Cyclic Behaviour of Laterally Loaded Monopiles in Sand Supporting Offshore Wind Turbines

Almost 90% of the current offshore wind turbines are supported by monopiles, which are steel hollow piles with a large diameter. Nevertheless, monopiles are currently designed with inappropriate methods that were developed by the oil and gas industry for slender piles that are different from monopiles, in terms of loading conditions and geotechnical behavior. In particular, the response of monopiles due to cyclic lateral loading is poorly considered in current geotechnical design methods, although monopiles are subject to millions of load cycles during their life time. In addition, cyclic lateral loading generates a number of issues on the monopiles behavior that still have to be addressed.
The research work presented in this PhD thesis is aimed to investigate the geotechnical behavior of monopiles in dense sand under cyclic lateral loading through experimental tests, single gravity and centrifuge tests. The main investigated issues due to cyclic loading are the accumulation of rotation, the variation in stiffness and the increase in lateral capacity of monopiles. These three issues have been investigated by observing their relation with different cyclic loading characteristics, such as number of cycles, load magnitude and load symmetry (e.g. one-way or two-way).