Materials Performance

JUN 2019

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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29 MATERIALS PERFORMANCE: VOL. 58, NO. 6 JUNE 2019 uted to sealant failure due to application of the isolating joint outside of its design operating temperature. 4 Failures associated with a properly designed MIJ can also result from current bridging due to internal coating damage, such as the result of scraping pig opera- tions or coating abrasion when sand pro- duction occurs with the produced fluid. This article describes the investigation of a leak that occurred on a buried three- layer polypropylene-coated oil transfer line just seven months after commissioning of the CP system. The leak was detected at the 6 o'clock position, ~1 m downstream of the isolating joint, at the receiving end of an oil transfer line. The 24-in monolithic isolating joint was installed above ground, on a horizontal plane, and was made of cold drawn seam- less (CDS) carbon steel (CS) (ASTM A106 5 ) w ith insul atin g sp ac ers mad e of GRE (grades G10-G11). 6 The sealing consisted of an elastomeric material suitable to meet the specified design conditions. Spark arrestors were installed across both isolating joints, located at the ends of the pipeline. The pipe section corresponding to the nonprotected side of the MIJ was cut longi- tudinally to allow the observation of the internal surface (Figures 1 and 2). The visual observation of the internal surface revealed: • The internal coating was in good condition and extended up to 90 cm from each side of the joint. • A section with several pits distrib- ut ed on an arc of circumference between the 4 to 5 and 7 to 8 o'clock positions on a strip delimited by the end of the internal coating and the circular weld connecting the isolat- ing joint to the CDS pipe side (Figure 3), w here the trespassing pit was located at the 6 o'clock position, 30 mm downstream of the coating. The pit distribution extended up to ~1 m of the coated section, and the num- ber as well as the depth of the pits faded as the distance increased, con- verging to th e 6 o'clo ck position FIGURE 1 General view of the internal surface of the monolithic isolation joint, at the transition from coated to noncoated sections. FIGURE 2 Picture illustrating the circumferentially distributed pits converging axially to the 6 o'clock position as the distance to the coated/uncoated pipe increased. (Figures 1 and 2). All pits presented a cylindrical shape, more evident as their number decreased. • Corrosion damage occurred at the steel/dielectric material interface, on the CDS (unprotected) side of the MIJ between the 4 and 8 o'clock positions, with through corrosion of the pipe thickness (Figure 3). No leak has been observed at this location, which was attributed to the geometry of the MIJ. Laboratorial testing of metallic sam- p l e s c o l l e c t e d f rom b o th pi p e sp o o l s revealed chemical and mechanical proper- ties in line with the requirements of ASTM D709. 7 Metallographic examination of the w e l d j o i n t s re v e a l e d m i c r o st r u c tu re s c o h e re n t w i t h a n a d e q u a t e w e l d i n g

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