PSI - Issue 36

V. Zapukhlyak et al. / Procedia Structural Integrity 36 (2022) 378–385 V. Zapukhlyak, Yu. Melnychenko , І . Оkipnyi et al./ Structural Integrity Procedia 00 (2021) 000 – 000

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The real state of gas pipeli nes is characterized by 12 impact factors affecting the pipelines’ technical condition, and as a result, their failure rate. Each of them has a certain proportion of impact on the intensity in percentage correlation, namely: 1) external mechanical effects (minimal depth of the pipeline burial; level of anthropogenic stress; security level of ground equipment; protected zone state; patrolling frequency) – 12 percent; 2) underground corrosion (corrosion activity of the soil; product temperature; available metal structures and power systems near the gas pipeline; type and state of insulation coating; available stations of electrochemical protection and their quality; ILI (in-line inspection) carrying out) – 12 percent; 3) stress-corrosion – 10 percent; 4) atmosphere corrosion – (corrosion of ground swages; environment corrosion activity; insulation state) – 2 percent; 5) inner corrosion and erosion (corrosion and erosion activity of the product; the gas pipeline inner surface protection against corrosion) – 3 percent; 6) production factors (pressure and temperature of the product; the pipes production technology and steel grade; the pipes service life) – 10 percent; 7) quality of construction-assembly works (section category according to construction works difficulty; quality performed control; welded joints quality control; material storage quality) – 18 percent; 8) main gas pipelines testing – 3 percent; 9) structural-technological factors (the wall real thickness; metal fatigue; the possibility of a hydraulic shock; adequate choice of materials and equipment) – 7 percent; 10) natural factors and impacts (possibility of soil movement; soil composition; available overhead line hardware and other ground facilities; preventive measures) – 7 percent; 11) technical maintenance level (LMS (line maintenance service) technical fit-out; maintenance inspection and PPM frequency; staff qualification) – 7 percent; 12) failures and accidents have taken place previously – 9 percent. Apparently, at the late stage of gas pipelines operation, it is not necessary to use all 12 factors to characterize the specified section of the gas pipeline in the case when it is going to be taken out of service. Since when we compare two gas pipelines, and only of them will continue operating in the system and will accomplish the main purpose – gas transportation, then such factors as: “technical maintenance level” or “production factors” will be of the same impact value on the pipeline integrity. On the contrary, such factors as: “external mechanical impact”, “corrosion”, “natural factors and effects” must be taken into account for sure. Along with the above-mentioned comparison, the pipeline material and insulation state examination is being conducted (defects of the pipe walls and welded joints caused by corrosion-erosion and stress corrosion impact are detected, mechanical properties of the long-term used steel are determined, the insulation state is checked). Moreover, the external natural factors influence on the insulation, and the pipe metal state is being determined. In particular, it is necessary to determine the soil environment’s influence on certain properties of gas pipelines and rank them according to the degree of impact. Besides, the pipeline position in the soil is very important, namely the depth of its burial. As far as we know, according to the data of gas pipelines examination, almost on each leg between the compressor stations there are some sections where the gas pipelines buried in the ground do not meet the requirements of the burial specified in the design project, namely: changes in the stress-strain state, increase of impact of anthropogenous factors, increase of natural factors negative influence (Yavorskyy et al., (2017), Vilkys et al, (2018)). After that, the obtained results have to be analyzed at the first stage, and after their consolidation into a single calculation complex, the state of certain sections of the gas pipeline has to be determined for their further sorting out. To provide the possibility of complex comparison of several line sections of the gas pipeline, a coefficient called the defect severity and denoted by DB has been introduced whose value is calculated by the formula