Experimental and numerical investigations on offshore wind turbine foundations in soft marine Taiwanese clay subjected to combined high-cycle and seismic loading

• Applicants: Professor Dr.-Ing. Luis Felipe Prada Sarmiento; Dr.-Ing. Merita Tafili; Professor Dr.-Ing. Torsten Wichtmann
• Subject Area: Geotechnics, Hydraulic Engineering
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 447999811

Offshore wind turbines (OWTs) are subjected to wave and wind loading with millions of cycles of small to moderate amplitudes (so-called high-cycle loading). Earthquakes are the source of another type of cyclic loading to be considered in the OWT design for seismic-prone areas like Taiwan. A seismic loading is characterized by a rather low number of cycles with relatively large amplitudes (so-called low-cycle loading). Since the seabed of the Taiwanese coast is mainly composed of soft marine clays, the safety and performance of OWT foundations rely on the response of these soils to the different kinds of cyclic loadings. A cyclic loading may cause an accumulation of strain under drained conditions, and a build-up of excess pore water pressure under undrained conditions. The accumulation of strain can lead to a loss of the serviceability of the OWT, considering the low tolerances with respect to the tilting of the OWT tower. A significant reduction of shear strength due to the pore-pressure build-up can even result in a foundation failure. A safe OWT foundation design must prevent both limit states. A previous high-cycle loading could degrade the performance of the OWT foundation during an earthquake, as a consequence of an accumulation of pore water pressure. On the other hand, the effect of the high-cycle loading may be also beneficial, if it causes compaction under almost drained conditions. Furthermore, under repeated seismic events the soil may become less resistant from earthquake to earthquake, or it may be the other way around, depending on the strain levels induced by the propagating waves and the progress of consolidation, determined by soil permeability and the time between both events. Additionally, a high-cycle loading between two earthquakes may also have either detrimental or beneficial effects, due to the reasons outlined above. The simultaneous occurrence of an earthquake and a strong storm is the worst case to be considered in OWT design. Despite its practical relevance, the combined effects of high-cycle and seismic loading on the behaviour of OWT foundations in soft marine clay have not been sufficiently investigated so far. In particular, sequence effects are not well understood. Thus, the proposed project aims to thoroughly investigate such effects in Taiwanese soft marine clay. Element tests in the laboratory will focus on the effect of bundles of cycles with different amplitudes, changes of the average stress and partially drained conditions. Two sophisticated constitutive models dedicated to either low- or high-cycle loading will be enhanced based on the experimental data. A finite element model will be build which enables predictions of the response of OWT monopile foundations under various combinations of seismic and high-cycle loading during the design stage. The model will be validated based on centrifuge model tests on monopiles subjected to high-cycle, seismic or combined loading.

• DFG Programme: Research Grants
• International Connection: Taiwan
• Partner Organisation: National Science and Technology Council (NSTC)
• Cooperation Partners: Professor Dr. Jui-Ching Chou; Professor Dr. Yuning Louis Ge; Professor Chi-Chin Tsai, Ph.D.