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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32 AUGUST 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 8 FEATURE ARTICLE The junction box, installed above the lower platform, connects the anodes to the structure. The monitoring system collects data from hydrogen sensors, a CD plate, pH probe, dissolved oxygen probe, and current input monitor to confirm the operation of a ventilation system. According to Tavares, the offshore CP retrofit trial had revealed an unexpected phenomenon—the introduction of alumi- num anodes inside a sealed wind turbine structure reduces the water's pH level very quickly. A low solution pH reduces the effi- ciency of the anodes, he explains, making it difficult for them to produce the calcareous deposits required for adequate CP and also increasing the risk of hydrogen production. Water pH levels at the values observed or the production of toxic and flammable gases (namely H 2 S) from the operation of a sacrificial CP system when using aluminum alloy anodes had not been previously reported, although trace hydrogen was expected. Aluminum anodes have been used extensively for decades in the offshore oil and gas sector and marine industry— within ballast tanks and platform concrete leg foundations—without any reported concerns relating to changes in water pH, Tavares adds. At the time of the CP trial, issues associated with the use of aluminum anodes in stagnant water conditions were unknown. The reduction of the water's pH is believed to be associated with hydrolysis (the chemical breakdown of a compound due to its reaction with water) of the sacrifi- cial anode's corrosion product. "At the time, we didn't appreciate what the phenomena was. We were putting more aluminum in the water by introducing the third anode string," Delwiche comments. He notes that the additional aluminum alloy anodes increased the amount of corrosion product, and the pH of the water decreased as the aluminum ion concentration increased. Although the CP's electrochemical process produced hydroxyl ions that should have neutralized the acidic conditions created by hydrolysis, the hydroxyl ions also reacted with the buffering salts in the stagnant water to form relatively insoluble calcare- ous deposits—magnesium hydroxide [Mg(OH) 2 ] and calcium hydroxide [Ca(OH) 2 ]. This reduced the concentration of free hydroxyl ions to a level that was not sufficient to offset the acidic conditions created by hydrolysis of the anode corro- sion product. 5 A number of options were considered to counteract the low pH within the monopile, including the introduction of chemicals into the stagnant water. Several concerns, however, prevented the project team from executing this solution. Conveying chemi- cals in either large or concentrated doses can pose a health and safety risk on sea ves- sels and the transition piece. Additionally, it would be challenging to manage a chemical mixture in an offshore environment, Delwiche says. Ultimately, the solution was to regularly add fresh seawater to the monopile's stag- nant seawater environment. This was done by drilling holes in the seals used on the J-tubes at the bottom of the monopile so the seawater inside the pile was continuously flushed with fresh seawater. The holes were carefully designed to allow a 5% change in water per day and drilled so that the tides, especially the spring tides, did not reach the monopile's lower platform. After applying this solution on the trial monopile, the flush- ing technique was implemented on four additional monopiles with favorable out- comes. The CP design was finalized based on the trial results, and work started on retrofitting the remaining wind turbines in the LID wind farms. Delwiche notes this solution has proved to be extremely effective, although constant monitoring is required to ensure the fresh seawater flushing continues. Ongoing porta- ble pH checks confirmed that most of the monopiles now have water pH levels above pH 6, which is considered satisfactory, with the majority exhibiting a water pH level of 7. The H 2 S, which was subsequently found only in monopiles that had a water pH level <6, abated over time when the pH level returned to normal. The trial was extremely valuable in that it established current output require- ments and longevity of the anode strings and confirmed the original CP design premise for the monopile, he says. This article is based on CORROSION 2017 paper no. 8955, "Retrofit Strategy Using Alu- minium Anodes for the Internal Sections of Windturbine Monopiles," by A. Delwiche and I. Tavares. Photos courtesy of Alex Delwiche. References 1 "Offshore wind operational report January- December 2016," The Crown Estate, June 2017, https://www.thecrownestate.co.uk/ media/1050888/operationalwindreport2017_ final.pdf ( June 29, 2017). 2 A. Delwiche, I. Tavares, "Retrofit Strategy Using Aluminium Anodes for the Internal Sections of Windturbine Monopiles," CORROSION 2017 paper no. 8955 (Houston, TX: NACE Interna- tional, 2017). 3 A.R. Black, L.R. Hilbert, and T. Mathiesen, "Corrosion Protection of Offshore Wind Foun- dations," CORROSION 2015, paper no. 5896 (Houston, TX: NACE, 2015). 4 DNV-OS-J101, "Design of Offshore Wind Tur- bine Structures" (Høvik, Norway: DNV, 2014). 5 A. Delwiche, P. Lydon, I. Tavares, "Concerns Over Utilizing Aluminium Alloy Anodes in Sealed Environments," CORROSION 2017 paper no. 8956 (Houston, TX: NACE, 2017).

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