In spite of the considerable development in the field of marine protective coatings, there is still significant room for improvement. Very long lasting (20 yrs) coating systems that offer reliable protection against corrosion/biofouling or corrosion/cavitation, and which also avoid the need for supplementary cathodic protection are not available. The need for such coatings is acute in the case of static offshore structures, where dry dockage is not an option, and the growth part of this market relates to new build structures ranging from OWT foundations to wave and tidal stream devices. The SMEs proposing the ACORN project are interested in an alternative, highly differentiated and potentially patentable technical solution to the above problem. ACORN will take advantage of the proven long-term corrosion resistance of thermally sprayed aluminium (TSA) to provide a matrix coating with a proven life of 20+ years in the sea. Into this porous TSA matrix, the ACORN research team proposes to introduce islands of new environmentally friendly antifouling substances which can then be gradually exposed at the active surface of the coating as the TSA corrodes away at a few μm per year. This will result in an entirely new, non-paint, approach to the protection of offshore structures.
The developments in this project will be applicable to a wide variety of offshore static structures including oil & gas rigs and docks, but the focus for ACORN is on Offshore Wind, Wave and Tidal (OWW&T) ocean energy systems. The SMEs in the ACORN consortium believe that these developing markets offer considerable growth potential, as the ACORN coating will be able to offer this market significant technical and economic benefits without having to challenge any entrenched commercial practices and relationships. The development of an enhanced coating system for these structures will make a direct contribution to their long term performance, and will, in turn, contribute to the efficiency and stability of future European alternative energy supply.
Offshore wind tower foundations and almost all surfaces of W&T devices need to resist long term sea water corrosion and biofouling. Established methods to achieve corrosion resistance include:
- Material selection (e.g. use polymers instead of metals):
Not always feasible due to UV/water degradation and strength/ stiffness/ stability requirements.
- Cathodic protection (CP) :
Not effective in the splash zone (waterline of semi-submerged device), and for submerged devices requires careful design, but often found to be expensive and inadequate for long-term protection.
- Protective organic paints/coatings:
These offer only limited protection (typically 5 years);
- Thermally-sprayed aluminium (TSA) coatings:
Used in the Oil & Gas industry to provide long-term protection for steel structures in the splash zone, but they do not resist biofouling. Current antifouling coatings are unproven in service of over 5 years and are designed for ships where the coatings are regularly replaced. Corrosion performance over the longer term is unknown and almost certainly inadequate for OWW&T. In addition, they tend to have a negative impact on the environment and can be toxic to marine life. Biofouling resistance has traditionally been achieved by the use of toxic substances including Sn and Cu compounds. These are no longer considered viable on ecological grounds. Alternatives including a wide range of organic and inorganic compounds have been and are being researched. In all cases, these materials are deployed via paint coatings and thus have only limited (3-5 year) effective life.
- For marine power generation, cavitation erosion is a consideration if devices are operating at optimum efficiency. Although component & system design to avoid cavitation in hydraulic pumps and propellers is well understood, a different approach is required for marine current turbines because of the larger plane/rotor area.
ACORN Innovation (main points)
ACORN will use a new fouling-resistant strategy called “post-settlement inhibition (PSI)”. It is a non-toxic fouling-resistance strategy. The PSI concept does not rely on the release of the bioactive substance, but only operates to inhibit organism settlement and growth. This has a big advantage as regards environmental considerations. UGOT’s work has shown that the leaching rate of the PSI substances can be very low as a result of the low loading, low water solubility, high affinity to the matrix and high molar volume of the fouling deterrant. The PSI material will be combined with corrosion resistant Al coatings in two different ways to produce coatings that have anti-corrosion and anti-fouling properties.
- Innovation 1: Doping a liquid carrier with traces of environmentally-friendly ‘post-settlement inhibition’ (PSI) active substances.
- Innovation 2: Further development of the existing thermal spray (TS) techniques to deliver coatings with controllable porosity, without compromising corrosion resistance or adhesion/mechanical strength.
- Innovation 3: Infusing the PSI-doped carrier into the porous TSA layer, producing a composite coating that can resist both fouling and seawater corrosion for 20+ years.
- Innovation 4: Doping of polymer flame spray (FS) powders with PSI.
- Innovation 5: Adapting the thermal spray process to co-deposit Al with polymers.
- Innovation 6: Similar to the 1, 2, 3 but using a novel low surface energy sol-gel-derived material to fill the pores, rather than a paint system.
There are significant research challenges involved in this development. Bio-fouling is a complex phenomenon that is not yet fully understood, and has not been fully overcome for long-term immersion despite large R&D investment.
The resultant coating is a corrosion resistant thermally sprayed aluminium (TSA) layer, with tailored porosity designed to be filled with an “on-contact” fouling resistant substance. The anti-fouling component is present throughout the coating thickness and is exposed to the sea water in a controlled manner as the aluminium corrodes away at a few μm per year, resulting in gradual exposure of a substance that inhibits fouling upon the organism’s contact with it. The composite coatings described above will be a completely new and unique type of coating never before tested in cavitation conditions. Hence, the post promising variant will be assessed in cavitation testing under conditions that are relevant to the offshore industry and in particular to the markets being targeted by the SME partners. In addition, for the W&T devices where only cavitation is important, the performance of several possible cavitation-resistant materials will be researched. These coatings might not resist fouling, but they are expected to resist a combination of seawater corrosion and cavitation very effectively. There are several promising alternative materials that will be considered.