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50 AUGUST 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 8 CHEMICAL TREATMENT N Leaks were found at girth welds in shale gas flowlines fabricated from API 5L Grade X52 steel. Cracks had ini- tiated on the inside surface at the lower quadrant, and were intergranu- lar, with features resembling amine stress corrosion cracks in refineries and external carbonate cracking in gas transmission lines. Low flow rates permitted spent hydrogen sulfide (H 2 S) scavenger comprised of mono- ethanolamine (MEA) to accumulate at low points in the flowline. A reaction between high-pH MEA and carbon dioxide (CO 2 ) from the process gas formed a carbonate/bicarbonate so- lution, and together with high residual stresses from welding, this caused in- tergranular stress corrosion cracking of the carbon steel. N o r t h A m e r i c a n s h a l e o p e r a t i o n s consist of several hundred well pads tied to central processing facilities by a network of gas and liquid pipelines. At the well pad, multiphase f luids from each well pass through a choke before entering a high- pressure (HP) separator. Liquids either pass directly to a liquids f lowline or to a low-pressure (LP) separator, heater treater, and into storage tanks. Overhead gas from the HP separator is metered before enter- ing the gas system, through 8-in (203-mm) flowlines. The normal operating pressure is 5.5 MPa. The carbon dioxide (CO 2 ) concen- tration is 1 to 5 mol%, but typically 2 mol%. The hydrogen sulfide (H 2 S) concentration in the gas can reach 80 ppm but is usually <20 ppm. The gas is not typically inhibited; but if the well contains H 2 S, it is chemically treated by injecting a monoethanolamine ( M E A ) tr i a z i n e H 2 S s c av e n ge r i n t o a purpose-built f low loop to reduce the H 2 S concentration so it meets the pipeline specification of <4 ppm. High-H 2 S wells are treated using an Ultrafab † H 2 S mitigation unit. Gas velocities vary but often drop to 0.1 m/s (0.3 ft/s) as the well declines. Flow from several wells can be directed into a single f lowline and compression can be used during the life of a well to boost production . The pipeline specification limits the gas temperature to 120 °F (49 °C) but most wells operate between 80 to 100 °F (27 to 38 °C). Materials of Construction The pipelines were fabricated from API 5L 1 Grade X52 PSL2 electric-resistance welded (ERW) carbon steel (CS). The girth welds were made in accordance with API 1104 2 using shielded metal arc welding (SMAW), E6010 welding rod for the root pass, and an E8010 electrode for the hot pass and f i l l . Th e mat eri al s were not ordered to sour service requirements for API 5L Annex H. The girth welding proce- dures were not qualified to NACE MR0175/ Internal Stress Corrosion Cracking of Shale Gas Flowlines Edward H E av E r, BHP Petroleum, Inc., Houston, Texas, USA † Trade name.