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Title: Monopile foundations under complex cyclic lateral loading
Author: Richards, Iona
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2020
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The vast majority of offshore wind turbines are supported on monopile foundations. These foundations experience lateral loading which is both cyclic and complex: continuously varying in amplitude, direction, frequency and eccentricity with the environmental and turbine operation conditions. The effect of cyclic loading on the accumulated rotation (ratcheting), strength and stiffness of the foundation must be considered, while advanced structural dynamic modelling requires prediction of the cyclic hysteretic response. However, there are currently no commonly accepted design methods for predicting the cyclic response of monopile foundations, particularly under the complex loading to which they are exposed. This thesis explores the response of monopile foundations to cyclic lateral loading through laboratory-scale physical modelling in dry sand, principally at 1g. The development of novel laboratory apparatus allows the application of complex, continuously varying cyclic loading. Regular, unidirectional tests complement previous studies and explore the impact of load amplitude and asymmetry in detail. Regular, multidirectional tests provide novel insight into the response to loading with multiple direction components. And irregular, multi-amplitude and multidirectional tests reveal the response to realistic loading, highlighting the dominance of large load events. Throughout, the monopile response is characterised in terms of ratcheting, evolution of hysteresis loop shape and the post-cyclic reloading response. Behaviour in very loose and dense sand is found to be qualitatively similar. A complementary study using centrifuge modelling explores the impact of stress-level on the monopile response, to establish the relevance of 1g modelling and inform comparison of monopile responses observed at different stress-levels. Qualitatively similar behaviour is observed at multiple stress-levels, although the rate of ratcheting and stiffening reduce with increasing stress-level. Together, the physical modelling results inform the design of monopile foundations under cyclic lateral loading. In particular, the results facilitate demonstration and development of numerical models in the Hyperplastic Accelerated Ratcheting Model (HARM) framework, which can capture ratcheting and evolution of the hysteretic response, and can respond to arbitrary loading. Models in the HARM framework are shown to capture key features of the response to complex - irregular, multi-amplitude and multidirectional - cyclic loading in dry sand of various densities and at multiple stress-levels. The results build confidence in the application of models in the HARM framework for full-scale monopile design.
Supervisor: Byrne, Byron ; Houlsby, Guy Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available