Checklist for Choosing Coatings for Offshore Wind Farms
- Framos Fabrications
- Jan 9
- 10 min read
Offshore wind farms face harsh marine conditions, making the right coating crucial for durability and cost-efficiency. Poor coatings lead to corrosion, structural fatigue, and expensive repairs. This guide breaks down key considerations for selecting coatings:
Corrosion Zones: Structures are divided into five zones - atmospheric, splash, tidal, submerged, and buried - each with unique challenges like UV exposure, wave impacts, and salt deposits.
Risks of Poor Choices: Failed coatings increase stress on structures, raise maintenance costs, and harm marine ecosystems with microplastics and heavy metals.
Durability Goals: Aim for a maintenance-free lifespan of 25+ years. Standards like ISO 12944-9 recommend a minimum dry film thickness (DFT) of 1,000 microns in harsh zones.
Material Options: Zinc-rich primers, epoxy mid-coats, polyurethane topcoats, and Thermally Sprayed Aluminium (TSA) are effective solutions, offering long-term protection.
Environmental Impact: Low-VOC and solvent-free systems reduce harm, while durable coatings minimise repair needs and waste over time.
Application and Quality Control: Proper surface preparation, accurate DFT checks, and adhesion testing ensure coatings meet performance requirements.
Choosing the right coating system isn't just about protecting structures - it also saves costs and reduces risks over the lifespan of offshore wind farms.
Performance Requirements for Offshore Coatings
Exposure Zones and Service Life Expectations
Offshore coatings are expected to meet stringent performance standards, especially as the demands for durability continue to evolve. For offshore wind farms, the goal is now a maintenance-free lifespan of over 30 years - well beyond the earlier standard of 15–25 years. This extended service life is driven by the high costs and logistical hurdles of repairing unmanned structures. To achieve what is classified as "Very High Durability" (over 25 years) in the splash zone - the harshest environment - modern guidelines specify a minimum Dry Film Thickness (DFT) of 1,000 microns applied in two coats.
Different exposure zones come with unique challenges. For instance, in the atmospheric (CX) zone, low-carbon steel can lose up to 1,500 g/m² in the first year. In contrast, Thermally Sprayed Aluminium (TSA) coatings showcase remarkable longevity, with corrosion rates of less than 0.01 mm per year after three months of immersion in artificial seawater. In submerged zones, TSA coatings can protect for over 25 years, even when up to 5% of the coating is damaged. Across all zones, adhesion strength is a critical factor, as it directly impacts the coating's ability to resist both chemical and physical wear.
Corrosion and Mechanical Resistance Properties
Beyond durability, offshore coatings must excel in resisting both corrosion and mechanical stress. Effective coatings create a barrier against water, oxygen, and chloride ions, while also offering galvanic protection through zinc-rich primers or metallic coatings like TSA. Furthermore, they must withstand cathodic disbondment, particularly in submerged and splash zones.
The splash zone poses some of the toughest mechanical challenges. Coatings here need to endure wave impacts, debris collisions, and even ice, making impact and abrasion resistance essential. In the atmospheric zone, coatings face different demands: they must resist UV radiation and weathering to maintain colour and gloss, even under harsh sunlight and exposure to marine aerosols.
However, performance in the lab means little if coatings fail during application. As noted by Sherwin-Williams experts, ease of application is just as important as the coating's inherent properties. A coating that is difficult to apply in the field may compromise the very protection it is designed to provide, underscoring the need for practical solutions alongside technical excellence.
Standards and Compliance Requirements
Relevant Standards and Regulations
To ensure effective corrosion protection, adherence to established standards is essential. These standards form the backbone of performance requirements for offshore wind coatings. One of the most critical frameworks is ISO 12944-9:2018, which outlines performance criteria for protective paint systems in corrosivity category CX (offshore) and immersion category Im4 (sea or brackish water). In the UK, this standard is adopted as BS EN ISO 12944-9 and applies to carbon steel structures with a minimum thickness of 3 mm. The standard is priced at approximately £304.64.
In addition to ISO standards, NORSOK M-501 (Edition 7, 2022) offers detailed guidance on surface preparation and protective coating requirements for offshore installations. Although originally developed for the petroleum sector, it has become widely recognised in offshore wind applications. Meanwhile, ISO/WD 25249-1, under development and expected by late 2024, aims to provide a dedicated framework for corrosion protection tailored to offshore wind structures, including foundations, towers, and nacelles. This progression highlights the industry's growing focus on wind-specific needs.
When specifying coatings, it’s crucial to select systems with a "Very High" durability classification, ensuring a lifespan of 25+ years. These coatings must perform reliably within temperature ranges of –20 °C to +80 °C in atmospheric conditions and up to 50 °C in submerged environments.
These standards set the groundwork for rigorous testing protocols, which are detailed in the next section.
Testing and Verification Procedures
Before coatings are approved for offshore use, they must undergo stringent pre-qualification tests. For atmospheric zones, the primary standards - ISO, NORSOK, and VGBE - mandate cyclic ageing tests lasting at least 4,200 hours to replicate the harsh conditions offshore. These tests include UV weathering (ISO 16474-3), neutral salt spray (ISO 9227 NSS), and –20 °C cold storage. This method offers a much closer approximation to real-world conditions compared to continuous salt spray tests, which fail to capture the complex chemical interactions of natural seawater.
For splash, tidal, and submerged zones, additional tests address their unique challenges. Cathodic disbondment (CD) tests are critical for ensuring compatibility with cathodic protection systems. Adhesion strength is another key factor, with ASTM D 4541 requiring a minimum of 5 MPa, as demonstrated in research by Sung-Hyun Eom et al.. Fracture type analysis (ISO 4624) is used to identify whether failures occur at the substrate interface (A/B), which poses a higher corrosion risk, or within the coating layers.
Impact resistance testing is particularly important for splash and tidal zones, where coatings must endure mechanical damage caused by floating debris or ice. Coatings are also evaluated for blistering (ISO 4628-2), rusting (ISO 4628-3), cracking (ISO 4629-4), flaking (ISO 4628-5), and delamination around a scribe (ISO 4628-8). These thorough evaluation criteria ensure that coating systems can meet the demanding, maintenance-free service life expected of offshore wind farms.
Coating Material and System Selection
Coating Technology Options
Choosing the right coating system starts with matching its capabilities to the specific exposure zone. For atmospheric zones, a combination of a zinc-rich primer, an epoxy mid-coat, and a polyurethane topcoat works effectively. The zinc layer offers sacrificial protection against corrosion creep, while the polyurethane ensures UV resistance and maintains colour stability. These coatings typically have a dry film thickness (DFT) ranging from 280 to 320 µm.
In splash and tidal zones - where corrosion rates for carbon steel can reach between 0.4 and 1.2 mm annually - more robust systems are essential. These areas often require a minimum DFT of 1,000 µm, applied in at least two layers. The industry is increasingly favouring performance-based coating selection over rigid compositional guidelines. As Joao Azevedo and Neil Wilds from Sherwin-Williams explain:
If a specific coating composition is forced on the operators... we close the door to many effective (potentially more effective), more modern, and more sustainable coating solutions.
Thermally Sprayed Aluminium (TSA) is another strong option, particularly for foundations and monopiles in submerged and splash zones. TSA provides both barrier and cathodic protection, offering durability for over 30 years. Studies show TSA keeps corrosion rates below 0.01 mm per year in artificial seawater - even with up to 18% surface damage. Applying a sealer (less than 40 µm thick) helps penetrate surface pores and prevents blistering when adding organic topcoats. A 20-year cost analysis highlighted the financial benefits of a duplex system (TSA combined with paint), averaging approximately £75,000 per year, compared to £260,000 for paint-only systems. These advanced technical choices are paving the way for long-lasting, low-emission solutions.
Environmental Impact and Low-VOC Options
Once the ideal coating system is selected, it’s crucial to assess its environmental impact. Beyond ensuring performance, sustainability plays a key role. Extending durability to 30 years reduces the need for steel repairs, which have a larger environmental footprint. Solvent-free or low-VOC (volatile organic compound) systems also help avoid issues like pinholing and solvent retention during application.
When specifying environmentally friendly coatings, look for systems with Type III Environmental Product Declarations (EPDs). These declarations provide verified and transparent sustainability claims. Water-borne coatings can often replace solvent-based options, lowering VOC emissions without sacrificing durability. TSA systems also contribute to reducing environmental harm by minimising the thickness of organic layers, which decreases the release of microplastics and harmful chemicals like bisphenol A into marine environments.
Experts advocate for solvent-free formulations and the exclusion of non-maritime ingredients to further lower environmental impact. These measures align with the overarching goal of achieving 30 years of low-maintenance, sustainable offshore operations.
Application, Fabrication, and Quality Control
Surface Preparation and Fabrication Compatibility
Steel surfaces must be treated to ISO 8501-1 Sa 2½ standards to remove contaminants like oil, dust, mill scale, and rust. This step is crucial for creating a strong bond between the surface and the coating system (A/B interface), especially since steel is the primary material used in offshore wind structures. As Sung-Hyun Eom from Daejin University points out:
Defects between the surface and the coating system (A/B) not only decrease the adhesion strength, but also act as a factor that significantly deteriorates the anti-corrosion performance.
For Thermally Sprayed Aluminium (TSA) coatings, achieving the right surface profile is essential to improve mechanical bonding.
Fabrication processes like CNC cutting and welding often result in complex areas, such as corners, bracket edges, and manual welds, which demand targeted stripe coats to achieve the required dry film thickness (DFT). Roller application is generally unsuitable, except for minor topcoat touch-ups. Features like flange holes and other machined areas should be thoroughly cleaned and protected with a zinc stripe coat to prevent localised failures.
At Framos Fabrications, we integrate meticulous surface preparation into our CNC laser cutting, bending, and welding services. This ensures optimal coating compatibility and long-lasting durability. Once the surface treatment and fabrication are completed, thorough inspections are critical to confirm the coating’s integrity.
Inspection Procedures and Quality Checks
After surface preparation, rigorous quality inspections are key to ensuring long-term performance. Adhesion strength testing, conducted according to ASTM D 4541, is a vital step. ISO 12944-9 specifies a minimum adhesion strength of 5 MPa. Additionally, fracture analysis as per ISO 4624 helps identify potential issues. Failures at the substrate interface (A/B) indicate a high risk of severe corrosion, while fractures within coating layers point to specific weaknesses, such as reduced barrier or UV resistance.
Splash zone coatings must meet specified DFT requirements. The "90-10 rule" is applied here: the average of all DFT measurements must meet or exceed the nominal DFT, and no individual measurement should fall below 80% of the nominal value. Furthermore, no more than 20% of measurements can fall between 80% and the nominal DFT. The maximum allowable system thickness is limited to three times the nominal DFT.
Humidity control between coating layers is essential to prevent oxidation, and applying a mist coat can help mitigate this risk. Coatings must be fully cured in line with the product data sheet before any handling or transport to avoid mechanical damage. These procedures are critical for maintaining the long-term reliability and durability of offshore protection systems.
11. Cathodic protection of offshore wind turbine monopiles: Dave Buxton, Intertek
Conclusion
Selecting the right offshore coating isn't just about protecting a structure from the elements - it's a smart move to manage long-term maintenance, structural integrity, and safety. With operations and maintenance costs making up 23% to 30% of total investment expenses, choosing the right coating can be one of the most cost-effective ways to safeguard your investment.
Opting for high-performance coatings can drastically lower lifecycle costs. For example, targeting "Very High" durability (25–30+ years) as defined by ISO 12944 standards isn't just a technical benchmark - it’s a financially sound decision.
The importance of proper coating selection goes beyond cost. It helps avoid risky and expensive repairs. As noted by Sherwin-Williams Protective and Marine experts:
The foundation's integrity is one of the top priorities for corrosion protection... aiming for a maintenance-free design life of more than 30 years to ensure a proper return on investment.
In extreme corrosive environments (CX), especially for unmanned structures, premature coating failure can pose serious safety risks. Addressing these issues after the fact is not only complex but also hazardous.
A comprehensive approach - including meticulous surface preparation, accurate DFT (Dry Film Thickness) checks, and thorough adhesion testing - ensures coatings achieve their intended longevity.
At Framos Fabrications, we integrate these principles into every project. From precise CNC laser cutting to expert welding, every detail contributes to a coating system designed to perform as expected over its service life. By aligning with the discussed performance and compliance standards, we remain committed to supporting the UK's renewable energy goals.
FAQs
What should you consider when choosing coatings for offshore wind farms?
When choosing coatings for offshore wind farms, it’s essential to consider the challenging marine conditions. These include high levels of salinity, exposure to UV rays, and the relentless cycles of wetting, drying, and splash zones. Beyond withstanding these elements, the coating system must deliver a service life of 25 years or more, while complying with standards like NORSOK M-501 and ISO 12944.
It’s also important to weigh the cost-efficiency and environmental considerations, as well as how well the coating integrates with any cathodic protection measures. Commonly used solutions include zinc or aluminium-rich coatings, thermal spraying, hot-dip galvanising, and polymeric systems. Selecting a coating designed specifically for the unique demands of offshore structures is critical to ensuring durability and long-lasting performance.
How do environmental zones impact the choice of coatings for offshore wind turbines?
Environmental zones on offshore wind turbines play a key role in selecting the right coating systems to protect against corrosion over time.
In the submerged zone, where structures are constantly in contact with seawater, coatings work in tandem with cathodic protection systems. Zinc-rich or aluminium-rich primers are commonly applied here. These primers not only provide galvanic protection but also act as a barrier to prevent chloride penetration. Moving to the tidal zone, which faces regular cycles of wetting and drying, the coatings need to be highly flexible. Epoxy or polyurethane coatings are often used in this area as they can handle the stress of expansion and maintain strong adhesion. For the splash zone, located just above the waterline, durability is key. Coatings in this zone must resist water spray, UV exposure, and periodic drying. A UV-stable topcoat with excellent water resistance is ideal here. Additionally, above the waterline, coatings may need to meet specific colour requirements, such as yellow, to adhere to marine safety standards.
Each of these zones presents distinct challenges, requiring coating systems that comply with standards like ISO 12944 or NORSOK M-501 to ensure they perform effectively. Framos Fabrications specialises in custom metalwork and finishing solutions, incorporating these protective coating strategies for offshore wind energy structures.
What are the environmental advantages of using low-VOC and solvent-free coating systems for offshore wind structures?
Using low-VOC (volatile organic compound) and solvent-free coating systems comes with clear environmental advantages. These coatings limit the release of harmful chemicals into the air, which not only improves air quality but also helps protect marine ecosystems from potential damage.
On top of that, they align with environmental regulations and sustainability targets, making them a smart and responsible option for offshore wind energy projects. By cutting down on emissions and reducing hazardous waste, these coatings promote eco-friendly practices while still delivering the toughness needed to withstand the challenging conditions offshore.
Comments