Study Of In-Service Welding Repair On X70 Pipeline

In-service welding repair technology is widely used in oil and gas pipeline industry with the advantages of economic,efficient,environmental friendly and so on.The defects of the old transportation pipeline such as X70,like corrosion or others caused by a variety of reasons promote the development of in-service welding technology to some extent.However,it is difficult to form a unified standard at present for in-service welding repair technology due to its variability of procedure upon welding.In this respect,after verifying the correctness of finite element method by combining simulation results with experimental data from literature based on ANSYS software,this paper firstly made a further study on the sleeve size and installation position which commonly used in in-service welding repair,and pointed out the shortcomings in referring to GB 50235-2010"industrial metal pipeline engineering construction standards"to determine the size of the sleeve and the installation position,then a new sleeve size and installation position was given;Also by comparing yield stress and Von-Mises stress through radical direction of pipe wall thickness under the first weld bead when welding,a new method of predicting burn-thought was proposed;Finally,according to Yurioka model,the highest hardness of heat affected zone was calculated combining with chemical composition of the pipe and temperature distribution during multi-pass in-service welding,then the risk of hydrogen induced cracking when performing in-service welding repair on oil and gas pipeline was identified.In this paper the uncoupled thermal-mechanical method was adopted in finite element analysis of welding.In solving stress,the second development of ANSYS using APDL to deal with the welding pool was involved.The specific research methods are:selecting different diameters,wall thickness and process conditions to clarify the residual stress affected zones of butt welds and stress concentration range of defects;establishing finite element sleeve repair models under different pipe size and operating conditions to determine the residual stress affected zones of fillet weld after sleeve repair;comparing Von-Mises stress on the outer surface of the pipe under different sleeve wall thickness after sleeve repair to determine the appropriate sleeve wall thickness;combining with a specific finite element sleeve repair model to predict the possibility of burn-though and hydrogen induced cracking which often faced in in-service welding repair.The results show that:1.The residual stress affected zone of pipe butt weld is only related to pipe diameter and wall thickness and has nothing to do with the heat input of weld;The stress concentration area around the pipe defect is obviously varied with pipe diameter,but it is not sensitive to wall thickness,defect size and internal pressure;Residual stress affected zone of fillet weld bead after sleeve repair is less than 100mm;the sleeve wall thickness should be selected between 1to 1.4 times of pipe wall the thickness.2.When conducting in service welding on the pipe which diameter is 273.1mm,thickness is 6mm,inner pressure is 1.5MPa,heat input is 0.6kJ/mm,it turns out to be that the pipe is unlikely to burn though and the remaining effective thickness is 2.6mm by checking Von-Mises stress and yield strength of the pipe below weld bead.3.With the help of ANSYS software,the welding temperature distribution in muti-pass in-service welding is achieved to calculate the cooling time t_8/5.According to the chemical composition of X70 steel,the highest hardness value of heat affected zone was calculated by the Yurioka model,then it was compared with the Vickers hardness 350HV to see whether hydrogen induced cracking occurred.The results show that under normal conditions,the risk of hydrogen induced cracking is relatively small.