PSI - Issue 20

A.A. Antonov et al. / Procedia Structural Integrity 20 (2019) 270–277

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A.A. Antonov et al. / Structural Integrity Procedia 00 (2019) 000–000

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After detecting the sections of the maximum bends and radii of pipeline curvature, the statistical data on the distribution nature of the stress concentration coefficient in the transition zones of welded joints of the underwater pipeline are determined experimentally to calculate the stress-strain state of welded joints. According to Gusenkov (1979), the radius of coupling of the base metal with the welded one is 0.25-0.5 mm for the trunk pipelines, the average values of the height of the welded joint collar and its width are 3 and 20 mm, respectively. The value of the stress concentration coefficient for the weld without joint displacement with the given dimension ratios is 1.5-1.65. It has been established that the stress concentration coefficient in the welded joint of the pipe siphons of the underwater gas pipeline from cap weld side is approximately 1.57, and it is the maximum of 3.7 from the side of the inner wall in the section of the coupling of the base metal with the root weld by Yakovlev et al. (2012). The obtained values of the stress concentration coefficient in a welded joint from the external wall of pipes practically coincide with the data of works by Ikrin (2004) Navrotsky (1968) and it considerably exceeds them from the internal wall of the pipes. Thus, according to the Neuber interpolation relation Makhutov (1979), the generalized theoretical stress concentration coefficient is approximately 2.41 by Yakovlev et al. (2012). Considering the population of data obtained by monitoring the planned-high-altitude position of the gas pipeline, the structural and technological features of the welding of pipe joints are assessed by the level of bending stress along the underwater gas pipeline route. In general, the bending stress is equal to: σ ���� � σ ��� � σ ��� � σ �� (4) where σ dis is the bending stress due to the joint displacement, σ ang is the bending stress due to the angularity of the welded joint, σ ov is the bending stress due to the ovalization of the pipe cross section obtained by Fokin et al. (1984). The intensity of the bending stress in case of the joint displacement can be calculated by the formula given by Makarov and Emelyanova: σ ���� � � ��( � � � �⁄�) σ ��� � � � � ��� �� �� � (5) where σ н is the nominal stress (pointless zone), Δ = h/ δ is the relative displacement, h is the value of the joint displacement, D in is the internal diameter of the pipe, Е is the modulus of elasticity. Due to the angularity in the area of the longitudinal pipe weld, an empirical formula can be used to calculate the bending stress. It gives the best convergence with the experimental data that shown by Gusenkov and Aistov (1975): σ ��� � � � � � tgβσ ��� (6) where β is the angle between the axis of the pipe wall and the axis of the weld. The effect of the initial ovalization of the cross section on the stress state of the pipes can be estimated using the formula obtained taking into account the change of the ovalization due to the internal pressure P given by Aistov (1973): σ �� � � ��� ��� (�⁄�)����(���) � (�⁄�)(� �) � ⁄ � (7) where R is the nominal radius of the pipe, U = (Dmax – Dmin)/2R is the pipe ovality. The local increase in stress from bending effects caused by the joint displacement, the angularity of the welded joint and the ovalization of the cross section corresponds to the zone of stress concentration of the weld. At the same time, ασ at tension and bending are almost the same as shown by Trufyakov et al. (1966). Taking this into account, the maximum stress in the weld zone can be calculated by the formula: σ ��� � � � (σ ��� � σ ��� ) (8)