A full three‐dimensional model for the estimation of the natural frequencies of an offshore wind turbine in sand

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Abstract

Abstract 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 OWT 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.

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