51 / 2025-04-24 12:55:18

Fourier series aided finite element analysis to predict the effect of scour on the modal properties of monopile-supported offshore wind turbines

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.