Effects of pressure fluctuation on the growth of stress corrosion cracks in an X-52 linepipe steel

Stress corrosion cracking of pipeline steels in near-neutral environment was investigated in the laboratory by testing full-scale pipes in actual soil and using a loading spectrum consisting of static and dynamic loading. The saw-tooth-shaped dynamic component was designed to evaluate the effects of pressure fluctuations in oil and gas pipelines. The pressure spectrum applied to the test pipes was characterized by the pressure level of the static part (the maximum pressure), the ratio of minimum to maximum pressure (R), and the duration of each saw-tooth and static period. For the sixteen cracks on an X-52 pipe, crack growth rates were measured at a maximum stress level of 93% SMYS (80% of actual yield) with R = 0.6 and 0.9. The majority of cracks showed growth rates in the range of 1*10^-8 to 1*10^-7 mm/s for R = 0.6. Two of the cracks showed growth rates in the order of 10^-6 mm/s after they had advanced through 70% of the wall thickness. After R was raised to 0.9, the growth rate of all the cracks decreased to below 5*10^-9 mm/s, which is the lowest rate measurable with the detection sensitivity of the DCPD system used and the test duration. The rates of crack growth during the 30-day testing period in which R = 0.9 was applied remained below the detection limit for all but one crack, which showed growth at a rate of 5.8*10^-8mm/s. The effects of pressure fluctuation on crack growth are discussed in terms of its role in maintaining a dynamic crack tip opening rate.