Coating Selection for Offshore Wind Structures

Written by Joao Azevedo and Neil Wilds, Sherwin-Williams Protective and Marine.

The Importance of Offshore Wind Generation in Meeting Our Carbon Footprint Reduction Goals

The world is craving renewable energy sources. At COP28 in Dubai 2023, governments agreed the historic goal to triple renewable energy capacity by 2028, and all three main economic blocs set in motion ambitious plans to deliver the needed reduction in carbon dioxide emissions, namely the US Inflation Reduction Act, the EU’s Wind Power Package, and China’s Five-Year Plan.

In wind energy alone, the 1 TW global installed capacity milestone was reached in 2023. This figure is expected to pass 2 TW before 2030. Onshore wind is still the main contributor for this installed capacity, but offshore wind is growing at a much faster pace and is needed to reach the objectives of renewable energy production. In 2023, 10.8 GW of new offshore wind capacity was connected to the grid, a 24% increase from the previous year globally. Annual new offshore wind installations are expected to increase at a compound growth rate of about 25% between 2023 and 2028 [1, 2].

New Offshore Wind Installations, Global (MW)

*Compound Annual Growth Rate | Source: GWEC Market Intelligence, June 2024

Figure 1: Projected growth of offshore wind installations [1]

Why Corrosion Protection Is Central to the Success of Offshore Wind

The accelerated growth of offshore wind energy means that more of these structures are being installed in the field, although they lack enough long-term experience in corrosion protection durability. The offshore wind turbines themselves are fixed on top of towers positioned on foundations, normally monopiles, sometimes jackets, or floating, which is expected to increase in the future. These assets, and most notably their foundations, are exposed to aggressive environmental conditions leading to protective coating breakdown by a combination of factors: 

1 - Exposure to ISO 12944-9-defined environments: CX extreme offshore atmospheric corrosiveness, Im4 seawater immersion with cathodic protection, or a combination of both in the cyclical immersion and splash zone areas [3].

2 - Unmanned – unlike in most Oil & Gas projects, meaning no crew present for regular maintenance.

3 - Structural movements are much more pronounced than in Oil & Gas offshore assets.

4 - Doubts about the usual Offshore Oil & Gas coating specification’s ability to provide extended durability in extreme conditions. 

Premature coating breakdown of the foundations of offshore wind structures has been detected in the field, where access for maintenance is extremely difficult. For obvious structural reasons, the foundation's integrity is one of the top priorities for corrosion protection. This causes uncertainty in an industry aiming for a maintenance-free design life of more than 30 years to ensure a proper return on investment. The offshore wind industry is thus facing two important needs:

1 - Redefine their new construction coating specification standards to ensure the required durability from the offshore wind foundation coating system.

2 - Develop maintenance cycle solutions to address the lack of control of environmental conditions and difficult access. 

This article covers the first need in the specific context of offshore tower foundations. A separate article addresses this second need (maintenance) [4]. A future article will cover the OEM coating needs of towers, nacelles, and blades.

The Search for Durable Corrosion Protection of Offshore Wind Tower Foundations 

Developers of offshore wind projects invest billions of dollars in assets that are required to function without maintenance for over 25 years, possibly in some cases aiming for over 30 years. In durability terms, the critical part of such assets are the offshore wind foundations. The industry has been on a long, bumpy road on the path to this “durability reassurance” goal. However, some good news has emerged - a new cathodic protection standard, ISO 24656:2022 [5], specifically designed for offshore wind foundations, has been recently issued and is a good step towards improved durability. However, despite several attempts, the same reassurance has not yet been gained for the coating selection sidea, as detailed below.

First Attempt

Past corrosion protection standards commonly used by the Oil & Gas industry for offshore assets prescribed accelerated cyclic ageing laboratory test criteria for coating systems pre-qualification and application guidelines for these installations, with the aim of delivering “High Durability” performance in offshore use. Examples are the ISO 20340 (later replaced by ISO 12944-9) and Norsok M501 Rev 6 standards. If it worked in Oil & Gas, why not in Wind? The problem was it did not work so well in Oil & Gas either, and the construction, structure, life cycle and maintenance of offshore wind foundations are different. Another issue with this approach is that both ISO 20340 / ISO 12944-9, and Norsok M501 Rev 6 only cover “High Durability” for this exposure environment (C5M in ISO 2030, CX in ISO 12944-9) – defined as “more than 15 years durability” by ISO 12944. Short of the offshore wind needs.   

Second Attempt

If old standards do not work well, why not seek new, better standards? A good example came from Germany, home of many recent offshore wind projects, where the joint VGB 9 (VGB Powertech e.V., now vgbe energy e.V.) and BAW (Bundesanstalt für Wasserbau – German Federal Waterways Engineering and Research Institute), initiative created a new standard for this specific need. The 4-part standard was recently upgraded: VGBE-S-021-01-2023-05-DE “Corrosion Protection for Offshore Wind Structures - Part 1 to Part 3 (2023), and Part 4 (2018) [6]. This vgbe/BAW Standard adds more stringent testing to that prescribed by the ISO 20340/ISO 12944-9 and Norsok M501 Rev 6 standards. However, tests required according to VGBE-S-021-02-2023-05 are based on ISO 12944. The cathodic disbonding is performed after a BAW-own test which lasts 15 months.

It should be noted that the costly and elaborate tests according to vgbe/BAW Standard are needed only for offshore projects in the German Exclusive Economic Zone.

However, recently, the compositional analysis testing to act as a ‘fingerprint’ has been shown to be flawed for solvent-free systems, as they are not compatible with the test methods.  This did narrow the field, making it more demanding for coatings’ systems to get pre-qualified, but it also brought benefits in prescribing more robust systems in terms of thickness and number of coats. It is still too early to see what this improvement will bring, but two challenges have been identified: 1) it is a national standard and not an internationally recognised one; 2) the field has eventually become too prescriptive, risking “false negatives” (systems failing to be approved but that can actually perform well in the field).  Norsok M501 has also been upgraded to Rev 7, in terms of the coating systems and testing requirements [7]. However, it is still designed for Oil & Gas, and is short of addressing offshore wind's very high durability requirements.

Third Attempt

If standards are insufficient to ensure good long-term performance, we should look at what has been working in the field “for 30 years” and specify that. Armed with this optimism, developers and designers took note of field performance data provided by coating manufacturers and classification societies. The outcome of this approach led partly to the ISO 24656:2022 standard. This cathodic protection standard defined five “types” of coatings systems, from I (faster breakdown rate) to V (slower breakdown rate), to help design cathodic protection accordingly (the better the system, the lower the requirements for CP). The “best” Type V systems were defined as “2-component epoxy or polyester, with a minimum of 20% lamellar glass flake in their composition”. This conclusion is “informative”, not “normative”. Still, some developers and designers took it as the new golden rule, and the ISO 24656 standard started being used to specify coating systems despite not being adequate for it. The association of a “20% glass flake compositional requirement” as the best performance for offshore wind conditions has several flaws:

A More Reasonable Approach to the Corrosion Protection of Offshore Wind Foundations

After reading this article to this point, any developer or designer of offshore wind foundations must consider the following: If old oil and gas coating pre-qualification standards are not meeting our needs, field feedback is unreliable, compositional requirements are misguiding and more recent standards are not internationally recognised, how do I achieve the 30-year durability needed?

The answer is a multi-pronged approach that dispenses with “wishful thinking”:

1 - Compositional coating requirements must be avoided, as this will lead to the selection of old materials based on their composition only, ignoring other critically important success factors listed below and the potential for using better coating systems.

2 - Coating pre-qualification and corrosion protection design standards should be refined to meet the specific needs of offshore wind foundations, focusing on performance (not compositional) requirements.

3 - The coating system’s ability to be applied and inspected easily must be part of the selection criteria. This means the ability to provide efficient protection, at the level obtained in the laboratory, on an industrial scale while minimising the installation effort and maximising installation speed must also be a criterion. Passing the standard performance criteria is worthless if the application is troublesome to the point of increasing the risk of faulty installation.

4 - Sustainability credentials must be part of the selection criteria. The pressure for performance at this level is especially important in an industry intended to contribute to the overall sustainability of the energy mix. This means the optimum balance of coatings, cathodic protection, and steel allowance should minimise the embodied carbon resulting from all contributions. Indications so far point to the benefits of maximising the contribution of coatings while reducing corrosion allowance [9] [10].

The pitfalls of coating compositional requirements have been abundantly described above; however, the remaining three recommended factors deserve further explanation.

Standards Pre-Qualification Tools

Past efforts to improve pre-qualification standards have not been wasted. The latest VGB BAW (2023) and Norsok M501 (revision 7) editions brought welcome upgrades to the prequalification criteria. The minimum requirements for coatings systems for offshore foundation splash zones - the most critical of all the exposure areas are now recommended as 1000 microns for the minimum dry film thickness, with at least two coats. Testing protocols are also closer to the needs, with the introduction of impact testing to the mix for splash zone areas. The current ISO 12944 is also under revision, and once the new edition is published (a few years from now) will be a better tool, with pre-qualification criteria for CX and Im4 systems for Very High Durability (> 25 years). Finally, a new standard project (ISO/AWI 25249) [8] has been proposed to the relevant ISO technical committees for the development of a future “Corrosion protection of offshore wind structures” standard. This was approved, and the new ISO/TC 107/JWG 6 (a joining ISO/TC 107 and ISO/TC35 experts) was created. The first meeting of this Committee, in December 2024, is a harbinger of a new important tool for coating system prequalification. Preliminary drafts point to three levels of durability (< 25 years / 25-35 years / +35 years), with design approaches considering coatings, cathodic protection, corrosion allowance, and maintenance strategies in combination, depending on the durability scenario. Furthermore, offshore wind end users are beginning to consider their own pre-qualification testing outside the previously described common test methods.

Application Features Criteria

Either formally (as part of pre-qualification standards), or in the context of project development (joining coating manufacturers with design engineers), the selected coating systems must be easy to apply under the specific conditions of monopile (or other type of foundation) fabrication. “Easy” may be defined in several factual ways, from the simplicity of spraying setup, to the ability of film forming /matching the specified thickness. This should also include aspects like speed of application, including the ability to easily control applied thickness by the painter, speed of curing, absence of solvents (to reduce risks of pin holing or solvent retention), and ability to be inspected quickly and more accurately (better rate of defects detection).  If all these aspects are optimised, the chances of good results in accelerated lab testing being replicated in the field during real-life exposure are maximised.

Sustainability Credentials

For the offshore wind industry, the carbon footprint of its activity is of critical importance. This importance is linked with the “green” image associated with the industry’s output, which must not be too negatively affected by the impact of the materials used on its assets. Given the large impact of steel in this footprint, this means preferring approaches that minimise corrosion allowance (which adds weight and cost both to the structure and the installation process) and maximising the use of coatings. It also means that the balance between cathodic protection and coatings used for submersed areas needs to be minimised; for example, the carbon footprint of a fully cathodic-protected bare-metal monopile is much higher than the one resulting from the combined use of a robust coating system and a more moderate cathodic protection level [9]. Finally, the criteria should prioritise the adoption of coating materials with reduced direct environmental impact, i.e. by using solvent-free materials, currently being utilised in some, but not all, European offshore wind structure construction yards, and avoiding or minimising the presence of ingredients not normally found in the maritime environment (for example metallic zinc).

Conclusion

Offshore wind is a relatively young industry facing the challenges of exponential growth with booming investment needs whilst operating in unchartered waters (literally). Design lives of over 30 years are becoming the common target, while history (mainly from the offshore Oil & Gas sector) does not provide enough reassurance about coating systems and other corrosion control tools’ ability to protect for the aimed durability.

Anxiety to cover for the “reassurance gap” has led the industry to over-rely on past standard tools and/or compositional requirements linked with a very limited number of old products in the market and based on a very limited and not representative set of field data, in conditions different from the reality of today’s offshore wind foundation’s fabrication/operation.

The good news is that replacing the “wishful thinking” from the past with a new reasonable multi-prong approach to offshore wind foundation coatings selection is possible. This approach avoids using compositional requirements and instead adopts existing and future prequalification standardisation tools that have been improved as described above and combines these with an intimate discussion with coating manufacturers during the design phase of their projects to select solutions that meet the best criteria in terms of application features and sustainability credentials.

References

1. Global Wind Energy Counsel GWEC),  https://gwec.net/

2. https://gwec.net/strong-2023-offshore-wind-growth-as-industry-sets-course-for-record-breaking-decade

3. ISO 12944-9 Paints and varnishes — Corrosion protection of steel structures by protective paint systems. Part 9, protective paint systems and laboratory test methods for offshore and related structures (2018)

4. Offshore Wind Farm Maintenance – A New Coating Toolbox, in publication

5. ISO 24656:2022 Cathodic protection of offshore wind structures

6. vgbe/BAW-Standard - Korrosionsschutz von Offshore-Bauwerken zur Nutzung der Windenergie (2022)

7. Norsok  M-501 Surface protection and protective coating – Rev 7 (2022)

8. ISO/NP 25249 Corrosion protection of offshore wind structures

9. “How do you take sustainability into account during design?” Birit Buhr, AMPP Italy, Genoa, 3rd conference & Expo 2024, June 2024

10. “Benefits of Corrosion Protection In Offshore Wind” Anthony Setiadi, ICorr CED Working Day, Teddington, 27 Apr 2023