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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
Category: Technical papers from the Journal of Pipeline Engineering
Downloadable: Yes 
Project No.:
Research Agency:
Catalog No.: 2116s
Date of Publication: December, 2007
Price: $25.00 US
Authors: Prof. Valerie Linton, Dr Erwin Gamboa, and Dr Michael Law
Abstract: A SECTION OF submerged-arc-welded 864-mm (36-in) 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 metallographically examined.

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 the DIN and DNV standards, respectively.

The amount of fatigue crack growth extension caused by pressure cycles equivalent to the 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 BS7910. The model tended to overestimate fatigue-crack growth because the shape of the stress-corrosion cracks was far from ideal and because of complex crack interaction effects. The stress-corrosion cracks were inclined away from the perpendicular and had a range of configurations and crack aspect ratios different from 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 stress-corrosion 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 two standard deviations) for a gas pipeline containing stress-corrosion cracks rendered dormant.

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