PSI - Issue 5

Jan Kec et al. / Procedia Structural Integrity 5 (2017) 340–346

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Jan Kec, Ivo Cerny / Structural Integrity Procedia 00 (2017) 000 – 000

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transport of the natural gas from its deposits to customers has shown to be very effective and its extent increases permanently. An extensive pipeline network passes through the european continent. Very important factor influencing the construction and operation of long-distance pipelines is especially their reliability. In spite of threefold increase of length of the gas pipeline network in Europe between 1970 and 2007, the accident frequency diminished five times. It results from a combination of a right construction, materials used and working procedures. Nevertheless, the destructions of gas pipelines appear from time to time (Richards (2013), Fazzini et al. (2007)). The most frequent cause of the pipeline failure (in Europe) is an external mechanical damage (Allouti et al. (2014)), even of a less important type (corrosion defect, gouges or dents). Dent in the pipeline is considered as a permanent plastic deformation of circular cross-section. It causes an important change of the pipe cross-section – Fig. 1. The dent depth is defined as the difference between nominal diameter and minimum diameter of an insribed circle. According to Cosham et al. (2004), five types of dents can be considered: Smooth dent dent causing smooth changes of curvature of the wall thickness Kinded dent dent causing total change of circular cross-section of the pipe Plain dent smooth dent that does not cause the change of the wall thickness and is crack free Unconstrained dent dent having a possibility of movement and not blocked by soil or stones Constrained dent dent blocked by soil or stones

Fig. 1 The dimension of dent In case of more important defects (cracks in welds, sharp crack or combination of a crack and a dents), the methods of fracture mechanics are applied. The destruction of the gas pipeline can result even from the soil movement during the flood. From the reasons mentioned above, the tests verifying the actual state of the operated pipeline are carried out. The obtained data are used for the assessment of its operability and as criteria for its construction. The most common method applied during gas pipeline operation is the cyclic loading of pipeline wall by a gas pressure. In a lot of works, the evaluation of fatigue strength is carried out with the help of standardized specimens. However, the values of fatigue strength measured in standardized specimens are higher than those of full scale specimens. The use of full scale specimens makes the consideration of the effect of residual stresses and dimensions (constrain) possible and gives a more objective view of actual situation (Fang et al. 2014). Cunha et al. (2009) propose the method of assessment of fatigue life-time of dented pipelines based on empirical formulation and numerical simulations as well. In this work, a new algorithm for the evaluation of fatigue life-time of plain dents outside the weld and heat affected zone is described. Fatigue tests were carried out by internal pressure on small scale dented steel pipe models. The tests were performed on pipes from AISI 1020 low carbon steel of external diameter Dn = 73 mm and wall thickness t = 3,06 mm. It follows from the results that, from the point of view of stress concentration and, therefore, from that of the fatigue behavour, the longitudinal dents are the most critical points of the tube. Longitudinal dents are those ones, the length of which in the longitudinal direction is at least two times greater than that in the circumferential direction. Further, it was found that the fatigue strength of the pipeline decreases with the increasing dent depth. Allouti et al. (2008) investigated the effect of the dent depth on critical pressure of pipeline. Pressure tests were carried out by internal pressure on five small scale dented pipes. Dent depths were 25 mm, 14 mm, 11,8 mm a two times 8,9 mm. The pipes were made from A37 low-carbon steel of external diameter Dn = 89 mm and wall thickness t = 3,2 mm. The difference in burst pressure values was only 2 % and the lowest pressure value at the destruction was reached on the dent of 8,9 mm depth. It follows from the results that, unlike the fracture strength, the burst pressure does not depend on the dent depth. 2. Experimental study During the internal inspection of the high pressure pipe line DN 300 originating from Olesna – Barchov section, two dents in the pipe wall were found. This pipeline system has been in operation some decades of years. It was impossible to find out from the internal inspection, whether the dents in the pipe wall originated from the construction of the pipeline or emerged during the recent time. Dent can arise from the ploughing or the earthwork near the gas pipeline. The present work deals with the assessment of