Dynamic Behaviour of Suction Caissons

Offshore wind energy is a promising source of energy in the near future, and is rapidly becoming competitive with other power generating technologies. The continuous improvement in wind turbine technology means that the wind turbines have increased tremendously in both size and performance during the last 25 years. In order to reduce the costs, the overall weight of the wind turbine components is minimized, which means that the wind turbine structures become more flexible and thus more sensitive to dynamic excitation. Since the first resonance frequency of the modern offshore wind turbines is close to the excitation frequencies of the rotor system, it is of outmost importance to be able to evaluate the resonance frequencies of the wind turbine structure accurately as the wind turbines increase in size. In order to achieve reliable responses of the wind turbine structure during working loads it is necessary to account for the possibilities of dynamic effects of the soil-structure interaction. 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 frequencydependent 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 analysis has been carried out for different combinations of the skirt length, soil stiffness and the Poisson's ratio of the subsoil. Subsequently, the high-frequency impedance has been determined for the use in lumped-parameter models of wind turbine foundations.

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. For that reason, the dynamic stiffness (impedance) for each degree of freedom have been formulated into lumped-parameter models with frequency independent coefficients, suitable for implementation in standard dynamic finite element schemes. 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 analysis of the wind turbine makes use of "Output-only modal identification" which is utilized when the modal properties are identified from measured responses only. The experimental modal analyses have shown that the approach is a useful tool to estimate the response of the wind turbine.