Natural Gas Industry B (Jun 2020)
Evolution features of riverbeds near underwater crossing line pipes: An experimental study
Abstract
It is of important significance to study the evolution law of the riverbed near underwater crossing line pipes for ensuring the safe operation of oil and gas pipelines and reducing the risk of damage by water flood disasters. In order to clarify the evolution law of the riverbed near underwater crossing line pipes and its negative effects, this paper conducted flume model experiments on underwater crossing line pipes. The physical process of riverbed evolution near pipes was observed and the effect of hydrodynamic conditions on the local pipe scour was studied. What's more, the formation mechanism of local scour at underwater crossing line pipes was revealed. And the following research results were obtained. First, when the water flow is slow, the riverbed evolution process near underwater crossing line pipes is mainly divided into six stages, including riverbed undercutting, pipe exposure, micro-pore formation, scour hole propagation, pipe suspension and scour equilibrium. Second, vortex and seepage flow are the reasons for the local scour of underwater crossing line pipes. Before pipes are exposed, the silt around the pipes is reduced by vortex. After pipes are exposed, micro-pores occur at the pipe bottom under the joint action of vortex and seepage flow. And thus, local scour is formed. Third, flow velocity and water depth jointly influence the riverbed scour duration of each stage and the maximum scour depth at the pipe bottom. When the Froude number (Fr) is in the range of 0.306–0.808, with the increase of Fr, water flow gets fast, the maximum scour depth at the pipe bottom increases, the duration for scour equilibrium decreases, the riverbed undercutting depth increases and the riverbed topographically gets flatter. The maximum scour depth at the pipe bottom is 0.9–1.6 times the pipe diameter, and the duration for scour equilibrium is between 1650 min and 2620 min. In conclusion, the experimental results provide important reference for predicting the burial depth of underwater crossing line pipes and ensuring their safe operation.
