Analysis of the results of the magnetic inspection of circumferential welded joints with allowable defects during pneumatic tests of Bovanenkovo-Ukhta gas pipeline
The paper presents the results of studies of possible structural changes detection in the metal of circumferential welded joints of large diameter pipes used in gas pipelines construction by applying a magnetic method of non-destructive testing. The conducted studies included computer simulation of the internal pressure loading process of a gas pipeline section with a circumferential welded joint, laboratory research of the samples made like two welded steel plates as well as route studies, which consist in measuring the coercive force on the surface of pipes and circumferential welded joints with allowable defects, carried out during pneumatic tests of Bovanenkovo-Ukhta gas pipeline section. Based on the computer simulation results, the possibility of structural changes in plastically deformed metal areas at the boundaries of allowable defects, which can be detected by the magnetic method, was established. Further on, in the course of laboratory research, the general nature of the change in the of coercive force level was established when the coercive force sensor was sequentially moved along the line crossing the welded joint. The field studies resulted in revealing some specific features of the coercive force change, measured before and after the pneumatic tests. In the absence of defects, the coercive force, measured over the circumferential welded joint, decreased by 3,0…4,5 % after the pneumatic testing. In case of a single non-critical defect occurrence, the coercive force after the pneumatic tests increased up to 2,5 %. In the presence of a group of non-critical defects, the coercive force increase made 1,5 %.