Soil modelling impact on the natural frequency of offshore wind turbines with reference to in-field measurements

Abstract

The natural frequency of the first bending mode of monopile-founded offshore wind turbines (OWTs) is decreasing along with the general trend of increasing turbine size. In addition, it is observed that in some cases the natural frequency starts to approach the rotor 1P frequency, which may trigger an increase in (wind) fatigue loading due to resonance effects. To mitigate this risk there are various options such as adjusting the support structure design by increasing the diameter and/or wall thickness, but this has a direct impact on the Capital Expenditures (CAPEX). Alternatively, enhancing soilstiffness in engineering models allows to minimise the reliance on additional steel in the design. Evaluation of in-field frequency measurements from various offshore wind farms indicates a consistent trend: natural frequencies estimated during the design phase are often lower than the frequencies measured under in-situ conditions. A portion of observed “frequency gap” can be attributed to uncertainties and conservative assumptions in geotechnical design aspects, such as soil interpretation, soil-structure interaction modelling methods, presence of a scour protection system, installation and pile-soil interface ageing effects. Using measurements from several installed OWTs, this study demonstrates how addressing these factors during the design process can help bridge the frequency gap. Furthermore, this paper aims to feed a community-wide discussion on the extent to which the reported findings can be incorporated in the design phase of offshore wind support structures. The overarching goal is to achieve more accurate estimate of the natural frequency, reducing design conservatism, optimising steel usage, and minimising associated project costs.