Abstract
Mini-pipe testing has been used to expose miniature low-alloy carbon steel pipes to one-sided and two-sided H2S combined with high internal pressures (20,000+ psi) and axial loads. This paper explains the types of tests that have been done and the results that have been observed. One-sided vs two-sided H2S exposure combined with stress gradients across the pipe wall is shown to lead to differences in pipe performance. Test results indicate a substantial increase in pipe material performance when one-sided H2S exposure is taken into account compared with two-sided H2S exposure. Tests of prenotched mini-pipes are introduced which indicate that a fracture mechanics formula using KISSC from the DCB test is nonconservative and overpredicts the pipe failure pressure due to sulfide stress cracking. Use of the limiting value, KISSC–Limit, is shown to improve the prediction of pipe cracking failure significantly, making it almost conservative. Use of KISSC-Limit combined with a design factor accounting for DCB uncertainty finally makes the fracture formula a conservative predictor of C110 pipe cracking. Tests of mini-pipes without pre-existing notches are shown to indicate that a fracture mechanics formula alone cannot reliably predict pipe cracking performance; i.e., the tests indicate that both a three-dimensional stress formula and an improved fracture mechanics formula are needed to conservatively predict pipe material performance in H2S. The tests of mini-pipes have been consistent with tests conducted by the API on full-size casing. Three-dimensional mini-pipe testing is shown to be a tool to better understand and quantify pipe performance in H2S. Results to date suggest that much understanding and benefit can be gained through additional three-dimensional testing by the sour testing community.
