation impedance and tower transfer functions for offshore wind turbines

摘要

This article investigates the dynamic interaction between a horizontal-axis wind turbine, modelled as a discrete Euler-Lagrangian system, and the soil-foundation system, modelled in a finite element framework. The model comprises a rotor blade system, a nacelle and a flexible tower connected to the soil-foundation system using a substructuring approach. The aerodynamic loading on the rotor blades is modelled using blade element momentum theory. The offshore wind turbine is founded on a monopod suction caisson foundation. The soil-foundation system is modelled using a finite element method (Plaxis 3D dynamics) and a ID strength of materials based on wave propagation approach (cone method) for comparison. A dynamic-stiffness matrix and a static stiffness matrix formulation are used to represent the soil-foundation behaviour at the mudline. Tower displacement response transfer functions are generated using each soil-foundation model at various soil stiffness. The effects of the soil stiffness on the displacement transfer functions and the wind turbine dynamics are discussed. Results from a static stiffness formulation (with coupling) are seen to match those given by frequency-dependent models, indicating that such simplified models maybe used for analysis under certain conditions.