Cyclic Behaviour of Monopile Foundations for Offshore Wind Farms

Offshore wind farms are gaining popularity in the UK due to the current interest in the need for greener energy sources, security of energy supply and to the public's reluctance to have wind farms on-shore. Offshore wind farms often contain hundreds of turbines supported at heights of 30m to 50m. The preferred foundations for these tall structures are large diameter monopiles due to their ease of construction in shallow to medium water depths. These monopiles are subjected to large cyclic, lateral and moment loads in addition to axial loads. It is anticipated that each of these foundations will see many millions of cycles of loading during their design life. In coastal waters around the UK, it is common for these monopiles to pass through shallow layers of soft, poorly consolidated marine clays before entering into stiffer clay/sand strata. One of the biggest concerns with the design of monopiles is their behaviour under very large numbers of cycles of lateral and moment loads. The current design methods rely heavily on stiffness degradation curves for clays available in the literature that were primarily derived for earthquake loading on relatively small diameter piles with relatively small numbers of cycles of loading. Extrapolation of this stiffness deterioration to large diameter piles with large numbers of cycles of loading represents the key risk factor in assessing the performance of offshore wind turbines. Further research is therefore required. The proposed project aims to understand the behaviour of large diameter monopiles driven through clay layers of contrasting stiffness and subjected to cyclic lateral and moment loading. Centrifuge model tests will be conducted taking advantage of recent developments at the Schofield Centre that include a computer-controlled 2-D actuator that can apply both force or displacement controlled cyclic loading to monopiles in-flight. In addition it is possible to carry out in-flight installation of the monopiles to simulate the insertion of these monopiles into the seabed. New equipment will be developed for the in-flight measurement of soil stiffness and dynamic response comparative to the state-of-the-art equipment which is now used in the field. The main outcome of the project will be a better understanding of the response of the monopiles in layered soil systems to large number of loading cycles (lateral and moment loads). The results will be directly compared to the current design practices and guidelines for improved design will be developed. The outcome of this project will allow an accurate estimation of the behaviour of offshore monopile foundations under very large numbers of cycles of loading, thus leading to a confident estimation of the life cycle of the foundation. This is critical in determining the economic viability of an offshore wind farm given that the capital costs are high and the revenue stream is relatively low but continues for the life of the wind farm.