Abstract
While the classic proof ring test is the epitome of the NACE Method A test for sulfide stress cracking, TM0177 makes clear that any apparatus may be used for SSC evaluation as long as the test method applies known sustained stress to a tensile specimen having gage section dimensions specified by TM0177 Method A exposed to the H2S-containing environment of concern. One of the several alternative apparatuses shown in NACE TM0177 Method A for applying sustained load to the specimen under test consists of a tensile specimen in a small test cell to contain the sour environment mounted in a rigid frame with a calibrated spring external to the test vessel supplying the sustained load. Like the proof-ring, the spring-loaded stress frame shown in TM0177 Method A is designed to expose a single tensile specimen under atmospheric pressure conditions. While the apparatuses illustrated in TM0177 Method A were clearly designed for testing at room temperature at atmospheric pressure, TM0177 Method A is also clear that all of these test apparatuses can be adapted to high-pressure, high-temperature applications by the use of small individual autoclaves by testing one specimen at a time per autoclave
The so-called “Window-Box” sustained load apparatus and methodology, a derivative of the spring-loaded rigid frame apparatus shown in TM0177 Method A, was developed as an alternative for conducting TM0177 Method A tests in HPHT systems while maintaining consistency with the requirements per TM0177 Method A. These requirements included those for minimum deflection, measurement of spring deflection and confirmation of applied load.
The Window-box methodology also incorporates procedures to document and minimize stress relaxation and thermal expansion effects when testing at elevated temperatures. The methodology has become an industry work horse to evaluate SSC / SCC in high pressure CO2/H2S systems at elevated temperatures.
