Christelle Abadie

Doctor of Philosophy, St Catherine's College, University of Oxford, Michaelmas Term 2015

Cyclic Lateral Loading of Monopile Foundations in Cohesionless Soils

The monopile is the dominant foundation type for offshore wind turbines, with current design guidance based on knowledge transferred from the oil and gas industry. Whilst there are some similarities between wind turbine and oil and gas pile design, there are also a number of key differences. Notably, offshore wind turbine monopiles are subjected to many cycles of large horizontal loads during their lifetime, whereas such loading conditions are not as prevalent in oil and gas design. As a result, the pile response due to this cyclic loading is poorly accounted for in current practice.

This thesis presents experimental and theoretical research, aimed at improving the understanding of the behaviour of rigid monopiles in cohesionless soils, when subjected to lateral cyclic loading. The experimental work involves laboratory floor model tests, scaled to represent a full-scale wind-turbine monopile. The test programme is designed to identify the key mechanisms driving pile response. It is divided into four main parts, investigating loading rate effect, hysteretic behaviour during unloading and reloading, as well as pile response to long-term single and multiamplitude cyclic loads. In particular, the results show that the pile response conforms to the extended Masing rules, with permanent deformation accumulated during nonsymmetric continuous cyclic loads. This ratcheting behaviour is characterised by two features: first, the ratcheting rate decreases with cycle number and depends on the cyclic load magnitude and secondly, the shape of the hysteresis loop tightens progressively, involving increased secant stiffness and decreased loop area. Tests investigating multi-amplitude loading scenarios prove that the interaction between these mechanisms describes the pile response. Finally, the continuous cyclic test results are interpreted using the p-y method combined with the Degradation Stiffness Model, and this shows a good fit to the observed pile deformation.

The key experimental findings are used for the development of a constitutive model that captures ratcheting while conforming to the observed Masing behaviour. The model, called HARM, is rigorous yet simple, and is framed within the hyperplasticity approach presented by Houlsby and Puzrin (2006). The model is tuned to capture the macro response of the pile under monotonic and cyclic loading, and is calibrated using the experimental data. The results demonstrate that HARM can successfully reproduce the main elements of the pile response with high accuracy. The method could easily be within common design approaches, such as the p-y method.

Thesis (19Mb, pdf)