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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Page 32 of 80

30 JUNE 2019 W W W.MATERIALSPERFORMANCE.COM technique. Hardness test results for the dif ferent zones of the weld were found below 245 HV 10, in line with the sour ser- vice requirement. 8 Review of the swing test results per- form ed during commi ssioning did not highlight any lack of isolation. Discussion Engineering design considered a pro- duced multiphase fluid with water content of 29%. A water resistivity of 5 Ω·cm was estimated based on a chloride content of 13.1% measured in the produced water. 9 As per engineering best practices, the electri- cal isolation required an isolating spool. A literature survey revealed there are several methodologies to calculate the required length of an internally coated pipe section to prevent bridging of current from the unprotected to the protected side of the MIJ. The industry-accepted best practice relies on an empirical correlation between the electrolyte resistivity and the pipe diameter 10 to determine the required spool length. This practice was compared with alternative models that rely on either an electrical attenuation model applied to the internal side of the pipe 11 or a model based on an electrochemical model considering activation polarization and large overpo- tential on the steel surface. Applying the engineering best practice, the estimated spool length was 73 m, the a tt e n u a t i o n - b a s e d m o d e l p r o v i d e d a required internal coating length of the iso- lating joint of 8.7 m, while the electrochemi- cal-based model required a coated length of 38 m. The total applied internally coated length of the isolating joint was 1.8 m (0.90 m to each side). As the comparison between this value and the previous calcula- tions has shown, regardless of the model used, the MIJ was not adequately designed, and current was expected to bridge across the MIJ, producing the corrosion observed downstream of the coated section of the MIJ (unprotected side). The corrosion observed immediately downstream of the internally coated section at the unprotected side of the MIJ was caused by current flowing between th e unprot e ct ed an d prot e ct ed si d e s through the internal conductive media, which is a well-known phenomenon. The step resulting from the weld root penetration associated to a low-flow condi- tion of the f luid contributed to a semi- circumferential distribution of the pitting upstream of the field weld joint and the conf luence of pitting distribution down- stream of the weld. A significant amount of corrosion was also observed at the steel/isolating mate- rial interface on the unprotected side of the isolating joint, indicating that the gap between the nonmetallic material and the face of the steel was compromised, leading to loss of isolation in this area, further reducing the isolation length. The break of isolation at the steel/nonmetallic interface by electrolyte ingress may have resulted from: • Bending stresses caused by improper installation of the isolating joint. In fact, inspection of the isolating joint while still assembled on the pipeline revealed that it was not properly installed. As per the manufacturer's recommendations, it should have been installed in a pipe support to avoid bending momenta. Lack of sup- port introduced a momentum caused by gravity, inducing stresses in the O-ring and/or the isolating elements, which was conducive to electrical contact at this location. • Tensile or bending stresses applied on the monolithic isolating joint caused by buckling of the line. • Deficiency/absence of the coating at the metal-to-polymer interface or sealant coating applied on the inter- nal surface for proper sealing of the isolating material. • Damage to the O-ring or the isolating element due to poor workmanship, notably excessive or uneven radial compression or excessive heating of th e non m etal lic elem ents during welding operations. • Poor design or workmanship of the nonm etallic resin . In th e present case, the glass transition temperature of the epoxy was ver y close to its design temperature, eventually con- tributing to the failure by compro- mising the dielectric properties of the joint and promoting an electric path across the MIJ. The damage observed at this interface has not yet been reported in the literature, although it has a direct impact on the per- formance of the isolating joint by reducing the isolated section by one half of the iso- lated path (as it is equally internally coated on both sides). Loss of isolation at the iso- lating material interface probably resulted from bending stresses or buckling of the pipeline, which caused a reduction of the already under-designed coated section of the pipe, enhancing the amount of CP cur- rent bridging across the isolation , and therefore, a high corrosion rate of the steel. The observation of the leak in the pipe, and not at the isolating joint itself, is justi- fied by the geometry and overall thickness FIGURE 3 Illustration of the corrosion observed at the pipe/isolating material interface, at the 4 and 8 o'clock positions. CATHODIC PROTECTION

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