PSI - Issue 30

Igor Sannikov et al. / Procedia Structural Integrity 30 (2020) 144–148

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Igor Sannikov et al. / Structural Integrity Procedia 00 (2020) 000–000

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1. Introduction Improving the safe operation of potentially dangerous high-risk facilities remains one of the most important scientific and technical problems of our time. Such systems include main gas pipelines due to the increased risk of technological accidents caused by insufficient operational reliability of the metal and welded joints. The main gas pipelines of the Republic of Sakha (Yakutia) are unique metal-consuming structures that have been laid and operated in the propagation zone of permafrost soils since 1970. The analysis of emergencies on the main gas pipeline of the Republic of Sakha (Yakutia) conducted by Chukhareva et al. (2011), Bolshakov et al. (2007) revealed that the risk of failures caused by degradation processes in the pipe metal during their long-term operation increases. Gumerov et al. (1995) shown that the tensile strength and hardness of pipe steels change insignificantly during operation, the values of yield strength slightly increase, and the relative elongation decreases. Makhutov et al. (2007), Lubensky and Yamnikov (2013) noted that during the long-term period of pipe operation as part of gas pipelines, the mechanical properties of the material correspond to the real characteristics required by the standard. However, as shown by Syromyatnikova (2014), Golikov and Litvintsev (2015) an increase in the service life leads to increase of the ductile-brittle transition temperature, develop the fatigue processed leading to a decrease in the critical size of permissible defects develop, and the risk of stress corrosion cracking. As pointed by Sosnovsky et al. (2003), the determination of mechanisms for changing the physical, mechanical properties and structure-phase states of technical devices during a long-term operation is relevant since it suggests a more reasonable approach to the problem of predicting durability during the designing process, construction, and operation of structures and facilities. The present work is aimed to study the degradation of the mechanical properties of the base metal and welded joints of sections of the main gas pipeline after a long-term operation in the North. Nomenclature YS yield strength TS ultimate tensile strength E elongation a impact strength WM weld metal FL fusion line The mechanical properties of the samples of pipe steels cut from the pipe fragments after long-term operation were investigated. The result was compared with the data indicated in the design documentation. The investigation was performed on the equipment of the shared core facilities of the Federal Research Center of the Yakutsk Scientific Center of the Siberian Branch of the Russian Academy of Sciences (SCF FRC YaSC SB RAS). The main test methods used are certified within the accredited laboratory of destructive and other tests of the IPTPS SB RAS (Accreditation Certificate No. IL / LRI-002860). Test samples were cut from fragments of Ø529 mm pipe with wall thicknesses of 7 and 9 mm from different sections of the Taas-Tumus-Yakutsk gas pipeline (Fig. 1). The Taas-Tumus-Yakutsk main gas pipeline has been in operation for over 50 years. The sample No. 2 consists of pipes of different thicknesses and has a field girth weld. The sample has significant visible deformations. The chemical composition of the steels is determined according to GOST 18895-97 using the Foundry-Master optical emission analyzer. The surface of the samples to be measured is cleaned and brought to a flat state with a grinding machine. The average content of alloying elements is obtained from the results of three measurements of each element. 2. Materials and methods