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>DEMONSTRATION OF HYDROGEN SULFIDE FUGACITY AS A PRIMARY FACTOR IN ENVIRONMENTALLY-ASSISTED CRACKING AND RECOMMENDATIONS ON RETAINING CONSERVATISM IN SOUR QUALIFICATION AND DESIGN
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DEMONSTRATION OF HYDROGEN SULFIDE FUGACITY AS A PRIMARY FACTOR IN ENVIRONMENTALLY-ASSISTED CRACKING AND RECOMMENDATIONS ON RETAINING CONSERVATISM IN SOUR QUALIFICATION AND DESIGN
Environmentally-assisted cracking (EAC) has long been recognized to result from the combination of a susceptible material under applied or residual stress in the presence of a contributing environment. In the oil and gas industry, some of the most critical EAC concerns revolve around the impacts of hydrogen sulfide (H_2S) in production environments which can lead to sulfide stress cracking (SSC) or stress corrosion cracking (SCC) and coincident loss of containment of produced fluids (oil and/or gas). In order to avoid these failures, the industry relies on an array of standardized test methods to qualify metallic components for specific H2Scontaining environmental service. Better understanding of the controlling environmental parameters in these exposures permits qualification testing to more accurately duplicate the field exposures in the laboratory. This paper summarizes a laboratory testing program that demonstrated that H_2S fugacity and related aqueous chemical activity is the primary H2S-related parameter contributing to SSC of carbon steel. Other parameters studied included the H_2S partial pressure and H_2S aqueous solubility and related resultant H_2S dissolved concentration. This finding indicates that many previous qualification tests for high-pressure sour service, when using H2S partial pressure as a defining parameter, may be several-fold more severe than the field exposure; contrary to this mismatch in sour severity between laboratory and field, in lower pressure sour service, that degree of severity is much less pronounced. These wide ranges of EAC-related severity in the laboratory when compared to severity in the field imply the degree of conservatism employed by the industry in qualification is highly inconsistent. This paper concludes with commentary on a possible path forward to aligning and quantifying conservatism in sour service qualifications.
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