Numerical Modelling of Wave-Induced Seabed Liquefaction Around Marine Structures: Assessment of Mitigation Measures

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Abstract
Seabed liquefaction poses a significant threat to the operational stability of marine structures resting on or embedded within the seabed. Cyclic wave loading of saturated granular soils leads to a reduction in strength and stiffness. Protective surface layers such as cover stones and rip-rap are routinely installed for scour protection, yet their simultaneous role as a geotechnical surcharge that suppresses residual liquefaction is rarely quantified, leaving a keymitigation mechanism unexploited in design. In this study, an OpenFOAM-based Biot-drift-flux model is extended and applied to investigate a range of seabed liquefaction case studies, such as Wave-Soil-Interaction (WSI), Wave-Soil-Structure-Interaction (WSSI), and Wave-Soil-Structure-Cover Stone-Interaction (WSSI-CS). The model uses Biot’s poro-elastic theory to govern the behaviour of non-liquefied soils, while liquefied zones are represented using thedrift-flux model, with the transition between the two defined based on a liquefaction criterion. Through validation against reference experimental data, the developed model is shown to accurately reproduce the onset of residual liquefaction. The results further demonstrate that cover stones are effective in reducing the pore pressure accumulation by increasing the initial mean normal effective stress through a geotechnical surcharge, by 42 % for a single looselayer and up to 129 % for three layers near to the mudline. Furthermore, when the model is applied to WSSI, it has been found that although structural weight can provide localised confinement to the seabed, dynamic mooring force of 600 N significantly accelerates the pore pressure accumulation, resulting in residual liquefaction within 60 s. However, the application of cover stones successfully maintains seabed stability, reducing the maximum normalised pore pressure ratio from a failure state of 1.0 back to approximately 0.12. The novelty lies in resolving progressive wave loading, and granular surface protection concurrently within a single residual liquefaction solver, offering valuable insight for the design and resilience of coastal and offshore infrastructure

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