The pipes of gas trunk line of more than 45 years lifetime have been investigated. The corrosion-active components in the transported gas in the initial period of the gas line operation caused structural defects — stratifications — in the pipe wall. The goal of investigation is to estimate operability of defective pipes based on a set of lab and bench tests.

At the gas line construction the foreign production electric-welded straight-seam pipes made of steel Х53 and Х56 were used. Investigation of chemical composition and mechanical properties of the pipe steel did not detect any significant deviations of the specified characteristics from the normalized values. The content of non-metallic inclusions is high but acceptable for 1020 diameter pipes.

On the basis of analysis of modes of gas transportation through the gas line for the last 10 years applying the special software, the model of its operation was established. The alternating combinations of amplitude and pulsating loading cycles were considered. In order to perform life tests, a research bench containing pipe fragments with extended zones of internal stratification was built. The bench was used for pipe cyclic tests by internal pressure to model 20 year operation of a given gas line.  It is established that plastic deformation of the pipe occurs when the design pressure is exceeded twice in the gas trunk line. The locations of plastic deformation are located between the stratifications and welding seams. This causes the need to repair the pipes where stratifications are located next to the welding seams. The additional factor of stricter requirements to pipe rejection can be the combined defect types — such as «stratification + an internal crack». Based on the results of tests, the stratifications up to 1,5 m² area and (or) up to 3,5 m length as a separate defect type do not significantly affect the pipe operability.