This project is a WEAMEC project with the funding from the CARÈNE & Pays de la Loire Region.
The design of an offshore wind turbine (OWT) founded on a monopile foundation is principally based on dimensioning criteria related to its fundamental frequencies. These frequencies must remain outside the excitation frequencies to avoid resonance. For the calculation of the OWT natural frequencies, several studies exist but few of them simultaneously consider both the real geometrical configuration of the OWT superstructure (tower, blades and transition piece) and the three-dimensional (3D) soil domain and its interaction with the monopile foundation. This paper aims at filling this gap. A rigorous 3D finite element method-based model of a 10 MW DTU offshore wind turbine installed in sand is developed. The aim is to perform a structural modal analysis of the wind turbine in parked condition. The obtained natural frequencies are compared with those corresponding to other simplified models available in literature for the foundation and the superstructure in the scope of giving an insight about how poorly the existing simplified models can predict the OWT natural frequencies. Finally, a parametric analysis is performed to study the effect of the water depth, the monopile dimensions (diameter, thickness and embedded depth), the transition piece height, and the sandy soil relative density on the system natural frequencies.