Safety Assessment Of Girth Weld Of X80 Gas Pipeline With Composite Defects

The safety of oil and gas pipeline has been widely concerned,which is closedly related to social public resources and people's lives.However,it has been found in recent years that pipeline failure accidents occur frequently,and most of them are grith weld failures.There are a lot of welding defects and structural misalignment defects in grith welds,which are the weak part of the whole pipeline.Therefore,it is urgent to evaluate the integrity safety for pipelines with composite defects in grith welds,which can minimize the economic losses casualties and environmental damage caused by accidents.The research object is the large-diameter X80 gas pipeline in the Heibe-Changling section of the China-Russia Eastern Route.Based on the basic theory fracture machines,the common types of defects in pipeline analyzed,and understand the distribution and its causes.The body defects in the weld are quantified as regular embedded cracks,and the defects in the internal surface of the pipeline are simplified to the internal surface cracks.The stress of the pipeline when the misalignment and cracks coexist in the grith weld were studied.Based on the working condition,the fracture model of "misalignment + crack" were analyzed,and a new method for calculating the stress intensify factor of "misalignment + crack" composite defects in grith welds is given.In order to quantify the influence of different parameters on the stress intensity fator,the finite element analysis model of pipeline with misalignment and circumferential crack defects was established by finite element software.The stress intensity fator at the three-dimensional crack tip and the stress distribution near the weld were caculated.Under this model,the variation relation of stress distribution near the weld and stress intensity fator K_I at the crack tip with various parameters were analyzed The results showed that the stress intensity fator at the crack tip of the internal surface is larger than that of the embedded crack when there exists the misalignment.The relationship between K_Iand the misalignment at the grith weld is complex.When the internal surface crack is shallow,K_I tends to decrease with the increase of the misalignment.When the crack is embedded crack and the depth a/t exceeds 0.2 of the internal surface crack,K_I increases with the increase of the misalignment.Based on this,combining with theoretical analysis and numerical simulation method to the K_I of finite element calculation results and the theoretical derivation calculation method,and the function Gm and Gb of crack shape and size in the formula were obtained,which provides a simplified and accurate engineering methods for calculating the crack-tip stress intensity factor in practical engineering.By comparing the engineering estimation results with the finite element calculation results,it is concluded that the results obtained by the calculation formula are reliable,and the K_I can be calculated by the engineering estimation formula in the process of engineering safety assessment,instead of the tedious finite element calculation.In order to evaluate the safety of the defects in the girth weld of the pipeline,the Charpy impact test and micro zone tensile test were carried out on the girth weld of the X80 actual service pipeline in China-Russia Eastern Railway.The fracture toughness K_IC=53.7 MPa·m^1/2 and stressstrain data of the weld material were obtained.Based on the true stress-strain transformation relationship provided in the selection curve 2 of CEGB-R6,a new failure evaluation curve for X80 pipeline girth weld was obtained,which was more secure than the general assessment curve.Taking the specific defect misalignment sizes of 3mm and 6mm pipelines as examples,combined with the K_I engineering estimation formula,the safety assessment were carried out in the failure assessment curve.It is concluded that the pipeline with 6mm misalignments containing embedded cracks needs repairment and replacement,while the pipeline with other defect sizes needs further safety assessment.