Foundation impedance and tower transfer functions for offshore wind turbines

Abstract

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 1D 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.

Keywords

Soil–structure interaction wind turbines Euler–Lagrangian model finite element method blade element momentum theory transfer functions

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