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51312-01095-UNS N07725 NICKEL ALLOY CONNECTION FAILURE

Picture for UNS N07725 NICKEL ALLOY CONNECTION FAILURE
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Product Number: 51312-01095-SG
ISBN: 01095 2012 CP
Author: Steve S. Shademan
Publication Date: 2012
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Recently a DW-GoM well operated by BP experienced the failure of a completion component manufactured from the material Inconel-725 (I-725). This precipitation hardenable nickel-based super alloy is recognized as one of the oil industry’s most reliable high-strength corrosion resistant alloys with no reported similar failures. The incident occurred after approximately 22 months of the well suspension required for commissioning of subsea infrastructure. The tubing string separated 11-feet below the tubing hanger in a 5-1/2-inch premium box connection. Examination of the failed I-725 box revealed a fracture morphology characteristic of hydrogen assisted cracking (HAC). The failure investigation systematically eliminated physical overload and metallurgical failure mechanisms through rigorous testing protocols to the degree that a single most probable root cause was identified. Results concluded that cleaning and doping chemicals sealed within the failed box during assembly reacted in a deleterious manner such that the box environment became a source of hydrogen. As the box threads exposed to this environment were highly stressed due to the prescribed connection make-up torque the combination of adsorbed hydrogen and high localized stress established conditions necessary for HAC failure of a susceptible material. Failure of the box occurred approximately 35 months after its assembly 12 months prior to running the completion string demonstrating the HAC process proceeded very slowly. Laboratory testing was conducted with multiple I-725 specimens that were ‘cleaned and doped’ using chemical solutions and compounds specified by the approved doping protocol. Testing compared box threads that had been copper plated typical of CRA surface treatment in the threaded region to prevent galling and those with no plating. Test results demonstrated that all I-725 specimens that were doped incorrectly failed within 27 days and demonstrated fracture morphology characteristic of HAC that very closely replicated the failed completion component. This Paper describes; the investigation process used to identify the root cause the proposed failure mechanism responsible for the completion failure and learnings that lead to a modified doping protocol.
Recently a DW-GoM well operated by BP experienced the failure of a completion component manufactured from the material Inconel-725 (I-725). This precipitation hardenable nickel-based super alloy is recognized as one of the oil industry’s most reliable high-strength corrosion resistant alloys with no reported similar failures. The incident occurred after approximately 22 months of the well suspension required for commissioning of subsea infrastructure. The tubing string separated 11-feet below the tubing hanger in a 5-1/2-inch premium box connection. Examination of the failed I-725 box revealed a fracture morphology characteristic of hydrogen assisted cracking (HAC). The failure investigation systematically eliminated physical overload and metallurgical failure mechanisms through rigorous testing protocols to the degree that a single most probable root cause was identified. Results concluded that cleaning and doping chemicals sealed within the failed box during assembly reacted in a deleterious manner such that the box environment became a source of hydrogen. As the box threads exposed to this environment were highly stressed due to the prescribed connection make-up torque the combination of adsorbed hydrogen and high localized stress established conditions necessary for HAC failure of a susceptible material. Failure of the box occurred approximately 35 months after its assembly 12 months prior to running the completion string demonstrating the HAC process proceeded very slowly. Laboratory testing was conducted with multiple I-725 specimens that were ‘cleaned and doped’ using chemical solutions and compounds specified by the approved doping protocol. Testing compared box threads that had been copper plated typical of CRA surface treatment in the threaded region to prevent galling and those with no plating. Test results demonstrated that all I-725 specimens that were doped incorrectly failed within 27 days and demonstrated fracture morphology characteristic of HAC that very closely replicated the failed completion component. This Paper describes; the investigation process used to identify the root cause the proposed failure mechanism responsible for the completion failure and learnings that lead to a modified doping protocol.
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