Seepage effects on local scour around twin tandem pipelines under wave loading

Offshore twin pipelines are increasingly deployed for oil and gas transportation, where wave-induced local scour poses significant threats to structural stability. When waves propagate over submarine pipelines, complex interactions between wave dynamics, seabed response, and sediment transport occur. The seabed generates excess pore pressures and seepage flows that modify sediment stability conditions under wave loading. Previous investigations have primarily focused on single pipeline scour under pure hydrodynamic conditions (Chiew, 1991; Fuhrman et al., 2014). Recent studies examined twin pipeline configurations, with Zhao et al. (2015) and Hu et al. (2019) investigating scour under current conditions, while Li et al. (2020) explored combined wave--current effects. However, these works neglected seabed response mechanisms. Limited research by Guo et al. (2019) demonstrated that upward seepage significantly influences sediment incipient motion around single structures, but comprehensive analysis of wave-induced seabed response effects on twin pipeline scour remains unexplored.

This study applies the PORO-FSSI-SCOUR model (Zhai and Jeng, 2024) to investigate seepage effects on scour around twin tandem pipelines. The computational framework couples hydrodynamic simulation with poro-elastic seabed analysis and incorporates seepage-modified Shields parameters for sediment transport. Parametric studies examine pipeline spacing ratios (G/D = 1-5) and soil properties, including permeability (ks) and the degree of saturation (Sr).

Results indicate that wave-induced seabed seepage substantially enhances scour development compared to conventional models without soil response. The seepage correction factor consistently exceeds unity, and upstream pipelines experience greater scour than downstream structures. Scour interactions intensify as G/D increases from 1 to 4, but twin pipelines exhibit independent behaviour at G/D = 5. ks influences scour magnitude more than Sr. Seabed response mechanisms significantly modify sediment transport around twin pipelines, emphasising the importance of considering soil--structure interactions in offshore pipeline design and stability assessment.