Elvis, John
(2019)
Centrifuge and analytical modelling of offshore wind turbine monopile foundations.
PhD thesis, University of Nottingham.
Abstract
Wind energy is an interesting prospect due to its availability and sustainability along the coast and offshore locations. Development of offshore wind farms is projected to increase very rapidly within the next decades, which is expected to efficiently improve the electricity challenges in areas located along the sea. While the offshore wind energy is collected by turbine structures, which are supported by towers and foundations, the challenge remains on the provision of cost-effective foundations to resist the lateral loads from wind, waves and dynamic actions of the wind turbine nacelle-rotor. The popular foundations used are monopiles, having a competitive advantage of stability, ease of installation and low cost of materials compared to other types. The loads acting on monopiles are cyclic in nature and can be resisted by earth pressure mobilised in the soil surrounding the pile. The cyclic loads can affect the strength and stiffness of both the soil and pile, leading to accumulated rotation and change of overall stiffness. Studies have been carried out regarding these effects, however, there has not been a clear understanding of the response of a stiff pile when subjected to many cycles.
The literature has revealed that the current method (p-y curves method) for analysing and designing the offshore monopiles is insufficient, tend to overestimate the stiffness of the rigid piles, and leading to interference between resonance and natural frequency of the wind turbines. The method usually regards the soil as a series of non-linear wrinkle spring and derives its base on the empirical relationships developed from full-scale tests on slender piles. The deficiency of the prevailing design approach, therefore, justifies a need for further research to develop a model which will monitor how the monopiles foundations respond to both monotonic and cyclic loading.
This thesis, therefore, presents an experimental and theoretical research approach that will improve the understanding of the response of monopile foundations in sands when subjected to both monotonic and cyclic loading. The experimental work involves a comprehensive design and development of a new mechanical loading system in a geotechnical centrifuge, with model tests scaled to represent full-scale wind-turbine monopiles. The test programme is designed to identify the key mechanisms driving pile response, including investigating the monotonic loading behaviour as well as the response of a pile to long term cyclic loads. The methodology developed and data collected from this thesis will provide a potential contribution to the establishment of a better understanding of monopile responses within the field.
The experiment was carried out by initially, scaling down the prototype monopile using a 1:100 geometric scale. Three monotonic tests were carried out at 100g to identify the responses under monotonic loading, estimate the ultimate capacity and determine the initial (tangent) stiffness of the pile-soil system. One monotonic test was conducted at 30g as a reference to the cyclic test results. The parameters extracted from the monotonic tests are used as the basis for the design of cyclic loading system and analysis of the cyclic test results. Available models from the literature were modified to capture the ground global response of the pile under monotonic loading. The models were employed in a kinematic approach, with soil being modelled as a series of spring elements and the pile as an elastic beam element. The model pile in the centrifuge was not instrumented, hence assumptions were made to match the global response of the monotonic centrifuge tests at the ground surface. With the absence of data along with the embedded depth of the pile, the fitting constants were used to estimate the ultimate capacity and the concept of the modulus of subgrade reaction. The model was also used to the published centrifuge test results of the past monopile research.
The methodology developed for cyclic load tests incorporated the effects of cyclic loading on the response of a monopile. The validity of the model is supported by centrifuge tests in which a stiff model pile, installed in cohesionless soil, was subjected to a series of load cycles with load amplitude and frequency. The tests were carried out to investigate the responses of the newly developed model. Due to technical challenges during the model testing, all cyclic tests were achieved at centrifuge acceleration of 30g instead of the 100g for the monotonic tests. Selected tests were used to examine the total cyclic pile-head response to gain an insight into the accumulation of pile-head displacement and change in cyclic secant stiffness. Power and logarithmic functions were used to predict the accumulated displacement and cyclic stiffness variation using the data from the centrifuge, respectively. The key experimental findings of the cyclic tests were then used to develop a theoretical model that captures the unload-reload hysteresis behaviour. The model function, called the modified Romberg Osgood (MR-O), is rigorous yet simple and is framed where the pile-head cyclic response is modelled as the hysteresis loops for the backbone, unloading and reloading curves. The model was calibrated against the cyclic centrifuge tests and successfully reproduce the main elements of the pile response with high accuracy. However, it does not predict precisely the cyclic accumulation and change in cyclic stiffness as shown in the experiment, thus further improvement will still be required.
Nevertheless, the newly developed model and suggested methodologies in this thesis can be used as a primary stage for research developers to understand the behaviour of foundations supporting offshore wind turbines, with scientific justification based on the centrifuge model scale tests.
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