Author(s): Titi Sui; Qi Yang; Chi Zhang; Jinhai Zheng; David R. Fuhrman
Linked Author(s): Jinhai Zheng
Keywords: Span shoulder migration; Three-dimensional scour; Pipeline; Wave plus current; Shields parameter
Abstract: When the pipeline is placed in the marine environment, the surrounding flow pattern will be changed relative to the undisturbed bed in the far field, which may in turn lead to scour as the result of increased local sediment transport capacity. Such scour propagation may potentially cause the sagging, the vortex induced vibrations and the fatigue failure of the pipeline. It is therefore of great interest to gain overall knowledge concerning the scour processes below the pipelines. Many previous researchers focus on the 2D scour which in this case the scour only propagates in the vertical direction. Investigations that involved 3D scour around pipeline is mostly with the pure current or wave conditions. In this study, new analysis involving the span shoulder migration in the three-dimensional scour beneath submerged horizontal pipelines are presented. Specific emphasis is on gaining a better understanding of, and ability to predict, the span migration velocity with the current, and the wave plus current conditions. For the pure current cases, consistent with previous experimental observations, both a primary (faster) and secondary (slower) span migration are observed. Process visualization of suspended sediment patterns are in line with prior speculation that this transition coincides with reduced local bed shear stress amplifications as the scour hole both deepens and widens. Dimensional analysis and physical insight are combined, leading to a new rational model for predicting the span migration velocity in both live-bed and clear-water regimes, with predictions naturally coinciding at the limit of far field incipient motion conditions. In both regimes the data cluster as predicted, and fitted closed-form expressions are provided for predicting the span migration velocity. The rational approach likewise includes a new and simple criterion for the transition from primary to secondary migration in the live-bed regime. In the clear-water regime the model incorporates primary dependence on the ratio of the Shields parameter to its critical value. The developed rational model can be used to quantitatively predict all known major features of the span migration velocity in the early stages of the three-dimensional (live-bed and clear-water) scour beneath submerged horizontal cylinders induced by perpendicular flow, and can hence be regarded as the first complete model for this evolution. For the wave plus current cases, it is found that the migration velocity is decreased if the wave components were considered. By adopting the coefficient of relative strength in current, one model was proposed which predicts the span shoulder migration velocity with wave plus current conditions. The dependence of non-dimensional velocity on the Shields parameters was found to 3/2 power, which is the same to that in the pure current conditions.
Year: 2023