Failure analysis: SCC limits of AISI 321 stainless steel grade for oil gas

摘要

While AISI 321 should not be used in offshore environment anymore, a production separator made of AISI 321 was used on a Total E&P asset in the Middle East for more than 20 years of service in presence of salts, CO_2 and H_2S at about 60°C; Recently, the process temperature raised to 70-75°C causing a severe cracking and leakage. The crack occurred close to the vessel inlet where both the temperature and the chloride content are very high (T=70-75°C and about of 150,000 g/1 Cl~-). In addition, at the crack location the calculated pH at 60-70°C range is above 5, as this location is close to liquid entrance and the presence of formation water is very likely. On the other hand, no crack was detected at the other extremity of the vessel where the temperature is around 60°C. The chloride content would be low on top of the vessel in contact with the gas phase. Due to condensation effects at this location, the estimated pH can be lower than 4 and CT concentration down to 1000 ppm. In this context, this paper deals with some laboratory tests in order to determine the threshold temperature for SCC initiation on AISI 321 alloy under high chloride concentration in formation and condensed water conditions and in presence of H_2S.