THE EFFECTS OF STRESS ON THE SEVERITY OF AXIAL SCC

The great majority of SCC failures in the Canadian system occurred as axial cracking, driven by the hoop stress caused by the operating pressure of oil or gas pipelines. In the NEB Report on Pipeline SCC [1], it is shown in Figure 1 that SCC colonies could form on pipe sections where the operating stress was as low as 64% of the specified minimum yield stress (SMYS). It should be pointed out that the operating pressure for this particular pipeline is generally quite stable, with most of the pressure fluctuation events being associated with R-values of 0.9 or greater (R=minimum pressure / maximum pressure) and only infrequent excursions to lower R-values. For such loading conditions (i.e., maximum stress 64% SMYS and R=0.9), it has been difficult to initiate stress corrosion cracks under laboratory test conditions. In fact, it was reported that even for a lower R-value of 0.85, cracks could only be grown in the laboratory at a stress level of 72% SMYS or higher [10].

Figure 1: Variation of SCC severity as a function of operating stress for one gas pipeline [1].

There could be a number of reasons for this discrepancy. For example, the presence of residual stress on the pipe, a result of pipe-making processes, can be quite significant and thus the true stress in the sites where SCC initiated can be much above the nominal applied stress level. The stress concentration effect of pipe surface features such as welds and corrosion pits are well known to act as stress raisers. In fact, a significant portion of SCC failures have been associated with the welds, as shown in Figure 2.

Figure 2 Distribution of axial SCC ruptures as function of pipe surface features

(based on data in [1] )

It should be noted that corrosion grooves, or "linear corrosion" as it is known, forms on pipe surface when the tape coating wrinkles to form long and narrow pockets of disbondment and the subsequent corrosion takes on the appearance of the coating wrinkles.

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