PhD Project: Dynamic Behaviour of Suction Caissons

The aim of this thesis is to evaluate the dynamic soil-structure interaction of foundations for offshore wind turbines, with the intention that the dynamic properties of the foundation can be properly included in a composite structure-foundation system. The work has been focused on one particular foundation type; the suction caisson.

The frequency dependent stiffness (impedance) of the suction caisson has been investigated by means of a three-dimensional coupled Boundary Element/Finite Element model, where the soil is simplified as a homogenous linear viscoelastic material. The dynamic stiffness of the suction caisson is expressed in terms of dimensionless frequency dependent coefficients corresponding to the different degrees of freedom. Comparisons with known analytical and numerical solutions indicate that the static and dynamic behaviour of the foundation are predicted accurately with the applied model.

The requirement for real-time computations in commercial software packages for performance and loading analysis of wind turbines, do not conform with the use of e.g. a three-dimensional coupled Boundary Element/Finite Element Method, where the foundation stiffness is evaluated in the frequency domain. The lumped-parameter models have been used to simulate the soil-structure interaction within a numerical finite element model of a Vestas V90 3.0 MW offshore wind turbine with a suction caisson foundation. The simulations of the soil-structure interaction by means of lumped-parameter model approximations of the impedance have shown that the concept is useful for use in applications where the performance of the wind turbine are to be analysed.

Experimental modal analyses have been carried out with the intention of estimating the natural frequencies of an existing Vestas 3.0 MW offshore wind turbine. The experimental modal analyses have shown that the approach is a useful tool to estimate the response of the wind turbine.