Coatings greatly reduce the cost of offshore wind energy

A recent TWI-led project has demonstrated that thermally sprayed aluminium (TSA) coatings, which have been used for more than 40 years to protect offshore oil and gas platforms from seawater corrosion, can also reliably and cost-effectively extend the life of offshore wind turbine foundations to 20-40 years. The biggest advantage is offered in areas of highest risk. This includes areas subject to alternate wetting by the sea, i.e. the splash and tidal zone, as well as subsea. The work was part-funded by the Regional Growth Fund (RGF) via the National Renewable Energy Centre (Narec).

Protection from the sea

With the cost of wind energy still double that of fossil fuels, a reduction in the through-life cost of wind farms is a high priority. Offshore wind turbine foundations represent 30% of the cost of a wind farm. They need effective protection from corrosion. This is usually provided by a combination of thick (>0.5 mm) organic paint/epoxy coatings, and cathodic protection. The coating helps reduce the load on the cathodic protection system. In the splash and tidal zone, however, where the corrosion rate is at its highest, cathodic protection is ineffective and the structure is totally reliant on the integrity of its coating.

One major drawback of organic coatings is their susceptibility to damage. Any local breach in the coating leads to severe corrosion of the underlying steel. A recent coating failure on a wind farm off the coast of Ireland cost over £2m to rectify: 20 times the cost of the original installation itself. In fact, the cost of remedying coating failures is 50-100 times higher offshore than it is for onshore installations.

Demonstrating viability

Working with project partners Scottish Power Renewables, OGN Group, TATA Steel, Alphatek

Hyperformance Coatings, Metallisation and Hodge Clemco, TWI demonstrated that the metal spray process could easily be incorporated into the supply chain, with production costs that compare favourably with those of organic coating systems.

Using established industry standards for applying and testing TSA coatings, the project team deposited coatings on hundreds of steel samples. They found that the application of arc-sprayed TSA is more tolerant of parameter variations in manual blasting and spraying than previously thought. In fact, the surface preparation requirements are more forgiving than those for organic systems. Coating adhesion and damage tolerance are better and unlike organic coatings, a thin <0.5 mm layer of TSA continues to protect the structure even if it is locally damaged.

TSA coatings could also mitigate production bottlenecks associated with painting, through the elimination of curing time. TSA can be economically applied to structural steel tubulars using an automated system, leaving the ends uncoated for welding. Welds can then be coated manually onsite.

Follow-on work is planned to investigate the damage tolerance of TSA in more detail, demonstrate onsite repair methodologies, develop a detailed through-life cost model and work towards the inclusion of TSA in offshore wind turbine industry standards.

For more information please email contactus@twi.co.uk

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