PSI - Issue 20

Yakov Mikhailovich Andreev et al. / Procedia Structural Integrity 20 (2019) 167–173 Yakov Mikhailovich Andreev et al. / Structural Integrity Procedia 00 (2019) 000–000

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A distinctive feature of permafrost soils is their thawing at the site of damage to the natural vegetation cover, which is inevitable during the construction of a gas pipeline. For this reason, the gas pipeline is exposed: due to the liquefaction of frozen soils, the embankment of the gas pipeline “drains” and exposes the pipe. Due to the variability of soils, it is often possible to find complete exposure of the gas pipeline with its sagging in such areas. During gas transportation stops in the summer, the surface temperature of the pipe in these areas under the influence of direct sunlight can reach 30 ° C. Also, in winter, in cases of a gas pipeline shutdown, the surface temperature of the pipes drops to sub zero –55 ° С . The annual temperature difference in these sections can reach 85 ° С , with the design position of the gas pipeline and without gas transportation stops, the difference is 55 ° С . Thus, with changes in the temperature of the gas pipeline in the positive direction, the pipeline in this section increases in length. Under the influence of longitudinal compressive forces in the pipes, a local loss of stability occurs, accompanied by a curvature of the pipeline, and so-called gas pipeline emissions are formed. The length of emissions in these areas depends on the stiffness of the pipes, in the case of partial exposure, on the longitudinal strength and bearing capacity of thawed soil. 3. Object study Let us consider in more detail the incident that occurred on the gas pipeline of the first hazard category with a working pressure of 4.55 MPa (Fig. 1). For the construction of the pipeline, steel pipes of 13G1S-U grade with an outer diameter of 0.720 m and a wall thickness of 0.008 m were used. At the time of the inspection, thinning of the pipe wall was not detected. As a result of defrosting of unstable permafrost soils, a change in the relief and watering of the area took place. The length of the wetland along the gas pipeline was 1 km, while about 10 m was exposed to the complete exposure of pipes with sagging (see Fig. 1). After some time, namely in the summer period, an incident occurred in the form of a horizontal discharge of a gas pipeline with partial exposure. The horizontal offset from the project axis was 6 m (Fig. 2), the vertical offset was 0.363 m (Fig. 3), while the partial exposure of the gas pipeline surface was 237 m (see Fig. 1).

Fig. 1. Scheme of the frost heave of the gas pipeline (plane view): 1 – bench mark for measuring the horizontal displacement of the axis of the gas pipeline; 2 – bench mark for measuring the vertical displacement of the gas pipeline axis; 3 – section with a sag of the gas pipeline, length is 10 m; R1 – maximum bending radius of 888 m; L – open section of the gas pipeline, length is 237 m; 4 – underground section of the gas pipeline; 5 – direction of the transported gas.