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Evaluation of the effect of defects and service reliability of welded joints in straight‐seam and spiral‐seam pipes

The current standard documents (SNiP 2.05.06.97) for the quality inspection of pipes of transmission pipelines, containing surface defects, including weld defects, are excessively conservative. Repair welding is an expensive procedure because the pipes, buried in the soil and laid on the bottom of the water containers, situated in the individual regions, must be brought to the surface. It is therefore necessary to carry out theoretical and experimental investigations which would make it possible, on the one side, to carry out the quantitative theoretical justification of the method, excluding inefficient repair of damage areas of pipes and, consequently, reducing the volume of repair welding, and on the other side, carry out the inspection of the structural strength of the pipelines. According to the standards valid in England, Japan and the USA, qualification tests of repair welding on the basis of the criteria of suitability for service of pipelines are carried out on the basis of the currently available strength criteria. In Russia, there are no such standard documents and, consequently, this problem is very urgent. One the most widely used strength criteria at present is the Irwin criterion: K < Kc, where K is the stress intensity factor (SIF), equal to <WTT/ (a is the maximum tensile stress in the pipeline, I is the depth of the crack); Kc is the fracture toughness, being the mechanical characteristic of the material. The Irwin criterion is suitable only for brittle fracture and the given equation is valid for calculation of the SIF only if the depth of the surface crack does not exceed 30% of the wall thickness of the pipe. However, the problem of the possibility of occurrence of the brittle fracture mechanism in the pipelines is disputable, although there are data on the decrease of the fracture toughness of pipes steels in certain corrosive media to 10 MPa. Ductile fractures have been reported even in pipelines subjected to stress corrosion. The mechanism, explaining the ductile nature of failure during corrosion cracking, has been proposed in Ref. 2. In the same study, the author discussed the possibility and efficiency of the application of the Soete strength criterion, based on an evaluation of the value of the accumulated plastic strain in the small vicinity of the tip of a crack-like defect. The strength criterion based on the crack tip opening displacement CTOD is well-known. The analytical model for calculating the curves of the permissible dimensions of the surface cracks is the model of the surface crack in a pipe with a correction for plasticity and the thickness on the basis of the critical crack tip opening displacement: