An experimental investigation of the nature of longitudinal cracks in oil and gas transmission pipelines

Oil and gas pipeline maintenance and monitoring are of utmost importance. Therefore, being aware of the pipeline conditions, predicting their failure, planning to manage emergencies and evaluating their conditions are essential. In this study, Magnetic Particle test inspection was performed on a 52-year-old, 6 inch steel pipeline, which was primarily used as an oil transmission pipeline but was repurposed to a gas transmission pipeline later. Longitudinal cracks were found on the pipes during the inspection. Several tests were conducted aiming to clarify the nature of these cracks. The tests included chemical analysis, tensile test, impact test, hardness measurement to determine the type of steel, soil tests to specify the elements present in the soil and to evaluate corrosion conditions of the soil, SEM imaging, optical microscopy, and Energy-Dispersive X-ray Spectroscopy (EDS). The steel was found to be of X46 grade and the tests did not suggest the presence of aerobic as well as anaerobic bacteria in the environment. Moreover, the EDS results did not show any signs of corrosion in the samples. The depth of cracks was measured to be 80 mm. No evidence of hydrogen induced cracking (HIC) and stress corrosion cracking (SCC) which are the main cracking mechanisms of steel transmission pipelines was appeared in the present study. Overall, it is safe to conclude that the longitudinal cracks were formed during fabrication. (C) 2018 Elsevier B.V. All rights reserved.