RESULTS OF COMBINED AXIAL TENSION/COMPRESSION AND INTERNAL PRESSURE TESTING OF MINI-PIPE SPECIMENS IN H2S ENVIRONMENT: DISCOVERIES OF MATERIAL RESPONSE TO TRIAXIAL STRESS WITH NON-UNIFORM EXPOSURE TO H2S

摘要

Results are presented from combined tension/compression-internal pressure, H2S testing of miniature, hollow pipe specimens cut from the wall of low allow, carbon steel coupling stock or casing. The H2S environment was applied selectively to the inside, outside, and both inside and outside of these minipipes. The influence of three dimensional stresses and the stress-gradient across the pipe wall are highlighted. Test results are correlated relative to the yield envelope and yield strength of the pipe. The test results show that the hydrogen-concentration gradient in the pipe wall interacts with the stress gradient and plays a key role that correlates with pipe failure. The one-sided H2S effect is quantified in terms of pipe failure and is seen to have significant impact for pipe performance in gas well casing service where exposure is one-sided. A distinction is made between testing for crack initiation (chemical and stress induced SSC to failure) and testing for crack propagation with low fracture toughness due to hydrogen from the H2S environment. A new test protocol is proposed using mini-pipes with pre-placed EDM notches, exposed to H2S at low stress and then rapidly loaded to failure in order to use mini-pipes to calibrate predictive crack propagation (fracture mechanics) failure models for the hydrogen embrittled material.