Research On Metal Magnetic Memory Signal Characteristics Of Welds On Long-distance Oil And Gas Pipeline

With the structural adjustment and the increasing demand for oil as well as natural gas,projects for constructing long pipeline in China are increasing day by day.In the process of pipeline welding,it is inevitable that there will be weld defects,among which the most dangerous defect are weld crack.As the internal pressure of pipeline is high during operations,cracks can be easily produced,which can lead to serious explosion and casualties.Therefore,it is a must to detect the weld to ensure the safety of pipeline.The Metal Magnetic Memory(MMM)technology can detect stress effectively,and has been widely applied because of its characteristics of non-contact detection,high efficiency and no excitation structure.It is of great significance in reducing the cost and improving the flexibility of pipeline testing.The MMM technology is very sensitive to the change of stress.Through detecting stress concentration,we can determine the stress damage or micro-crack on the weld,which has good potential for further application in the field of weld detection of long-distance oil and gas pipeline.Based on the J-A theory,this paper establishes a numerical model for determining the relationship between stress and magnetization intensity.The residual stress of pipe weld and weld crack is calculated by Finite Element Method(FEM).Next,a 3D model of weld and weld crack is built,with which this paper analyze the effect of weld thickness and width on MMM signal,study the influence of crack width and depth on MMM signal,as well as reveal the effect of crack and internal pressure on the MMM signal of weld.A systematic experiment is carried out to verify the FEM results.The results show that the MMM signal of crack at weld is the combination of the weld signal and the crack signal.The axial component of the MMM signal of crack at weld has a peak value,and the radial component has two sinusoidal fluctuations.The inner sinusoidal fluctuation is caused by the crack.By comparing the MMM signal characteristics of weld and crack at weld,it is clear to know whether there is a crack on the weld.The thickness and width of the weld have a certain influence on the MMM signal of the weld,and the MMM signal characteristics of weld increases with the increase of weld width and thickness.The width,depth and position of the crack on the weld have a certain influence on the MMM signal of crack at weld,and the MMM signal characteristics of crack at weld increases with the increase of crack width and depth,which is corresponding to the position of crack on the weld.The crack on weld does not affect the amplitude of the MMM signal of weld.Both the MMM signal characteristics of weld and crack increase with the internal pressure of pipeline.Moreover,the MMM signal of crack is much more sensitive to internal pressure than that of weld.