Materials Performance

AUG 2017

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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51 NACE INTERNATIONAL: VOL. 56, NO. 8 MATERIALS PERFORMANCE AUGUST 2017 ISO 15156 3 hardness requirements. The longitudinal welds had a postweld heat- treatment (PWHT) but girth welds were as-welded. Gas Leak A leak occurred in an 8-in gas flowline at a low point at the bottom of a decline where the pipe passed through a bore made for a dry river crossing. A circumferential crack passed through the pipe wall, causing the pipe to part when it was removed from the bore (Figure 1). There were multiple crack initiation sites on the inside of the pipe close to the girth weld. There was no significant wall loss du e to corrosion , although shallow pitting was obser ved . There was an accumulation of solids close to the fracture, and some liquids were recovered. Girth welds upstream and downstream of the leak site were examined by wet fluo- rescent magnetic particle testing (WFMT). Twenty gir th wel d s were found to b e cracked. While the fractured weld was at the low point, the other cracked welds were all on the downstream side of the low point as the pipe rose up the incline. No welds were cracked on the downhill portion of the line that was upstream of the leak. At the low point, cracking was almost completely around the full circumference of the pipe; howe ver, th e cracking was reduced in extent and restricted to the lower quadrant on the incline. The pattern of cracking was consistent with liquid slugging up the incline. Sectioning revealed multiple branched intergranular cracks typical of environ- mentally assisted cracking. All cracks were in the weld area. The majority of cracks were in equiaxed parent metal close to the heat-affected zone (HAZ), but weld metal and the HAZ were also cracked. The longi- tudinal ERW seam, which had a PWHT, was not cracked and neither was the parent metal away from the weld. The maximum hardness recorded in the cracked welds was 201Hv10. All values were well below the NACE MR0175/ISO 15156 hardness limit. The H 2 S partial pres- sure of untreated gas at the maximum allowable operating pressure (MAOP) was 0.19 kPa. The flowline typically operated at half of the MAOP. If H 2 S scavenging was interrupted for some reason, the service condition would not be sour. Magnetite (Fe 3 O 4 ), goethite [FeO(OH)], siderite (FeCO 3 ), and wustite (FeO) were found close to the cracks, but no macki- nawite or iron sulfide by arsenide test was seen . Solids found at the fracture site included organosulfur compounds and ammonium salts. Sulfur and phosphorous were detected in the cracks. Percent levels of MEA and carbonate were found in pig inspection returns. Sulfur and phosphorus implicate the chemical treatment package and its reaction products. Ammonium car- bamate (CH 6 N 2 O 2 ) is a reaction product of MEA and CO 2 . Eight other pipelines operating with low f low were destructively examined by sectioning at a low point. All showed inter- granular stress corrosion cracking (IGSCC) at girth welds. Cracks found in seven of the pipelines were shallow (<200 µm), but IGSCC had penetrated 40% through the pipe wall in one pipeline. IGSCC was also found in flow loops that were destructively FIGURE 1 Shown are the low-ductility failure, multiple crack initiation parallel to the girth weld, and multi-branched intergranular cracks. FIGURE 2 An example of multi-branched intergranular stress corrosion cracks found close to recovered girth welds.

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