21 January 2014
Offshore Wind Farms are expecting a life of at least 30 years with minimal maintenance. This includes the coatings to the Towers and Transition Pieces, which are above and below the water line.
There are two basic methods to stop corrosion. The first is to apply a ‘Barrier Coating’, which is a thick coating that prevents salt water getting through to the steel.
The second method is to apply a much thinner ‘Anti-Corrosion Coating’, which has built-in anti-corrosion properties, for example, Thermal Sprayed Aluminium (TSA).
There are few paint coatings that can reliably even approach a 30-year performance period.
In practice, at Barrier we have used Glassflake-Filled Epoxy (GFE) coatings from various manufacturers at between half and 1mm thick.
The 1mm coating is used in the most aggressive area, known as the ‘Splash Zone’ which is affected by waves, and is normally applied in three layers.
A more long-lasting coating is TSA. This has been used successfully for military ships for over 60 years worldwide. It has also been used on North Sea Oil & Gas platforms for over 40 years. It has a proven track-record when applied correctly.
The coating is applied by melting aluminium in a flame, or by arcing two wires, which is a process similar to welding.
The coating is usually 1/5th of a millimetre-thick and must be applied in a single layer as it will split if applied in two layers. However, it ‘cures’ immediately as it cools, which is unlike paint that can take a day to cure per coat. Therefore, it is an ideal coating for mass production.
The detailing of the structure is just as important to long-life. There should be no sharp edges, for example, all edges should be rounded to a radius of at least 3mm.
It is also important to eliminate water traps, and small boxes where corrosion could accelerate. Even welds should be smooth to avoid weak spots in the coating.
These areas break down first, as do repairs to the coatings. Therefore, it is essential to carry out all welding before coatings are applied. If attachments must be made, then bolting should be considered.
Overall, the coating is 50 percent of the answer. The rest is down to design and eliminating coating repairs. In addition, it is essential that a competent supplier is engaged to apply the coatings to company specification through a thorough inspection of the process.”