PR378-173601-R01 Effect of Pressure Fluctuations on the Growth Rate of Near-Neutral pH SCC

This report summarizes the work completed in PRCI SCC-2-12A project: The Effect of Pressure Fluctuations on the Growth Rate of Near-Neutral pH SCC, which is Phase 3 of the work on the same subject of investigation. The following insights from the current phase of the PRCI SCC-2-12A project are thought to be the most important:

1. Near neutral pH crack initiation is pressure-fluctuation dependent. Severe pressure fluctuations accelerate the fracture and spallation of mill scale on the pipeline steel surfaces, making it harder to initiate SCC cracks from the bottom of pits that are developed at flawed mill scale sites. On the other hand, the presence of a primer layer before application of the protective coating preserves the mill scale on the pipe steel surface and promotes crack initiation.
2. The early-stage crack growth primarily features crack length extension on the pipe surface but limited crack growth in the depth direction. Three different mechanisms of crack length extension have been identified, including that determined by the geometry of coating disbondment, a chaotic process of crack coalescence, and the ability of existing cracks to induce further crack initiation and growth. This latter process is pressure-fluctuation sensitive.
3. A complete set of equations governing crack growth in Stage 2 has been established based on experimental specimens with surface cracks under mechanical loading conditions realistic to pressure fluctuations during the operation of oil and gas pipelines.
4. The contribution to crack growth by direct dissolution of the steel at the crack tip has been determined, which has been found to be crack depth-dependent and pressure-fluctuation-sensitive. Gas pipelines operated under high mean pressure show higher rates of dissolution.
5. The severity of crack growth and the accuracy of the predictive model can be significantly affected by crack tip morphology, either sharp or blunt, and this would yield different threshold values for Stage 2 crack growth and therefore different lengths of remaining life.
6. Full scale testing was performed and has validated the crack growth models contained herein.
7. The PipeOnline software has been revised to incorporate the new experimental results obtained from the current PRCI SCC 2-12A project. This PipeOnline software was previously developed from the two earlier phases of the PRCI project.