Centrifuge tests of monopile foundations under cyclic lateral loading
Publication Date
July 29, 2025
Creator
Abstract
Wind energy has attracted growing interest due to the global energy crisis. This has led to an emphasis on the development of offshore wind turbines (OWTs). Monopile foundations have been widely adopted to support OWTs due to their construction simplicity. The OWT foundation generally undergoes a large number of load cycles as the result of wind, wave and turbine blade rotation. On sustaining such load, pile head rotation must be less than 0.5 degree. In order to meet this requirement, engineers must understand the monopile behaviour under monotonic and cyclic loading.
The design of monopile foundations is based on p-y method, in which soil is treated as a series of non-linear springs. Research on monopile response to lateral loads shows the current design guidance API (2011) overestimate the p-y curves for stiff piles. This may result in uncertainty about the safety of OWTs.
The majority of previous research on this topic has focused on short-term pile response under to cyclic loading and the applied cycles was limited to low thousands. In this study, cyclic loading at least 20,000 cycles was applied to a 60mm model pile, equivalent to a 6m prototype foundation, in sand to investigate the long-term behaviour of pile under cyclic loading. The cyclic loading can be generally classified as one-way (force with a constant direction) and two-way (force direction reversed) load and the magnitude and the bias of the load can vary. A new loading device was developed to apply cyclic loads with different characteristics in the centrifuge. In addition, the pile was instrumented with Fibre Bragg sensors, which allowed the measurement of bending moments along the pile.
The study shows that the pile head displacement accumulates and then levels off in the long term for one-way tests. For two-way tests, it decreases after the first cycle and then levels off. This suggests that the one-way loading scenario is more damaging for monopile foundations. It was also observed that the stiffness of the pile increases and then stabilise for both one-way and two-way tests. A hyperbolic model was proposed to predict the pile stiffness development. The change in bending moment along the pile during cyclic tests was examined. The results show a reduction in bending moments for both one-way and two-way tests. Cyclic p-y curves were derived based on the bending moment profile of the pile. It is shown that the cyclic p-y curves become stiffer with cycles, which indicates a stiffening soil-pile system.
In addition, a pushover test was performed on the pile and the results confirm that the API method over-predicts the soil response for monopile foundations. Furthermore, the displacement force curve from monotonic tests can provide useful information for the cyclic test study.
Item Type
ethesis
Thesis Type
PhD
Subjects (LC)
Associated Schools / Departments
Department of Civil Engineering (UK)
eprints ID
81429
UoN Repository URI
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Thesis_kunkun.pdf
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Full-text
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Examined. Final submission
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19.86 MB
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