The purpose of this paper is to present a condensed summary of the work done by Dolphin Energy on chloride stress corrosion cracking (Cl-SCC). Stress corrosion cracking poses a serious threat to the integrity of a natural gas plant and the company responded to this threat by launching a comprehensive initiative to determine the cause(s) and to adopt best practices for preventing future failures.
This paper provides a valuable engineering insight into the ways and means adopted by the company for tackling Chloride Stress Corrosion Cracking issues in an operational plant without jeopardizing safety while maintaining the production targets at the same time. Included in this paper are various monitoring tools as well as interim and permanent preventive measures that were employed by Dolphin Energy following the first reported instance of chloride cracking in one of the stainless steel pipe works carrying hydrocarbon mixed with H2S.
One of the sources of the chlorides responsible for the corrosion/cracking failures was believed to be from hydrolysis of Ca/Mg/Na chloride salts generated in the hot stabilizer bottom and/or from left over hydrochloric acid (from well stimulations) in the well head production fluid. This condition was not considered during the design phase.
Interim and permanent preventive measures were identified for tackling the Cl-SCC issues. Key recommendations to address Cl-SCC included a reduction in operating pressure / temperature at vulnerable sections of the unit, use of various non-destructive testing (NDT) techniques to identify and monitor the cracks and generation of an Integrity Operating Window (IOW) to keep the key operating parameters within thresholds. It was determined that a material upgrade was the permanent solution for avoiding Cl-SCC in the Condensate Stabilization Unit.
A metallurgical investigation confirmed the mode of cracking to be intergranular as well as transgranular unlike conventional chloride cracking, which is primarily transgranular in nature. The reasons for observed mixed-mode cracking could be ascribed to certain combination of Chlorides, H2S, CO2 and partially oxidized sulfur species that are present in the service apart from the degree of cold work that material has undergone during fabrication.
It is important that within one year of commissioning the unit, a detailed review is made to verify process and operating parameters (design assumption versus actual operating conditions).