Failure Risk Analysis Of Buried Natural Gas Pipelines In Central Sichuan Oil And Gas Mines

In recent years,with the increasing demand of domestic energy consumption,oil and gas energy has become the main type of energy supply in China.As the main mode of transportation,pipelines began to be laid in large quantities.However,due to the inflammable,explosive,toxic and harmful characteristics of the transportation medium,the safety of pipelines directly affects the safety of surrounding areas.The failure accident rate of the three gathering and transportation pipelines from Moxi Station 9 to Lei-1 gas gathering station,Guang 'an Well 002-21 to Guang 'an 51 and Longgang Well 001-2 to Longgang Gas gathering Station in the jurisdiction area of Sichuan Oil and Gas Mine has been continuously high since 2013.Although relevant measures have been taken in the mining area,little effect has been achieved.Therefore,this article through to the three pipeline field investigation,the accident statistics and data collection,analysis of pipeline in the actual operation of various types of failure factors,using the quantitative risk analysis method for screening type main factors,in combination with pipe type service state analysis of main factors on the influence degree of the pipeline under actual conditions,pipeline residual strength values by calculation to determine the safety status of pipeline,and security maintenance measures are put forward in order to ensure the safe operation of pipeline.Its main research contents and results are as follows:(1)Based on the analysis of typical pipeline failure accident reports,a Bow-tie failure factor model was established and a detailed list of pipeline failure factors was summarized.Entropy weight-improved analytic hierarchy process(IAHP)was used to carry out quantitative risk analysis of the three pipelines according to the list factors,and the hierarchical ranking method was used to screen the main factors.The results show that the major failure factors of the three pipelines are the excessive internal compressive stress and thermal stress with temperature difference.The improved Kent method was used to re-evaluate the risk of the pipeline.The results showed that the three pipelines were at a high risk level,which was slightly different from the mine evaluation results.(2)Then,according to the risk analysis and evaluation results,magnetic flux leakage testing was carried out on the pipelines.The results showed that the three pipelines all had dents with different depths and a large number of defects,and the stress concentration phenomenon occurred when the pipelines were subjected to internal pressure and temperature loads.Therefore,five typical sag depths of 2%OD,4%OD,6%OD,8%OD and 10%OD were selected for modeling to analyze the stress-strain law under the coupling action of internal pressure and temperature load.The results show that: 1)When the pipe only bears internal pressure load,the equivalent stress-strain value at the sag increases with the increase of internal pressure and sag depth,and the stress concentration area gradually shifts from the center of the sag to the edge of the sag.2)There is an overlapping effect between the temperature load and the internal pressure load on the pipeline,and the equivalent stress value in the depression is larger than that under the internal pressure load alone,and its variation range is also more obvious;3)6%OD pipeline suffers yield failure under the action of coupling stress,which is inconsistent with the present provisions on the existence of sag with the allowable depth of mining area not exceeding 6%OD?(3)At last,the residual strength of the three kinds of depressed depth pipelines(2%OD,4%OD and 6%OD)was calculated to verify the conclusion of thermal coupling.The results showed that when the internal pressure load of 6%OD pipeline reached 6.3mpa,its residual strength value under the action of coupling stress was lower than 0,indicating that failure had occurred.At the same time,according to the calculation results of the residual strength of 4%OD pipeline,the safety analysis is carried out by simulating the fatigue life state under the actual working conditions.The results show that: 1)When 4%OD pipeline operates at an internal pressure lower than 4Mpa,the sag is basically in a safe state.The probability of fatigue failure is low.2)When the internal pressure rises to 5Mpa,the fatigue life value in the depression begins to decrease and the safety factor begins to decrease;3)When the internal pressure reaches 6.3Mpa,the safety factor in the depression is basically below 1,with a relatively low safety factor and a high possibility of fatigue failure.Therefore,combined with the actual operation state of the pipeline,safety measures and maintenance Suggestions are proposed to reduce the accident rate of the three pipelines and ensure the safe operation of the natural gas pipeline network.