Abstract: “Many countries are now planning to build new wind farms with high capacity up to 5MW. Consequently, the size of the foundation increased. This kind of structures are subjected to fatigue damage from environmental loading mainly due to wind and waves as well as from cyclic loading imposed through the rotational frequency (1P) through mass and aerodynamic imbalances and from the blade passing frequency (3P) of the wind turbine which makes their behavior dynamically very sensitive. That is why natural frequency must be determined with accuracy in order to avoid the resonance of the system. This paper presents a numerical procedure which combines the finite element method based on the 2D discretization in the radial plane and the Fourier series based on expansion of displacements and forces in the circumferential direction. Firstly analytical expressions of stiffnesses of foundations with large diameter embedded in soils with different stiffness profile are established. Secondly and in ord”

Keywords: Offshore Wind Turbines; Semi-analytical FE analysis; DNV/Risø; Monopiles foundations; Natural frequency.