The design of an offshore wind turbine founded on a monopile foundation is principally based on a dimensioning criteria related to its fundamental frequencies. These frequencies must remain outside the excitation frequencies to avoid resonance.

For the design standards for offshore wind turbines, soil structure interaction is taken into account as described in IEC 61400-3, Design Requirements for Offshore Wind Turbines. The general method in the IEC standards relies on the API (American Petroleum Institute) design approach. In the API approach, the soil py curves are developed for slender flexible piles, which are widely used for offshore oil and gas platforms. However, the monopiles supporting offshore wind turbines are typically rigid with large diameters (3.5m – 8m) and length (30m – 60m). Therefore, the API approach developed for slender piles may not be applicable for the monopiles.

In this paper, a full 3D model for the DTU-10MW wind turbine is developed using the commercially available finite element code ABAQUS/Standard. The main objective is to perform a rigorous modal analysis of the wind turbine considering the entire soil-foundation-structure system, thus avoiding the use of the unsuitable p-y curves available in literature (API approach). The tower was discretized using shell elements. Each blade was modeled as a beam with specific stiffness properties for each section and a hinge connection was employed to simulate the rotation of the blades with respect to the rotor. Finally, the soil was modeled as a three-dimensional solid continuum and the soil-monopile interface was simulated using small sliding surface-to-surface master/slave contact pair formulation.

Numerical results showing the effect of the monopile geometrical parameters and the soil spatial variability on the wind turbine natural frequencies are presented and discussed.