S.L. Golofast, Dr. Sci. (Eng.), Prof., trasser@inbox.ru OOO Gazprom Proyektirovaniye, St. Petersburg, Russia
At assessment of the trunk pipelines strength and service life, the deterministic models are most widely used, which allow to determine the safety margin on destructive loads and to compare them with the maximum permissible (recommended) values. The main difficulty in the implementation of this approach at assessment of the strength reliability is that as such, all the input values included in the calculated dependencies of these models are of the random nature, but assumed to be constant when performing calculations. Due to the random nature, the input values are subject to dispersion to a greater or lesser extent, their samples have, as usual, different lengths and are limited in certain limits determined by the physical meaning of each specific value. Restored on the basis of such samples the distribution laws are cut off (trimmed) both from the left and from the right that leads to the change in the statistical characteristics of the distribution. The limits of dispersion of one and the same random variables, and, as a consequence, the laws of its distribution can vary with time. In a number of cases, the samples of the random variables included in the calculated dependences is impossible to refer to any of the known parametric laws of classical mathematical statistics. The frequency distributions of the majority of values are not unimodal, so for correct processing of such input data it is required to develop and apply the special mathematical apparatus.
The assumption that the input values despite their random nature are constant, leads in a high probability of serious errors at calculating the quantitative indices of strength reliability and, as a result, increases the risk of operation of the potentially hazardous objects, which the trunk pipelines are related to.
These errors can be avoided if when calculating the strength criteria the random variables and the corresponding laws of their distribution are used as input data. Implementation of such calculations requires improvement of existing and development of new probabilistic methods that allow to assess the strength reliability of the trunk pipelines both at the stage of design and operation taking into account the specifics of the objects under consideration.
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