Monopile-supported offshore wind turbines (OWTs) are dynamically sensitive structures, in which their design is principally based on a dimensioning criterion related to its fundamental frequencies. Therefore, an accurate estimation of the natural frequency is essential to assess the working lifetime of the OWT.

For the calculation of the OWT natural frequency, several studies exist but few of them simultaneously consider both the real geometrical configuration of the OWT superstructure (blades, nacelle, tower, and transition piece) and the three-dimensional (3D) soil domain and its interaction with the foundation. The aim of this paper is to investigate the suitability of (i) the different simplifying assumptions used in literature for the superstructure and (ii) the recently proposed PISA soil-foundation interaction model, when calculating the natural frequency. This is performed by comparing the results obtained using a rigorous three-dimensional (3D) finite element-based model recently developed by the authors of this paper with those obtained based on these simplifications. The comparison results have shown that both the simplified superstructure models and the newly developed soil reaction curves of the PISA project provide very good results of the OWT natural frequency.

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