Stress corrosion cracking (SCC) in near-neutral pH environment remains a major concern for high pressure pipelines transporting oil and gas in Canada since its first discovery in 1980s. A variety of laboratory experiments and models have been developed to address different aspects of this complex problem. Full-scale pipe SCC testing using soil box that mimics the condition in the field can directly assess crack growth in terms of pressure level, range of pressure fluctuation, soil conditions, etc. This type of test also offers the most direct validation of SCC models.
A state-of-the-art full-scale SCC pipe testing facility has been established at CanmetMATERIALS Hamilton Laboratory. The facility includes a new hydraulic power unit (HPU), an upgraded 500 kN fatigue frame, and a new 2000 psi (14MPa) pressurization system. In addition, a 24-channel direct current potential drop (DCPD) unit has been refurnished for in-situ monitoring of crack growth.
The full-scale pipe SCC testing facility has been successfully used to measure crack growth in an X-70 (Grade 483) large diameter (914 mm or 36” OD) spiral seam-welded pipe. Six axial cracks were made using saw cutting and fatigue pre-cracking in the base metal and across the spiral-weld metal. All cracks were buried under two types of soil boxes with soil obtained from a near-neutral pH SCC pipeline failure site mixed with distilled water or NS4 solution. The pH of the solution was maintained between 6.9 and 7.2 throughout the testing. Several loading conditions were tested and DCPD was used to monitor SCC growth rate during all the tests.
No detectable growth was observed in the cracks of weld area during all the tests mainly due to over-matching strength. Crack growth was also not detected for the base metal until the maximum pressure was raised up to 95% SMYS with R = 0.7. The threshold of the range of stress intensity factor, (ΔK)th for SCC is thus estimated to be between 11.53 to 13.52 MPa m1/2. The measured average crack growth rate was 5.98×10−7 mm/s.