Analysis Of Mechanical Properties Of Buried Gas Transmission Pipeline In Seasonal Frozen Soil Area Under Fault Action

In order to achieve the goal of carbon neutrality in 2060,the country has vigorously promoted the use of clean energy in recent years.Natural gas,as a low-carbon clean energy,has ushered in an important development opportunity period,and is an energy bridge for transitioning from a high-carbon society to a zero-carbon society.Due to the distribution characteristics of my country’s oil and gas resources,pipeline transportation has become an important means of natural gas transportation in my country.The environmental conditions of long-distance gas pipelines in my country are complex,and many gas pipeline projects pass through or are located in seasonally frozen regions.Research on the operation of buried gas pipelines in the case of large temperature changes,the seasonal changes of soil in seasonally frozen soil areas are completely different from that of permafrost and annual non-frozen soils.It is very necessary to analyze the mechanical properties of buried pipelines under different conditions,especially for the analysis of the state of buried pipelines under the motion of large deformation sites such as faults,to prevent leakage of buried pipelines and increase the resistance of buried pipelines to disasters.It is of great significance to ensure the smooth operation of pipelines during disaste.The main research contents and work of this thesis are as follows:(1)Considering the contact nonlinearity between the pipeline and the soil during the fault dislocation process,a pipe-soil nonlinear contact model with a size of 20 m × 20 m ×180m was established,In order to accurately reflect the local buckling deformation that may occur after the pipeline is subjected to fault.the buried pipeline is drawn using shell elements,and the accuracy of the model is verified by comparing it with the normative method and the previous results.(2)Determine the thermal boundary conditions in the seasonal frozen soil area,simulate the temperature change of the seasonal frozen soil area to obtain the soil temperature field of the seasonally frozen soil area,and use the sequential coupling method to import the temperature field into the mechanical analysis step to achieve soil frost heave.The three-dimensional thermal coupling nonlinear finite element model is proposed,with strike-slip faults as the research object,analyzing the deformation law of the buried pipeline under the action of the fault.It can be seen that the buried pipeline with the crossing angle of 90° fails mainly due to local compression buckling under the action of strike-slip fault.(3)Among the influencing factors,the pipeline wall thickness has the most obvious improvement on the mechanical properties of buried pipelines.The critical displacement of pipeline faults with 22 mm wall thickness is 4.9 times the critical displacement of pipelines with 6mm wall thickness;high-grade steel pipelines are used to improve pipeline mechanical properties,X80 pipelines Compared with the X56 pipeline,the critical displacement of the fault increases by 39%;if the diameter of the pipeline exceeds 800 mm,the mechanical properties of the pipeline are almost no longer affected by the diameter of the pipeline;the deeper the pipeline is buried,the critical displacement of the fault decreases relatively.The displacement is reduced by 5%;the thermal strain of the pipeline caused by the increase of the temperature of the natural gas in the pipeline increases the compressive strain of the pipeline,and the probability of the pipeline being damaged by pressure increases;when the internal pressure of the pipeline is increased,the risk of the pipeline being damaged by pressure increases,and the internal pressure of 12 MPa increases.Compared with the 4MPa internal pressure pipeline,the mechanical properties of the pipeline are reduced by 50%;in addition to the above factors,uneven frost heave on both sides of the fault will cause the mechanical properties of the pipeline to fluctuate with the ambient temperature field.(4)Compare the calculation results of the model considering the soil temperature field with that of the model without considering the temperature field,and determine the influence law of the soil temperature field on the buried pipeline.If the change in the interaction between the pipe and soil caused by the soil temperature field is not considered in the analysis process,the critical displacement of the fault will have a deviation of 5-10%,and the deviation can reach 50% in the case of poor mechanical properties of the pipeline.The thinner the pipe wall,the shallower the buried pipeline and the larger the diameter of the pipeline,the more attention should be paid to the effect of soil frost heave on the mechanical properties of the buried pipeline.Through the research of the above content,the influence of various factors on the buried gas pipeline in seasonal frozen soil area under the action of fault is accurately analyzed,which shows the importance of considering the influence of the temperature field in seasonally frozen soil area on the buried pipeline,and provides a reliable basis for the construction,operation,post-disaster maintenance and reinforcement of buried gas pipelines in seasonally frozen soil areas.