OVERVIEW OF RECENT THREE-DIMENSIONAL, SMALL-SCALE, MINI-PIPE TESTING OF CARBON STEEL WELL TUBULARS WITH INTERNAL PRESSURE, AXIAL LOAD, AND INTERNAL/EXTERNAL H_2S EXPOSURE

摘要

Mini-pipe testing has been used to expose miniature low-alloy carbon steel pipes to one-sided and two-sided H_2S 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 H_2S 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 H_2S exposure is taken into account compared with two-sided H_2S 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 H_2S. 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 H_2S. Results to date suggest that much understanding and benefit can be gained through additional threedimensional testing by the sour testing community.