Currently, the technical status of the linear part of the main gas pipelines is mainly determined on the basis of in-line technical diagnostics data, which, along with defects of pipe walls and circumferential welded joints, enables detection of sections with substandard bending radius. In accordance with PJSC Gazprom regulations, in the next 2 to 3 years (pigging frequency) ~ 68 % of the total quantity of circumferential welded joint defects are subject to identification in Gazprom transgaz Ufa LLC main gas pipelines’ holes. Based on PJSC Gazprom standards, the hazard degree of such defects is assessed and classified by categories (“A”, “B”, and “C”) separately, i. e. one document addresses the hazard of the defect itself, and the other – the hazard of the bending radius. Gazprom transgaz Ufa LLC resources allow for inspection of ~ 30–40 holes annually. Therefore, we needed to develop a methodology which would allow us to comply with regulations, and reduce these figures to practicable limits at the same time.
Engineering basis of the methodology is as follows: if a defect is located in an area where there are no dangerous tension stresses, or the only stresses present are compressive, the breakdown process slows down or stops altogether. Using this approach, we can limit the assessment of circumferential welded joint defects to the areas of dangerous tension stresses, and focus our resources on their elimination in accordance with GOST 6966–66 (state standard) requirements. According to that standard, hazardous defects occur under the effect of tension stresses.
Studies of linearity and altitude of main gas pipelines’ positioning show that during construction, at the stage of achievement of design temperature and pressure, and during multiyear operation, an underground pipeline is mostly affected by compressive stresses forming bending radii. It means that there are conditions which may result in substandard bending radii. This, in turn, means that in such areas, circumferential welded joint defects of “B” and “C” categories (including those which lengths exceed 1/6 of the pipe perimeter) are not considered hazardous. This allows us to postpone their hazard assessment until the next scheduled in-line technical diagnostics campaign.
Implementation of this methodology in the Gazprom transgaz Ufa LLC enabled the company to justify the reduction of the scope of defect identification in holes by ~ 98 %.

R.M. Askarov, DSc in Engineering, Associate Professor, Ufa State Petroleum Technological University (Ufa, Russia), Askarov1943@mail.ru

O.A. Baulin, PhD in Engineering, Ufa State Petroleum Technological University, rector@rusoil.net

A.M. Shammazov, DSc in Engineering, Professor, Ufa State Petroleum Technological University, ashammazov@mail.ru

B.N. Mastobaev, DSc in Engineering, Professor, Ufa State Petroleum Technological University, mastoba@mail.ru

S.V. Kitaev, DSc in Engineering, Professor, Ufa State Petroleum Technological University, svkitaev@mail.ru

I.M. Islamov, PhD in Engineering, Gazprom transgaz Ufa LLC (Ufa, Russia), mildarislamov@mail.ru

A.V. Yarovoy, Gazprom transgaz Ufa LLC, yarovoia@mail.ru

V.V. Gaisin, Engineering and Technical Center, branch of Gazprom transgaz Ufa LLC (Ufa, Russia), gaysin_v@icloud.com