TNO: Keep it simple, sustainable and predictable…

Currently most of the licences for offshore wind farm development zones are granted with a lease period of 20 to 25 years. As a consequence the turbines have to be built to last for at least that period. There are several elements that can affect the life span of a turbine once it is installed and in operation. They need, for example, to be strong enough to endure the movement stresses that take place on an operational turbine. However, the biggest challenge comes from nature itself.

The harsh winds offshore, while needed to get the best energy output, are at the same time a turbine’s enemy due to their corrosive nature. UV also plays a role but the biggest enemy is the sea, with its salt content, which damages the steel foundations. The splash zone is especially vulnerable as it is here where air, water, salt and UV are all attacking the steel components and the protective coating at the same time with aggressive and varying types of corrosion.

Studies on the corrosive effects of water on offshore structures are on going throughout the world. On a sunny but icy day in March Offshore WIND travelled up to Den Helder to visit Dutch research facility TNO’s Maritime Materials Performance Centre (MMPC), situated on the Dutch Royal Navy base in Den Helder.

It is not a coincidence though that this particular research group of TNO is based here. In 2002 TNO, already located elsewhere in Den Helder, took over the Navy’s maritime corrosion lab. The combined corrosion and anti-fouling experience of TNO goes back over 40 years. As an independent research laboratory TNO is also very active for the Royal Dutch navy.

Harald Van der Mijle Meijer, Business Developer at TNO, and working for the organisation since 2002, and Johan van Malsen, Material Scientist, working here since 2010, gave a tour around TNO’s facilities. In this TNO office 14 people are working on several research fields. There is a team of micro biologists analysing bio fouling and the effects of bacteria on corrosion. Another team is looking at corrosion and electrochemistry and there is a team of materials scientists that are active in consulting activities, for instance on damage caused by mechanical and/or corrosion related failures.

We started off in the main entrance hall where a range of objects in a glass display explains precisely what TNO is doing – innovation for customers in the offshore and maritime industry. Development of fundamental knowledge in order to continuously keep broadening and deepening their expertise is essential to be of benefit to the industry. However, clients in the offshore wind industry are reluctant to pay for fundamental research as they wish to apply the result in a shorter time frame.

For this reason TNO is involved with many funding research programs such as European Framework programs and National funding programs like the TKI ‘Wind op Zee’ (Wind at Sea), a funding programme by the Dutch Government for knowledge and innovation at sea, and also cooperates with different universities. At TNO MMPC activities in offshore wind related projects are equally divided in 33% fundamental research, 33% industrial research and 33% consultancy.

Research on corrosion

Mr Van der Mijle Meijer: “There are around 35 types of corrosion, in the offshore industry a smaller number is relevant. In the case of an offshore wind turbine foundation it is mainly uniform corrosion and mechanical load related corrosion fatigue that plays an important role.” Throughout the years the focus of research on corrosion has changed. While initially it was more focused on detecting and prevention, nowadays the aim is also on condition health monitoring to predict maintenance.

For offshore structures corrosion can be influenced and accelerated by bacteria and micro-organisms in the seawater. This is called biocorrosion or Microbial Influenced Corrosion (MIC). Prevention for MIC attack can be done by either killing the devastating micro-organisms or preventing them from attaching to the steel structure.

Mr Van der Mijle Meijer is clear on one thing though: “Any material has to resist the conditions exposed to. Nature is flexible and adaptive enough to find its own way. So corrosion will always be there, finally the seas will always win. The challenge is to delay it as far as possible and have insight in its predictability.” It is therefore important to test the coatings on quality and durability. A protective coating functions as a barrier to isolate the steel from the corrosive environment. Quality and durability of such protective coatings need to be properly tested in order to assure their performance. Electrochemical Impedance Spectroscopy (EIS) is a suitable technique to characterize the protective properties of coatings. In the past this was done using large electrochemical measuring equipment set ups. Nowadays much smaller portable set ups are available that can also be used in so-called in-situ tests, i.e. measurements on the spot.

The next step would be continuous monitoring of coating properties using a measurement system permanently installed. “At TNO we are investigating such measurement systems in the project World Class Windturbine Maintenance to see if a remote sensor is suitable in a maintenance system,” explains Mr van Malsen.

Test facilities

TNO in Den Helder has several rooms where different types of research take place. We visited a few to gain insight into the work being done. In the metallographic lab optical and electron microscopes are used to look at corrosion processes, and to investigate
mechanical failures. Metallography and fractography are vital aspects in failure analysis.

In another room electro chemical tests are carried out. With these techniques it is possible to check whether corrosion is taking place on a coated object at a stage that is not yet visible to the eye. In one of its projects TNO has tested the performance of some commercially available coating solutions in partnership with the German Fraunhofer IWES and the Norwegian SINTEF.

Another room contains a pressurised container in which bacteria are grown for process analysis at pressures up to 800 bar – equalling a water depth of 8km. Does corrosion behave differently in waters twice as deep now that wind farms are being planned further out at sea? Mr van Malsen, “The deeper you go the conditions change, even oxygen is present at large sea depths. Also underwater current flows occur at varying depths which may influence the material degradation behaviour like erosion.”

In the room next door bacterial studies are being done on degradation and corrosion of materials in different environmental conditions. Here also advanced facilities for DNA studies on micro-organisms are available.

At the back of the TNO building the large seawater hall contains several custom made test set ups, including a scale sized monopile foundation placed in natural seawater. The seawater is pumped up from the harbour and after some filtration steps fed into the experimental set ups that simulate exposure to relevant marine conditions. During a full year various types of corrosion processes were simulated and process parameters on inside and outside of the monopile were measured and monitored. On the outside of the monopile various types of coatings were applied and investigated on protective properties. This test set up was designed for the Dutch Offshore Wind Energy Services (D OWES) project, aiming at development of an innovative asset management control system which can be used to operate and monitor a wind farm at sea.

Why is it needed to investigate corrosion on the inside of the monopile?

Mr van Malsen explains: “Corrosion also takes place inside the monopile. When the inside is sealed off the air slowly looses oxygen and that’s when the corrosion process will change from predictable uniform corrosion attack to more aggressive local corrosion attack by MIC, since the area inside is also a good place for bacteria to grow.” Wind farm developers and operators encountered this and in some cases decided to apply a protective coating also on the inside of the monopile from now on.

The project with the monopile is completed now and the results are being analysed and will be published later this year. TNO is exploring options to continue with a second phase. “We would like, for example, to test the solution of applying a thermal sprayed zinc layer as protective system with or without a coating system on top of this. Zinc functions as a sacrificial anode called cathodic protection. In April TNO will apply for a tender in the TKI wind project for this idea. We are still trying to attract more sponsors for this project.”

More focus needed from the industry

You would think that offshore wind developers will be knocking on the door at research centres such as TNO to ensure that appropriate materials and material protection techniques are being used, but up till now this is not the case. TNO actively needs to go to the market to raise awareness. Both men think that currently this topic does not get the attention that is needed from the industry itself. The main problem is that in many cases the developers are not the operators of the wind farm and therefore do not put as much importance on the aspect of durable protection as would be needed.

“The offshore wind market is largely dictated by cost of energy (CoE), reduction and unfortunately corrosion protection normally comes last on the budget planning list. Contractors are generally looking to find the cheapest solution as they are more concerned about the overall CAPEX. The corrosion protection must be integrated in the LCA and early stage of the design process,” explains Mr van Malsen. A relatively small investment in a more durable corrosion prevention system for this specific application will be more cost effective over the life time when repairs on site can be limited. Research has calculated that the cost of repair of 3% of the coating area of a turbine at sea equals the cost of applying the coating on a complete turbine on shore.

Another obstacle is the existing legislation and standardisation. The offshore wind industry is following the current standards as outlined by the classification bodies. There are specific offshore wind industry guidelines but also standards from the conventional offshore industry or modified on shore wind energy standards. Mr Van der Mijle Meijer explains: “Guidelines are of course needed but these are often rigid with no room to invest in innovative concepts. In some cases they can be too strict and in some cases they can turn out to be insufficient. Therefore there is a need for offshore wind specific guidelines. This is difficult to achieve as it takes a lot of time for this to be processed and it requires full cooperation of the classification bodies.” Mr van Malsen adds, “We foresee in the future that companies will make their own guidelines that they find to be appropriate to their specific project and have these officially checked by classification bodies.”

QA check

A lot of conservation problems can be prevented by ensuring selection of suitable coating products and ensuring appropriate application procedures and conditions.

Coating application takes place in varying settings. They are not always applied in covered halls with adequate climate control, application outside also occurs quite frequently. The temperature, air humidity, and conditions such as salt and dirt content have strong influence on the coating process and the quality of the protective layer.

“A small degree of damage to the foundation might not lead to an extreme dangerous situation in the oil & gas industry but it does in the offshore wind industry where the turbine and foundation structure is subjected to varying loads. A local defect like a corrosion pit may result in a fatigue fracture. The challenge is to make sure that the coatings are of high quality and that they are properly applied, ideally resulting there being no need for inspections once the turbine is in operation.” Mr Van der Mijle Meijer continued, “Unfortunately there is no coating solution in the market at this moment that can really guarantee a life endurance of the 20 to 25 years of the wind farm’s life span.”

However, both men see a shift in responsibility where the developers will also be the operators and they hope this will lead to a more responsible management where quality control on site will be as important as during the manufacturing stage of the steel components.

TNO is trying to create awareness on this issue. In June last year it organised a workshop where the complete supply chain; operators, wind turbine and foundations OEMs, coating suppliers, and those who apply the coatings were invited to join and discuss this topic. According to Mr Van der Mijle Meijer the meeting was surprisingly well attended with good interaction between all parties. One main thing became clear, they all agreed that there should be more communication between all of them. The results of this workshop will be used for initiating a corrosion prevention research program for offshore wind with industrial partners.

Best solution?

So what is the best solution at the moment according to TNO? They agree that there is no such thing as a perfect system, but they do think that contractors should choose more carefully which solution they should use. Mr van der Mijle Meijer adds: “Keep it simple, sustainable and predictable! We are still in a learning curve and a lot more research is needed to be done on this topic especially on extending the life span of coatings.”

Fortunately the sun was shining on this icy cold winter’s day as we finished the tour outside to look at the testing site of coatings in a maritime environment. Here coatings on steel samples have been enduring exposure to the maritime environment and UV influences.

At the end of the rows of these steel samples there was a large steel structure coated with a bare thermal spray aluminium layer. Mr Van der Mijle Meijer explained “This is currently not used in the offshore wind industry but could be a good alternative to the existing coatings as it has a longer durability and can be applied in different environments like offshore. This was once placed here by the Ministry of Infrastructure and Environment and has been out here for more than 20 years now with no corrosion attack of the steel. This is one of the solutions that we would like to test also in an offshore environment.”

This is just an example of the many concepts that can be tested to improve the corrosion protection on offshore wind farms and for sure a topic that will be returning in a future edition of Offshore WIND.

Sabine Lankhorst