some in clay [3] and the others in a gravel-type soil [4], and then loaded hydraulically under a cyclic saw-tooth loading spectrum comprised of realistic R-values, frequency and in some cases realistic maximum stresses. The load spectrum was designed to include the fluctuating loading conditions in real pipelines, although with some acceleration in terms of a somewhat accelerated load cycle and higher than design maximum stress. A direct current potential drop (DCPD) technique with a detection resolution of about 30 mm was used to monitor the crack depth [3].

At a pressure equivalent to 96.6% of the SMYS of the X-60 pipe used (1450 psi), the crack grew when R=0.6. However, growth could not be produced when the loading changed to static (R=1.0) or quasi-static (R=0.97). Results for the X-52 pipes are similar with respect to the effects of pressure fluctuation. Significant crack growth was measured for stress levels up to 80% of actual yield with R in the range of 0.6 to 0.9 [4].

When stress corrosion cracks are found on a particular pipeline, it is likely that cracks of various lengths and depths exist on the system, with the maximum crack depth being determined by the age of the pipeline and the yearly crack growth rate. The effects of a particular pressure fluctuation event would vary with crack size. One approach that allows a systematic evaluation is to examine the general dependence of the crack growth rate on the mechanical crack driving force, and then the effect of a pressure variation can be assessed on the basis of the change in the driving force associated with this variation, and the rate at which this change occurs. In the CANMET studies on crack growth behaviour, the crack growth rate data such as those included in Figure 4 are analyzed and plotted as a function of the time rate of J. A typical result of this transformation is shown in Figure 5.

Figure 5 Variation of crack growth rate as a function of the time rate of J. ("S" - static hold period (min.) and "Dyn" - Dynamic load period (min.))

It is shown that the growth rate varies almost linearly with the time rate of J on the log-log plot. It implies that when the severity of pressure fluctuation is reduced, by reducing the magnitude of the excursion in the pressure, the growth rates of existing cracks would decrease. Assuming a continued linearity, under the extreme circumstance of a static load, cracks should tend to become effectively dormant.

THE EFECTS OF STRESS ON THE GROWTH OF CIRCUMFERENTIAL SCC

In the case of circumferential cracking, the driving force is in the axial direction of the pipe, which is largely due to such secondary stresses as bending stress or axial tension caused by ground movement. The characteristics of circumferential cracking are described in a review by Sutherby [12], which includes a rupture on a SNAM pipeline located in southern Italy [13], and in a recent CANMET investigation report on the St. Norbert (Manitoba, Canada) failure [14], which occurred at a river-crossing.

The failure in southern Italy [13] and near St. Norbert [14] both involved axial stress in excess of the SMYS level of the respective linepipe. In the seven failures that occurred in Alberta, it was believed that the axial stress at the failure sites was also close to or greater than the SMYS of the steel. Indeed, over half of the cases were associated with denting or buckling of the pipe in the cracked region [12].

One common feature of these circumferential cracks, as shown in the pertinent fractographs contained in the failure analysis reports, is that the overall crack growth was distinctly discontinuous. That is, the cracks grew for some distance, became dormant for a considerable length of time, and then grew again, creating visible "crack arrest markings" on the surface. Figure 6 is a close-up view of the transition point between the end of the previous growth cycle and the most recent growth.

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