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.

The renewable energy sector is rapidly expanding, with offshore wind energy gaining global significance. Designing bottom-fixed offshore wind turbines (OWTs) with monopile foundations in seismically active regions, particularly in coarse-grained soils, presents challenges due to the risk of soil liquefaction during earthquakes. Conventional design practices address seismic effects by reducing soil shear stiffness to account for excess pore water pressure (𝛥𝑢) buildup. This study proposes a procedure for predicting Excess Pore Pressure build-up in coarse-grained soils using the cyclic contour diagram framework (CDF) under seismic loading. In this study, the PM4Sand soil model is employed to generate cyclic contour diagrams for a representative coarse-grained material. Site response analyses (SRA) are conducted in DEEPSOIL, and the resulting shear stress time histories are transformed into equivalent loading parcels to predict excess pore pressure using the CDF. Predictions are validated against PLAXIS 2D simulations employing the PM4Sand model. Finally, the proposed method is applied to assess the impact of seismic pore pressure build-up on monopile embedment depth. Results indicate that the proposed procedure offers a reliable alternative to conventional methods for evaluating liquefaction potential, providing improved insights for engineering practice in seismic design.

Sound evaluation of soil damping is of great importance for the optimised design of offshore wind turbine support structures. As practice indicates, design optimisation of the support structure often leads to fatigue-driven structural components, especially at the monopile foundation. Saving steel material is important for the economic feasibility of the project, while satisfying fabrication, transportation, and installation requirements. Conventionally, the baseline/background damping consists of steel material, hydrodynamic and soil damping. Design experience indicates that soil damping contribution to the overall baseline damping is significant, especially in case of strong non-linear soil response. This study employs an analytical and a numerical method to evaluate soil damping under realistic project conditions. Results indicate that the analytical method offers a sound basis for fatigue-oriented soil damping assessments, especially at the initial stages of the project.