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28 AUGUST 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 8 O FEATURE ARTICLE Retrofitting Wind Turbine Monopiles with Cathodic Protection Strategy Developed to Protect Submerged Internal Spaces with Aluminum Anodes Kathy Riggs Larsen, Editor Offshore wind power has gone from a marginal in- dustry to a major renewable energy source in North- ern Europe within the last decade. According to the latest offshore wind operational report prepared by The Crown Estate, 1 the technology that barely existed 10 years ago in the United Kingdom has multiplied 10-fold, with over 30 wind farms supplying ~5.4% of the United Kingdom's total estimated electricity con- sumption in 2016. At the end of December 2016 there were 29 offshore wind farms with a total of 1,463 fully operational offshore wind turbines on the U.K. sea- bed that generated over 5.1 GW of operational capac- ity. An additional 14 offshore wind farms with a total of 830 wind turbines are now under construction in the United Kingdom and expected to add new opera- tional capacity of 5.3 GW. Lynn and Inner Dowsing (LID) is a combination of two wind farms located off United Kingdom's East Lincolnshire coastline in the North Sea. Project con- struction started in 2006 and the wind farms became operational in 2009. LID is comprised of 54 monopile- foundation wind turbines with an output capacity of 3.6 MW each for a total maximum output of 194 MW. The monopiles and their steel transition pieces were installed in 2007 and the turbines in 2008. 2 The monopile foundation is currently the most common type used on wind turbines due to its ease of installation in shallow to medium water depths (up to 30 m deep). 3 A typical monopile foundation consists of a hollow, ~4- to 6-m diameter steel pile with ~50-mm thick walls that is driven ~25 m into the seabed. Usually 1 to 2 m of the pile structure is above the water line and a portion of the monopile's interior remains flooded. The transition piece, which supports steel components such as boat landings, ladders, and platforms required for accessing the wind turbine, is installed over the top of the mono- pile with an overlap of ~6 to 9 m. The annular space between the monopile and the transition piece is then filled with high-strength grout that cements the two pipes together. While in service, the transition piece will settle slightly from vibration of the turbine or broken bonds between the steel and the grout. The bottom end of the transition piece (called the lower platform) is sealed, and sits inside the top of the monopile. For a large number of designs, internal J-tubes are used to house the turbine's electrical con- nection cables. They enter the base of the monopile near the sea floor and run up the inside of the pile into the transition piece. Specially designed seals are used at the interface between the monopile wall and the J-tubes to create an airtight space within the monopile. According to NACE International member Alex Delwiche with Deepwater EU, Ltd. (Surrey, United Kingdom) and Isaac Tavares with Centrica plc (Windsor, United Kingdom), the early version of a design standard for offshore wind turbines prepared by DNV 4 noted that corrosion protection on the inside of the monopile is not required if the pile is airtight (i.e., there is no or very low oxygen content) and the structure is non-corroding. Construction of the LID structures followed this design standard, Delwiche notes, so cathodic protection (CP) or any