Advanced Numerical Modelling of Offshore Wind Turbine (OWT) Foundations in Sand

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This research project aims to investigate the behaviour of offshore wind turbine foundations in sand, in particular monopile foundation of an OWT in sand (or silty sand). Numerical analysis has played a crucial role in predicting behaviours of monopile foundations, especially in conditions that cannot be replicated accurately by experimental means. Estimated behaviours in such conditions can then be used to optimise current approaches in practical design. With numerical analysis, multiple realistic conditions can be replicated, providing more explicit and detailed trends. This project falls within the EPSRC engineering design research area. In this study, investigations will be carried using advanced 3D finite element (FE) simulations with coupled hydromechanical analysis. As a preliminary study, cases explored will include simple dynamic loadings from wind and wave. Hydromechanical analyses are then further extended to a combination of both dynamic and wind loading. Under the complex interplay of these different realistic dynamic loading types, the structural response of a monopile (large-diameter steel pipe) and geotechnical soil response are compared and examined. Hydromechanical behaviours of soil like pore water pressure accumulation and dissipation will be a major focus in this study. For sandy soils, loading conditions are usually modelled as drained. However, in intermediate layers (like silty sand), fast cyclic loading may result in partially drained behaviours. From the perspective of geotechnical numerical analysis, current soil constitutive models used in the modelling of monopiles in OWT will be tested for their behaviours under cyclic loadings and partially drained responses. One objective here is to validate the existing capabilities of known soil models (under the loading conditions mentioned above) when hydromechanical analysis is applied. At the current moment, it is definite that no one constitutive model is able to capture a complete and accurate foundation response for a range of loading conditions and drainage responses (in OWT). By understanding the limitations of existing soil constitutive models (in the study of monopile foundation), this study hopes to refine and build upon the current models. Another objective of this study is to identify the current of limitations and performance of known constitutive models under cyclic loadings and partially drained conditions. The objective/aim for these "refined" models would be to 1) Capture the proper hydromechanical behaviours under partially drained and cyclic loading conditions 2) Easy and practical applicability for practicing engineers. Another possible objective would be to test these "refined" soil models and evaluate the extent of its performance. Lastly, preliminary FE analyses will be carried out using commercialised software, in hopes of building a novel software incorporating new and refined soil models for partially drained conditions and cyclic loadings.