High Strength Cold Worked Corrosion Resistant Alloy CRA Materials Deployed in HPHT Wells: Strength Uncertainties Due to Anisotropy and Associated Considerations
Authors: Dhawan Sandeep , Jensen Søren Lundgren 
Published: November 2017

Deployment of corrosion resistant alloys (CRA) materials in HPHT wells is becoming more common as the quest for reserves replacement drives the Industry to deeper depths. These environments are hostile due to the presence of acidic gases, high pressure and temperature. CRA alloys developed to mitigate sulfide stress cracking, stress corrosion cracking and other corrosion risks are cold worked for high strength. The objective of this paper is to look into various strength uncertainties introduced due to anisotropy of cold worked CRA materials and related considerations.

A range of high strength cold worked or cold hardened CRAs referenced as type 2, 3 & 4 of ISO 13680/API 5CRA have been developed by a few manufacturers to cater for the variety of corrosive down hole conditions encountered. Material selection guidelines are available in NACE MR0175/ISO 15156 and with the mills. Further, fit-for-purpose material testing to characterize the corrosion and environment assisted cracking (EAC) resistance of the material can be carried out. This paper covers a stepwise look into the issues due to lack of characterization of anisotropy and its impact on various areas like strengths, material testing and qualification, connection qualification etc.

While the mechanical properties of CRAs are covered in the ISO 13680/API 5CRA standard under groups 2, 3 & 4, the characterization of anisotropic behavior exhibited by such materials and its impact on the mechanical properties is lacking in the literature. The failure mechanism(s) of such materials can be compounded by anisotropy induced through cold working as cracking is also a function of the direction of applied stress and orientation of crack plane. Anisotropy can introduce inaccuracies for material test qualification due to improper threshold stress values. The fracture path in the test specimen should agree favorably with anticipated results. The latest API RP 5C5 which covers connection qualification procedures does attempt, to some extent, to address the anisotropy effects into the connection evaluation envelope construction and testing. On the other hand, ISO 10400/API TR 5C3: 2008 does not cover the issue of tri axial yield, collapse and axial limits of anisotropic materials. Therefore, this paper also attempts to identify and highlight gaps in various API subcommittee 5 (SC5) standards, further delving into potential impact on several aspects of CRA material selection and for material testing qualifications.

Following from the above, it is evident that anisotropic characterization of high strength cold worked or cold hardened CRA materials is an evolving subject, and, very scarce experimental evidence is available throughout the industry. Therefore, this article attempts to provide insights to various strength uncertainties that can exist due to anisotropy while offering recommendations for associated considerations pertaining to manufacturing process, material specifications, materials testing and qualification, strength envelope and connection qualification.