Dynamic response to a porous seabed and the associated liquefaction around a piggyback pipeline system

Seabed liquefaction induced by simultaneous wave and current loads poses significant threats to the stability of offshore infrastructures such as piggyback pipelines. This work focuses on an integrated numerical model (PORO-FSSI-FOAM) that couples OlaFlow solver with Biot's poro-elastic theory is employed to investigate soil response around piggyback pipeline systems subjected to wave-current loading. Validation of the present model against previous experimental measurements reveals consistent agreement for the distribution of wave-induced pore pressures in the vicinity of the pipe. Parametric studies examine the influences of pipeline configuration on seabed liquefaction. The numerical results reveal that piggyback pipelines generate more pronounced vortex patterns than single pipelines, leading to enhanced pressure gradients in the seabed. The spacing ratio G / D = 0.2 consistently produces the maximum pore pressure amplitudes and liquefaction depth ratios under all wave conditions. The diameter ratio d / D = 0.3 emerges as the most critical configuration, creating substantial velocity gradients and enhanced seabed instability. The analysis of position angle demonstrates that the placement of the auxiliary pipeline at α = 0 ∘ or 180 ∘ significantly increases the depths of liquefaction compared to the single pipeline system, attributed to leaking of pore water seepage pathways. The wave height exerts the most pronounced influence on the liquefaction potential across all pipeline configurations, while the wave period shows minimal sensitivity. • The piggyback pipelines generate enhanced vortex patterns compared to a single pipe. • The spacing ratio G / D = 0.2 produces the maximum depth of liquefaction under waves. • Diameter ratio d / D = 0.3 emerges as the most critical configuration for instability. • Position angles α = 0 ∘ or 180 ∘ increase liquefaction through the seepage paths. • The wave height affects liquefaction, while the wave period shows minimal impact.