Strength properties of the pipe material are random variables and have a statistical straggling that differs for pipe products of different grades. Distribution of these properties significantly effects on the results of calculating the current level of strength reliability and risk of linear sections for the main pipelines. The reliability of such calculations results depends not only on the distribution that obeys the strength properties of the pipe material at the surveyed section, but also on the dispersion limits of these properties. At present, the main pipelines have significant service lives. Due to this, when making assessment of the technical condition, safety, and risk level of linear sections, the issue is raised of how stable the distribution is related to the pipe steel strength properties with an increase of the service life for the main pipeline. The results of studies are presented concerning the actual distribution and limits of dispersion for the strength properties of pipe steel grade 17G1S after a long-term operation of the main pipeline. It is established that the regularities and strength properties dissipation limits for the material of a pipe made of the above steel are changing with time. Based on the results of the study, a practical example is considered related to the assessment of the effect of the identified changes on the probability of failure and the risk level for one of sections at the linear part of the main oil pipeline.
The results obtained in the work substantiate the need in considering changes in the distribution of strength properties that arises with an increase in the main pipelines service life. The outlined approach allows to increase the reliability of risk assessment results for the main pipelines having long-term service life.
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