Keywords: pipes, corrosion, structural integrity, complex defects, remaining strength, failure pressure, semi-empirical methods.

This paper considers the prediction of the limit load and remaining strength of pipelines under complex corrosion defects. Semi-empirical methods for isolated defects such as B31G [1], modified B31G [2], RSTRENG and the DNV RP-F101 method [3] are reviewed. Numerical models employing the finite element method both with shell elements and 3D solid elements are constructed, and numerical results are compared to those obtained using semi-empirical methods. Non-linear analyses are carried out to represent the constitutive behaviour of materials.

Pipes containing isolated defects with different geometric parameters are analyzed and the results are compared with experimental results provided by Choi et al. [4]. Results show that values provided by finite element models are more accurate than the results provided by the semi-empirical methods.

The weighted depth difference method (WDD) [5] for predicting the failure pressure of complex corrosion defects is also presented. This method considers an irregular profile of the corrosion defect instead of a smooth lower surface as the other methods do. The influence of multiple defects is treated and some examples illustrate the interference of two close defects.

First pipes containing three longitudinally aligned rectangular defects are analyzed. It is seen that both numerical models; namely, shell elements model and three-dimensional solid elements model provide results which are very close. The influence of the distance between the defects is also investigated in the sense that there must be a distance for which defects may be considered as isolated. It is found that such a distance must be twenty four times the value of the thickness of the pipe walls.

An attempt to determine the effectiveness of the WDD method is made by comparing the results provided by the WDD method to results provided the three-dimensional solid finite element analyses. No pattern has been found. The results are problem-dependent.

Finally, pipes with three circumferential defects with different angular distances between them in the same cross-section have been considered. It is found that the influence between defects is minimized when the angular distance is 70°.

1

ASME, "Manual for Determinig the Remaining Strength of Corroded Pipelines - A Supplement to ASME B31 Code for Pressure Piping", The American Society of Mechanical Engineers, New York, 1991.

2

Kiefner J.F., Vieth P.H., "A Modified Criterion for Evaluating the Remaining Strength of Corroded Pipe", Final Report on Project PR 3-805, Pipeline Research Committee, American Gas Association, 1989.

3

DNV, "DNV Recommended Practice - DNV-RP-F101 - Corroded Pipelines", Det Norske Veritas, Norway, 1999.

4

Choi J.B., Goo B.K., Kim J.C. et al., "Development of Limit Load Solutions for Corroded Gas Pipelines", Int. J. Pres. Ves. Piping, v.80, p.121-128, 2003. doi:10.1016/S0308-0161(03)00005-X

5

Cronin D.S., Pick R.J., "Prediction of the failure pressure for complex corrosion defects", Int. J. Pressure Vessels and Piping., v. 79, p. 279-287, 2002. doi:10.1016/S0308-0161(02)00020-0

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