The world´s first offshore wind farm Vindeby will be decommissioned after a service time of more than 25 years. The duplex coatings on the poles of these windmills have given it a proven effective and excellent corrosion protection over its full economic lifetime.
Two of the oldest Danish wind farms, 15 and 22 years old respectively, were recently given a thorough inspection by paint supplier Hempel. The Horns Rev 1 wind farm was installed in 2002, its towers are protected by 60 microns of thermal sprayed zinc followed by a 4-part Hempel paint system. The Thunoe Knob wind farm was installed in 1995 using 80 microns of zinc followed by a 4-part paint system. The exterior and interior surfaces were judged to be in prime condition. The zinc coating prevented the spread of corrosion away from areas of mechanical damage. The Thunoe Knob system is predicted to provide at least 25 years of complete protection with a residual durability of 5-10 years. The Horns Rev 1 system is also predicted to provide 25 years of complete protection with a residual durability of 15 years.
Mabey Bridge, a specialist supplier of high quality steel bridges, infrastructures, wind turbine towers, and heavy plated structural steel work, recently metal sprayed a number of Endurance Windpower X-Series wind towers to protect them from corrosion.
Based in Chepstow, Wales, Mabey Bridge has extensive experience in the production of wind turbine towers, with a specialist factory designed to produce up to 150 towers per year. The company manufactures wind towers from flat sheet to final fit out at its Chepstow factory. The whole process from rolling, welding, coating and fitting out a tower takes around seven to ten days.
Wind turbine towers are exposed to harsh external environments, particularly those in an offshore location. Mabey Bridge is well-established in the wind energy market, serving several major manufacturers and increasing the number of UK-made components in wind energy installations.
The metal spraying process used is a typical specification for this type of structure. The surface of the wind towers is prepared by blasting with a mix of GH25 and GH40 steel grit using a 12.5mm nozzle at nine bar pressure. The areas to be sprayed with zinc are grit blasted to standard SA3.
The blasted sections are then transferred on the blast rotators to the metal spray booth. The zinc coating cures instantly so the tower sections can be rotated directly on the outside diameter. For a land based turbine, various areas are zinc coated with typically 60-120 microns of zinc, The metal sprayed zinc coating is applied to areas more susceptible to corrosion due to assembly damage and normal usage. Specifically, these areas include the studs, brackets, internal fixing areas, the connection flanges located at both ends, plus 300mm internally and externally from the flanged ends. The base section of the tower has an access door, where the internal and external surfaces of the tower are zinc metal sprayed to the top of the access hatch plus 300mm.
After metal spraying is complete, the tower sections are transferred to dedicated paint spray booths. A typical paint specification would now see all areas of the tower section, except the flanges, being sprayed with around 50 microns dry film thickness (DFT) of a 2-component zinc rich epoxy primer coat. The flanges are left unpainted, as the profile of the metal sprayed zinc aids the strength and friction of the bolted joints. There is an approximate curing time of two hours at 40 to 50 degrees Celsius. The mid-coat, commonly a 2-component high build epoxy, is then sprayed at around 120 microns DFT for the internal surfaces of the tower and 150 microns on the external surfaces.
There is a further two hours curing time required at 40 to 50 degrees Celsius. The final top coat to be applied is around 50 microns DFT of 2-component gloss acrylic polyurethane to the external surfaces of the tower. There is approximately four hours curing time required at this stage, again at 40 to 50 degrees Celsius. This specification of zinc metal spray and paint is quite typical for land based wind towers.
This case study is courtesy of Metallisation.