It is known that large and small pressure fluctuations can promote both high-pH and near-neutral-pH SCC, but a quantitative understanding of what kinds of fluctuations should be avoided, if possible, is lacking. In addition, recent research has raised the possibility that two different cracking mechanisms in near-neutral-pH environments may re-activate dormant cracks:
1) Conventional SCC under low-frequency, low-amplitude cyclic stresses, and 2) Corrosion fatigue under high-frequency, high-amplitude stress cycles.
This project is specifically designed to address the effect of pressure fluctuations on the growth rate of near-neutral pH SCC. In particular, there is an interest to determine what kinds of pressure fluctuations represent the greatest risk for increasing the potential for SCC to form and propagate; and the different types of mechanisms that are operative in the near-neutral pH environment, and what factors control and determine the extent and degree to which the mechanisms will occur. It is expected that findings of prior studies in combination with this research will help to clarify how the crack growth rate is affected by the nature of the pressure fluctuations, both in terms of average growth rate and the dormancy issue, thus helping to determine whether it is feasible to prevent SCC by controlling pressure fluctuations.
This study was divided up into a multi-year effort with various deliverables for each phase. The investigation in year one has been primarily focused on the validation of the software being developed for crack growth and remaining life prediction using SCADA Data. A total of 9 pressure spectra, 4 for oil pipelines and 5 for gas pipelines, have been collected and used as inputs for the software. It was found that a pressure spectrum can be quantified using a parameter termed an Acceleration Factor to represent the severity of load/pressure interactions in terms of crack growth rate. The value of the acceleration factor can indicate whether the crack growth is enhanced or retarded. The oil pipelines are characterized with more frequent and larger pressure fluctuations, and crack growth is directly caused by these cycles. The gas pipelines are characterized with minor cycles with high stress ratio and the subsequent under-loading cycles with low stress ratios. The software allows the SCADA/pressure fluctuation data in an excel spreadsheet format to be directly analyzed producing a projected remaining life of the pipeline based on the past pressure fluctuations and assumed future pressure fluctuations.