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Controlling Corrosion on Offshore Turbines with Coatings

WEDNESDAY, APRIL 11, 2018

By Lee Wilson


The renewable sector is rapidly becoming one of the biggest energy sectors in the world, with farms breaking records for output and size. More and more countries are developing wind farms and the world is becoming more reliant upon their environmentally friendly energy in order to fuel their economies. However, with these farms come many of the same corrosion and engineering challenges that have been faced by the oil and gas sectors for many years.

Offshore wind turbines
© iStock.com / Grassetto

Corrosion—and specifically corrosion protection—has long been a thorn in the side of many fabrication companies and is often the last step in the fabrication process prior to installation and commissioning.

Corrosion is a major risk for offshore wind foundations: Its effects could mean costly offshore retrofit work, result in the loss of generation and expose operatives to additional health and safety risks. Corrosion protection is of vital importance to assure the integrity of offshore foundations, minimizing exposure to these risks. In this two-part blog entry, I'll look at the risk for corrosion in offshore wind applications, the best ways to prevent it, and some of the standards and systems available to protect offshore turbines.

When and How

Corrosion—and specifically corrosion protection—has long been a thorn in the side of many fabrication companies and is often the last step in the fabrication process prior to installation and commissioning. The same has often been the case in the oil and gas sector, with many punch-list items from painting and coating programs carried out during installation. This is usually because construction and delivery fell behind, or because of pressure on the surface-treatment and paint shops to load out for delivery even though the assets are not complete, as fabrication management, owners and operators continuously push for installation.

It is estimated that the cost of painting an offshore structure in an autonomous painting facility can be as low as one to 25 percent less per square meter in comparison with carrying out coating onsite. In addition, costs brought about by repair work to a new coating system carried out onsite have been estimated by one coating manufacturer to cost up to 5 to 10 times more per square meter in comparison with repairs made in the shop.

It simply makes sense, then, to complete the corrosion protection of any structure in a paint shop or facility straight after fabrication—including painting, inspection and any necessary repairs. Corrosion protection should be integrated automatically, and an appointed sensible time scale made available in order to successfully execute a strategic painting program prior to delivery.

It is widely accepted that should premature failure of the protective coating system occur offshore, the cost can significantly increase to over 100 times the cost per square meter in comparison with coating in the paint shop. It’s a rather simple concept: In order to increase quality and reduce costs, owners must see to it that a dedicated site team with a corrosion-control specialist is present during construction and fabrication in order to ensure that the work is carried out on time and to specification requirements.

Corrosion Causes

In general, root causes for coating failures occurring on offshore wind foundations include:

  • Mechanical damages occurring before, during and after installation;
  • Areas not coated before transportation offshore, often occurring due to delays in production; and
  • Environmental breakdown over prolonged exposure.
Offshore farm
© iStock.com / Ian Dyball

Offshore wind farms are typically located in harsh corrosion environments with corrosivity evaluations generally ranging from C3 Middle to C5 M as depicted in the International standard ISO 12944-2.

Planning and executing painting projects offshore is far more expensive as opposed to onshore or site-based projects. Extremely high repair costs (up to 3,000 euros per square meter and higher) have been recorded, primarily due to offshore circumstances. This is due to a combination of factors, which include:

  • Logistics: physically getting manpower, materials and equipment to the turbine;
  • Safety: The risks increase and so do preventive measures;
  • Actual work time: The environmental conditions often do not allow for continuous work (lots of down time/standby); and
  • Wave action.

Offshore wind farms are typically located in harsh corrosion environments with corrosivity evaluations generally ranging from C3 Middle to C5 M as depicted in the International standard ISO 12944-2. We have to remember that the current trend is toward installations that are deeper and further offshore, resulting in even higher repair costs and the installation of turbines in C5 M environments. Some factors that are encountered in this classification include:

  • Flowing sea water;
  • Sea salt spray;
  • Temperature variations;
  • Biofouling; and
  • Deaerated sea water (inner side).

This accumulation of corrosion accelerators ultimately results in a number of different corrosion risks such as:

  • Uniform corrosion;
  • Local corrosion;
  • Microbiologically influenced corrosion (MIC);
  • Pitting; and
  • Bi-metallic corrosion.
Corrosion categories

Offshore environments usually incorporate long-term exposure of the turbine structure to extreme high humidity levels with high salinity, wave action and high UV exposure. The presence of a splash zone area adds to this with high corrosive stress, giving rise to dramatic, very fast corrosion at holidays, weak points and damaged areas of the coating system.

And the corrosion loss is rather rapid:

Mass loss

As you can see, mass loss in a C5-M environment can quickly reduce the thickness of steel. The above table is based upon only 1-year exposure to that environment. With this in mind, it is clear that a highly robust protective coating system has to be applied in order to provide long-term adequate protection from exposure to such harsh corrosion environments.

Standards in Use

There are a number of standards in force in Europe today that regulate the corrosion protection of offshore structures; these were adopted from the oil and gas industry and have been implemented with success in the renewable sector. These include:

Standards

Many of these standards have been used in the oil and gas industry for many years. In turn, oil and gas professionals have been instrumental with fabrication and maintenance of the renewable sector. The skills are fortunately easily transferrable and because of vast familiarity with the above standards, particularly the ISO standards and NORSOK M-501, coating projects have been improved since the early days of turbine corrosion control via protective coatings.

One major problem, though, has come from manufacturers: Due to the mass production and quick turnarounds of turbines, owners expect short/reduced cure times in protective coatings. This has in turn led to an abundance of challenges from coating manufacturers, who have strived to keep up with the renewable energy company production requirements. This has seen varied levels of success, as formulating quick-drying coatings and applying them successfully is a very challenging task that requires years of painstaking testing!

There have been numerous reported coating failures and transportation damage primarily due to load out of uncoated turbines and lack of curing of the protective coating system, and thus we being the very expensive maintenance merry-go-round.

Next Post

Now that we’ve established the importance of corrosion control on offshore wind structures and some of the challenges the structures bring about, next week’s continuation of this blog will look at the makeup of offshore turbines and the coating systems used today to protect them—as well as the importance of quality control on offshore projects.

ABOUT 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.

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Tagged categories: EMEA (Europe, Middle East and Africa); Engineers; Inspection; Institute of Corrosion (ICorr); Lee Wilson, CEng, MICorr; NACE; North America; Quality Control; Quality control; SSPC; Asia Pacific; Corrosion; Corrosion protection; Exposure conditions; Galvanic corrosion; Immersion service; Latin America; Microbiologically Induced Corrosion (MIC); Offshore; Power; Program/Project Management; Protective coatings; Salt exposure; Seacoast exposure; Wind Towers

Comment from Lars Lichtenstein, (4/12/2018, 2:26 AM)

Hi Lee, nice overview. Just want to point out that ISO 12944 has just been revised, so some of the tables and numbers are not up to date anymore, e.g. C5, not C5-M, and CX for offshore inviroment, ISO 20340 is now ISO 12944-9. Also valuable guidance is given within DNVGL-RP-0416: Corrosion protection for wind turbines, but here again the latest revisions of ISO 12944 have not yet been included, but will be soon.


Comment from Andreas Momber, (4/12/2018, 4:33 AM)

For all intersted parties, please see the recent torough review paper: Momber, A.W., Marquardt, T.: "Protective coatings for offshore wind energy devices (OWEAs): a review", Journal of Coatings Technology and Research, 2018, 15 (1) 13-40. Also, for offshore wind power transmission platforms, see the following paper: Momber, A.W.: “Quantitative performance assessment of corrosion protection systems for offshore wind power transmission platforms”, Renewable Energy, 2016, 94, 314-327 For repair coatings for offshore wind power devices, see the following paper: Momber et al.: “Statistical effects of surface preparation and coating type on the corrosion protection performance of repair coatings for offshore wind power constructions”, Materials and Corrosion”, 2018, 69(4), 460-471 There is plenty of information in the papers. Do not hesitate to ask for details. Andreas Momber


Comment from Lee Wilson, (4/16/2018, 7:19 AM)

Hi Lars Yep some changes have indeed occurred and the changes were under review during the construction of the blog hence the main reason why I didn't include it in the standards table its my plan to point out the changes and the mistakes made in a later blog! Thanks for the comments!


Comment from Marco Fabio Ramenzoni, (4/16/2018, 2:02 PM)

Thank you Lee, nice blog! So, one can conclude that what is desirable is a ZINC coating that dries fast, accepts recoating, has unlimited shelf life, has a strong content of metallic pure ZINC, easy to apply (high RH, mostly any temperature) low application and repair cost, and yet complies to Norsok M501 and ISO 12944 (its updated standards, that is). I would love to offer that to our more open-minded offshore wind clients here in South America!


Comment from Andreas Momber, (4/17/2018, 4:24 AM)

Dear Marco, please see the first reference in my previous comment. It contains a list of coating systems applied to OWEA structures for different corrosivity categories. This list is based on specifications, meaning the systems are applied in reality. The reference also contains a number of requirements for OWEA coatings in addition to plain corrosion protection (e.g. gloss, color stability, abrasion and impact resistance). Regards. Andreas


Comment from Marco Fabio Ramenzoni, (4/17/2018, 6:59 AM)

Thank you, Andreas!


Comment from Martin van Leeuwen, (4/18/2018, 11:35 AM)

I would like to make you aware of a set of guidelines which have been issued by the German Society of Corrosion Protection (GfKORR). Of interest is Part 1 for Corrosion Protection of offshore Windmill Tower Pieces with Duplex Systems - 2015. These duplex systems are a proven solution for adequate corrosion protection over the lifetime of the wind towers, as can be concluded from the residual coating quality of the first offshore wind tower park Vindeby which has recently been dismantled due to end of its economic lifetime. And for Marco, a ZnAl15 thermally sprayed coating has instant drying, no curing and unlimited shelf life, with a better corrosion protection than pure zinc in a maritime environment. It complies with NORSOK M-501 and its own ISO 2063 standard.


Comment from Lee Wilson, (4/19/2018, 2:22 AM)

Hi Martin, Some good comments the use of metallic coatings is addressed in part 2 of the blog. I do believe that it is due out sometime this week!


Comment from Mario Colica, (4/19/2018, 4:07 AM)

I agree with Martin Van Leeuwn and Marco Ramenzoni comment . The Zn/Al 85/15 coating is long lasting measure against rust in a maritime environnement


Comment from bart de cremer , (4/20/2018, 2:42 AM)

Another proven coating system is a Belgium solvent free epoxy to be applied in 1 layer and Norsok M-501 approved.


Comment from Andreas Momber, (4/20/2018, 4:35 AM)

Dear all, we compared plain paint systems and duplex systems (metallizing + organic layers) some years ago under laboratory conditions (ISO 20340) and site exposure conditions (3 years offshore, North Sea) and found the duplex systems to provide a better performance. We quantified that trend. Please see the following publication: Momber, A.W., Plagemann, P., Stenzel, V.: "Performance and integrity of protective coating systems for offshore wind power structures after three years under offshore site conditions", Renewable Energy, 2015, Vol. 74, 606-617. You can also check my ResearchGate account for this and other papers. Andreas


Comment from Gunnar Ackx, (12/18/2018, 2:17 AM)

Much respect for your blog, Lee, but I am going to respectfully disagree with the corrosion rates you use to substantiate your statement "and the corrosion rate is rather rapid" YES, they come straight out of ISO 12944, the European Bible of Corrosion Protection (and yes, it got revised earlier this year). But what MANY people in our industry FAIL to read & understand is that this values are SHORT term (fresh) exposure rates meant to determine the corrosivity of an environment and NOT to predict the long term consumption of steel or zinc. It does NOT by any means imply that these corrosion rates are constant beyond year 1 (which is typically the length of these exposure tests). In fact, if one reads ISO 9223 & ISO 9224 you'll see that the corrosion rates for zinc, for example, are MUCH lower over longer periods, even in a C5 or CX environment. Dividing the thickness of HDG by the "4 - 8 µm/year" corrosion rate as mentioned by ISO 12944-2 to determine the lifetime (as 99% of our industry seems to do) is plain wrong. So, on that bombshell, I would like to invite any coating professional to actually go out there and obtain the standards involved, read them & apply them, steps all too often overlooked in these times of information overflow and instant solutions. Merry Xmas to all and wishing you all MANY standard reading for 2019 ;-)


Comment from David Grove, (1/14/2019, 2:35 PM)

All of the above comments are very good however, I strongly agree with Mr. Gunnar Ackx about the corrosion rates and that a full life-cycle evaluation is still required. Also, the designers are faced with other considerations, which include, impact or tolerance to the application or installation process of the corrosion protection, the impact of the weight of the corrosion protection to the wind turbine, and of course the installation deadlines. As far as rapid cure or instant cure coatings, there has always been the problem of undercreep corrosion with/and delamination. If the surface prep was correct, these generally depend upon the substrate, the actual coating and the mechanical impacts, such as vibration or just plain physical damage. With the manufacturers having to deliver coatings that may be based upon shorter testing periods to will meet the ever-increasing demand, there will be lots of fun for everyone involved. Good luck, and to all, have a safe and prosperous 2019!


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