Conventional Russian and foreign strength calculations of pipelines transporting liquid and gaseous hydrocarbons are presented as a deterministic estimation of wall thickness based on the given design pressure, the selected pipe diameter and mechanical properties of the pipe steel. In deterministic estimates, basic strength calculations are performed using a force-determined strength condition expressed as nominal rated stress vs. permissible stress. A set of safety and reliability factors is introduced into these calculations to account for basic design, process and operational factors.
The article provides a quantitative assessment of the role of statistical factors of operational loading, the breaking point and the yield point, mechanical tolerance for wall thickness and pipe diameter, and pipe material degradation. It links the safety factors used in the international standards (API 579 and DIN 2470) and the Russian Construction Code SNiP 2.05.06-85*. The article suggests considering the statistical data on operational loading of oil pipelines by recording pressure at the inlet and outlet of the given oil pump station and measurement of local pipe stress in representative pipeline sections. The paper also contains proposals to consider that the strength properties of pipes may be affected by pipe steel degradation, especially in surface layers, in the course of long-term operation. Strength reserve values were obtained to account for other factors such as welds present, manufacturing defects in pipes and pipe steel, local corrosion, loading cyclicality and cyclic crack extension). These values should be further considered with advent of the new basic and check calculation methodology, to account for their stochastic nature.
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