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31 NACE INTERNATIONAL: VOL. 56, NO. 8 MATERIALS PERFORMANCE AUGUST 2017 Retrofitting Wind Turbine Monopiles with Cathodic Protection Two months after CP commissioning, the water had a blue sheen. A trial anode string is lowered into the monopile's contained water. The Cathodic Protection System Design The CP design had to consider several issues, including the weight and manage- ability of the anodes for offshore installa- tion, options for mounting the anodes, and hydrogen evolution in a sealed environ- ment. Anode weight was a key concern for the CP system from a personnel-handling standpoint because this installation would require loading anodes onto a sea vessel; lifting them onto the wind turbine's transi- tion piece by a davit crane; placing them inside the monopile; and lowering them into position from the hang-off location on the lower platform. Aluminum alloy (Al-Zn- In) anodes were selected because they have two to three times more electrochemical capacity (i.e., the amount of material con- sumed over time as electric current flows) than zinc anodes—up to 2,500 ampere hours per kilogram. Since one aspect of the trial was to determine the total anode weight needed to meet the CP minimum protection require- ment of –800 mV or more negative vs. a sil- ver/silver chloride (Ag/AgCl) reference electrode, four anode strings comprised of six 55-lb (25-kg) aluminum alloy anodes cast on galvanized steel wire rope were installed. Although the CP designers expected that two anode strings would pro- duce the necessary current for CP protec- tion, the other pair of anode strings would be readily available during the trial if needed. For safety reasons, connections between the anode strings and structure were made in a junction box located above the sealed lower platform so the anode con- nections could be made without entering the airtight column. Initially, only two anode strings were connected to the structure. Six Ag/AgCl dual reference electrodes, located in two positions at three elevations in the monopile, monitored the structure- to-seawater potential. An additional refer- ence electrode monitored an anode string. Hydrogen sensors and a 1- by 1-m steel plate to measure current density (CD) were installed at the top of the sealed monopile column. The monitoring system also included a pH probe, dissolved oxygen probe, remotely accessible datalogger, and current input monitor to confirm the oper- ation of a ventilation system, which was switched off after a short time period to determine the presence of accumulated hydrogen, one of the major concerns with installing a CP system in an airtight environment. Trial Results According to Delwiche and Tavares, a high cathode CD is expected when the CP system is first initialized, then over time the current output from the anodes is reduced and the CD decreases as the structure polarizes. For this CP system, the structure started to polarize as expected, with a high current output initially and then a reduc- tion in current output as the structure achieved more protection (as measured by the CD plate, which was a clean sheet of steel vs. the uncoated, corroded monopile structure). After two weeks of operation, however, the structure unexpectedly started to depo- larize and the structure-to-electrolyte potentials became less negative, which meant the structure was no longer ade- quately protected. The anode current out- put and CD also increased, which was unusual because the anode current output and CD were expected to show a time- dependent decrease as gradual structure polarization occurred. The ventilation sys- tem had been switched off so hydrogen lev- els could be monitored, and only insignifi- cant changes in hydrogen were detected. At this stage, a risk of hydrogen build-up in the monopile, which could create a hazardous area within the monopile, was not a concern. To boost the CP current, Delwiche says, the third anode string was connected to the CP system; however, that only made the results worse. The anode current output and cathode CD continued to increase, which raised concerns about the function- ality of the CP system. Two months after commissioning the trial CP system's two anode strings, personnel were sent offshore to assess the monitoring equipment and take water samples. The monitoring system was checked and found to be fully func- tional and the readings were verified to be accurate. When the airtight platform was opened to take water samples, the findings were surprising. The hydrogen sulfide (H 2 S) alarms were set off and a strong rotten egg smell was noted, which confirmed the alarms were, in fact, measuring H 2 S. The water had a distinctly blue sheen, Delwiche notes, with whitish deposits seen around the monopile walls that were later thought to be early signs of calcareous deposits. Tavares comments that water samples tested in a laboratory showed very low pH levels—as low as 4.5. Within a 10-week period, the seawater pH inside the mono- pile had changed from a near neutral pH of 8 to an acidic pH of <5, which was much lower than that for typical seawater.