Full-Scale Investigation of Crack Growth Behaviour Under Variable Pressure Fluctuations in Near-Neutral pH Environments

• Author / Creator

  Knechtel, Joshua J

• The goal of this investigation was to study hydrogen-facilitated fatigue crack growth behaviour on pre-existing cracks in a full-scale specimen exposed to a near-neutral pH (NNpH) environment. The full-scale specimen used in this study was a 4.77 m segment of NPS 18, SCH 10, X-60 gas transmission steel pipeline. The segment was cut-out after 46 years of service when multiple stress corrosion cracking (SCC) colonies were discovered during an integrity assessment. A novel corrosion cell was developed to encapsulate portions of the pipe segment containing some of the SCC colonies; the remainder were exposed to atmospheric conditions. The corrosion cell was sealed, and the bulk solution was purged with a 5 % CO2 + 95 % N2 gas mixture for the duration of the test to maintain a pH of 6.29 and an anaerobic environment. To eliminate the effects of corrosion, − 130 mV of cathodic polarization was applied to the fully exposed steel pipe, delivered by a custom impressed current cathodic protection system.
  Constant amplitude and underload-type variable amplitude loading schemes were designed to simulate conditions commonly seen in the field. The maximum stress throughout the full-scale test and the frequency of the major loading events in each loading scheme were set at fixed values of 310 MPa (75 % SMYS) and 10−3 Hz, respectively. After completion of the 91-day test, destructive analysis found that 2.5 % of the total cracks examined exhibited re-initiation and growth. It was found that the threshold stress intensity factor (Kth) required for re-initiation and growth in the depth direction for the NNpH and atmospheric environments were ~ 18 and ~ 25 MPa√m, respectively. The average crack growth rate in the former environment was determined to be 2 – 3 times greater than the latter. It should be noted that this threshold is only valid for the loading conditions of the full-scale test. Four dents were machined into sections of the pipe with no pre-existing cracking in an opportunistic study of NNpHSCC initiation in dents. After test completion, magnetic particle inspection was conducted in the areas surrounding the dents and no cracking was found.
  The total measured crack growth attributed to the full-scale test was significantly less than expected. The primary cause was attributed to conservative reporting of the pre-test crack depths, where the depths reported by ECHO-3D were at least 30 – 40 % deeper than what was found by destructive analysis. This error was significantly larger than the maximum allowable manufacturer specification and the pressure fluctuation design did not account for such a large error in the pre-test crack depth measurement. Consequently, crack re-initiation did not occur at the start of the test and significant uncertainty regarding the moment of crack re-initiation was introduced; thereby impacting the discussion of this study and severely limiting the findings related to the original research objectives.

• Subjects / Keywords

  • pressure fluctuations
  • cathodic protection
  • full-scale testing
  • carbon dioxide corrosion
  • variable amplitude
  • stress corrosion cracking
  • SCC
  • mechanical fatigue
  • X60
  • pipeline integrity
  • environmentally assisted cracking
  • NNpHSCC
  • hydrogen embrittlement
  • near-neutral pH
  • non-destructive examination
  • NNpH corrosion fatigue
  • combined factor model
  • CO2 corrosion

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-wng9-fx95

• 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.