Numerical modeling of residual pore water pressure under cyclic loading: Application to offshore foundation design

Abstract

This dissertation investigates wave-induced pore-pressure responses and liquefaction potential in saturated, medium-dense sands supporting two critical offshore structures: marine hydrokinetic (MHK) structures and suction anchor foundations. Although these systems differ in geometry and loading mechanisms, both are subjected to intense cyclic loading from waves, making pore-pressure buildup and soil-strength degradation key concerns for long-term stability. In order to address these challenges, finite element (FE) models based on Biot’s coupled flow–deformation theory were utilized in COMSOL and improved through a modified pore-pressure source term incorporating both cyclic stress ratios (CSR) and static shear-stress ratios (SSR). This formulation captures phase-resolved oscillatory shear stresses and their cumulative contribution to residual pore-pressure generation. The models were calibrated using particle swarm optimization (PSO) and validated against wave-flume experiments and analytical solutions, ensuring reliable prediction of seabed behavior under cyclic loads.For the MHK structure, the results show that maximum residual pore pressures develop beneath the structure due to lower values of static shear stress ratios and limited drainage. Liquefaction phenomenon was not triggered for the studied wave–soil conditions, but strength degradation and serviceability concerns remain relevant. In case of suction anchor, the combined effect of cyclic tensile loading and geostatic stresses were considered. Oscillatory pore pressures alternate between compressional and extensional cyclic loading, while residual pore pressures rise sharply in early cycles and dissipate slowly, particularly inside the caisson where drainage is restricted. Residual pore-pressure ratios may exceed unity at shallow depths, indicating liquefaction susceptibility under certain loading scenarios. Progressive residual pore-pressure accumulation leads to a pronounced reduction in frictional resistance and a corresponding decline in pullout capacity. Overall, this research provides a unified, experimentally validated framework for evaluating oscillatory and residual pore-pressure behavior around offshore structures. The findings provide an understanding of seabed instability mechanisms and offer practical insights for the design and performance assessment of MHK systems and suction anchors in wave-dominated environments. Finally, a set of recommendations for future work is provided.