PSI - Issue 20

Nikolay I. Golikov / Procedia Structural Integrity 20 (2019) 161–166 Nikolay I. Golikov / Structural Integrity Procedia 00 (2019) 000 – 000

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denudation of about 52 meters in length and sagging up to 430 mm. The pressure was 0.4 MPa at the moment of destruction (the standard tubing pressure is 2.0 MPa). The flow rate of the river during the flood period was 1.5-2.0 m/s. Fragments of the pipes with damaged welded joints obtained from the underwater crossing section of the gas pipeline were investigated. According to the results of the impact bending tests, it has been established that the KCV of the WM and the HAZ of the field joints are significantly inferior to the KCV of the BM at negative test temperatures. The impact strength of BM is 1.6 times more than KCV of HAZ at temperatures of 0 ° ... + 4 ° C and 2.7 times more than WM. Thus, the sections of the weld metal and heat-affected zones are characterized by a low level of resistance to the initiation and growth of cracks under dynamic loads (Fig. 3). The results of the fractographic and metallographic analysis revealed that the development of the destruction of both welded joints of the underwater crossing occurred from the inside, from the root seam (Fig. 4). The nature of the development of the cracks indicates the fatiguing nature of the fracture, which is confirmed by electron microscope studies. The site of the destruction of both welded joints is the sections of the fusion of the root weld with the base metal.

Fig. 4. Fatigue fracture with the initiation zone of the destruction of the welded joint.

The underwater pipelines are exposed to the river flow. Subsequently, they are subjected to the impact of a constant and varying in magnitude and direction force, which can cause pipeline oscillations with significant amplitude leading to the destruction of pipes. When the underwater pipeline is exposed to the water flow with significant velocity, pipeline vibrations are possible, and this leads to its rupture. This is due to the alternating vortices forming behind a cylindrical body when the water flows around it. The frequency of the vortices breakdown depends on the cylinder diameter and the flow rate. In this case, the pipeline is affected by variable forces in the direction perpendicular to the flow direction. When the frequency of action of these forces coincides with the period of resonant oscillations of the pipeline itself, excessive vibrations occur, which are the most frequent cause of the pipeline damage, according to Borodavkin et al. (1979). The following initial data were used to calculate the scour section of the underwater crossing of the main gas pipeline: the pipe diameter is 530 mm, the wall thickness is10 mm, the weight of cast-iron weights is 450 kg, the distance between the weights is 2.5 m, the sagging length is 46 m, the river flow rate is from 1.7 m/s, the gas pressure is 2 MPa. According to the calculation results, the critical dimensions of the length of the unprotected section of the underwater gas pipeline and stresses arising in the pipe wall depending on the speed of the river flow were obtained. Analysis of the applied forces shows that the scoured section of the underwater crossing of the main gas pipeline was subjected to cyclic loads when exposed to the river flow. Occurring loads on the scoured section of the pipeline are greatly enhanced by increasing rate of the river flow. The inspection revealed that the length of the sagging section of the underwater crossing was 46 meters. Hence, the underwater pipeline may experience resonant oscillations at the river flow rate close to 2 m/s. According to the calculations, the stresses arising at resonance reach the level of 173 MPa. This level of stress is insufficient for the destruction of the pipeline base metal as a result of fatigue loading. For the pipes studied, the ratio is r/t = 26.5 . According to the research results, it can be affirmed that tensile RWS comparable to the yield strength of the base metal is formed in the girth welds of the underwater gas pipeline from