PSI - Issue 16

Ivan Shtoyko et al. / Procedia Structural Integrity 16 (2019) 148–152 Ivan Shtoyko, Jesús Toribio, Viktor Kharin, Myroslava Hredil / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Gas main pipelines have been operating for a long time under complicated service conditions. The pipes in service are subjected to combined action of service stresses and impacts of the environment. The problem of ensuring the safe operation of gas pipelines becomes increasingly important over time. Based on the results of gas pipeline safety assessment made by Student et al. (2018), the stress corrosion cracking is the main cause of pipeline accidents. Cracks mostly initiate from corrosion pits on the pipe external surface in areas where the protecting cover is damaged, whereas the pipe is subjected to prolonged combined action of operational stresses caused by gas internal pressure. Besides, Tsyrul'nyk et al. (2008) and Hredil et al. (2010) showed that some components of natural gas should be considered as corrosive agents. This leads to the phenomenon known as operational degradation. It manifests itself first of all in sharp decrease of the steel resistance to brittle fracture, as it was shown by Tsyrul'nyk et al. (2004), Gredil (2008), Maruschak et al. (2014), Meshkov et al. (2015). It also worsens other important steel parameters. For instance, Zvirko (2017) revealed electrochemical activation of steel, whereas Andreikiv et al. (2012), Kryzhanivs’kyi et al. (2015) proved crack growth facilitation. Comprehensive study of the degradation and its experimental estimation showed that the fracture toughness defined by J-integral is more sensitive to changes in material state caused by operation than the impact toughness of Charpy specimens does. Besides, corrosive environment influence, and subsequent metal hydrogenation were reported to increase steel sensitivity to in-service degradation (Hredil et al. (2012), Andreikiv et al. (2016), Tsyrul’nyk et al. (2018), Zvirko et al. (2018)). Therefore, although a crack starts from the external pipe wall as a result of soil corrosion in the place of damaged protection, after a certain period of steel operation it propagates in the material with mechanical properties essentially worsened comparing to the non-operated steel. Many researches have been devoted to the problem of residual lifetime determination of the gas pipelines with cracks. In these researches, possible changes of metal properties during operation time were not considered. However, the problem of operational degradation should not be neglected. For more accurate assessment of the residual lifetime of gas pipelines, it is important to introduce certain parameters in models to reflect the changes in material’s state during service and, in this way, to take into account negative factors (in -service stresses, corrosion action) influencing the pipeline serviceability. Accordingly, the aim of the present research is to develop the model for residual lifetime assessment of gas pipelines considering the effect of soil corrosion and material degradation. The object of research is the Х52 steel pipe having the wall thickness h = 12 mm and the pipe diameter 2 r = 1420 mm, which was subjected to constant pressure of natural gas p = 8 MPa and is weakened at its external surface by the semi-elliptical crack with semi axes 0 , 0 (Fig. 1). The chemical composit ion of the Х52 steel and its mechanical properties are presented in Tables 1 and 2. It is assumed that the soil environment fills the crack and causes its corrosion-mechanical propagation. As it has been mentioned above, long-term operation of the pipe leads to material degradation, i.e. its resistance to the fatigue crack growth decreases. The task consists in the determination of the time t = t * necessary for crack propagation through the pipe wall until b = h , when its decompression starts. 2. Materials and methods

Figure 1. Loading scheme of the pipe with an external crack under soil environment action.