Considering The Influence Of Welding Residual Stresses Containing Cracks Pressure Pipe Safety Assessment And Sleeve Reinforcement

The pressure pipeline,which carries oil and natural gas,is connected by welding pipe segments to form pipelines.However,the welding area is susceptible to defects,resulting in residual stress up to yield strength.Therefore,national standards consider the influence of welding residual stress in safety assessments of pipes with defects.However,there are differences in calculation methods and safety assessment provisions between domestic and foreign standards.When defects affect pipeline service or evaluation results are unacceptable,steel epoxy sleeves can be used for reinforcement.Currently,there is no reinforcement theory related to steel epoxy sleeves.This paper focuses on researching high-grade X80 pipelines and presents the main findings as follows:(1)A numerical simulation was performed to evaluate welding residual stress in an X80 pipeline,with the residual stress distribution determined.The girth weld residual stress was measured using a blind hole method and compared against simulation results.The numerical analysis encompassed a comparison of residual stress calculation models from various specifications,leading to the proposal of a model suitable for X80 pipelines of higher steel grade.The simulated values were found to align well with the measured values,with circumferential residual stress exerting greater impact upon the pipeline than lateral stress.Differences were observed between the simulated and actual circumferential residual stress distributions along the pipe wall thickness,while the simulation results closely approximated the British BS7910 and oil and gas SY/T17 specifications regarding transverse stress.However,differences arose relative to the GB/T19 specification.(2)With the help of the FRANC3D software,fatigue crack growth simulation was performed on surface cracks of X80 pipelines,and the influence of welding residual stress on fatigue life was analyzed.The minimum crack size that leads to crack propagation in pipes with residual stress and the maximum crack size that leads to pipe fracture were studied.For different crack sizes,crack positions(inner/outer surface),and crack types(axial/radial cracks),the stress intensity factors calculated by different codes were compared,and the differences among various codes in the safety assessment of pipelines containing cracks were further analyzed.It was found that the maximum residual tensile stress could reduce the life of the pipe by 43%.The minimum crack size for axial cracks on the outer surface of the pipe and the maximum crack size for radial cracks on the inner surface were found to induce crack propagation.During the pipeline safety assessment,it is recommended to use the British BS7910 standard to evaluate axial and radial cracks on the outer surface of the pipeline and axial cracks on the inner surface,while the GB/T19 standard is recommended for evaluating radial cracks on the inner surface.(3)For defective pipelines,steel epoxy sleeves can be used for repair.Based on the material mechanics and elastic mechanics,and considering the stress characteristics of the repaired section of the pipeline after reinforced with steel epoxy sleeves under internal pressure and bending moment,the calculation formulas for circumferential stress,bending stress,and shear stress of the pipeline-colloid-sleeve under different forces were established and numerically verified.Based on these formulas,the influence of parameters such as the thickness of the steel sleeve and the elastic modulus of the colloid on the reinforcement effect of the steel epoxy sleeve was explored.The theoretical values of each part of the reinforced section were found to have a maximum error of not more than 5% compared to the simulation values,indicating good agreement.The thickness of the pipeline wall and the thickness of the steel sleeve have a significant impact on the reinforcement effect,while the thickness of the colloid and the elastic modulus have a minor impact.It is suggested that the elastic modulus of the colloid in the steel epoxy sleeve should be 2.5GPa,and the thickness should not be greater than 20 mm,the thickness of the steel sleeve should not exceed 50 mm.