Research On Mechanical Behavior And Failure Mechanism Of Cross-fault PE Pipe

Polyethylene(PE)pipes have gradually replaced steel pipes in gas pipeline network due to their excellent performance.With the rapid development of PE pipes for gas,an increasing number of attention has been paid to the safety of PE pipes.According to the statistics of the United States Pipeline and Hazardous Materials Safety Administration(PHMS A),there were 10798 pipeline accidents from 1992 to 2012,with 383 deaths and 1584 injuries,of which 78.3%were gas pipeline accidents and 74.6%were casualties.Among the reasons affecting its safe operation,the characteristics of earthquake disasters,such as difficult to predict,sudden and destructive,should be paid more attention.Existing seismic damage data illustrate that the permanent large surface deformation caused by fault dislocation is one of the main factors leading to the destruction of buried pipelines.Simultaneously,buried pipelines are prone to buckling failure and the seismic performance of pipelines is relatively weak under the action of reverse faults.Therefore,it is necessary and instructive to systematically study the mechanical behavior of buried PE pipelines under the fault action.By studying the strength and failure mechanism of buried PE pipes under various geological hazards,a systematic safety evaluation method of geological disasters for gas PE pipes will be gradually formed.Based on the mechanical behavior and failure mechanism of steel pipelines under fault disasters,considering the mechanical properties and failure criteria of PE pipelines,the numerical calculation model of PE pipeline-soil system under typical seismic fault conditions was established by using finite element software,and the mechanical behavior and failure mechanism of buried polyethylene pipelines were systematically analyzed,and safety assessment and risk prediction methods for PE pipes were established preliminarily.The details are as follow:(1)The characteristics and hazards of the failure of buried steel pipelines and buried polyethylene pipelines under earthquake fault disasters were briefly introduced.The necessity and feasibility of studying the mechanical behavior and failure mechanism of PE pipelines under fault disasters were put forward.After summarizing the research at home and abroad,the main research contents and technology roadmap of this paper were put forward.(2)Based on the analysis of the load on the pipeline,and mechanical characteristics of stress,strain and deformation about PE pipelines,mechanical model of buried PE pipelines were established.Studied the damage mechanism of PE pipeline and analyzed causes of pipeline failure.According to the mechanical characteristics of PE pipeline,the failure criteria of PE pipeline under the cross-fault deformation are compared and determined.The non-linear finite element method was used to analyze the buried PE pipe material,soil models and the pipeline-soil system under the cross-fault deformation.Considering the way of imposing fault dislocation displacement load,the boundary conditions of the model were established,and the non-linear finite element model of buried PE pipeline crossing the fault disaster was improved.(3)Based on the mechanical experiment of PE pipes,and according to the stress and strain data obtained from the tensile properties test of PE 80 pipes,the hyperbolic constitutive model of PE pipes was fitted.Meanwhile,the strength and toughness at different strain rates were obtained.(4)The numerical calculation model of buried PE pipelines under the strike-slip fault was established.Studied the mechanical behavior of non-pressurized and pressurized buried PE pipelines,and discussed the causes of local deformation of pipes.At the same time,the effects of fault dislocation,types of surrounding soil,inner pressure,thickness of covering soil and dimension parameters of pipes on buckling behavior and stress-strain of pipeline were analyzed,and the flatness of 20%was selected as the criterion for PE80 pipes failure.(5)The numerical calculation model of buried PE pipelines under the reverse fault was established.And then simulated and analyzed the stress,strain and deformation of buried PE pipelines under the influence of multiple parameters.The mechanical behavior of PE pipes under the reverse fault was discussed under the different of parameters,such as fault dislocation,inner pressure,angle of reverse faults,types of surrounding soil and thickness of covering soil.(6)According to the safety evaluation process of PE pipe,the damage of buried PE pipe caused by the fault movement was measured with a group of dynamic parameters of PE pipessoil system.The conceptual framework of early warning software for buried PE pipeline in cross-fault disaster area was proposed based on the post-accident analysis of gas PE pipeline damage.Combining with the previous research,the protective measures to reduce the damage of buried PE pipes caused by critical level of earthquake fault disaster were put forward,which can provide reliable guarantee for the safe operation of PE pipelines.Finally,it was concluded that the types of surrounding soil in the fault zone have a great influence on the PE pipes.The change of normal working pressure has little effect on the mechanical behavior of PE pipes.PE pipes with larger SDR and smaller diameter are more likely to be damaged.In the design stage of buried gas PE pipeline,three measures are taken to reduce the dangerous accidents of buried PE pipeline caused by sudden earthquake fault disasters of critical level,such as choosing suitable size PE pipelines,ensuring appropriate thickness of overlying soil above buried PE pipeline and designing backfilling of soil.