Pipeline SCC in near-neutral pH environment: Results of full-scale tests

Stress corrosion cracking of pipeline steels in a near-neutral soil environment was studied by testing full-scale pipes in actual soil. Crack growth has been reproduced in both grades of steel used, i.e., X-52 and X-60. Dynamic loading was found to be a key factor in affecting crack growth rate. Growth rates in the order of 10^-8 mm/s were measured for most of the cracks on the X-60 pipe at a stress level of 98% SMYS (81% of actual yield), with R=0.6. Under this loading condition, the total J integral (elastic and plastic) for the sixteen cracks of various depths ranged from 4 to 12 kJ/m². Crack growth from the tips of fatigue precracks on the X-52 pipes was measured at an applied stress of 78% SMYS (67% of actual yield). This loading level was very close to that associated with normal pipeline operation. The stress intensity factor required for the initiation of crack growth from the tips of precracks was considerably lower for the X-52 pipe than for the X-60 pipe; J values are being calculated for comparison. Pressure fluctuation was found to be a key factor in the growth of the cracks reproduced in the full-scale test. No crack growth was observed under a static load of 97%SMYS (80% of actual yield). This indicates that the dynamic component of K or J should be used in describing the crack tip driving force. The applicability of crack tip opening rate as the crack tip mechanical parameter is being explored.