Study On Structural Behavior Of In-serving Welding Onto Urban High-pressure Gas Pipelines

During long-term operation of high-pressure gas-pipelines, local corrosion ordamage will occur because of natural or human-made reasons, so gas pipelines needstrengthening and repairing. In-serving welding is a technique that directly weldsonto a gas pipelines in the active operation, and is frequently employed in the repair,modification or extension of gas pipelines. It has significant economic, social benefitsand application prospect for the gas transmission continuity of supply to the customer,modification rapidly, pollution-free. But once a gas pipelines which containshigh-pressure gas in the high speed flow burn-through occur, a gas fire and explosionaccident will happen, and people injuring and property loss will be caused.Consequently, study on in-serving welding by finite element simulation has guidesignificance for controlling the burn-through and residual stresses.In this paper, thermal field, the transient stress field and residual stress field ofurban gas pipelines for Grade Q345B steel have been analyzed by ANSYS. A newapproach to the problem of burn-through has been developed based on the analysiscombining the thermal filed and transient stress field, and the effect of gas pressureand gas velocity on peak temperature and stress distribution has been also discussed.With the heat input of this paper, following is the main conclusions:1) The highest temperature of gas pipelines inner surface is322℃, which islower than the982℃,proposed by BMI.2) The gas pressure and gas velocity have an important impact on the highesttemperature of inner surface. Results show that the highest temperature of innersurface decreases with the raising of gas pressure and gas velocity, respectively.3) The thermal field, the transient stress field and residual stress field have beenstudied with the4MPa gas pressure and20m/s gas velocity in this paper. Resultsshow that gas pipelines is considered relatively safe by combining the thermal filedand transient stress field during in-serving welding, and the highest residual stresscould achieve yield strength, and local region has been undergone plastic deformation4) The gas pressure and gas velocity have an important impact on the transientstress field and residual stress field of inner surface. Results show that the girth stressof inner surface under the weld decreases with the raising of internal gas pressure and gas velocity separately, and while the axial stress increases with the raising of internalgas pressure and gas velocity, respectively. That the compression deformationgenerating near the weld joint on the high temperature and the shrinkage whilecooling are the prime reasons for the transient and residual stress of inner surface.