The renewable energy sector is experiencing rapid growth, with offshore wind energy gaining significant global importance. Designing bottom-fixed offshore wind turbines (OWTs) with monopile (MP) foundations in seismic-prone regions, particularly in coarse-grained soils, presents challenges due to the risk of soil liquefaction during earthquake excitations. Current design practices for soil-structure interaction modelling address seismic pore pressure effects by accounting for the net decrease in soil shear modulus resulting from the development of excess pore water pressure.
This thesis examines the cyclic contour diagrams framework (CDF) for its capacity to predict excess pore water pressure (EPP) build-up in coarse-grained soils under seismic conditions. The framework is assessed by employing PM4Sand in PLAXIS2D to produce the cyclic contour diagrams for a coarse-grained material. This material is then used to perform site response analysis (SRA) on a soil column in PLAXIS2D. Subsequently, a total stress SRA is conducted using DEEPSOIL V7.0 to obtain the equivalent cyclic stress ratio (CSR) and the number of cycles by employing a best-match soil material similar to the one utilized in the PLAXIS analysis. By analyzing the cyclic stress history obtained from the SRA, the equivalent number of cycles and cyclic stress ratios are applied to the material's cyclic contour diagrams. Finally, the predictive capabilities of the method to predict the EPP calculated in the PLAXIS model are assessed.
Two case studies illustrate the framework's practical application. The first case focuses on Akita Port, Japan, which experienced significant damage during the 1983 Nihonkai-Chubu earthquake. The CDF was able to accurately predict liquefaction severity, validating the method. The second case study involves an offshore wind farm in the Netherlands, examining how liquefaction impact monopile foundation designs. The study highlights the importance of incorporating soil degradation because of liquefaction into the design of offshore wind turbines.
The findings indicate that the CDF is a viable tool for evaluating liquefaction potential in coarse-grained soils under seismic loading in engineering practice.