Laboratory investigation of the effect of installation parameters on the lateral response of offshore monopiles

Details

Supervisors

Rattez, Hadrien

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil des constructions, à finalité spécialisée

Abstract

As Europe strives to become a climate-neutral continent, interest and investment in the renewable offshore wind energy sector has grown considerably over the past decades, particularly in the North Sea region. In this evolving landscape, technological advancements play a critical role in driving down costs and facilitating the deployment of larger wind turbines in deeper waters. The field of foundation design holds here a particularly crucial position, especially for monopiles, which are the predominant foundation type for offshore wind turbines. Various complex phenomena, impacting the lateral capacity of the soil, have to be taken into account in the design of monopiles. However, the thorough understanding of how the parameters involved in the installation process influence the lateral response, and consequently the design of offshore monopiles, remains incomplete. It is within this particular context that this thesis seeks to make a contribution: through reduced-scale laboratory testing, examine those parameters and investigate how they affect the pile response to monotonic lateral loading. The research focuses on the impact hammering installation technique, chosen due to its widespread use in the industry. In this regard, novel 1g laboratory apparatus was designed, enabling representative pile drivings in dense dry and saturated sand samples. A set of parameters of the impact driving process were varied throughout the tests, and the corresponding pile response to a subsequent monotonic horizontal point load application was studied. This response was characterised by the measurement of the pile displacement and strains induced by the applied load. The key experimental findings suggest a limited impact of the ram mass on the lateral soil resistance, whereas the pile's diameter and its ratio with the embedded pile length exert significant influence. Results also highlighted the potential importance of the specific hammer configuration on the energy transmitted to the pile during driving, as well as the consequences of applying a monotonic loading at a different pile height. Finally, tests conducted in saturated soil conditions emphasised the need for further investigation in the field. A detailed strain analysis of the pile using optical fibre, also demonstrated the consistent rigid behaviour of the pile under monotonic lateral loading.