PSI - Issue 26

Olha Zvirko et al. / Procedia Structural Integrity 26 (2020) 219–224 Zvirko et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 3. Relative changes in hardness of the 17H1S steel (API 5L X52 strength grade) after 28-40 years of operation.

a

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Fig. 4. Relative changes in electrochemical characteristics of pipeline steels with different strength caused by in-service degradation: (a) corrosion current density and (b) polarisation resistance.

parameters can be considered as degradation indicators for evaluation of degradation degree of pipeline steels under operation. For high-strength steels (API 5L X70 strength grade and higher) usage of impact toughness can be limited due to possible increasing fracture energy caused by dissipated damaging and delamination. Nevertheless, impact toughness is one of the most important characteristic of pipeline steels, it is regulated by normative documents (DSTU EN ISO 3183:2017 / API 5L, and others). Corrosion current density and polarisation resistance, indicated corrosion degradation of steels, are the most sensitive to in-service degradation among electrochemical characteristics, as it was reported by Zvirko (2017) and Zvirko et al. (2019b). They can be used as degradation indicators for pipeline steel during long-term operation. A usage of polarisation resistance is enough prospective for evaluation of degradation degree of steels because of its determination is enough easy in the field. In natural gas transmission system, great attention is being paid to safety, reliability and maintainability of pipelines. In this respect, data on current state of operating pipelines have become of increased importance. In-bulk degradation of material is usually evidenced by destructive mechanical testing of specimens cut out of pipe. However, evaluation of state of operated pipeline steel non-destructively is very important. The recent trend is quantifying steel degradation by usage of non-destructive methods based on indentation tests (Bolzon et al. (2015, 2018) and Bolzon and Zvirko (2017)). It makes possible to predict, first of all, operational changes in yield limit and strength characteristics. It is based on correlation between the evolution of basic mechanical 4. Evaluation of operated pipeline steel state considering degradation stage