Magnetic Flux Leakage Detection And Simulation Of Weld Defects In Oil And Gas Pipelines

Pipeline transportation is one of the five traditional transportation modes,pipeline has a very widely application in the field of oil and natural gas.Although pipeline transportation can create huge economic benefits.At the same time,pipeline leakage,explosion and other accidents also threat life and property safety.Therefore,in order to ensure the safety of transportation,Pipelines should be inspected regularly.In recent years,pipeline nondestructive testing technology has been developed rapidly.Magnetic flux leakage internal detection is one of the effective methods of pipeline nondestructive examination.The method can judge locations of defects and pipeline condition accurately.However,the method is unsuitable for quantitative analysis of weld defects.In order to ensure the safety of pipeline transportation,evaluation and identification of weld defects should be lucubrated.In this article,the magnetic flux leakage characteristics of pipeline defects and pipeline girth weld defects were analyzed using finite element method combined with the pull test based on axial magnetic flux leakage principle and COMSOL software.By exploring the effect of pipeline defects,weld reinforcement and weld defect on leakage magnetic field,a method for stripping magnetic leakage signals of weld defects was obtained.The results show:The results of pull test show: Incomplete-penetration weld,defects near the weld,pipeline defects can be detected.With the increase of defects width,magnetic leakage signal becomes stronger.Most of fusion line defects can be detected.For weld center,only the larger defects can be detected.The sequence from strong to weak magnetic leakage signals of defects at different positions:near the weld > Fusion line >weld center.Magnetic leakage signal analysis of pipeline defects show: the results of pull test are consistent with simulation results.The simulation model is accurate and reliable.According to the curve variation characteristics,with the increase of defect axial length,peak-to-peak value of radial component of circumferential defect flux density can be divided into two stages: First,there is a positive correlation between peak-to-peak value of radial component and the axial length of defects.Then,the growth rate of peak-to-peak value is lower when the defect axial length greater than1 mm.With the increase of defect axial length,peak-to-valley value of axial component of circumferential defect flux density can be divided into three stages:First,there is a positive correlation between peak-to-valley value of radial component and the axial length of defects.Then,the growth rate of peak-to-valley value is lower when the defect axial length between 1mm and 7mm.Peak-to-valley value starts to decrease when the defect axial length between 7mm and 13 mm.Moreover,the valley of the magnetic flux density axial component distribution curves is replaced by a new peak.For axial defects,peak-to-peak distance of radial component is equal to the axial length of defect approximately.For axial defects,The distance between the peaks is approximately equal to the axial dimension of the defect.Magnetic leakage signal analysis of pipeline girth weld defects show: The leakage magnetic field distribution of pipeline girth weld appears thickening-type characteristic when there is no defect in the weld center.The radial component peak-to peak value and axial component peal-to-valley value of magnetic flux density increase linearly.With the increase of pipeline thickness,magnetic leakage signal becomes weaker.Peak-to-peak value of radial component declines at the same weld reinforcement.Magnetic flux leakage signals show the magnetic field distribution characteristics of the weld and ellipsoid defect,that is,the superposition of the thickening by weld and the thinning by defect.Moreover,with the increase of weld reinforcements,the thinning type signal characteristics of the weld defect become weaker.Based on simulation and theoretical analysis,this research propose a method of processing magnetic leakage signal.These curves can be analyzed by weld defect curves minus weld curves.The method can remove the effect of weld reinforcement on magnetic leakage signal effectively.The sequence from strong to weak magnetic leakage signals of defects at different positions:near the weld > Fusion line >weld center.This means that weld defect is the most difficult to detect.The result is consistent with pull test.The thesis has 63 pictures,19 tables,99 references.