Advanced Coatings for Offshore Renewable ENergy

Final Report Summary - ACORN (Advanced Coatings for Offshore Renewable ENergy)

Executive Summary:
Steel structures in a marine environment are subject to many forms of degradation. Corrosion and biofouling are major problems. Cavitation damage is also relevant where local water velocity is high, for example in tidal turbines. Structures requiring protection include docks and buoys, oil and gas rigs and the structures in the emergent ocean energy sector, including wind, wave and tidal generation systems. Collectively the cost of protecting these structures represents a significant economic load on critical areas of the European economy, with impacts ranging from transport operations, through to energy production.

The ACORN project takes advantage of the proven long-term corrosion protection of thermally sprayed aluminium (TSA) for steel substrates. Using the TSA as a matrix coating, which has a proven life of 20+ years in the sea, the ACORN research team introduced islands of environmentally friendly antifouling substances into the TSA coating. The active antifouling substance is embedded in the coating in very tiny concentrations (<1%). The antifouling mechanism is not reliant on dissolution and release of the antifouling substance into the sea, but depends only on barnacle cyprids coming into contact with the substance when they try to settle on the surface. Release of the antifouling substance into the seawater is negligible.

ACORN has developed methods that can be implemented by the SME partners involved in the project, and which combine the corrosion protection behaviour of TSA with these eco-friendly biocides. A number of coating variants were trialled and test coupons of both flat and welded sections were installed in a marine corrosion test facility in Northern Spain, where the corrosion and biofouling performance was evaluated over a 7 month period. Despite low overall levels of barnacle activity at the test site in 2015, promising results were obtained, with ACORN coatings exhibiting excellent corrosion protection and minimal barnacle settlement.

Detailed laboratory evaluation and characterisation was performed on the coatings to enhance understanding of performance and estimate coating design life. Such examinations included microscopy studies, chemical depth profiling, active-component release rate measurements, adhesion and electrochemical estimates of corrosion rate. Based on the data obtained, the ACORN coatings should be able to provide offshore structures with corrosion and fouling resistance a 20+ year design life. Therefore, the ACORN project has developed an entirely new, non-paint, approach to the long term protection of offshore steel structures.

In parallel to the development of corrosion/biofouling resistant coatings, the ACORN project team also developed and proved a corrosion and cavitation resistant coating with a 10+ year design life, suitable for tidal energy generators. Candidates for cavitation resistant coatings were selected and deposited on coupons for both corrosion testing and cavitation studies. The corrosion resistance of coatings was ranked using salt spray testing, while cavitation resistance was studied via experiments ranging from simple laboratory-based ultrasonic trials through to scaled testing of 2D and 3D components within a cavitation tunnel. Cavitation studies were backed up by detailed computational fluid dynamics modelling using a shear stress transport model. A cermet HVOF coating was finally selected as the most appropriate coating to provide long-term cavitation and corrosion resistance.

Project Context and Objectives:

Summary

The ACORN project has developed two coatings for the offshore renewable sector to address surface degradation mechanisms that have been experienced in prototypes and will present problems as the ocean energy industry grows in the future. The first comprises a new and long-lasting solution to the problem of marine biofouling, offering specific advantages for static offshore structures such as wind turbine foundations and ocean energy devices. The second consists of a cavitation resistant coating that does not degrade in seawater and is suitable for tidal energy turbine components.

The Challenge

Offshore settings present one of the harshest environments for steel structures, with seawater causing rapid corrosion of unprotected metal. Structures requiring protection include ships, docks, buoys, oil and gas rigs and the emergent renewable ocean energy generation industry. Collectively the cost of protecting these structures represents a huge economic load of critical areas of the European economy, with impacts ranging from transport operations, through to energy production.

The accumulation of marine growth on offshore structures can cause a myriad of different problems, increasing drag forces, reducing hydrodynamic efficiencies, adding weight, blocking components and making structural inspection problematic. Marine organisms can settle on almost all structures placed offshore and regular maintenance is required to inspect and remove growth. Barnacles in particular represent one of the most harmful organisms, as they are able to cut through protective paint systems and expose underlying steel to corrosion.

While control of corrosion for shipping is an enormous market, ship coating systems are the subject of intense development by paint companies and other researchers and specific needs of this sector are intensely cost sensitive. Coatings developed within ACORN are initially targeted towards static structures, where periodic dry docking is often impossible and in-situ maintenance is difficult and costly. Such structures require a long term solution to corrosion and biofouling and coating systems used for shipping are too short lived to be entirely suitable.

Fast moving components of ocean energy generators, such as tidal turbines, can suffer from a wear mechanism known as cavitation. In regions of fluid subject to rapid changes in velocity, bubbles can form as a result of pressure differences. As bubbles repeatedly implode against surfaces, very high local forces can be generated which over time, can erode the surface and damage components. The ACORN project also investigated coatings that can be applied to such components to prevent erosive wear. Such coatings also need to be resistant to corrosion by the seawater environment to which they are exposed.

The State of the Art

Current solutions to combat corrosion offshore include marine paints supplemented by sacrificial anodes. Organic paint coatings have limited lifetimes (<10 years) before needing substantial maintenance and repair. Any damage to the coating results in rapid corrosion of the underlying steel. Sacrificial anodes can provide added protection in immersed regions, but are costly to install and replace and provide no protection in splash and tidal zones where corrosion is most severe.

Current antifouling paints for static offshore structures use similar formulations that are used in the shipping industry and rely either on biocides that gradually leach out of the coating (damaging surrounding ecosystems and only providing a limited time period of effective bio-activity) or on so called ‘self-polishing’ systems that require a certain water velocity to remove accumulated growth (unsuitable for static structures).

Cavitation erosion is currently a poorly understood phenomenon and literature searches reveal some dispute surrounding the most important material properties needed to provide cavitation erosion resistance. No single coating has yet emerged to best combat this type of wear.

The ACORN Concepts

Corrosion and Biofouling Resistance

The ACORN project has developed a new coating based on thermally sprayed aluminium. Pioneered by the oil and gas sector, this coating consists of a layer of aluminium deposited onto the surface of the steel to be protected. This coating primarily forms a barrier between the seawater and the steel and prevents corrosion. The aluminium corrodes at a very low and predictable rate of <10µm/yr, with a typical coating thickness of 300µm easily providing 20+ years of protection. Unlike paint, where any damage exposes underlying steel to the corrosive environment, damage to TSA coatings results in the coating corroding preferentially. As the coating corrodes preferentially, the formation of insoluble calcareous material is promoted on the surface of the exposed steel, which seals off damaged regions from the environment. This ‘self-healing’ action results in a robust coating capable of providing excellent long-term corrosion protection.

Using TSA a basis, the ACORN consortium has incorporated regions of environmentally friendly, low-release rate biocides. Unlike traditional antifouling compounds, these chemicals do not significantly leach out of the coating and can inhibit barnacle colonisation at concentrations 100-1000 times lower than alternatives. By interrupting barnacle life-cycles as they settle on the surface, coatings can be formulated to resist biofouling while providing excellent corrosion protection.

Cavitation and Corrosion Resistance

The ACORN project evaluated a range of thermally sprayed coatings including a ceramic, a cermet and an alloy coating.

The ACORN Objectives

•Develop a coating to resist seawater corrosion and prevent barnacle settlement for 20+ years on large offshore structures.
•Develop a coating to prevent cavitation erosion damage to turbine components that does not suffer from seawater corrosion.

Project Results:
Corrosion and Biofouling Resistant Coatings

Having developed a novel method for combining TSA with antifouling compounds, detailed laboratory characterisation and evaluation was carried out and field trials were undertaken in two locations.

Corrosion of ACORN coatings

ACORN coatings are based on TSA and therefore provide a conductive metallic coating that provides sacrificial protection. Electrons within different metals will have different levels of energy. When metals are brought into contact with each other, free electrons flow such all electrons have the same energy. This results in a potential difference between two dissimilar metals. When the metals are placed in an electrolyte, this results in the more active species corroding preferentially.

In the case of ACORN coatings, this means that any region of damage in the coating will result in dissolution of the aluminium layer, rather than the underlying steel (Figure 1). However, as the aluminium corrodes, the local pH near the region of exposed steel changes as dissolved oxygen is converted to OH-. This change in local pH promotes the formation of calcareous deposits (CaCO3 and MgOH2), which form over the exposed area. This seals of the exposed steel and reduces the corrosion rate of the system (Figure 2).

Laboratory Studies

Corrosion was assessed by quantitative electrochemical assessments based on Linear Polarization Resistance (LPR) measurements. Corrosion occurs as a result of electrochemical reactions and therefore electrochemical techniques can be used to study this phenomenon. Careful measurements of current and potential can yield information regarding corrosion rates, passivity, pitting and other important data.

If a metal sample is immersed in a corrosive medium, both reduction and oxidation reactions occur on the surface. In general corrosion occurs as the specimen is oxidised and the medium is reduced. When the specimen is not connected to any measurement devices (i.e. as it would be ‘in service’), the sample assumes a potential (relative to a standard reference electrode) that is referred to as ECORR. When the specimen is at this potential, both anodic and cathodic reactions are occurring and electron transfer is equal in both directions. As this electron transfer (current) is equal in both directions, no net current flows that can be measured. When the sample is at ECORR, the system is at equilibrium with the environment and corrodes freely.

In order to measure corrosion currents, the system must be displaced from equilibrium by imposing an additional potential. This is known as polarising the specimen. If the sample is polarised to be more positive than ECORR, the anodic current dominates at the expense of the cathodic current. As the potential increases further, this cathodic current becomes negligible. The reverse is true when the specimen is polarised more negatively than ECORR. By plotting the current response of the system as the polarisation is swept by a small amount either side of ECORR, it is possible to estimate the corrosion current at equilibrium, using a quantity known as polarisation resistance. Full details of how this is calculated are deemed to be outside the scope of the present report.

If the corrosion current is known and the number of electrons each metal atom must lose during oxidation, estimates can be made for the rate at which metal atoms are being oxidised. If there is a well-defined area over which the sample is corroding, this can be converted into an estimate of corrosion rate in terms of mm lost per year. This was the method employed during laboratory corrosion studies of ACORN coatings.

Such electrochemical measurements of ACORN coatings showed a higher initial corrosion rate over the first 10 days of measurements as the aluminium coating passivated. This then decreased to a steady rate of ~2µm/yr (Figure 3).

Field Studies

Coupons of coated and uncoated S355 steel measuring 390 x 100 mm were placed into a marine corrosion test facility at ‘El Bocal’ near Santander, Spain on 20-03-15 (Figure 4). Coupons were placed in three different environments corresponding to splash, tidal and submerged zones having been weighed. Coupons were exposed for a total of 7 months, being removed on 29-10-15 and re-weighed.

Uncoated coupons showed obvious signs of corrosion and within the splash zone lost 162g, which corresponds to a corrosion rate of 0.45mm/yr (Figure 5). In comparison, coated samples lost a negligible mass (and in some cases gained mass due to accumulation of insoluble aluminium corrosion products and calcareous material).

Biofouling of ACORN coatings

While laboratory assays can provide fast results in comparing the efficacy of various coatings, they do not necessarily reflect service conditions in practice. Therefore biofouling was assessed through two field studies where coupons of ACORN coating variants were exposed to the marine environment. Real-world field trials provide a more reliable measure of biofouling resistance.

Field trial in Sweden

A series of ACORN variants were exposed off the coast of Sweden at Tjarno to provide initial fouling assessment. Few barnacles were found on ACORN coatings after 11 months. However, few barnacles were also found on control samples that contained no biocide. However, barnacle settlement was observed on the reverse side of samples that contained no TSA (Figure 6).

Corrosion performance was not as good as expected, with significant build-up of TSA corrosion products visible on the coating surface. As the reverse side of the samples was inadequately sealed off from the environment, a large area of steel was exposed, which drove the corrosion of the TSA coating. It is postulated that this enhanced corrosion had some effect on barnacle settlement that prevented large scale colonisation of control samples.

Field trials in Spain

A second set of ACORN coatings were exposed at ‘El Bocal’ near Santander for a total of seven months. In this case, the total surface of the coupons was sprayed with TSA and avoided the enhanced corrosion rate of the trials in Sweden.

However, similar effects were observed, with no barnacle colonisation of the front surface of coupons and only minimal colonisation of reverse surfaces (Figure 7). It is still uncertain as to whether this was due to the inclusion of biocide additives, some other structural characteristic of the ACORN coatings (e.g. roughness, electrochemical behaviour, etc) or an environmental effect (e.g. water velocity, light levels, etc). It appears that at the two locations in question, overall levels of barnacle settlement were particularly low for the 2015 season. Levels of barnacle activity vary both seasonally and annually and 2015 appears to have been an uncharacteristically poor year.

The ACORN consortium is returning some samples to the test site for further testing in 2016 and beyond.

Cavitation and Corrosion Resistant Coatings

Corrosion of cavitation resistant coatings

Corrosion of cavitation resistant coatings were ranked using salt spray testing, alternate immersion testing and humidity testing using synthetic seawater. Coatings were tested on both steel and bronze substrates.

Results showed that cermet and ceramic coatings performed well, but the alloy coating was inadequate in seawater service (Figure 8).

Ceramic, alloy, cermet and uncoated steel samples after 480hrs of exposure to synthetic seawater spray. Each coating contains a scribe to simulate the effect of a coating defect.

Cavitation Modelling

Cavitation studies were carried out in four phases as shown in the flow chart in Figure 9. Firstly small scale ultrasonic homogeniser tests were performed to rank coatings in terms of cavitation erosion damage. Damage was assessed using both weight loss measurements and 3D optical scanning of wear damage (Figure 10). These results showed the cermet coating to be the most resistant to cavitation erosion.

A shear stress transport model was selected as the most appropriate to model cavitation and numerical calculations were performed based on a 2D hydrofoil design. Predictions of performance behaviour were made and validated using experiments run in a cavitation tunnel at SVA Potsdam (Figure 11). After validation of the model, a 3D model was setup based on the proprietary designs of Tocardo’s turbine blades. Again simulations were run to predict performance data and validated using experiments run within a cavitation tunnel at SVA (Figure 12).

Cavitation Erosion

During 2D hydrofoil and 3D blade experiments, the three coatings under consideration were applied and 3D optical scans were taken before and after cavitation experiments in an attempt to measure the cavitation erosion resistance. However, even after prolonged exposure to cavitation conditions, no coating showed significant damage from cavitation erosion.

Potential Impact:
Impact

A new corrosion and biofouling resistant coating has been developed with commercial and technical benefits over existing methods. This allows ACORN SME partners to develop new business, not only in established offshore markets (oil and gas, offshore and coastal infrastructure), but also in the emerging offshore renewable energy market.

Coatings have been evaluated for cavitation performance and shown to provide good resistance to cavitation erosion.

Both coatings will reduce maintenance requirements for ocean energy devices and increase lifetimes of structures and components. As the technology is adopted, the Levelised Cost of Energy (LCOE) for renewable power generated by ocean energy will decrease. Lowering renewable energy costs enables uptake of clean power generation and allows it to displace traditional fossil fuel sources. This brings a number of societal benefits such as, lowered CO2 emission and reduced dependency on imported fuel price volatility.

The development of the two ACORN coatings will bring economic benefit to the consortium partners in terms of increased sales revenue from device manufacturer seeking improved solutions to problems with surface degradation of structures and components. By providing solutions to corrosion, biofouling and cavitation, energy generators will suffer from reduced maintenance downtime and O&M costs will be reduced.

Dissemination

ACORN results have been presented through scientific posters at:

•Bilbao Marine Energy Week 2015
•International Conference on Ocean Energy 2016
•International Congress on Marine Corrosion and Fouling 2016 (abstract submitted)

Scientific presentations have been made at:

•Biofouling, Benthic Ecology and Marine Biotechnology Meeting 2015
•International Conference on Ocean Energy 2016

Marketing and dissemination activities have been carried out at:

•International Tidal Energy Summit 2014
•All Energy 2014
•EWEA Offshore 2015
•THETIS 2015
•All Energy 2015
•EWTEC 2015
•EUROCORR 2015

Two industry specific workshops were also held to disseminate the outputs of the project and gain valuable feedback from the market regarding the use of coatings in offshore devices.

Exploitation

The main opportunities can be summarised as follows by coating type:

1. Anti-Corrosion/Anti-Fouling Coating

The project has developed an innovative coating system that offers the necessary corrosion protection and, potentially, the desired anti-fouling properties. Alphatek and DEGIMA will conduct further development in the coming 12 months to improve and refine the coating characteristics and performance. Alphatek and DEGIMA will also validate the coating systems performance on actual OWW&T applications as part of pursuing the necessary approvals and industry acceptance to enabling use of the developed product. Following this, Alphatek and DEGIMA will be able to approach new lucrative markets, including major spin-off markets in addition to wave and tidal.

Although the ACORN TSA-based antifouling coating development is primarily aimed at wave and tidal devices, the wave and tidal market is not yet mature. Whilst DEGIMA is planning to apply the ACORN coating onto selected areas of a single Wave Energy Converter (Powerbuoy) in 2016, to serve as a real-world demonstrator of the long-term behaviour and performance of the coatings, the main potential within the wave and tidal market is more likely to emerge with in the next 3-8 years and is a longer-term prospect for commercialising the coating, although naturally the SME partners will remain very close to this industry and will ensure that the coatings are marketed and/or trialled at every available opportunity, and especially during early-stage feasibility trials of such devices.

Several secondary (spin-off) markets are of greater immediate interest; these comprise Offshore Wind Turbine (OWT) Foundations, Meteorological Masts, dry dock furniture, fish farms, mooring chains, etc.) where fouling causes localised accelerated corrosion, damages paint layers, reduces the lifetime and/or impedes the functionality of structures and makes it difficult to inspect the structures in situ. The short-term commercialisation strategy therefore is to take advantage of this immediately addressable market in order to build up a track record for the ACORN coatings.

The consortium plans to protect the IPR via a strategy that is still evolving. The initial IPR protection route for the ACORN Anti-Corrosion/Anti-Fouling Coating will be based around obtaining an exclusive supply arrangement with the manufacturer of the active anti-fouling agent.

2.Cavitation Resistant Coating

•The project identified and verified the performance of a cavitation resistant coating that can enhance the performance of the energy converters designed and manufactured by Tocardo and Wave Dragon.

•Whilst the identified coating system cannot be patented, the derived IPR will be of value to both SMEs to penetrate their respective Wave and Tidal energy converter markets enabling them to demonstrate better performance and reliability of critical components.

List of Websites:
www.acorn-project.eu

Final Report Summary - ACORN (Advanced Coatings for Offshore Renewable ENergy)

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