Safety evaluation for prolonged gas pipeline operation in areas of high seismicity

Abstract

The article considers questions of changes in pipe steel properties of strength category K50-K56 during operation under conditions of the action of seismic loads. The limiting permissible number of loading cycles is shown for pipelines made from test steels with earthquakes of different intensity.Within the Russian Federation regions with seismicity of seven points and higher embrace about 2.3 million km 2 of area, which comprises more than 13% of the territory. The territories that are most critical in a seismic respect relate to Kamchatka, Kuril Islands (more than nine points); the Transbaikal and Baikal regions, southern regions of the Krasnoyarsk and Altai rims (six-nine points), Dagestan (eight points), the resort regions of the Black Sea coast of the Caucasus, and the Caucasus Mineral Waters (seven-eight points).The seismic resistant planning of structures, including objects of pipeline systems, within Russia is controlled by a number of standard documents [1][2][3]. For systems of pipeline transport, consideration of seismic activity is carried out for building regions with seismicity of seven to nine points [1]. Earthquakes with an intensity lower than seven points are not considered for pipeline objects since it is assumed that as a rule the loads created by them are not definitive in the level of stress-strained state, formed in the wall of a pipeline [1]. In addition, it should be noted that the frequency of occurrence of earthquakes of low intensity is considerably higher than for high-point occurrences. The total number of cyclic loads created by them in the operating time of a pipeline is very significant and may have a marked effect on promoting the occurrence of crack defects in a pipeline wall.It should also be considered that calculation of the limiting stressed state of a pipeline, including additional stresses, which arise in the walls of a pipeline under the action of a seismic wave, are carried out proceeding from the level of strength properties of new pipe material [2]. However, during operation of a pipeline and cyclic loads, which arise with low intensity earthquakes, there may be a marked change in metal strength property indices as a result of the occurrence of strain aging. Embrittlement of metal under the action of these factors leads to an increase in the sensitivity to the action of alternating loads of low intensity and may promote crack generation, which it should be considered in evaluating the level of operational safety for gas pipelines, planned in areas of increased seismicity. In this article, an estimate is provided of the probability of gas pipeline failure made of low-alloy steels of strength categories K50, K56 during their operation in a seismically active area of the Caucasus region (in the region of Grozny).