O&G industry is more and more cautious about the risk of Sulfide Stress Cracking when low alloyed steels are in contact with aqueous environments saturated by high partial pressures of H2S. According to NACE MR0175/ISO 15156 part 2, material tested in NACE TM0177 Solution A saturated by 1 bar H2S above 80% Actual Yield Strength is qualified for all regions of NACE environmental severity diagram. Basically, this lab test condition is considered severe enough for widening the admissible field exposure conditions up to 10 bar H2S partial pressure. However, recent works highlighted the existence of a threshold partial pressure of H2S below 10 bar for TMCP seam welded pipes. In the present work, similar situation has been observed for girth welds on X65 seamless pipes: acceptable welds based on hardness data passed at 1 bar H2S NACE Solution A but failed in a fit-for-purpose condition of 6.8 bar H2S. Improved welding procedure has been defined for reaching the appropriate SSC resistance at this H2S partial pressure. It is recommended to evaluate the SSC resistance of girth welds at the field H2S partial pressure when superior to 1 bar.
NACE MR0175 / ISO 15156 document part 2 provides material requirements for selecting cast irons and low-alloy steels for sour environments.1 Figure 1 is reproduced from this standard and describes different regions of environmental severity for Sulfide Stress Cracking (SSC): service in region 0 being the least susceptible to cracking, even for very sensitive materials, and region 3 being the worst. This diagram is built from two chemical parameters related to the environment: pH in the y-axis and partial pressure of H2S in the x-axis. Lower pH and higher partial pressure of H2S results in higher environmental severity for H2S cracking. More specifically, NACE TM0177 defines NACE Solution A,2 which presents a pH below 4, saturated by 1 bar of H2S, as a reference environment for assessing the susceptibility of steels to cracking. According to Annex B of NACE MR0175 / ISO 15156,1 a material can be qualified as SSC resistant in all regions of Figure 1, meaning up to 10 bar of H2S, if it presents a threshold stress higher than 80% Actual Yield Strength (AYS) in this reference environment, as evaluated by uniaxial tensile, Cring or four-point bend test.
