Experimental Study On The Evolution Process Of Local Scour And Suspension Of River Bottom Crossing Pipeline

Due to the increasing emphasis on social economy and scientific and technological innovation in China,the rapid development of social economy and the remarkable progress of scientific and technological innovation,the demand for oil and gas energy is increasing day by day,so that the construction scale of the national oil and gas pipeline network is expanding rapidly,and the coverage of long-distance oil and gas pipelines is becoming wider and wider.Because of the diversity and complexity of the transportation environment of long-distance oil and gas pipelines,especially in the face of various complex water environments such as mountain rivers,the pipeline adopts the underwater crossing transportation method as the first choice for practical engineering.However,oil and gas pipelines in various waters are prone to be exposed and suspended under the action of natural factors,human factors and other factors.When the suspended pipeline breaks under the impact of water flow,the daily life of surrounding residents is seriously affected,and with the leakage of oil and gas energy,it will cause damage to the natural ecological environment that is difficult to recover.Through many years of investigation on the fracture events of oil and gas pipelines,it is found that the main cause of pipeline fracture is natural factors,and flood erosion is the main factor.Therefore,it is necessary to study the evolution process of local scour and suspension of the pipeline under the action of unsteady flow,and to explore the actual scour situation in each stage of the whole process from exposure to fracture.The analysis of the influence of water flow conditions,pipeline conditions and bed sand parameters on the pipeline will provide important theoretical support for the construction,operation and maintenance of underwater crossing pipelines.In this thesis,through the hydraulic physical model test under the action of unsteady flow,the evolution process of local scour and suspension of pipeline under different flow conditions,different pipeline conditions and different bed sand parameters is observed,recorded and analyzed.The main research results are as follows:(1).Through the establishment of hydraulic physical model,the moving bed test was carried out,and the high-definition camera was used to record the process of local scour and suspended evolution of the pipeline under the action of unsteady flow under the conditions of flood peak flow,flood peak type,pipeline diameter,pipeline laying angle,pipeline buried depth,bed sand density and bed sand particle size.The local physical scour characteristics of the pipeline in the process of unsteady flow scour,the scour duration of each stage,the maximum scour range and maximum scour depth of the scour pit around the pipe after reaching the scour balance and the change value of the sand volume of the model sand in the test section before and after the scour balance were observed and counted.(2).According to the maximum scour range and maximum scour depth data of the scour pit,the scour duration data of each stage of the local scour,the residual volume of the sand body in the test section after reaching the scour balance and the scour removal data,the main factors of the local scour are analyzed and judged.The test results show that the main factors affecting the scour stage from the beginning of the test to the riverbed are : bed sand particle size;the main factors affecting the riverbed undercutting to the pipeline exposed scouring stage are : bed sand density;the main factors affecting the erosion stage of pipeline exposure to micropore formation are :bed sand particle size;the main factors affecting the formation of micropores to the stage of fully suspended scouring are : flood peak type;the main factors affecting the upstream side of the maximum scour range of the scour pit are : flood peak type;the main factors affecting the downstream side of the maximum scour range of the scour pit are : flood peak type;the main factors affecting the maximum scour depth of the scour pit are : flood peak type;the main factors affecting the residual amount and scouring amount of the sand body in the test section are : flood peak type.(3).The data of all test conditions are counted,and the difference diagram of the maximum scour range and the maximum scour depth of the local scour of the pipeline under each condition,the difference diagram of the scour duration of each stage,the volume diagram of the residual sand body and the volume diagram of the sand body taken away by the flow scour are obtained.The sensitivity analysis of the local scour evolution process of the river bottom crossing pipeline under the action of unsteady flow is carried out to determine the primary and secondary influencing factors of the local scour of the pipeline.The test results show that the flood peak type factor is the main influencing factor of the maximum scour range and maximum scour depth of the scour pit around the pipe,the volume change of the model sand in the test section and the scour duration from the formation of micropores to the complete suspension of the pipe.The parameters of the bed sand mainly affect the erosion duration from the beginning of the test to the stage of micropore formation.The pipe diameter factor and the bed sand density factor are the main secondary factors of the local scour and the scour duration.