Materials Performance

APR 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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14 APRIL 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 4 The relatively low resistance of seawater means interference can pose major problems on FPSO units. Model Predicts Interference Impact on Offshore Cathodic Protection D esigning and operating cathodic protection (CP) sys- tems to protect oil and gas structures from corrosion in offshore marine environments is a com- plex task. Modern developments fre- quently consist of connected assets like well casings, seabed f low lines, pipelines, wellhead structures, risers, and surface vessels on f loating production, storage, and off loading (FPSO) units. A lthough these structures are often separated by large distances, the rela- tively low resistance of seawater means they pose the potential to interfere with each other and signif icantly affect the overall performance of a CP system. Interference between anodes can degrade their ability to supply the required current, while interference can also occur between structures— sometimes resulting in a reduction in CP system life. External electrical sources can also cause interference, particularly for onshore pipelines and tanks. In recent years, however, computer modeling technologies have enabled bet- ter predictions for CP system perfor- mance, while also identif ying the risks of subpar protection or accelerated anode depletion due to interference. The effects of anode interference can be most clearly seen when anodes are closely grouped—for example around a monopile—or in an array of anodes on a sled, 1 where the output of anodes can be radically reduced when compared to that expected by historical anode resistance calculations. Meanwhile, a lesser-known interference possibility exists between multiple sacrif icial CP systems. Such interference can occur under several cir- cumstances, and could arise when differ- ent design approaches are used for sub- structures, which are then integrated together. A lthough internal resistance path- ways within structures—such as attenua- tion along pipelines—are not normally considered as a source of interference, they can have a similar impact. In a previous study, 2 it was demon- strated that signif icant interaction can occur between the impressed current CP (ICCP) system and subsea systems and f low lines if the CP system is not designed and operated correctly. Interactions could lead to accelerated depletion of the sacrif icial anodes on the subsea systems and reduced protection of the FPSO unit later in life. The ICCP systems in the prior study also exhibited faulty controls, resulting in overprotection and coating damage. Since then, computer modeling has developed over many years and is now widely used to verif y CP system perfor- mance in the maritime environment and onshore. One system using these modern techniques is the BEASY CP software package. In this package, the computer model simulates the physics of galvanic corro- sion and the features of a CP system. The model simulates the electrode kinetics on the metallic surfaces, the coating barrier, the electroly te (seawater) resistivity, the internal resistance paths in the struc- tures, and the three-dimensional geom- etry of the electroly te and the metallic structure immersed in it. To predict how a design will perform over the life of the structure, the software package performs a time dependent sim- ulation—where assumptions regarding the rate of degradation of the coatings over time are considered. These data can MATERIAL MATTERS

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