Wind turbine structures in offshore windfarms are subjected to aggressive environments that include humidity with high salinity, and intensive ultraviolet radiation from the sun. At the tidal/splash zone, the structures endure stress from mechanical impacts, such as waves and service boat collisions, and strain from corrosion created by saline seawater with high oxygen levels. To protect the wind turbine, particularly the foundation and transition piece (TP), long-term corrosion resistant protective coating systems are essential, since offshore repair is costly.
In CORROSION 2071 paper no. 9364, “New Developments in Coatings for Extended Lifetime for Offshore Wind Structures,” authors C.E. Weinell, T. Mathiesen, A.R. Black, and P.K. Nielsen describe a coating system that failed on offshore wind turbine TPs in the North Sea’s Horns Rev 1 wind farm after 14 years of service, and an alternative coating system for offshore wind turbines that has demonstrated good corrosion resistance.
The wind farm owner selected a two-coat, ceramic-reinforced epoxy system, applied wet-in-wet with a total dry film thickness (DFT) of 350 μm for the TP and upper part of the submerged monopile. The paint system was approved after a successful testing regime in accordance with Norwegian protective coating standard NORSOK M-501. As a trial, however, TPs of the wind farm’s last five wind turbines were painted with a two-coat, solvent-free epoxy coating system with a total DFT of 1,000 μm.
Within the first two years of service, pinpoint rusting was observed in both the atmospheric and splash zone areas of the TPs painted with the 350 μm DFT two-coat epoxy system. A forensic investigation determined the corrosion initiated as blistering on the coated surface. The blistering was caused by the thin DFT combined with insufficient grinding of the coating’s ceramic extenders. Oversized agglomerates in the thin film permitted salt water to access the steel surface through pinholes.
A 2014 inspection of one wind turbine with the 350-μm DFT two-coat epoxy coating system, after 12 years of exposure, found extensive rusting on most of the areas exposed to the splash zone. Two years later, the same wind turbine was inspected, and the structure showed almost complete loss of coating. Although the 350-μm DFT coating system passed the tough testing regime of NORSOK M-501, it did not pass service in real life. The 1,000-μm DFT two-coat solvent-free epoxy coating system on the last five TPs in the Horns Rev 1 wind farm, however, showed good, long-lasting corrosion resistance after 14 years of service. Apart from minor damage from supply boat impacts, the coating system appeared intact with no visible corrosion.
Based on the experience from Horns Rev 1, a revised coating system was introduced for the wind turbine TPs and foundations. It comprises one 250-μm nominal dry film thickness (NDFT) coat of high-build epoxy primer, two 250- μm NDFT coats of high-build epoxy intermediate coating, and one 80-μm NDFT coat of polyurethane (PU) topcoat for a total NDFT of 830 μm. The coating system has shown excellent durability on various projects in Europe. Based on the positive experience with this coating system, as well as new developments for these types of paints within the last five years, paint manufacturers recommend modifying this four-coat coating system to a three-coat, 660-μm NDFT system comprised of one 300-μm NDFT coat of high-build epoxy primer, one 300-μm NDFT coat of high-build epoxy intermediate coating, and one 60-μm NDFT coat of PU topcoat.