Burn-through Failure Mechanism And Safety Evaluation Of In-service Welding Of Long-distance Pipeline

In-service welding is a maintenance and emergency repairing technique for operating pipelines which features safe, environment protection, economic, high efficiency and suitable for pipeline proper maintenance, rehabilitation, rush to repair for sudden accident (such as emergency repair under pressure, replace the corroded pipe sections, fix additional devices and off-take pipeline etc). The pipeline medium can be crude oil, product oil, chemicals, natural gas etc. This in-service welding repair can maintain continuous operation of pipelines and has good economic return and extensive prospect of application.There are two primary concerns with welding onto in-service pipelines. The first is for burn-through, where the welding arc causes the pipe wall to be penetrated, allowing the contents to escape. The second concern is for the integrity of the pipeline following repair. Preventing burn through is the first problems need to be solved during in-service welding of pipeline. In this dissertation, based on the actual operating condition of in-service welding onto pipelines, an in-service welding burn-through test device, which was designed and established by the design method of bead-on-plate welding on chamber, was used to conduct the in-service welding experimental study of burn through onto X70 pipelines. The temperature distributions and the size of failure's molten pool were measured. The morphology and microstructure of the burn-through joints were observed by optical microscope and scanning electron microscopy. The mechanism of bulge and burn through failure during in-service welding and its affecting parameters were studied carefully. The heat transfer process and characteristic between in-service welding joint and pipeline contents were analyzed. Heat transfer formula adopted by former researchers was modified and new formula was deduced based on considering the thermophysical parameters of pipeline steel and pipeline contents varying with temperature. A three-dimension finite element (FE) model, which is integrated into commercial finite element analysis (FEA) software SYSWELD by means of user subroutines, was established to simulate the in-service welding of X70 pipeline steel and predict temperature distributions. The calculated transient'volume-scale defect'in the pipe wall representing the loss of strength during in-service welding, which can be calculated from the results of the simulated temperature and the yield strength of the material. The maximum allowable operating pressure (MAOP) of a pipe with a volume-scale defect in its wall can be used to evaluate the safe of in-service welding of long-distance pipeline.The experimental results show that the pipeline contents create a large heat loss through the pipe wall, resulting in accelerated cooling of the weld. The size of heat affected-zone (HAZ) and high temperature region is relative small during in-service welding. The parameters of welding thermal cycle such as t_H, t_8/5 and t₁₀₀ for in-service welding are smaller than traditional welding. The main failure modes of in-service welding are bulge deformation and burn through. The welding deformation of in-service welding joint is external convex for the effect of pipeline content's pressure. The typical appearance of burn through is only a small hole in the high temperature region of molten pool.The thermal stress causing by welding temperature, the load stress causing by the content's pressure and the material critical failure stress have an effect on bulge deformation during in-service welding. If the former two types of stress are bigger than the material critical failure stress, the welding bulge deformations will occur. Burn through depends on the pipe's internal pressure and the localized high temperature, which creates a local reduction of pipe-wall strength in the region of the welding pool. The pipe wall may burst if the effective strength of the wall in the region of the welding pool can't carry the pipe's internal pressure during the welding process. The reduction in strength in the weld can be represented by an effective reduction in thickness of the pipe wall, at its original strength. The reduction of strength was changed into a local thinning region in pipe-wall. The in-service welding pipe joint can be viewed as a pipe with local volume-scale defect. The MAOP of a pipe with a volume-scale defect in its wall can be calculated by using a procedure specified in the Standard DNV-RP-F101. Then the safety of in-service welding can be predicted. This approach only requires a thermal field calculation which can improve the efficiency. In addition, this technique accounts for internal pressure which can enhance the precision of prediction. The evaluation system of the safety of in-service welding onto long-distance pipeline was design on VB6.0. The system can call SYSWELD soft to calculate the temperature during in-service welding, and Matlab soft to calculate its MAOP. Then the safety of in-service welding under specific situation can be predicted.The MAOP increases with the increase of pipe internal pressure at 0.1MPa～5MPa. The MAOP decreases with the increase of heat input. While carrying a safety of in-service welding on pipeline, relative higher pipe's internal pressure and smaller heat input should be taken for decreasing the effect on pipeline. Based on orthogonal experimental design, pipe wall thickness, heat input and internal pressure have much more great effect on the safety of in-service welding, while the influence of pipe diameter is relative small.