Cyclic Behaviour of Monopile Foundations for Offshore Wind Farms

海上风电场在英国越来越受欢迎，这是由于目前对绿色能源的需求，能源供应安全以及公众不愿意在岸上拥有风电场的兴趣。海上风电场通常包含数百台涡轮机，支撑在30米至50米的高度。这些高层建筑物的首选基础是大直径桩，因为它们易于在浅至中等水深中施工。除了轴向载荷外，这些支柱还承受大的循环、横向和力矩载荷。预计这些基础中的每一个在其设计寿命期间将经历数百万次负载循环。在英国附近的沿海沃茨中，这些碎石通常会在进入较硬的粘土/砂层之前穿过较软、固结较差的海洋粘土浅层。设计中最大的问题之一是它们在大量横向和力矩载荷循环下的行为。目前的设计方法在很大程度上依赖于文献中提供的粘土刚度退化曲线，这些曲线主要是针对相对较小直径桩的地震荷载得出的，荷载循环次数相对较少。将这种刚度退化外推到具有大量载荷循环的大直径桩，是评估海上风力涡轮机性能的关键风险因素。因此，需要进一步研究。该项目旨在了解大直径碎石桩穿过不同刚度的粘土层并承受循环横向和弯矩荷载的行为。将利用斯科菲尔德中心最近的发展成果进行模型试验，这些成果包括一个计算机控制的2-D致动器，该致动器可以对飞行中的飞行器施加力或位移控制的循环载荷。此外，还可以在飞行中安装桩柱，以模拟将桩柱插入海床。将开发新的设备，用于在飞行中测量土壤刚度和动态响应，与目前在该领域使用的最先进的设备进行比较。该项目的主要成果将是更好地了解分层土壤系统中的桩对大量荷载循环（横向和弯矩荷载）的响应。结果将直接与目前的设计实践进行比较，并将制定改进设计的指导方针。该项目的成果将允许在非常大量的载荷循环下准确估计海上可移动基础的行为，从而导致对基础的生命周期的可靠估计。这对于确定海上风电场的经济可行性至关重要，因为资本成本很高，收入流相对较低，但在风电场的寿命期间持续存在。

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.