The development of scour around monopile-supported offshore wind turbines presents an important operational risk. Loss of foundation stiffness due to scour can lower the support structure’s natural frequency, potentially reducing the structure’s lifetime due to increased fatigue damage. A novel numerical modelling procedure is proposed for computing the modal properties of monopile-supported offshore wind turbine structures following scour development. The objective is a computationally-efficient method that provides greater accuracy than existing rapid methods, reducing uncertainty during design optimisation or back analysis. The substructure and soil domain are modelled using Fourier series aided finite element analysis and are coupled to a one-dimensional beam representation of the superstructure. Fourier series aided finite element analysis is a method for modelling axisymmetric bodies undergoing non axisymmetric loading, with the computational burden reduced to a radial plane analysis. The soil continuum is fully modelled, meaning scour and scour protection systems can be explicitly included. The proposed approach is validated against experimental model test data. The results demonstrate that the approach can accurately model the change in natural frequency of monopile-supported offshore wind turbines following the development of scour, with a mean absolute percentage error of 1.85% on the scoured experimental data. The modal analysis calculation achieved a factor 17.5 speed-up compared to an equivalent three-dimensional finite element analysis.

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