To ensure the safe operation of main oil pipelines, their periodic diagnostics by in-line inspection tool is carried out with the identification of a full range of various defects, determining their sizes, performing calculations for the strength and durability of pipe sections with defects, assigning the number and timing of repairs. The most dangerous are defects of the planar type of the factory weld and the base metal of the pipes, oriented in the longitudinal direction. The date of the next in-line inspection should be determined from the condition that none of the identified defects will lead to a failure during the inter-inspection period, or that the probability of such a failure should be sufficiently small. In this regard, it is important to assess the probability of failure of a pipeline section with in-plane defects identified by in-line inspection tool during the inter-inspection period. Failure is understood as the achievement of the dimensions of a planar defect during its fatigue growth of the limiting dimensions. The limiting dimensions of the defect are determined using a two-criteria fracture diagram according to the given design pressure for each defective section and the mechanical properties of the metal of pipes and welded joints.
This article presents a method for assessing the probability of destruction of a pipeline section with surface-type defects of a factory weld and the base metal of pipes, detected by an in-line inspection tool during scheduled in-line diagnostics. The technique was developed under the assumption that the initial depth and growth rate of defects are random variables. Thus, the parameter of cyclic crack resistance for pipe steels is also considered as a random variable. Empirical and calculated distributions of the error in measuring of the defect depth by in-line inspection tool are constructed. All defects are considered as surface semi-elliptical fatigue cracks with dimensions determined during in-line inspection. The fatigue crack growth kinetics is described by the Paris equation. The deterministic values are the length of the defect, the limiting depth of the defect at the design pressure, the reduced pressure cycling and operating pressure in the defective section, the pipe wall thickness. The results of the calculation according to the developed method of the dependence of the probability of failure on the value of the inter-inspection period for a section of the main oil pipeline containing 301 flat-type defects in the factory seam, oriented in the longitudinal direction. The length and depth of the defects were determined from the data of the in-line inspection tool.
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