PAINTSQUARE BLOG

In the first part of this two-part blog, we looked at corrosion control on offshore wind structures, and some of the environmental factors that make it a challenge. In our concluding section, we’ll discuss coating systems currently in use on these structures and the steps I think will help the industry to more effectively and efficiently control corrosion.

There are a number of different turbine types and structural configurations and designs, and these all have an impact on the protective coating system applied. The offshore wind turbines typically consist of the following designs:

• Jacket: Sub Structure
• Monopile: Sub Structure
• Gravity Based: Substructure

On any type of structure, an offshore wind turbine has several zones, all requiring different protective coating systems and different levels of corrosion protection. The upper part of the tower is in atmospheric exposure; the "splash zone" near the water is subjected to off-and-on wetting due to waves and tides; the submerged section of the structure is completely underwater, and part of the structure is normally buried in the sea floor. (Except in the case of structures like the more recently developed "floating wind farms," which don't have a buried zone.)

© iStock.com / Ian Dyball

On any type of structure, an offshore wind turbine has several zones, all requiring different protective coating systems and different levels of corrosion protection.

Regarding the coating systems used for corrosion protection, Karsten Mühlberg of Hempel wrote in JPCL:

The majority of steel towers for wind turbines located offshore are metallized and painted on their outer surfaces. Inside the towers, only pure paint systems are typically used, except for the lower part where some specifications call for metallization plus paint.

• Metallization (e.g. Zn/Al, 85/15) 60-100 μm

• Epoxy paint 2 coats 100-120 μm (including flash coat)

• Polyurethane paint 50-80 μm

However, due to the demands for less time spent on painting, cost reduction, and good experience over years with paint systems, wind turbine structures may be metallized less often. More and more, high quality paint systems without metallizing (according to DIN EN ISO 12944, C5- Marine), at 320 μm dry film thickness (dft) for external protection, are going to be used. An example of such a paint system is an epoxy zinc dust primer applied at 60 μm dft; an epoxy mid- coat at 200 μm; and a polyurethane topcoat at 60 μm. Inside of steel towers, paint systems at 240 μm dft (C4) are going to be used. For example: epoxy zinc dust primer at 60 μm; epoxy paint at 180 μm; and another coating system used: epoxy zinc dust primer at 60 μm; epoxy midcoat at 120 μm; and a polyurethane topcoat at 60 μm.

Mühlberg goes on to add:

External (immersion and splash zone) use: specialized epoxy coat- ing, 2 to 3 coats, each at 200–250 μm*; polyurethane topcoat at 50–70 μm; specialized epoxy coating, 2 coats at 500 μm*; and polyurethane topcoat 50–70 μm.

• For permanently immersed areas: Epoxy coating, 2 coats at 200– 250 μm*

* The epoxy and specialized epoxy coatings above, depending on the design of the relevant structure, must be compatible with Impressed Current Cathodic Protection.

• For foundation structures insides (not air tight closed): epoxy coating, 2 coats, each at 200–250 μm are normally used.

Mühlberg, who wrote on the use of protective coating systems way back in 2010, was simply basing his requirements upon historical data and the Norsok M501-1 specification requirements for zonal areas—again adopted from the oil and gas industry.

I must add there has been very little change with regard to materials used since then, with a vast reduction in metallization. The DNV recommended code of practice DNV-RPO-0416 states that requirements for corrosion-protective coatings shall be specified in a dedicated document or in a section of some other relevant design document. At least for primary structural parts, generic types of coating systems and requirements to the qualification of manufacturer-specific materials (as defined in the data sheet by the coating supplier of the intended protection system) to be used for such systems shall be specified in the project design basis. Pre-qualification of these coating systems in accordance with a recognized standard (i.e., NORSOK M-501 or ISO 12944, which now includes what previously was known as ISO 20340) with the designated use for designated environment is mandatory.

Organic Coatings vs. Metallizing

There has been a trend to lean toward organic coating materials with a reduction in the use of metal spraying and galvanic protection through TSA and TSZ coatings. It has been clearly demonstrated that thermally sprayed aluminium and zinc coatings, which have been used for more than 40 years to protect offshore oil and gas platforms from seawater corrosion, can also reliably and cost-effectively extend the life of offshore wind turbine foundations to 20-40 years. The biggest advantage is offered in areas of highest risk. This includes areas subject to alternate wetting by the sea, i.e., the splash and tidal zone, as well as subsea. This has been overlooked my many owners and operators who have deemed this process to be time-consuming and ultimately expensive; however, this initial cost including production time is far less that the cost of offshore maintenance.

As stated by The Welding Institute:

With the cost of wind energy still double that of fossil fuels, a reduction in the through-life cost of wind farms is a high priority. Offshore wind turbine foundations represent 30% of the cost of a wind farm. They need effective protection from corrosion. This is usually provided by a combination of thick (>0.5 mm) organic paint/epoxy coatings, and cathodic protection. The coating helps reduce the load on the cathodic protection system. In the splash and tidal zone, however, where the corrosion rate is at its highest, cathodic protection is ineffective and the structure is totally reliant on the integrity of its coating.

With this said, owners and operators need to be looking at high-end, robust and durable products applied with full supervision and meticulous inspection and quality control in order to ensure long-term corrosion protection and reduce maintenance fees. In recent years unexpected corrosion-related issues have emerged, especially in relation to the monopile structures. It has to be remembered that offshore wind farms are typically designed with an anticipated service life of 25 years.

Earlier on, it was assumed that the internal side of foundations below the lower working platform was airtight. If airtight, corrosion was assumed to cease when the oxygen present inside the foundation was consumed. This assumption has been shown to not be fully valid, as both sea water and oxygen have access to the inside of the monopile under certain conditions—not the least on sites where large tidal variations exist (up to approximately 10 meters). This may result in active corrosion, which can compromise the durability of the wind farm and reduce the service life if not prevented.

© iStock.com / teaa1946

The industry is lacking the experience in wind turbine fabrication and good inspectors are being overlooked for offshore wind farm maintenance and onshore fabrication due to a lack a lack of wind turbine experience.

Externally the corrosion in general is well understood and rather similar to the challenges observed across multiple offshore industries. Internally, in the closed compartments, the guidelines and standards, however, are inadequate—but data from inspections and surveys are becoming available. Issues such as fatigue life, internal (and external) cathodic protection design and internal cathodic protection and corrosion need to be considered. The same cannot be said for internal corrosion, as Kathy Riggs Larsen recently noted in Materials Performance:  

Industry experience has shown, however, that it is difficult in practice to completely seal compartments and render them airtight. If the closed-compartment structure is not properly sealed, direct ingress of air is possible. Seawater and oxygen ingress have been detected in foundations that are two to 10 years old, which increased the rate of corrosion and localized corrosion attacks. Furthermore, in some cases, the increasing water level has led to the internal aluminum ladder acting as a sacrificial anode.

These unexpected contradictions to design expectations were discovered during inspections and surveys related to grout failures that caused the settling of the transition piece on the monopile in a large number of northern European wind farms. In the case of oxygen ingress, the corrosion rate in the atmospheric zone initially may be high, but it will decrease in time. Below the water line, corrosion is caused by aeration differences between the upper layer of water and the active steel surface beneath. If the water level is completely stagnant, corrosion will be highly localized with limited coverage, so widespread corrosion is not expected at greater depths in the foundation or on the portion of the structure buried in sediment. 

There is also a risk of microbiologically influenced corrosion (MIC) in a closed-compartment foundation, with localized corrosion attacks on the submerged surface and in the portions of the monopile buried in the upper region of the sediment. Alternating aerobic and anaerobic conditions may also favor bacteria growth, and the risk of MIC depends on the bacteria species and the environmental conditions present. Sulfate-reducing bacteria (SRB) are expected to be present, and if growth conditions are favorable, then sulfide production can occur.

In my opinion, the offshore wind industry still has many challenges to face with regard to corrosion mitigation, control and protection. There must be more awareness and focus on long-term protection; a better understanding of corrosion mechanisms and deterioration of wind turbines; and better standards, codes of practices and specifications coupled with high-end engineering and quality control for assets.

I have recently attended several coating failure analysis investigations of a number of high-profile wind farms. Upon inspection it was determined that the vast majority of the failures were due to a lack of adhesion and inadequate curing of the initial protective coating system. This adds weight to my theory of mass-produced and rushed fabrication of turbines. The repair costs of these farms are to be substantial—and to think that these problems and issues could have easily been prevented. Subsequently the field owners now have to implement an unplanned and unscheduled maintenance/retrofit program.

I have seen an increase in the demand for QA/QC Inspectors for the offshore sector, which is promising from a quality perspective—however, I am well aware that the industry is lacking the experience in wind turbine fabrication and good inspectors are being overlooked for offshore wind farm maintenance and onshore fabrication due to a lack of wind turbine experience. This is a major mistake, as the specifications and standards all originated from the oil and gas sector. Oil and gas inspectors have vast experience with these systems specification and standard requirements. These are fairly straightforward and, in my opinion, easily transferable skills.

Owners really need to stress corrosion control at the design stage and ensure that their projects are well supervised, managed and inspected from front-end engineering through to engineering, procurement, construction and installation, or they will unfortunately keep hemorrhaging money and carrying out unnecessary offshore reworks—which is not an economically or viable solution to the world’s environmental problems.

ABOUT THE THE BLOGGER Lee Wilson Lee Wilson, CEng, FICorr, is a NACE Level 3-certified CIP Instructor, NACE Corrosion Specialist, NACE Protective Coating Specialist and Senior Corrosion Technologist, as well as an ICorr Level 3 Painting Inspector and Level 2 Insulation Inspector. The author of the best-selling Paint Inspector’s Field Guide, Lee was named one of JPCL Top Thinkers: The Clive Hare Honors in 2012. Contact Lee. SEE ALL CONTENT FROM THIS CONTRIBUTOR