Mechanical Behavior Of Oil And Gas Pipelines Under Typical Geological Disasters

There is a bigger demand for oil and natural gas with the development of national economy and the industrialization level in China.Exploration and exploitation cannot meet the needs of of oil and gas resources.External dependency of oil is more than 60%for the first time in 2015,Pipeline is the most efficient,economic and reliable transportation way of oil and gas,and it is called lifeline engineering.Pipelines' structure safety and reliability have been widely concerned.By 2014,total mileage of global oil and gas pipelines is 196×104km,and total mileage of China's oil and gas pipelines is 10.7×104 km.Ground permanent deformation caused by geological disasters is one of the main reasons for buried pipeline failure according to the survey.Fault movement,landslides,ground subsidence,perilous rock collapse,surface overload that along the long-distance pipelines seriously threats to pipelines'safety operation,then results in pipeline failure and causes the significant safety and environmental accidents.Researches on mechanical behavior of buried pipelines under geological disasters have completed by scholars based on the elastic theory and elastic foundation beam model.But there are few studies on the local buckling of the pipeliones.Deformation of the pipeline is a nonlinear process because it is a thin-walled cylindrical shell structure.When instability of the cross section occurs,pipeline failure modes are difficult to study by the beam model,rope model and other analytical methods.Pipeline-soil interaction under geological disasters is relatively complex,so it is must be considered in solving the pipeline mechanics.With the development of pipeline industry and its application,geological conditions are more and more complex.Therefore,research on the mechanical behavior and failure mechanism of oil and gas pipelines is especially necessary.Failure forms and causes of oil and gas pipelines under the typical geological disasters were investigated in this paper.The six pipeline-soil coupling numerical models were established based on elastic-plastic mechanics,elastic foundation beam and finite element theory.Mechanical behavior and failure mechanism of oil and gas pipelines were systematically investigated,and a warning software of buried pipelines under geological disasters was developed.Finally,pipeline protection devices were designed and their feasibility were analyzed.The details are as follow.(1)The main geological disaster characteristics and their harms along the long distance oil and gas pipelines were briefly introduced.The necessity and feasibility of mechanical behavior and failure mechanism of buried pipeline were put forward,and the research status at home and abroad was summarized.Then,the research content and ideas of this paper were put forward.(2)Numerical models of buried pipelines under fault movement were established,mechanical behavior and buckling phenomenon of the pipeline in soil and rock stratums were investigated.Reasons of local collapse and wrinkle for the pipeline were discussed,and effects of fault displacement,internal pressure and diameter-thickness ratio on thebuckling behavior were studied.Critical strain and the maximum strain of the pipelien and the limit fault displacement were gained by multivariate fitting method.(3)Stress-strain responses of the buried pipeline under lateral landslide were studeid.The results show that tensile strain of the pipeline in landslide bed is greater than compressive strain.So,tensile strain is the main reason of pipeline failure.Effects of the pipeline parameters,landslide parameters and surrounding soil parameters on the pipeline's mechanical perfonnance.Relationship between the critical landslide displacement and other parameters was received based on CSA design strain criterion.(4)Responses of the overhead pipeline under the impact of spherical and cubic rockfalls were investigated.Simplified mechanical model of the impact force of the buried pipeline cuased by rockfall was established,whose reliability was validated by the comparison with the numerical simulation.After the failure analysis of buried pipelines in soil and rock stratums,the results shows that thin wall non-pressure pipelines in shallow area are prone to failure.Semi-empirical solution of buried pipeline's dent rate under rockfall impact was reached after the sensitivity analysis of surrounding soil parameters,pipeline parameters and rockfall parameters.(5)The pipeline mechanics model in mining subsidence area was established,and the analytical solution of deformation and strain were reached.Effects of pipeline parameters and surrounding soil parameters on the pipeline's mechanical properties were discussed based on the numerical simulation.Then,calculation formula of buried pipeline's displacement was revised.The prediction formulas of pipeline's maximun strian and limit surface subsidence were put forward.(6)Numerical model of a buried pipeline under ground load was established,effects of loading area,load pipeline parameters and surrounding soil parameters on the stress-strain of the buried pipeline were discussed.Ovality is as pipeline failure criteria,forecasting fonnulas of the allow maximum ground load were solved when the ovality are 3%and 6%.(7)Loads,failure phenomenon and reasons of the directional crossing pipelines were analyzed.The dent and collapse processes of the pipeline were simulated,and the mechanical properties in different stages.The deformation rules of non-pressure pipeline and pressre pipeline under external load were revealed.Under surrounding soil's pressure,the corss section of the pipelines experience the change in the shapes of ellipse,crescent,calabash and"8".(8)A warning software of the buried pipelines under geological disasters was developed based on the semi-empirical solutions of pipeline's maximum strain,ovality and dent rate.It can be used to evaluate the pipeline's safety status.Protective devices were designed toprotect the buried pipelines and directional crossing pipelines.They are reliability and they can reduce the pipelines' failure probability.Based on the investigation of oil and gas pipelines's failure,mechanics behaviors of the buried pipelines under fault movement,landslide,roakfall,goaf subsidence,ground subsidence and ground overload were studied.Numerical simulation of the two failure forms for directional crossing pipelines were done.A warning software of the buried pipelines under geological disasters was developed,and protective devices were designed.Therefore,failure mechanism of the pipelines were explored,a new safety assessment method was formed and a new protective device was designed.That can provide reliable guarantee for safety operation of oil and gas pipelines.