PSI - Issue 59

Roman Hrabovskyy et al. / Procedia Structural Integrity 59 (2024) 112–119 Roman Hrabovskyy et al. / Structural Integrity Procedia 00 (2024) 000 – 000

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Keywords: gas pipeline; accident; low-alloyed steel; fracture toughness; fracture pressure; hydrogen concentration; environmental safety.

1. Introduction The safe operation of main gas pipelines, more than 90% of which are underground, first depends on the durability of the corrosion-resistant coatings, which during long-term operation lose elasticity, becomes brittle and usually delaminate from the pipe surface. At the same time, because of the groundwater influence, two-dimensional defects (pitting, corrosion pitts, microcracks) appear on the gas pipe surface (Kryzhanivs’kyi et al . (2013); Panasyuk et al. (2014); Dmytrakh et al. (2021); Syrotyuk and Dmytrakh (2014, 2015)). The intensity of these defects initiation and development depends on the stressed state of the metal of the gas pipe, the metal electrode potential and the pH of the environment. It should be emphasized that the action of asymmetric cyclic load significantly accelerates the development of crack-like operational defects along the gas pipe thickness (Kryzhanivs’kyi et al. (2013) ; Dmytrakh et al. (2019, 2023)), which leads to emergencies. The gas release due to the formation of a fistula with a diameter of 8 – 25 mm in the pipe wall before the elimination of leakage, occur in 90% of cases, when the gas pipeline is depressurized. A particular danger for the environment causes the gas pipeline destruction with the gas inflammation, which accounts for almost 10% of the total number of accidents (Kryzhanivs’kyi et al. (2013)) . As a result of the emergency rupture of the gas pipeline, the following dangerous influences on the environment occur:  Flying-off of fragments and splinters of the pipeline;  Air-shock wave during the transformation of compressed gas energy;  Combustion with a thermal effect (under the discharge inflammation). The estimation of the conditions of the pipe failure of main gas pipelines with external axial two-dimensional defects is relevant in this regard. 2. Research Materials and Methods Using the criteria (approaches) of fracture mechanics, the fracture conditions of pipes made of 17H1S (1220×12.0 mm) and 10H2BТ (1420×15.7 mm, steels were studied. These steels were under long-term operation with operating pressure p max =5.0 MPa for 45 years and operating pressure p max =7.4 MPa for 42 years, respectively. The chemical composition of 17H1S steel is as follows (mass%): up to 0.17 C; up to 0.87 Si; up to 1.39 Mn; up to 0.19 Ni; up to 0.034 S; up to 0.031 P; up to 0.29 Cr; up to 0.12 V; up to 0.011 N; 0.02 – 0.05 Nb; 0.07 – 0.09 Ti; up to 0.31 Cu; up to 0.08 As, Fe balance. The chemical composition of 10H2BТ steel is as follows (mass%): up to 0.12 С; up to 0 .38 Si; up to 1.64 Mn; up to 0.22 Ni; up to 0.035 S; up to 0.037 P; up to 0.19 Cr; up to 0.12 V; up to 0.012 N; 0.02 – 0.05 Nb; 0.07 – 0.09 Ti; up to 0.27 Cu; up to 0.08 As, Fe balance.

2 с

а

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Fig. 1. Schematic representation of an external axial semi-elliptical crack-like defect under the internal pressure.

At the same time, axial two-dimensional (crack-like) defects of the two most typical semi-elliptical shapes with a given ratio of semi-axes ( a / c ) were considered on the outer surface of the investigated pipes. A calculation scheme (Fig. 1) was