The seabed stability in the vicinity of submarine pipelines under wave-plus-current conditions is one of the major issues in offshore projects. Previous works mainly focus on the evaluation of the seabed response around a single pipeline. In this study, the previous model (PORO-FSSI-FOAM) will be adopted to investigate the effect of the gap ratios (G/D) of twin pipes on the wave & current-induced transient seabed response. Based on numerical examples, the following conclusions were found: (i) the effect of different G/D between twin pipes on the distribution of excess pore-water pressure and resulting seabed liquefaction cannot always be ignored; and (ii) as the gap ratio (G/D) is 1.25, the soil response beneath both pipelines is significant than in the condition of a single pipeline.
Pipelines are one of the essential installations for the oil and gas industry, which have been used for the transportation of oil and gas from offshore to onshore. Since the first offshore pipeline was built by Brown Root to carry oil in 1954, the development of the submarine pipeline networks has across the globe, even were regarded as "lifelines" of the oil industry (Sumer and Fredsøe, 2002). The existence of the submarine pipeline does not only alter the nearby flow morphology but also lead to the surrounding seafloor instability (including soil liquefaction, scour and shear failure) and ultimately cause damage or failure of the pipeline (Sumer, 2014). In general, when the seabed is exposed to the wave, the generated dynamic pressures along the seafloor can induce the pore pressures, effective stresses and soil displacements within the seabed (Jeng, 2012). Then, the seabed in the vicinity of the pipeline could become unstable or even liquefied due to the increasing excess pore pressures and the reducing vertical effective stresses. Once the liquefaction occurs, the soil will behave like a kind of heavy fluid without any shear resistance, which greatly enhances the risk for the pipeline to sag.
