Response of monopiles subjected to combined vertical and lateral loads, lateral cyclic load, and scour erosion in sand

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Although wind energy capacity has increased significantly in the last few decades, the installed capacity of offshore wind turbine still lags far behind that of onshore wind turbines due to the installation and foundation cost. The aim of this research project has been to clarify the influence of combined vertical and lateral loads, lateral cyclic load, and scour erosion on monopile foundations, in order to achieve more realistic and cost beneficial solutions for offshore wind turbine foundations and thereby increase its competitiveness when compared with other energy sources. Monopiles are the most popular foundation system today for offshore wind turbines installed in shallow to medium water depths. These relatively light structures (low vertical load), need to resist substantial lateral and moment loads. There have been a dearth of studies conducted to investigate the influence of vertical load on the lateral response of these rigid monopiles and the few available have drawn contradictory conclusions. In addition the lateral and moment loading exerted on monopiles due to wind, wave, and water currents is cyclic in nature. This type of loading can lead to the accumulation of lateral displacement/rotation and possible degradation of soil resistance over time. This evolution of pile head displacement and the change in soil stiffness with increasing cycles of load is poorly understood. Cylindrical structures, like monopiles, founded in offshore regions are commonly subjected to scour erosion caused by flowing water and currents, which induces loss of soil support around the pile, reducing the lateral load capacity and causing increased pile displacement. As a result, the system dynamics of the structure might be adversely affected. The results of numerical models suggest that the shape of the scour hole affects the loss of pile lateral capacity, however, there is a shortage of experimental test data that measure this effect. More than 60 centrifuge tests which are categorized into three groups are presented in this thesis, which consider the interaction of combined vertical and lateral loads, lateral cyclic load and scour erosion on the behaviour of rigid monopiles. The tests have been performed in homogeneous dry Geba sand in order to mimic simplified drained offshore soil conditions.