Improvement of the toolset for diagnosing underground pipelines of oil and gas enterprises considering changes in internal working pressure

A new criterion of strength and a set of informative parameters have been devised for modelling the stressed-strained state (SSS) of the underground metal pipeline (UMP) taking into consideration the system of defects of the cavity type, atop of which there is a crack. We have inspected the surface of pipes made from structural carbon steel 20, which are exposed to the internal hydrostatic pressure. It has been proposed that the strength criterion of a pipe's metal, which is in contact with the soil electrolyte, should take into consideration the stages of elastic and plastic deformation. The strength criterion has been supplemented with ratios for a corrosion current (the Kaesche type) and internal pressure, which acts on a cylindrical pipe, taking into consideration the inelastic energy characteristic of the surface layer. For a pipeline, in a neutral soil environment, we have measured polarization potentials and corrosion currents using the PPM (polarization capacity meter) and CCM (contactless current meter) equipment. Results measure respective defects of the cavity type (pitting), formed at the outer surface of an underground pipeline. For five variants of internal pressure pi=5.5¸7.5MPa, the CCM and PPM devices determined currents and voltages for characteristic surface defects and, based on them, we have estimated the effective time it takes for a crack to reach critical depth (a pipe resource), as well as the reliability parameter b (a safety characteristic). By comparing results from experimental study and appropriate calculations it has been established that the relative changes in the rate of corrosion Vcoris 2.8 times, and, accordingly, the UMP resource parameter tRis 3.1times, larger, while the reliability parameter b is 6.9 times less, than the relative changes in internal pressure changes pT. Based on analysis of the parameter tR, which characterizes the UMP resource, it has been found that this dependence tRon internal pressure pTis nonlinear and tends to saturation. The specified information is important for improving the methods of control over UMP at oil and gas enterprises, specifically, procedures for correct estimation of anode current density in metal defects at the outer surface of an underground pipeline, taking into consideration changes in internal hydrostatic pressure