RESULTS OF COMBINED AXIAL TENSION/COMPRESSION AND INTERNAL PRESSURE TESTING OF MINI-PIPE SPECIMENS IN H{sub}2S ENVIRONMENT: DISCOVERIES OF MATERIAL RESPONSE TO TRIAXIAL STRESS WITH NON-UNIFORM EXPOSURE TO H{sub}2S

摘要

Results are presented from combined tension/compression-internal pressure, H{sub}2S testing of miniature, hollow pipe specimens cut from the wall of low allow, carbon steel coupling stock or casing. The H{sub}2S environment was applied selectively to the inside, outside, and both inside and outside of these mini-pipes. 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 H{sub}2S 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 H{sub}2S environment. A new test protocol is proposed using mini-pipes with pre-placed EDM notches, exposed to H{sub}2S 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.