Soil-structure interaction of foundations for offshore wind turbines

The interaction between the foundation and the wind turbine is normal modeled in an internationally recognized aeroelastic code as HAWC or FLEX. At present, the foundation in this codes is modeled simply by beam ele-ments, and, as such, the foundation stiffness becomes frequency independent. The present modelling, is known to by an insufficient and not realistic approximation to the frequency dependent foundation concepts, e.g. pile, gravitation or bucket foundations. Furthermore, it is not possible to formulate the damping of the foundation in the present model. Modal analysis of an existing wind turbine installed on a bucket foundation has shown highly damped eigen-modes of the structure which the present aeroelastic codes is unable to model. The purpose of this project has been to extend the foundation module in HAWC and FLEX to be able to simulate the frequency dependent stiffness and damping of the foundations. The aim has been to implement the foundation module as a 'black-box', described by its dynamic stiffness in the frequency domain. By taking such a general approach, the foundation module is capable of modeling a range of possible foundation types, ranging from monopile via bucket to gravitation foundation types. The approach used to formulate the foundation module is described in listed papers. The interaction between the foundation and the wind turbine is implemented in the aeroelastic code HAWC and FLEX. The foundation module has been implement the as a 'black-box', described by its dynamic stiffness in the frequency domain. By taking such a general approach, the foundation module is capable of modeling a range of possible foundation types, ranging from monopile via bucket to gravitation foundation types.