Multi-Field Coupling Mechanics Analysis Of Buried External Corroded Pipelines In Cold Regions

As the lifeline of oil industry,oil and gas pipelines play an important role in the development of the national economy.Due to the harsh environmental of the cold regions,not only oil and gas pipelines bear the failure risk which was caused by aging of pipeline and corrosion,but also will face the threat of soil frost heaving,melting and some geological disasters.The strength of corroded pipelines in cold regions is affected by the load of temperature,internal pressure and frost heave force,and it belongs to the multi-physics coupling problem.Once the pipelines become invalid,they will cause a series of consequences,such as casualties,economic losses,the destruction of natural ecological environment,and so on.Therefore,it is very significant to study the multi-field coupling strength of corroded pipelines in cold regions.For the buried hot oil pipelines with corrosion defects in cold regions,the mechanics models and analysis methods which are much more comprehensive and closer to the actual working condition,are adopted to study the multi-field coupling strength calculation and the law of related influencing factors.The main contents of the thesis include four aspects:Firstly,considering the nonlinear properties of pipelines and the nonlinear frost heaving characteristics of soil,a two-dimensional thermo-solid coupling dynamic model consisting of pipeline,heat preservation layer and soil was established for the single corroded pipeline which was under the loads of internal pressure,temperature and frost heave force.The numerical solution is obtained by numerical analysis and compared with the failure pressure solutions obtained by the Modified ASME B31.G method,the RPA method and the DNV-RP-F101 method.The results show that the theoretical results are conservative and it is necessary to do multi-field coupling analysis.In the analysis of influencing factors for the mechanics properties of corroded pipeline,degree of influence from strong to weak in the four influencing factors are: fluid pressure,relative corrosion depth,fluid temperature and the atmospheric heat transfer coefficient.Secondly,according to the thermo-solid coupling analysis method,a three-dimensional thermo-solid coupling dynamic model consisting of double corroded pipeline,heat preservation layer and soil was established for the double corroded pipeline which was under the loads of internal pressure,temperature and nonlinear frost heave force.The influence laws of various influencing factors on the mechanics properties of double corroded pipeline were discussed.The results show that the pipeline strength is affected obviously by fluid pressureand relative corrosion depth,the fluid temperature affects it secondly,while it is affected relatively weakly by the other factors.Thirdly,on the basis of three-dimensional thermo-solid coupling analysis method,considering the effect of fluid-solid coupling between the fluid and the corroded pipeline,as well as the influence of the soil frost-heave environment on the mechanics properties of the corroded pipeline,the three-dimensional thermo-hydro-mechaical coupling mechanics models consisting of double corroded pipeline,fluid,heat preservation layer with the single frost heaving soil and differences frost heaving soil were established respectively for the double corroded pipeline which was under the loads of internal pressure,temperature and nonlinear frost heave force.The influence laws of various influencing factors on the mechanics properties of double corroded pipeline were discussed.The results show that the influence of fluid pressure and relative corrosion depth on the mechanics properties of pipeline is strong,the influence of fluid temperature is second,and the other influencing factors are relatively weak,no matter what the soil environment the corroded pipelines are located.In addition,for the corroded pipelines located in the differences frost heaving soil,the spacing between the corrosion defects and the soil boundary has a significant effect on its mechanics properties and should be given sufficient attention.Finally,for the double corroded pipelines which was under the loads of internal pressure,temperature and nonlinear frost heave force,a three-dimensional thermo-hydro-mechaical coupling model consisting of double corroded pipeline,fluid,heat preservation layer and soil was established to do the seismic time history analysis.The influence laws of various influencing factors on the mechanics properties of double corroded pipeline were discussed.The results show that the relative corrosion depth and fluid pressure have a strong influence on the mechanics properties of the pipeline,the fluid temperature and relative corrosion width affect it secondly,the relative corrosion length has little effect on it.The results of this paper can provide an analysis method and theoretical basis for the integrity evaluation and maintenance measures of the buried pipelines in cold regions.