Abstract
Offshore Wind Turbine Structures (OWTs) are dynamically sensitive due to their shape and form (slender column supporting a heavy rotation mass) and also due to the different forcing functions (wind, wave, and turbine loading) acting on the structures. Designers need to ensure that the first Eigen natural frequency is not close to forcing frequencies to avoid dynamic associated effects such as resonance and fatigue damage. Such damages may result in higher maintenance costs and a lower service life. Therefore, it is crucial to get the best prediction of the first natural frequency during the early stages of a project. Other design requirements include the Serviceability Limit State (SLS) criteria which impose strict pile head deflection and rotation limits. These calculations require foundation stiffness and the aim of this chapter is to provide practical methods to predict the stiffness of the foundations for any ground profile (nonuniform or layered soils) through the use of standard methods. The foundation stiffness values can then be used as an input to predict the first natural frequency of the OWT system as well as check SLS requirements. An example problem is taken to show the application of the method.