51314-3787-Failure Investigation of a C110 Connection from a High Pressure High Temperature Well

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Product Number: 51314-3787-SG

ISBN: 3787 2014 CP

Author: David Hillis

Publication Date: 2014

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A High Pressure High Temperature (HPHT) well in the UK sector of the North Sea experienced the failure of a coupling component in the 10¾" production casing string. The coupling was manufactured from a 110ksi sour service grade chromium-molybdenum low alloy steel which was selected to mitigate against the risk of sulphide stress cracking on exposure to reservoir gas. At the time of the failure the well had been suspended for more than one year with pressure hydrocarbon gas in the production and intermediate annuli. The failure was found to have occurred due to a stress corrosion cracking mechanism unprecedented for this material and application.The paper describes the analysis work undertaken on the casing string to determine the root cause of the failure. This included 100% non-destructive examination to detect and classify cracking present and identify trends with depth. Comprehensive metallographic and fractographic examination was performed in order to determine the mechanism conclusively and augmented by extensive surface analysis using EDS and XPS techniques. Complimentary work on assessing mechanical properties hydrogen uptake and thread dope condition from connections from different depths in the well was also conducted. Furthermore 3D finite element analysis was utilised to understand the magnitude and distribution of stress in the connection from make-up and through the various loading scenarios experienced in service. This allowed correlation between stress state and observed orientation and distribution of cracking to clarify this aspect of the mechanism. Lastly laboratory studies on the specific corrosive environment and mechanism simulation were undertaken in order to better understand this novel case of stress corrosion cracking.