Effects of Hydrostatic Testing on the Growth of Stress-Corrosion Cracks

Abstract

Two hydrostatic tests were carried out on an X-52 (Grade 359) pipe containing sixteen cracks of depths up to 55% wall thickness. Stress corrosion cracking (SCC) growth rates were measured in full-scale tests performed before and after the first hydrotest in order to demonstrate the effects of hydrostatic testing on subsequent crack growth rates. The effects on crack tip deformation were investigated by metallographic examination of crack cross-sections immediately following the second hydrotest. The SCC tests were performed using a saw-tooth type load spectrum with the maximum stress set at 95% of the actual (as opposed to specified minimum) yield strength of the linepipe and R = 0.8. During the first hydrotest, the maximum applied stress was 108% of the yield stress; the total hoop strain in the pipe body reached about 0.2%, which is less than would have been reached in a uniaxial tensile test at this stress level because of the effect of the biaxial stress state in the pipe. The highest SCC growth rate measured before the first hydrotest was about 0.88 mm per year (2.4 * 10−3 mm/day), and the growth rate of the same crack after this hydrotest was about 0.37 mm per year (0.79 * 10−3 mm/day). The other cracks all showed varying degrees of reduction in growth rate. Post-mortem examination indicated that the hydrotests did not cause significant crack blunting. The beneficial effects of hydrotests are attributed primarily to the presence of compressive residual stresses in the heavily deformed region in front of the crack tip. The majority of cracks showed some growth during the first hydrotest. SCC growth and growth during the hydrotests are associated with distinctly different microscopic features on the fracture surface.

Full‐scale testing of near‐neutral pH stress corrosion cracking growth behavior of a vintage X52 oil pipe

“…To simulate the fluctuation of the pipeline internal pressure, a saw tooth-shaped loading spectrum (see Figure 1C) that has been extensively used in previous full-scale SCC growth tests carried out at CanmetMA-TERIALS 17,18 was employed. A single load cycle consisted of a dynamic component with the duration (t dyn ) of 240 s and a static component with the duration (t sta )…”

“…To compare the growth rates of NNpHSCC on pipelines manufactured at different times, crack growth rates obtained from another X52 pipe specimen in a test program conducted by CanmetMATERIALS between 1993 and 1996 (referred to as Canmet Test 1993 hereafter) 3,17,18 were evaluated from the corresponding DCPD data. The specimen was extracted from an oil transmission pipeline in Alberta, Canada, that had been in service for 25 years at the time of the extraction.…”

“…To simulate the fluctuation of the pipeline internal pressure, a saw tooth‐shaped loading spectrum (see Figure 1C) that has been extensively used in previous full‐scale SCC growth tests carried out at CanmetMATERIALS 17,18 was employed. A single load cycle consisted of a dynamic component with the duration ( t dyn ) of 240 s and a static component with the duration ( t sta ) of 350 s. The static holding time allows the crack front to be in adequate contact with the NNpH solution, thus promoting the hydrogen diffusion process and crack growth 34.…”

“…Extensive research has been carried out targeting different aspects of NNpHSCC, such as the failure mechanism, [5][6][7] crack initiation, [8][9][10][11][12] and crack growth quantification and modeling. [13][14][15][16][17][18] Full-scale testing 10,19 using soil boxes that simulate the field condition provides a viable means to directly evaluate the SCC crack propagation behaviors at different internal pressure levels, ranges of pressure fluctuation, and soil conditions. This type of test also allows a direct validation of SCC growth models.…”

“…Unlike high pH SCC, the morphology of NNpHSCC is transgranular in nature. Extensive research has been carried out targeting different aspects of NNpHSCC, such as the failure mechanism, 5–7 crack initiation, 8–12 and crack growth quantification and modeling 13–18 …”