Local scour around two pipelines in tandem in steady current

• The effects of the gap between two pipelines on local scour is studied using numerical method
• The numerical model are validated against the laboratory test results
• Both live bed and clear water scour conditions are considered
• The vortex shedding from the upstream pipeline increases the scour below the downstream pipeline significantly
• The effects of the gap on the scour in the clear water condition are found to be similar to that in the live bed condition.

In this study, local scour around two identical pipelines in a tandem arrangement are investigated numerically. The two-dimensional Reynolds-averaged Navier–Stokes (RANS) equations, together with the conservation equation of the sediment mass are solved to simulate the flow around two pipelines and the evolution of the local scour profile below the pipelines. The gap ratios between the two pipelines (G/D) range from 0.5 to 5 with an interval of 0.5, where G is the gap between the two pipelines and D is the diameter of the pipelines. The focus of the study is to investigate the effects of the gap ratio on the scour depth and scour time scale. It is found that the scour depth below the downstream pipeline is greater than that below the upstream pipeline under both clear water and live bed conditions because of the effect of the vortex shedding from the upstream pipeline. The maximum scour depth below the downstream pipeline occurs at G/D = 2.5 under both clear water and live bed conditions, which is about the critical gap ratio beyond which vortex shedding from the upstream pipeline occurs. As the gap ratio exceeds 2.5, the scour depth below the downstream pipeline decreases with increasing gap ratio due to the weakening effect of the vortex shedding from the upstream pipeline on the scour. The scour under the clear water condition is simulated at a velocity 25% lower than the critical velocity for live bed scour. The variations of the scour depths below the two pipelines in the clear water and live bed conditions are found to be similar to each other. Strong positive mean lift coefficient on the downstream pipeline was observed during the scour process, which is believed to have a negative effect on the stability of downstream pipeline. The RMS lift coefficient on the downstream pipeline increases significantly when the vortex shedding from the upstream pipeline occurs.