Page 57 - Well Control for Completions and Interventions

48 Well Control for Completions and Interventions

Figure 1.21 National Association of Corrosion Engineers (NACE) standard MR0175/
ISO 15156 limits for sour corrosion.

combines to form H 2 at the metal surface, or diffuses into the metal
matrix. As the sulfides on the metal surface inhibit hydrogen recombina-
tion, the amount of atomic hydrogen that recombines to form H 2 on the
surface is reduced, and more migrates into the metal. Once in the metal,
and away from the sulfides, the H 2 recombines to form hydrogen mole-
cules at dislocations and grain boundaries in the steel structure. Under
conditions of low stress, blistering can occur below the surface of the
steel. If the steel is stressed, it can crack and fail. High yield and inher-
ently hard materials are more prone to stress cracking.
The most widely used definition of sour service is provided by the
National Association of Corrosion Engineers (NACE) in standard
MR0175. In the United States, this standard is legally enforceable, and
equipment intended for sour service must comply with the service limita-
tions (Fig. 1.21).
Sulfide stress cracking can occur at a very low concentration of H 2 S,
especially at high pressure. Completion engineers and intervention super-
visors should be aware of the partial pressure of H 2 S, and ensure that any
pressure retaining equipment used in the completion, or for interventions
is suitable for the intended service.