Study On In-service Welding Of Long-distance Oil Pipeline

At present, China has more than twenty thousand kilometer long-distance oilpipeline. During its operation, due to the occurrences of pipeline leakage accidentsresulting from corrosion, oil stolen by drilling and pipeline branches addition formodification etc, long-distance oil pipeline often requires in-service welding in itsmaintenance. Meanwhile, traditional welding requires pipeline shutdown, decreasepressure, clean pipeline,which will cause a lot of economical loss to oil and gascompanies and make environmental pollution, and influence the company’sproduction. The implementation of in-service welding repair and emergencymaintenance in complex long-distance oil pipelines and the guarantee of safe, reliableand qualified repair is very worth studying.In the present paper, the common-used welding conditions in pipe welding repairand emergency maintenance are systematically researched and analyzed at first. Afterthat, great experimental tests and welding process evaluation tests are made, and SCCare conducted on the basis of numerous experimental analysis, which makefoundation for the parameter set and optimization of welding process, and burningthrough wall and hydrogen cracks.The main research contents and conclusions are as below:1) The relations among welding heat input, medium, medium velocity andpenetration depth are obtained both quantitatively and qualitatively through the fieldpenetration experiment and thermal simulation analysis; to find out, quantitatively andqualitatively, the impacts of heat input, wall thickness, medium, medium velocity onthe back temperature of pipe welding, through the back temperature experiment andthermal simulation analysis; to quantitatively and qualitatively identify the elementsthat influence the cooling speed of welding joints through the thermal simulationanalysis.2) The simulation analysis shows that even in the pipeline shutdown (mediumvelocity is zero), the maximum flow layer thickness of the internal pipeline of whichthe medium temperature influenced by the welding temperature field, is less than7mm. This proves that the application of pipeline flow adjusting device with thecasing pipe in welding process test is reasonable. 3) This paper also quantitatively studies on the control methods for hydrogeninduced cracking and burning through wall, the two big problems affected the pipelinein-service welding quality. It finds out from a large number of tests and simulationdata analysis that the preheating temperature is not the main factor that influences thehydrogen cracks and burning through wall, instead, welding parameters (includingwelding current, voltage and welding speed) and the welding sequence are the twomost important elements.