PSI - Issue 40

Z.G. Kornilova et al. / Procedia Structural Integrity 40 (2022) 245–250

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Z.G. Kornilova at al./ Structural Integrity Procedia 00 (2021) 000 – 000

Nomenclature PHAP the planned-high-altitude positions TGL the trunk gas pipeline

1. Introduction: In the territory of the Republic of Sakha (Yakutia), main gas pipelines are laid in the area of permafrost. Gas pipelines operate in harsh natural-climatic conditions, such as a wide range of temperatures from + 40  C in summer to -60  C in winter, the cryogenic processes, and transitions across water barriers that create an unstable stress-strain state. A specific hazard in the floodplain sections of underwater crossings of main pipelines lies in the migration of moisture during freezing and thawing of frozen soils and the deformation of the earth's surface, which affects the assessment of the strength and reliability of the structure as a whole by Markov and Pulnikov (2018), Burkov et al. (2013), Lisin et al. (2012), Permyakov et al. (2020). A widely used method for monitoring the condition of an underground pipeline is the measurement of PHAP. It is taken at several points. If the pipeline position deviates from straightness, for example, only in one section, then using the point data, it is possible to evaluate the pipeline deflection by calculating the bend radius and the resulted stress by the formula by Ikrin (2004), Pisarenko (1988): = 2 , (1) where E is the elastic modulus, D is the pipe diameter, r is the bending radius. Under permafrost conditions, the underground pipeline changes its position twice a year – at the moments of freezing and thawing of the surrounding soil. Frost heaving can be inhomogeneous at a distance of several meters. Anon-uniform load acts on the pipeline locally and causes large deformations. Scouring and sagging of the pipelines are observed in the trench during their operation. They lead to stresses in the pipe wall, the level of which increases with the increasing length of the scoured section. In addition to static stresses, dynamic ones occur due to sagging caused by fluctuations of the denudated section in the water flow by Permyakov et al. (2013), Ammosov et al. (2019). The riverbed erosion in the area of the underwater pipeline trench is one of the typical factors that cause a life loss during trench laying of a siphon across the watercourse. When the bottom is scoured, and the siphon of the underwater pipeline is denuded, significant hydrodynamic loads not specified by the project are added to the operational loads. Torsional, bending, and overturning moments, as well as the phenomena of hydro-abrasive erosive wear, develop by Ivanov et al. (2020). They lead to fatigue failure of individual sections of the underwater pipeline on the base metal and welded joints. 2. Research materials and methods According to the monitoring results for 2016-2018 on the Hatasskaya channel of the river Lena, significant transverse vertical displacements of the pipeline in the ground occur during seasonal ground motion on the slope at the entrance to the channel. Fig. 1 demonstrates the high-altitude positions of line-II of the underwater pipeline across the river Lena on the Hatasskaya channel measured for four seasons in a row. The measured points are indicated by a square and a small rhombus. The lines show the estimated position of the pipeline between the points obtained by interpolation with a cubic spline. The horizontal axis indicates the distance from the benchmark in meters. The vertical axis shows the high-altitude position of the pipeline from the benchmark height in meters. A triangle and a large square stand for the measured points of the earth's surface, and the dotted line shows the level of the earth's relief. On the left, the pipeline approaches the channel along the floodplain. It descends to the Hatasskaya channel at a distance of ~ 150 meters. Approximately, at an elevation of 200 m, the slope merges into the channel bottom. A