Widely used in the offshore oil and gas industry, thermally sprayed aluminium provides an alternative to the use of conventional coatings and anodes on foundations in the offshore wind industry if barriers to adoption can be overcome
Corrosion protection and management of foundations for offshore wind turbines is a potentially costly undertaking in terms of initial capital outlay and in long-term maintenance. The current solution of paint and sacrificial anodes not only slows foundation construction time but has a limited lifetime that requires frequent inspection and maintenance activities.
Anodes require additional secondary steelwork to be added to a foundation, and paint requires many days to dry, slowing production times and adding cost. Typical marine paints will degrade over time (particularly in the splash and tidal zone) under the action of UV radiation and marine fouling and can be damaged by impact. Organic paint coatings provide no protection to the underlying steel when damaged and corrodes rapidly.
The Welding Institute (TWI) in the UK believes that a more cost-effective solution could be at hand in the form of thermally sprayed aluminium (TSA), which, it believes, has the potential to entirely replace paint and anodes, providing robust, long-term corrosion protection that saves money not only in O&M but also in initial capital outlay.
Although TSA is used extensively in the offshore oil and gas industry on platforms and in pipeline applications and successful use of TSA has been reported on tension leg elements and production risers in deep water, it has yet to be embraced by the offshore wind industry. Now, however, an industry consortium of which TWI is a part, which is funded by Innovate UK, intends to change that.
“Replacing conventional corrosion protection, based on paint and sacrificial anodes, with a single coating of TSA can provide a highly reliable coating that requires less maintenance and repair and also the cost of manufacturing foundations,” it says. “While often seen as an expensive coating option, TSA can offer significant savings during fabrication as there is no need for sacrificial anodes, secondary steelwork or extended curing times for multiple coatings of paint.”
As Henry Begg, a senior project leader at TWI, told the Offshore Wind Energy 2017 conference, apart from its perceived cost, another challenge of using TSA in the offshore wind industry that the consortium is addressing is quantifying its ability to provide local cathodic protection in regions of damage. If sacrificial anode use is to be reduced or eliminated, a high level of confidence is required in the ability of the coating to prevent corrosion of the underlying steel.
To gain further insight into the potential of TSA coatings TWI used electrochemical methods to provide quantitative data on their corrosion rates in a range of conditions. The effect of coating damage was investigated, including data on area ratio of acceptable damage, geometric distribution of any through-thickness defects (holidays) present and absolute size of holiday areas. The interaction of TSA with imposed cathodic protection was also investigated to provide data on current drain of any galvanic anodes when used in conjunction with TSA coatings, allowing cathodic protection systems to be efficiently designed.
Mr Begg said electrochemical studies showed that TSA was able to tolerate damage in excess of 15 per cent of the total area, while still providing protection to the steel substrate. Local pH changes in the region of damage as a result of the cathodic protection action of the coating result in the precipitation of calcareous material from seawater. Such deposits act to reduce the area of exposed steel, enabling the coating to continue to provide protection without being consumed at a high rate. The interaction of imposed cathodic protection was combined with damage tolerance studies, providing industry with the data needed to design cathodic protection systems in cases where sections of structures must remain uncoated (such as for inspection purposes).
“By addressing barriers to TSA adoption in the offshore wind sector, costs associated with the installation and maintenance of foundations can be significantly reduced,” Mr Begg said. “Providing foundation designers and developers with data on the corrosion performance of TSA under a range of conditions will afford the required levels of confidence in the technology and facilitates its more widespread adoption.”