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Offshore and Structural Mechanics

The Behavior of Shallow Cracks in a Pipeline Steel Operating in a Sour Environment

[+] Author and Article Information
C. M. Holtam, D. P. Baxter

Structural Integrity Technology Group,  TWI Ltd., Cambridge, CB21 6AL, UK

I. A. Ashcroft

Wolfson School of Mechanical and Manufacturing Engineering,  Loughborough University, Leicestershire, LE11 3TU, UK

R. C. Thomson

Institute of Polymer Technology and Materials Engineering, Department of Materials, Loughborough University, Leicestershire, LE11 3TU, UK

J. Offshore Mech. Arct. Eng 131(3), 031302 (May 29, 2009) (9 pages) doi:10.1115/1.3058703 History: Received June 24, 2008; Revised September 18, 2008; Published May 29, 2009

Setting conditions for the avoidance of in-service crack growth in aggressive corroding environments has long been a major challenge due to the number of variables that have a significant effect on material behavior. One area where both experimental data and a validated assessment methodology are lacking is the behavior of shallow cracks. This paper describes the early results of an ongoing research program aimed at addressing the shortfall in experimental data to characterize material behavior in the shallow-crack regime, with the long-term aim of improving the understanding and assessment of the early stages of environment assisted cracking. There is an industry need for a better understanding of material behavior under these conditions and for the development of a more robust assessment methodology. API 5L X65 pipeline steel parent material was tested in a sour environment with initial flaw sizes in the range 1–2 mm. Fatigue crack growth rate tests have been performed to investigate the influence of crack depth on crack growth rate (da/dN). Initial results suggest that crack growth rates for deep flaws can increase by a factor of 5–100 compared with air depending on the applied stress intensity factor range (ΔK). Shallow cracks have been shown to grow up to 130 times faster in a sour environment than in air and up to an order of magnitude faster than deep cracks in a sour environment at the same value of ΔK. Constant load tests have also been performed to investigate the influence of crack depth on the threshold stress intensity factor for stress corrosion cracking (KISCC). Preliminary results suggest that in this case there is no crack depth dependence in the range of flaw sizes tested. While further experimental work is required, the results obtained to date highlight the potential nonconservatism associated with extrapolating deep-crack data. Guidance is therefore provided on how to generate appropriate experimental data to ensure that subsequent fitness for service assessments are conservative.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

FAD incorporating cutoff for EAC (after Ref. 11)

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Figure 2

Two-parameter approach to stress corrosion cracking (after Ref. 11)

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Figure 3

Increasing and decreasing ΔK test results

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Figure 4

Constant ΔK test results (ΔK∼300 N mm−3/2)

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Figure 5

Static test results in a sour environment; the arrow indicates a step load test

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Figure 6

Region of crack extension on fracture face of specimen M02-09 (constant load, a=7.92 mm)

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Figure 7

Log-log plot of stress versus flaw size (Kitagawa-type diagram)

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Figure 8

FAD using KISCC as a measure of material’s toughness

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