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Sequencing & Agglomeration of Cycles in Variable Amplitude Underload Spectra: Effects on Surface Crack Growth in a Near-neutral pH Environment
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- Author / Creator
- Li, Rui Ning
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Stress corrosion cracking (SCC) is one of the biggest integrity threat facing pipelines. Large numbers of SCC defects are detected every year, but scheduling the repair work for these defects is an enormous challenge. Pipeline companies have limited resources to take on the repairs. Therefore the repairs for these defects are prioritized using SCC growth models. However, current SCC growth models often make inaccurate life prediction and cannot meet the needs of the pipeline operators.
One of the reasons these models perform poorly is because they do not consider the variable amplitude pressure fluctuations experienced by a pipeline. It is known throughout the pipeline industry that pressure fluctuations in the pipeline drives SCC propagation, but what is not known is how the different cycles within a pressure spectrum interact with each other and affect crack growth rate. Pipeline operators are interested in determining the relative severity of different spectra. The biggest difference between these spectra is how the individual cycles are sequenced within.
The objective of this thesis is to investigate how the sequencing and agglomeration of cycles within a spectrum can affect crack growth rates. SCC tests were conducted on surface crack samples made from X 65 pipeline steel in an anaerobic NNpH environment purged with 5% CO2/95% N2 gas. Traditionally, SCC studies were conducted using thru-wall crack specimens such as the compact tension (CT) specimen. Surface crack samples were used in this study because they simulate SCC more accurately. Additional sample preparation techniques were developed specifically for surface crack samples during this study since they are seldom used by other researchers. Six load spectra with different sequencing and agglomeration of cycles were applied to the samples. The basic building blocks of all the test spectra were: minor cycles with high R ratios (minimum stress/max stress), severe underload cycles with low R ratio, and mild underload cycles with medium R ratios. The proportion of the different types of cycles remained constant for all spectra, and only the sequencing was changed for each spectrum. The spectra tested simulate underload-type spectrum experienced by an oil pipeline at pump station discharge, where most SCC’s have been found.
The crack growth rates obtained from the experiments were compared to determine the relative severity of the spectra. It was found that increasing the agglomeration level in a spectrum (i.e. grouping together the same type of cycle) increased the hydrogen embrittlement effects by allowing more hydrogen to build up in front of the crack tip during the minor cycles, and this tend to increase crack growth rate. However, increasing the agglomeration level also decreased the number of interaction events (i.e. how many damage causing cycles are enhanced by hydrogen embrittlement), which tend to decrease crack growth rate. Therefore the resultant crack growth rate is determined by the competition of these two effects. The most severe spectra balance these two opposing force to optimize the crack growth enhancement.
Sequencing of damage-causing underload cycles is also important. For spectra with high agglomeration level where multiple underload cycles are grouped together, the more aggressive underload cycle should not be the first cycle in the group because the hydrogen embrittlement effect is the strongest right at the start of the underload group. Spectra that ‘shielded’ the more aggressive underload cycle behind other mild underload cycles experienced slower crack growth rate.
Pipeline operators could use the finding from this study to help them determine the relative severity of their pressure spectra and make improvements to their existing SCC model. -
- Graduation date
- Fall 2018
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- Type of Item
- Thesis
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- Degree
- Master of Science
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- License
- Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.