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30 AUGUST 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 8 FEATURE ARTICLE Potential and current output both started to increase about two weeks after CP start-up, which was surprising and a sign that there was a problem with the CP system. Potential becomes more negative and current output is reduced after flushing with fresh seawater, indications that the CP system is functioning as it should. Images courtesy of Alex Delwiche. type of coating was not applied inside the LID monopiles or the transition pieces below the sealed lower platform. Since the area underneath the lower platform was considered to be a sealed environment, DNV 's early guidance indi- cated that oxygen ingress would not occur if the water inside the monopile was not refreshed, Delwiche says. The expectation was that corrosion would not take place in the submerged or dry portion of the mono- pile because existing oxygen would become depleted over time, and new oxygen would not enter the structure. Operators, however, were reporting concerns of a potential corrosion risk inside the sealed monopile compartment, Delwiche adds, commenting that DNV later revised its standard with a statement that monopiles should have CP with coating around the splash zone. In 2011, inspection of the wind turbines at LID determined that corrosion was occurring inside the mono- piles, particularly below the water line. He comments that some of the monopiles had not stayed airtight. In these instances, the transition piece had settled, and the result- ing motion exerted enough force on the J-tubes to push out the seals around them and allow seawater—and oxygen—to enter the monopile. Coupons were used on the wind tur- bine structures at LID to measure the severity of the corrosion both above and below the water line in the sealed mono- piles as well as the monopiles that were no longer airtight. For all monopiles tested, the results indicated that corrosion rates were minimal near the sealed lower plat- form at the top of the monopile, became increasingly larger closer to the water line, and were most significant in the sub- merged areas. In the monopiles that had remained sealed, general corrosion was detected, but the corrosion rates were generally less than those found in the unsealed monopiles that were being flushed with fresh seawater, Delwiche says. For those no longer sealed, corrosion levels were higher than expected for the design, Tavares adds. Although the corrosion rates were within the range suggested by the standard and didn't pose a failure concern, they indi- cated that the structures were freely cor- roding and action needed to be taken. Fatigue curves used to calculate the life expectancy of the monopile were based on a corrosion-free state, Delwiche explains. Since this was no longer the situation, leav- ing the monopiles without any corrosion prevention system would greatly reduce the theoretical service life. A strategy was pushed forward to pro- vide the monopiles with a corrosion pre- vention system, which included resealing the J-tubes to prevent seawater flushing and installing a galvanic CP system inside the submerged section of the monopile. A galvanic anode system was preferred over an impressed current CP system based on practicality and safety. Although designing a CP system can be relatively straightfor- ward, Delwiche notes there were no specific standards or published guides available at the time that addressed the application of CP inside offshore monopile structures. Because so many questions regarding a ret- rofit CP design for this type of application were unanswered, a trial design was devel- oped and implemented on one monopile in the LID wind farms. This project is one of the first galvanic CP installations inside a submerged section of a wind turbine monopile.