Dynamic response of offshore foundations - from pile installation to seismic performance

Abstract

The growing demand for renewable energy has resulted in a rapid, worldwide expansion of offshore wind installations in deeper waters and in areas of medium to high seismicity (e.g. Taiwan, China, Japan, USA) with structures of higher energy capacity. The first generation of offshore wind farms were located in areas of low seismicity, mainly in northern Europe, and therefore there is limited experience worldwide on how this type of structure would respond to seismic loading or to combined environmental and seismic loading. Design performance requirements and code provisions with explicit reference to offshore structures under seismic loading conditions are yet to be developed, while the existing guidance for onshore foundations is not always transferable to offshore conditions. This lack of guidance also applies to the assessment of liquefaction at offshore sites. Most offshore wind turbines are founded on monopiles with geometries (length to diameter ratios) and functionality which differ substantially from end-bearing piles traditionally used in many onshore conditions with significant seismic design considerations. Furthermore, as monopiles are a relatively new type of foundation the assessment of their driveability at different ground conditions poses challenges. Most of the existing driveability assessment methods, which are critical in the selection of a suitable hammer and driving system, were developed based on driving records for piles of much smaller in diameter than the monopiles used in the offshore wind industry today, leading to unreliable predictions. The first part of the lecture will discuss the use of dynamic analysis of pile driving as a tool for the improvement of drivability predictions for monopiles. The discussion will extend to driven piles supporting jackets, where dynamic analysis of driving data combined with the analysis of restrike data can gives estimates of their long-term axial capacity. The second part of the lecture investigates the seismic response of monopiles in liquefiable soils, showing the impact of liquefaction on shifting the modal response the combined system (i.e. structure-pile-soil) to a frequency range in which resonance phenomena can occur. Finally, it will be shown that the traditional separation of seismic loading into kinematic and inertial components, which is often adopted in simplified pile analysis, is not necessarily applicable to monopiles.

Biography

Stavroula Kontoe is Associate Professor at the University of Patras and Visiting Reader at Imperial College London, specialising in the development and application of numerical methods to study the performance of geotechnical structures under static, dynamic and seismic loading. She has authored over 100 peer-reviewed publications, receiving the 2008 BGA Medal, the 2012 Computers & Geotechnics outstanding reviewer award, the 2017 Shamsher Prakash Foundation Excellence in Geotechnical Teaching prize and the 2019 and 2021 David Hislop ICE awards for the best paper on offshore matters. She has served as Chair of the UK Society for Earthquake and Civil Engineering Dynamics (SECED) (2020-2022), is co-editor for Computers and Geotechnics and serves on the editorial boards of Soil Dynamics & Earthquake Engineering and of the Journal of Earthquake Engineering.