Numerical procedures for calculation of reference stresses for pipelines with circumferential defects, based on simulation of global and local ultimate plastic states are proposed. There are some peculiarities of proposed procedures.

First, the schematic analysis of the deformation process of pipe with surface defects is suggested, based on which two critical cases have been specified: global (the Net-Section-Collapse (NSC) criterion) and local, which are applicable for a very wide surface defect and for a sharp crack, respectively. The global solution also describes behaviour of through wall defects.

Second, within the framework of the available Net-Section-Collapse criterion the unified algorithm for determination of reference stresses (global solution) for irregular-shaped circumferential defects under multifactor loading (internal pressure, axial force, bending moment) is proposed.

Third, according to the local modelling, the restricted capability to resist plastic deformations is takes into account by inserting of artificial symmetrical defect. The unified procedure for calculation of reference stresses (local solution) for pipe with irregular-shaped circumferential defects under multifactor loading is developed.

Forth, an analysis of proposed procedure by comparing with results of full-scale burst tests.

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