Numerical simulation of the small pipeline placement on hydrodynamics and local scouring of the piggyback pipeline

Piggyback pipelines are wildly used in the offshore oil industry. However, the impact of small pipeline location on local scouring remains unclear. In this study, the hydrodynamics and local scour around a piggyback pipeline under unidirectional flow were investigated using a validated numerical model. The results indicate that significant shifts in the position of stagnation point along the large pipeline as alpha (the small pipeline position angle) varies between 0 degrees-75 degrees and 315 degrees-360 degrees. Specifically, the position of the highest stagnation point occurs at alpha = 60 degrees, while the lowest occurs at alpha = 318 degrees. When the small pipeline is installed below the large one, the scour hole around the piggyback pipeline is smaller than that on the single pipeline, while the minimum local scour at the piggyback pipeline occurs at alpha = 222 degrees. The scour hole is larger when the small pipeline is directly above the large pipeline compared to being installed below. Similarly, the scour hole is larger when the small pipeline is positioned at lower intake surface of the large pipeline (alpha = 270 degrees-360 degrees) instead of lower back surface (alpha = 180 degrees-270 degrees). These findings offer valuable insights for optimizing the structural configuration to minimize the local scour at piggyback pipeline.