Scour Characteristics and Equilibrium Scour Depth Prediction around a Submarine Piggyback Pipeline

Local scour around a submarine piggyback pipeline in combined waves and current is investigated experimentally. Based on the experimental results, the scour evolution and scour morphology are firstly analyzed. Then, a comparison with the equilibrium scour depth S-eq between the present experimental data and predicted results is conducted. After that, the correlation between the dimensionless scour timescale T* and the maximum Shields parameter theta(cw) is investigated, and a formula is obtained to describe the variation trend between T* and theta(cw) for different gap ratios G/D. Furthermore, the parametric study is carried out to study the effects of Reynolds number R-ed and theta(cw )on S-eq, respectively. The results indicate that the S-eq below the piggyback pipeline increases when the gap ratio G/D increases from 0 to 0.1, and it gradually decreases when G/D > 0.1. For a given KC, the S eq increases with the increase of the ratio of velocities U-cw. In addition, when U-cw is fixed, a higher KC results in a greater S-eq. The T* is closely related to theta(cw) and G/D. The higher R-ed and theta(cw) both tend to result in the greater scour depth below a piggyback pipeline in combined waves and current.