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Sour Service Qualification of Carbon Steels (SMYS 450 MPa) and Their Welds Considering Field H2S Fugacity / Dissolved H2S Concentration - Qualification at Design and Atmospheric Pressure

A Sulfide Stress Cracking (SSC) qualification program was conducted for carbon steel components (piping, flanges, and fittings with SMYS 450 MPa) and its welds for an offshore facility piping system that may carry off-spec gas in the event of a temporary malfunction of the dehydration and H2S removal units. The off-spec sour gas may contain 22% mol CO2 (pCO2 73 bara) and 6,700 ppm H2S (pH2S 2.22 bara) at a total pressure of 332 bara and temperature of 24 °C with condensed water pH of 3.2. The SSC qualification program included two parts. Part I involved testing at 332 bara using batch-loaded high-pressure autoclaves based on H2S fugacity (fH2S) as permitted by NACE MR175/ISO 15156-2. Part II involved testing at atmospheric pressure by matching the predicted dissolved H2S concentration of the aqueous phase (cH2S) and by matching the predicted fH2S of the gas. Part I and Part II tests were conducted using the same set-up (NACE TM0177 Method A tensile) and stress (75% AYS). Hardness readings taken at the base material (BM), heat affected zone (HAZ) and weld metal (WM) were below 250 HV10 near the root and mid-section of the rings (with couple of single reading exceptions along the HAZ for the rings involving flanges and fittings). Hence, most of the tested carbon steel rings could have been considered suitable for sour service through compliance with NACE MR0175 /ISO 15156-2. Nevertheless, post-examination of test specimens revealed susceptibility to SSC in both Part I and Part II tests. These lab results suggest that NACE MR0175 /ISO 15156-2 guidance on carbon steel SSC resistance, which is based on hardness, is not always conservative.
Product Number: 51324-20609-SG
Author: Jiangnan (Hugh) Zhang; Ramesh Rao; Felipe Ramirez; Adam Darwin
Publication Date: 2024