The expansion of the offshore wind industry in areas with high seismicity has led to engineering challenges related to the design of the offshore wind turbines (OWTs). Monopiles, i.e., tubular steel piles of large outer diameter, low aspect ratio (penetration depth over outer diameter), and relatively thin pile wall, are traditionally the preferred foundation type for OWT due to fabrication, transportation, and installation standardization. For all bottom-founded systems, soil–structure interaction (SSI) plays a crucial role in the system's response. Additional challenges arise in the case of seismic SSI as, not only the system's response, but also the seismic ground motion itself are affected by the soil characteristics. Furthermore, uncertainties related to soil properties, as derived from the soil testing campaign and interpretation, need to be thoroughly considered for OWT load calculations and the design of the support structure. The uncertainty in soil interpretation may have a large impact on the characteristics of the input seismic motion. Subsequently, SSI will affect the seismic loads acting on the support structure and the OWT. This knock-on effect of the interpretation of the soil parameters is unknown, but may be significant to account for. In fact, when a “best estimate” soil parameter set is used, the resulting seismic load may not necessarily correspond to the most probable load for the assumed seismic event. This paper investigates the influence of the uncertainty in soil parameters, as they may result from the soil interpretation, on the seismic loads. It demonstrates the skewed distribution of OWT seismic loads using a realistic design case study on a commercial OWT. Results are presented in the form of transfer functions, response spectra at mudline and normalized bending moments along the support structure. Three distinct structural components of interest are selected to evaluate the results. It is concluded that, for the analysis of OWT under seismic loading conditions in particular, it cannot be decided a priori which soil properties would result in conservative or progressive design. Based on the obtained results, recommendations are given which aim to de-risk and enhance the current design practice.

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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.

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Single large diameter piles, known as monopiles, are, nowadays, the most common foundation type for offshore wind turbines (OWT). Several research initiatives have been focusing on improving the conventional design practice. The PISA (Pile Soil Analysis) joint industry research project resulted in a proposal for a new design methodology for OWT pile foundations, focusing on laterally loaded monopiles, with length-to-diameter ratios (L/D) between 2 and 6. The proposed methodology defines a one-dimensional (1D) model, based on the use of Timoshenko beam theory, which overcomes certain limitations in current design practice. Soil reaction components can either be generated through mathematical formulations (rule-based design method) or calibrated from 3D Finite Element (FE) models (numericalbased design method). The present paper examines the suitability of the numerical-based PISA design method for analyzing the pile response in sandy soil conditions. A concept design study for monopiles in dense sand is considered. Soil properties are based on realistic conditions encountered in the North Sea. The PISA methodology is compared with the methodology followed by the current design practice. Results are discussed and several conclusions are drawn providing factual insight into the applicability of the PISA design methodology for the design of monopiles in sand. @en

3D non-linear finite element analyses are proving increasingly beneficial to analyse the foundations of offshore wind turbines (OWTs) in combination with advanced soil modelling. For this purpose, the well-known SANISAND04 bounding surface plasticity model (Dafalias & Manzari 2004) is adopted in this work to incorporate key aspects of critical state soil mechanics into the analysis of monopile foundations in sand. The final 3D soil-foundation-OWT model is exploited to simulate the response of an 8 MW OWT to a long loading history of approximately 2 hours duration. The scope is to investigate/explain the drops in natural frequency observed in the field during storms, as well as its subsequent recovery. The numerical results point out a strong connection between transient frequency drops and pore pressure accumulation, whereas the original OWT natural frequency seems to be restored as a consequence of post-storm re-consolidation.@en