Numerical investigation of the influence of the small pipeline on local scour morphology around the piggyback pipeline

This paper presents the results from a numerical simulation study to investigate the effect of the position angle (alpha) of small pipeline on the local scour and the hydrodynamic force around the piggyback pipeline in steady current conditions. Results show that the local scour depth around the piggyback pipeline increases first and then decreases with the increase of alpha. The scour depth and width reach the maximum values as the small pipe locates at the top of the large pipeline (i.e. alpha = 90 degrees). The scour around the piggyback pipeline is accelerated when alpha ranges between 30 degrees and 165 degrees, while for alpha = 0 degrees-30 degrees and 165 degrees-180 degrees, the local scour around the piggyback pipeline is inhibited. Furthermore, the small pipe placed in front of the large pipe has slightly larger effect on the scour hole morphology than that when it is placed behind the large pipe. The drag force coefficient increases first and reaches the maximum value at alpha = 75 degrees, and then decreases with the increase of alpha. Eventually the drag force coefficient approaches roughly a constant. The lift force coefficient is approximately a V-shaped with the variation of alpha and has the maximum value at alpha = 90 degrees.