To Assessing the Depth of Inclined Cracks in a Main Gas Pipeline Based on the Results of in-Pipe Magnetic Flaw Detection and Additional Diagnostic Testing

The paper deals with the reason for the discrepancy in the assessment of the depth of inclined stress-corrosion cracks of the main gas pipeline, that were discovered based on the results of magnetic in-pipe flaw detection, eddy current flaw detection, and additional diagnostic testing (ADT). Based on the current model of the crack, it was established that the amplitude of the magnetic field of an inclined crack does not exceed the amplitude of an equivalent straight crack with the corresponding effective depth, which is the reason for the appearance of a methodological error in estimating the depth of inclined stress-corrosion cracks of main gas pipeline during magnetic in-pipe flaw detection. In order to eliminate methodological errors and correct estimates of the depths of inclined stress-corrosion cracks in main gas pipeline, a correction factor is proposed, depending on the angle of inclination of the crack. The results of correction of the depths of real stress-corrosion cracks in main gas pipeline obtained from the results of magnetic in-pipe flaw detection are presented. Based on a mathematical model of an inclined crack, based on the magnetic moments of eddy currents in the metal, it is established that the amplitude of the signal of an applied eddy-current transducer (ECT) above an inclined crack exceeds the magnitude of the ECT signal above an equivalent straight crack with the corresponding effective depth. It has been shown that this leads to overestimation of the effective depths of inclined stress-corrosion cracks in main gas pipeline during ADT using eddy current flaw detectors.