Wind Energy Network Magazine

Ewan Clark, Director at Aquarius Marine Coatings, discusses how copper epoxy can provide outstanding long-term protection for transition pieces, both inside and out
The majority of offshore wind turbines in sea areas with depths less than 30m use monopile foundations. These typically consist of a tubular steel pile driven deep into the seabed, a tower supporting the turbine itself, plus a steel transition piece between the two.
While the monopile must be strong enough to withstand the enormous lateral loads placed on it by the tower, it is shielded from the worst of the marine environment by the seabed itself. Similarly, the tower, while subject to spray and other adverse atmospheric conditions, is not immersed and spends much of its working life relatively
dry. Both can be protected using corrosion coatings, applied during the manufacturing process.
The same cannot be said for the transition piece. The interface between saltwater and the atmosphere is always the worst place for both fouling and corrosion. Air promotes corrosion through oxidisation, and fouling thrives in the light and warmth of the upper water strata.
UNUSUAL SET OF CIRCUMSTANCES For transition pieces, fouling presents a specific and unusual set of circumstances. Whilst the outside of
the steel tube is subject to the usual influences on a static structure – tidal scour, wave erosion and side loadings exacerbated by increased area from fouling growth – the inside of the tube has its own problems.
The pieces are not sealed, but neither are they open to significant flow: instead, once installed they fill with water which will then remain largely static. Left untreated, fouling organisms will colonise the benign environment within the tube, beginning a cycle of growth and decay which slowly fills the tube with a ‘microbial soup’. This accelerates ageing of the
structure by developing a corrosive environment within the transition piece.
MINIMISING FOULING Minimising fouling attachment is therefore essential to avoid excessive force on the exterior structure and to reduce corrosion inside the transition piece. Water flow is too far reduced for a fouling release coating to be effective, which leaves biocides as the only viable alternative.
The problem is that all biocidal antifouling coatings work through erosion, whereby the matrix containing the active biocides gradually reduces to ensure that new material is always exposed. This process, although accelerated by water flow, will occur even in still water if the coating is well designed. According to tests carried out by Plymouth Marine Laboratory in a tidal stream off Orkney, most anti-foulings lose between 90% and 100% of their material over the course of five years. While erosion inside the transition piece will be lower, this is an unacceptably short time between recoating, given the complexity and cost of access.
Equally, most anti-foulings require application within a fairly short time frame before immersion, with limits typically between three and six months. They are also slightly soft, requiring extra care when handling to avoid compromising the coating. As transition pieces may well be in storage for longer periods, this adds another layer of logistical complexity by making it difficult to apply the coating at the manufacturing stage. The alternative – applying the coating on shore shortly before installation – requires significant additional infrastructure and cost.
SOLUTION Recent tests suggest that a coating originally launched to the leisure market may have the answer. Coppercoat comprises a waterbased epoxy loaded with a powder made from recycled copper and has had notably good results on vessels from small pleasure craft through to superyachts.
In the aforementioned test by Plymouth Marine Laboratory, Coppercoat eroded by just four percent in the same five-year period. It was also the most effective, the test panels showing minimal fouling attachment to the areas treated with the product. Based on the leisure recommendations, Coppercoat is effective for a minimum of ten years, but the real period is significantly longer, with customers citing as much as twice that figure. To leach all the copper in an application would take a staggering 86 years.
EFFECTIVE FOULING DETERRENT Coppercoat provides a means of applying a continuous copper coating while retaining the convenience and application flexibility of paint. The net result is an effective fouling deterrent with an exceptionally low copper leach rate. Additionally, as it’s effectively a sheet of copper, it can be applied at any time. The epoxy base is comparatively hard and resilient to manual handling, making it well suited for application in the factory.
This gives a controlled environment where film thicknesses can be accurately monitored and cleanliness of the substrate assured, before the transition piece is exposed to the rigours of transit or storage. Ensuring a contiguous coating also makes
Coppercoat an effective treatment against corrosion.
LONG-TERM PROTECTION These factors alone make Coppercoat an ideal choice for the long-term protection of transition pieces. But there is an additional factor, which is becoming increasingly important in a market with increasing environmental awareness and regulation. With its exceptionally low leach rate, Coppercoat is better for its environment than any other biocide-based competitor and as such is unlikely to fall foul of tighter regulations in the foreseeable future.
SUMMARY For transition pieces, then, it makes sense: a long-term, effective, easy-toapply and environmentally responsible coating which will protect for years to come.
Coppercoat-Commercial