Crack Growth Modeling and Life Prediction of Pipeline Steels Exposed to Near-Neutral pH Environments: Stage II Crack Gro
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THE safety of pipeline transportation is a matter of paramount importance because of the hazardous nature of the transported substances. Pipeline failures are associated with extensive losses both economic and environmental caused by the release of transported content, potential human injuries and casualties, huge repair and excavation costs, clean-up costs, and loss of pipeline content (gas or oil). Since the integrity of a pipeline can be threatened by various phenomena, threat mitigation and verification of fitness for service are challenges that the pipeline operators must overcome. Near-neutral pH stress corrosion cracking (SCC) constitutes a major integrity concern to buried pipeline steels worldwide. In Reference 1 crack initiation and early-stage crack growth have been modeled. The life JIAXI ZHAO, WEIXING CHEN, MENGSHAN YU, and REG EADIE are with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, T6G 2G6, Canada. Contact e-mail: [email protected] KARINA CHEVIL is with the Department of Chemical and Materials Engineering, University of Alberta, and also with the TransCanada Pipelines, 450 1st Street SW, Calgary, AB, T2P 5H1, Canada. JENNY BEEN and RICHARD KANIA are with the TransCanada Pipelines. GREG VAN BOVEN is with Spectra Energy Transmission Ltd., Suite 1100 1055 West Georgia Street, Vancouver, BC, V6E 3P3, Canada. SEAN KEANE is with the Enbridge Pipelines Inc., 10201 Jasper Avenue, Edmonton, AB T5J 3N7, Canada. Manuscript submitted January 27, 2016. Article published online January 12, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
time spent in Stage I[1] can vary depending on the corrosion behavior of pipeline steels in near-neutral pH environments. Pipeline integrity maintenance incorporates a dual protective mechanism from the soil environments using a protective coating and cathodic protection (CP). Despite these protective measures, damage of coatings and establishment of corrosion conditions are not entirely avoidable so that the occurrence of crack initiation in a large pipeline becomes almost inevitable. Stage I crack formation and growth[1] usually lead to dormant cracks with a depth of about 1.0 mm and since these cracks are dormant they may not form any concerns from an integrity management point of view. However, there are still less than 5 pct of cracks that are able to grow out of the dormant state.[2] It has been extensively studied that continuous growth beyond Stage I is possible if all of the following conditions are met: (1)
(2)
Sufficient mechanical driving forces, which would make a crack propagate as governed by the principle of fracture mechanics although fracture mechanics considerations, are negligible in Stage I crack growth, as modeled in Reference 1. The presence of residual stresses which would act together with the applied mechanical driving forces to either extend the period of dormancy if they are low or compressive, or shorten the dormancy when they are tensile and high, espeVOLUME 48A, APRIL 2017—1641
(3)
cially
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