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03129 OFFSHORE NICKEL ALLOY TUBING HANGER AND DUPLEX STAINLESS STEEL PIPING FAILURE INVESTIGATIONS

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Product Number: 51300-03129-SG
ISBN: 03129 2003 CP
Author: S. Huizinga, B. McLoughlin, W.E. Liek, J.G. de Jong
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In the offshore oil and gas industry, extensive use is made of corrosion resistant alloys. Two failure cases are described, one with the Cr and Mo containing nickel base alloy UNS N07718 and one with the 22%Cr duplex stainless steel UNS S31803. A UNS N07718 nickel alloy tubing hanger failed at the box end of the hanger at one of the upper threads, where stress was at a maximum. The cause was found to be the presence of delta phase, which rendered the material sensitive to hydrogen embrittlement, as was shown in a laboratory study. Hydrogen could originate from the manufacturing process, in which case it could be prevented, or from in situ corrosion, which may be enhanced by galvanic coupling to carbon steel. UNS N07718 accessories should therefore be produced in such a way as to minimize the formation of delta phase and evaluation of the presence of delta phase should be part of the qualification procedure. In a UNS S31803 duplex stainless steel hot condensate piping system, a leak was detected. It was found to have resulted from cracking from the inside at a location where pressure drop could lead to water evaporation. A laboratory study confirmed that at elevated temperature (140 °C) chloride stress corrosion cracking occurs, even in essentially oxygen free conditions, when the steel is exposed to the highly concentrated brines with reduced pH that result from evaporation. The role of oxygen appears to be an acceleration of the cracking process. In cases where brine concentration cannot be avoided, construction materials need to be upgraded. Keywords: nickel alloy, tubing hanger, duplex stainless steel, piping, embrittlement, corrosion, cracking
In the offshore oil and gas industry, extensive use is made of corrosion resistant alloys. Two failure cases are described, one with the Cr and Mo containing nickel base alloy UNS N07718 and one with the 22%Cr duplex stainless steel UNS S31803. A UNS N07718 nickel alloy tubing hanger failed at the box end of the hanger at one of the upper threads, where stress was at a maximum. The cause was found to be the presence of delta phase, which rendered the material sensitive to hydrogen embrittlement, as was shown in a laboratory study. Hydrogen could originate from the manufacturing process, in which case it could be prevented, or from in situ corrosion, which may be enhanced by galvanic coupling to carbon steel. UNS N07718 accessories should therefore be produced in such a way as to minimize the formation of delta phase and evaluation of the presence of delta phase should be part of the qualification procedure. In a UNS S31803 duplex stainless steel hot condensate piping system, a leak was detected. It was found to have resulted from cracking from the inside at a location where pressure drop could lead to water evaporation. A laboratory study confirmed that at elevated temperature (140 °C) chloride stress corrosion cracking occurs, even in essentially oxygen free conditions, when the steel is exposed to the highly concentrated brines with reduced pH that result from evaporation. The role of oxygen appears to be an acceleration of the cracking process. In cases where brine concentration cannot be avoided, construction materials need to be upgraded. Keywords: nickel alloy, tubing hanger, duplex stainless steel, piping, embrittlement, corrosion, cracking
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