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Type: Journal article
Title: Strategies for the repair of stress-corrosion cracked gas transmission pipelines: assessment of the potential for fatigue failure of dormant stress-corrosion cracks due to cyclic pressure service
Author: Linton, V.
Gamboa, E.
Law, M.
Citation: The Journal of Pipeline Engineering, 2007; 4(1):207-217
Publisher: Scientific Surveys Ltd.
Issue Date: 2007
ISSN: 1753-2116
Statement of
VM Linton, E Gamboa, M Law
Abstract: A section of submerged arc welded 864mm diameter API 5L grade X65 gas pipeline containing known stress corrosion cracks (SCC) was selected for fatigue testing to determine whether SCC shallower than a certain depth could be recoated without the need for grinding and returned to service without the risk of later failing by fatigue. The SCC containing pipe section was removed from service, instrumented with strain gauges, fitted with proprietary glass reinforced plastic (GRP) repair sleeves (Clocksprings) at the location of some of the cracking, and hydrostatically pressure cycled under automatic control. After pressure cycling, SCC cracks were extracted from the pipeline and examined metallographically. The fatigue testing was terminated by a small leak after the equivalent of approximately 8000 years of daily and maintenance related pressure cycling for the pipeline concerned. This life exceeded the fatigue life of defect free as-welded joints for pipelines and structures according to the fatigue design rules of e.g. DIN and DNV standards respectively. The amount of fatigue crack growth extension caused by pressure cycles equivalent to 50 years of service proposed for the pipeline concerned was negligible on bare pipe and minuscule under the GRP sleeves. The measured fatigue crack growth rates were compared to a computer model based on the Standard BS7910. The model tended to overestimate fatigue crack growth because the shape of the SCC cracks was far from ideal and because of complex crack interaction effects. The SCC cracks were inclined away from the perpendicular and had a range of configurations and crack aspect ratios different to typical fatigue crack characteristics. This added further complexity to the issue of predicting fatigue crack interaction and growth. The results of the testing show that if the SCC causing environment can be excluded from the pipe surface by effective re-coating, it is likely that SCC cracks small enough to be safely left in the pipeline under static pressure will not need to be removed by grinding and will not grow to critical levels by fatigue in normal gas pipeline service. This work has also shown that BS7910 can provide a conservative fatigue crack growth estimate (when using the recommended values plus 2 standard deviations) for a gas pipeline containing SCC cracks rendered dormant.
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