NACE MR0175-2003 Impact on API 6A Equipment and Customers

Abstract

Abstract API 6A "Specification for Wellheads and Christmas Tree Equipment" requires compliance with NACE MR0175 for equipment supplied for use in sour (Hydrogen Sulfide-H2S) service. Major changes to the NACE standard in the 2003 revision created the need for substantial changes to the API specification in terms of how both users and manufacturers specify equipment. New environmental restrictions have been imposed by NACE on many commonly used materials. Consequently, new material and temperature classifications have been created within the API document in order to permit compliance with these restrictions. This paper reviews the background to the changes in the NACE standard, their effect on API 6A and some precautions that users of this equipment should consider. Additionally, upcoming changes to NACE MR0175 2003 with the 2004 publication of NACE MR0175/ISO 15156 will be discussed. NACE MR0175 2003 Revision NACE MR0175 prescribes the requirements for production equipment to be used in sour (hydrogen sulfide bearing) hydrocarbon service. Prior to 2003, it only addressed sulfide stress cracking (SCC) as a possible failure mode. Recognizing this, NACE formed the CRA (Corrosion Resistant Alloy) Rewrite Committee to address some of the perceived shortcomings of the earlier revisions. These included:Catastrophic failure mechanisms other than (SSC) were not included in the scope of the document, but other mechanisms such as chloride stress corrosion cracking (SCC) may be the limiting factor in a sour environment.Many environmental parameters important to a material's susceptibility to environmental cracking (SCC or SCC) were not adequately addressed. Depending on the specific alloy, these could include temperature, partial pressure of H2S, chloride concentration, presence of free sulfur, temperature, and other factors.In the last ten years or so, there has been a growing trend to include environmental restrictions that define the safe operating limits for new alloy additions to MR0175. Many of the materials included in earlier editions have a limit on maximum hardness and possibly heat treat condition, but no environmental restrictions. This gave the wrong impression that these materials were safe to use under all environmental conditions.The balloting and approval process to add new alloys was unrealistically long - in some cases years. The resultant work of the CRA Rewrite Committee formed the basis for the changes found in NACE MR0175- 2003: Standard Material Requirements: Metals for Sulfide Stress Cracking and Stress Corrosion Cracking Resistance in Sour Oilfield Environments". Historically there have been few revisions to MR0175 that have impacted the acceptability of previously approved materials. Typically, the revisions have added new alloys, modified test environments, etc. The 2003 revision, however, presents a new approach for selecting materials for use in sour service.