Site liquefaction analysis via the contour diagram method: implications for offshore monopile design

The renewable energy sector is rapidly expanding, with offshore wind energy gaining global significance. Designing bottom-fixed offshore wind turbines (OWTs) with monopile foundations in seismically active regions, particularly in coarse-grained soils, presents challenges due to the risk of soil liquefaction during earthquakes. Conventional design practices address seismic effects by reducing soil shear stiffness to account for excess pore water pressure (𝛥𝑢) buildup. This study proposes a procedure for predicting Excess Pore Pressure build-up in coarse-grained soils using the cyclic contour diagram framework (CDF) under seismic loading. In this study, the PM4Sand soil model is employed to generate cyclic contour diagrams for a representative coarse-grained material. Site response analyses (SRA) are conducted in DEEPSOIL, and the resulting shear stress time histories are transformed into equivalent loading parcels to predict excess pore pressure using the CDF. Predictions are validated against PLAXIS 2D simulations employing the PM4Sand model. Finally, the proposed method is applied to assess the impact of seismic pore pressure build-up on monopile embedment depth. Results indicate that the proposed procedure offers a reliable alternative to conventional methods for evaluating liquefaction potential, providing improved insights for engineering practice in seismic design.