Session: MF-08-01/10-01 Development of Stress Intensity Factor Solutions (Joint with C&S) and Pipeline Integrity

Paper Number: 61640

Start Time: Wednesday, July 14, 2021, 09:00 AM

61640 - Analytical Limit Load Formula and Procedure for Strength Calculation of Axial Complex Shaped Defect in Pipe 

Analysis of acceptability of corrosion defects in pipelines is a key stage in pipeline integrity management. It is very responsible decision, which should rely on verified procedures. The defected place in pipe is often a 3D colony of defects, which can have a very complex shape. So traditionally, to perform their reliable assessment one should give answers at least on three questions. First, how to present the colony as the simplified projection on some axial line? Second, how to reduce the analysis of axial complex-shaped defect to repeating procedures of calculation for some commonly accepted forms (usually an rectangular defect)? The usual answer to it is application of artificial so-called RSTRENG procedure (named here as R-procedure) which consists in repeated calculation of various rectangular defects composed from neighboring parts of complex defect. Third, which formula from seemingly huge variety of existing in literature approaches should be used for defect of rectangular form?

This paper deals with two last questions by considering the original model published by first author in 1990. It is based on classical lower bound limit load theorem of theory of plasticity which consists in construction of statically admissible solution (distribution of stress which satisfies to equilibrium equation). Its main advantage is possibility to explicitly take into account a lot of geometrical (complex shape) and physical parameters (additional axial loading, various properties of material along the axis), while still retaining its relatively simple application and appearance. So, contrary to other approaches, the rectangular defect here is only a particular case of application of the general procedure, which allows after some simplification to draw out a simple analytical formula.

So, the main focus of the paper is description and justification of numeric algorithm of this general procedure (named here as O-procedure) for axial complex defect in pipe under internal pressure, which in fact is an optimization process to get the most favorable stress distribution.  

Four different approaches for complex shaped defect are compared. First, ASME formula (A-formula) for rectangular defect combined with R-procedure, A-R approach. Second, PPCORC (R-) formula with R-procedure, P-R approach. Third, Orynyak (O-) rectangular formula with R-procedure, O-R approach. Fourth, Orynyak universal procedure (as goal of the paper), O-procedure. The comparison is performed for some artificial complex defect, for example, two closely spaced equal rectangular defects, as considered in DNV RP F101. Very detailed analysis is made for known full scale tests perfomed in Ontario University by Cronin. It is shown that O-procedure usually gives the less conservative results.

Further directions as to requirements to experimental verification and development of the theory are made.

Presenting Author: Igor Orynyak National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”

Authors:

Igor Orynyak National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”
Yulia Bai National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”
Roman Mazuryk National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”