Numerical investigation on critical length of impermeable plate below underwater pipeline under steady current

Abstract

This study proposes a new approach in which an impermeable plate is placed under the pipeline to prevent the local scour around the pipeline. In order to understand the performance of this approach, the finite volume method (FVM) and volume of fluid (VOF) method are adopted to simulate the flow field around the pipeline. The pressure distribution along the sandy bed surface is obtained by considering the variation of water surface. Furthermore, the effects of water depth, unidirectional and bidirectional impermeable plates on pressure difference are discussed. The seepage flow field of sandy bed near underwater pipeline is numerically simulated using the laminar and porous media model. On this basis, the effect of the impermeable plate length on hydraulic gradient is investigated and the critical length of impermeable plate is obtained. The simulated results show that when the water depth is smaller than 5.00D (D is the diameter of pipeline), the effect of the water depth on the pressure difference is remarkable. The pressure differences between two endpoints of both the unidirectional and bidirectional plates decrease with the increase of the plate length. The variations of the pressure differences for both the unidirectional and bidirectional plates are similar. With the increase of plate length, the hydraulic gradient decreases and the piping at the seepage exit is avoided effectively as long as it reaches a certain length. Such a critical length of the plate decreases with the increase of the water depth. When water depth is larger than 4.00D, the effect of the water depth on the critical length is small. For the same water depth, the critical length of impermeable plate increases with the increase of the dimensionless flow parameter. Numerical simulation results are in good agreement with the available experimental measurements.