Abstract
Eight downhole flow control (DHFC) valves were installed between 2007 and 2012, two in each of four deepwater wells in the Gulf of Mexico (GoM) by BP. Five of the eight valves failed to function correctly when the wells were subsequently commissioned within two to six month after installation. After the first three failures, down hole filters were implemented in the hydraulic control lines as a part of mitigations and remedial actions. The changes appeared to have solved the problem—until two further failures occurred in 2012. This paper presents the physics-of-failure (PoF) based root cause analysis (RCA) conducted to investigate the causes of the last two valve failures. Key elements of the investigation process are highlighted. These key elements can help to make any investigation efficient, effective, informed and objective. The investigation showed that even when a failed part is not available for failure analysis, symptoms of the failure can be used to identify the failed part(s).
The final two failed valves were in a single well that had been tested operable at the time of installation and shut in with completion brine in the tubing. Two months later, during commissioning of the wells, the valve sleeves failed to shift. Based on an examination of the few recoverable parts from the inoperable flow control systems and a careful review of the fluids to which they were exposed, the failure mechanism for the DHFC valves was identified as deposition of corrosion products on the sleeve that interfered with its sliding. The sleeves with surfaces hardened using quench-polish-quench (QPQ) nitriding process of 13% Chromium (13Cr) stainless steel corroded upon exposure to completion brine with trace oxygen and hydrocarbons with dissolved carbon dioxide from the pay zone.
The performance of DHFC valves with QPQ-nitrided 13Cr surfaces in deepwater wells can now be explained in light of the identified failure mechanism. Wells that were produced shortly after completion had operable DHFC valves since the produced hydrocarbon fluids were not sufficiently corrosive to attack the QPQ-nitrided 13Cr. However, wells which were not produced for at least two months after well completion had failed DHFC valves, since the completion brine along with hydrocarbons had sufficient time to corrode and impair the valves function.
Ten DHFC valves made from Inconel 718 have subsequently been installed, two each in five deepwater wells, and all ten have functioned satisfactorily. The recurrence of a similar incident has been systematically prevented. Further, use of QPQ-nitrided 13Cr equipment on other downhole applications has been reviewed for suitability.
