GUARANTEED STRENGTH OF PIPES WITH TRANSVERSE CRACKS UNDER STRESS

Abstract

Background One of the priority tasks to ensure reliable operation of main gas pipelines is to identify defects in pipe walls. Modern intratubal flaw detectors provide reliable finding of defects such as «transverse cracks» and «transverse cracks zone» at a level of at least 20 % of the pipe wall thickness. This circumstance implies that pipes with these defects types (with a depth of less than 20 %) remain in operation, but they can lead to accidental failures of gas pipelines. In this regard, there is a need to assess the cracks risk in terms of brittle fracture, as the most unpredictable and rapid process. Aims and Objectives Based on the numerical calculation of the stress intensity factor, to determine the level of longitudinal stresses, which guarantees the strength of the pipeline section with transverse cracks under stress. Methods A stretchable strip of infinite length with a transverse crack is taken as the computational model. To account for the viscous component in the fracture of the pipe wall, the problem was considered in the elastoplastic formulation, with the calculation of the Cherepanov-Rice integral. Results In the absence of data, the following values are used to estimate the growth rate of developing defects: the growth rate of the stress-corrosion defect: 1.0 mm / year in depth, 4.0 mm / year in length and width. A theoretically undetected crack 3.14 mm deep over 2 years of operation (the frequency of in-line inspection in areas with identified stress-corrosion cracking defects) may increase to a depth of 5.14 mm and actually lead to destruction over a given period of time. The presented calculation results show the need to determine the pipe steel ductility characteristics: fracture toughness, elastoplastic fracture toughness, and also allow to determine the longitudinal stresses level for guaranteed strength of a section of the main gas pipeline.