Safety Evaluation Of Buried Bend Section In The Natural Gas Gathering System

The nature gas is flammable and easy to spread,so explosion is easy to be caused if leakage or rupture happens in pipeline of the gathering and transferring system.Many gas pipeline accidents were caused by failure of bend.Bend pipe quality and bearing capacity directly affect the safety of pipeline system.Bend can't be researched without the straight pipe on both sides that have an effect on bend.Bend and the straight pipe on both sides that have an effect on bend are collectively referred as bend section.So studying of the safety about buried bend sections is crucial.From two aspects: flexible anti-bending hinge method and the finite element simulation,the buried bend sections are analyzed by force analysis,displacement analysis and stability analysis.Influence factors and stress rules of bend sections are researched from three aspects as strength,stiffness,stability.The dangerous pipes of Puguang gas field are identified and predicted.The factors effect research of buried bend section indicated: bends between 5 degree and 30 degree are in high stress area.The radial horizontal displacement of bends between 10 degree and 30 degree is in a larger value,these bends are easy to be jammed.Based on wall thickness of actual bend existing non-uniform along the longitudinal and circumferential,a new model is put forward.This model solves problems of additional edge stress caused by discontinuous wall thickness.The new model is applied to the bend under the combined action of internal pressure and in-plane bending moment.By the stress separation technology,the distribution characteristics of membrane stress and bending stress are pointed out.To simulate pipe soil interaction,the nonlinear contact model of buried bend sections is established.For example of buried bend sections in Puguang gas field,the mechanical properties of L360 steel have been tested.In the running condition,the security of strength,stiffness and stability are checked;the wall thickness safety allowance of overall bend sections in Puguang gas field is 11%.Through changing the internal pressure,temperature and wall thickness reduction,the failure ratios of bend sections are given.The risk levels of all bend sections are determined based on the failure condition.High risk bend sections locate in the six pipelines,including 4 ?508 pipelines and 2?406 pipelines.The angle of high risk bend sections is in between 10 degree and 25 degree.For safety,the monitoring and surveillance to these high risk bend sections should be strengthened.Some specific measures to reduce risk are put forward,such as increasing the wall thickness,the radius of curvature,and adding thickness of covering soil.