Prevention is better than cure, say corrosion expertsTaken from: Offshore Wind Journal June 2012
Peter Kronberg Nielsen, a corrosion expert at Force Technology in Denmark, said that each of the different components of an offshore turbine has its own corrosion protection issues, but by far the most challenging areas are the ‘splash zone’ and the underwater parts of the turbine – that is, the foundation.
The standards commonly used to ensure adequate corrosion protection are NORSOK M501, a standard originally developed for the offshore oil and gas industry, which gives the requirements for the selection of coating materials, surface preparation, application procedures and inspection for protective coatings; and DNV-OS-J101, DNV’s standard for the design of offshore wind turbine structures. The latter was introduced in 2004, and has been updated for the third time recently, including Section 11, covering corrosion protection, which was restructured and expanded, stating requirements for which types of corrosion protection should be applied in the different corrosion zones. It also states requirements for corrosion allowance, and cathodic protection and coating, and uses a new definition of the splash zone based on the definition used in IEC61400-3.
As Mr Nielsen explained, the underwater parts of a wind turbine (foundations) are usually protected with sacrificial anodes or, more recently, with impressed current cathodic protection (ICCP) systems. In the early days of the industry, he explained, some anodes were poorly applied and fell off foundations, and needed to be replaced, but these problems have largely been recognised and overcome.
“Anodes today will last 25 years,” he explained, “but it is important to ensure that they are correctly placed. Experience has shown that they function best if they are attached just below the splash zone. In the early days anodes were often placed close to the seabed, but were found to be less effective there and did not provide such good protection.” As he also explained, on more complex structures, such as jackets, it is important to ensure that the entire structure is adequately protected, for which careful modelling may be required.
One of the most corrosion-prone areas is the area of the turbine around boat landings. The impact of windfarm service vessels on the structure can cause mechanical damage to coatings, and allow corrosion to start.
“For the most part, the coatings applied to wind turbines are the same type applied to other offshore structures, such as oil and gas platforms,” Mr Nielsen told OWJ, “mostly epoxies, and particularly epoxies that are resistant to mechanical damage.
“Quality control during application is especially important. Sometimes, in order to keep costs down, the application process isn’t properly overseen by a coatings inspector. If the application process is sloppy and work is not inspected, problems can arise,” he explained, noting that he has seen cases of transition pieces experiencing corrosion in as little as two to three years because of poor application.
Occasionally, corrosion is found inside a transition piece. This has usually been associated with where cabling leaves a turbine through a gasket in a monopile. If the gasket does not form a proper seal or becomes loose, water ingress can occur and can cause corrosion which can be difficult to deal with. One solution, which has been adopted on the Walney and London Array windfarms in the UK, is to run the cable on the outside of the tower.
As Anders Voldsgaard Clausen, group segment marketing manager at coatings manufacturer Hempel in Denmark, explained, a typical coating system for offshore wind turbines would be two layers of an epoxy coating and a polyurethane top coat.
Hempel is one of the largest suppliers of coatings to the market, and has been a supplier of corrosion protection to the offshore wind energy industry since 1991. It has long experience in coatings for offshore oil and gas platforms, pipelines and ships. The company is playing an important part in the WindFloat project, a semi-submersible floating structure for offshore wind turbines that has been deployed with a 2MW Vestas turbine off the coast of Aguçadoura, Portugal. The sections of the WindFloat for which Hempel is providing the test products include permanently immersed areas, the splash zone and the above-waterline areas. In addition to supplying coating systems, Hempel is also contributing to the project with technical support and advice.
The project is being overseen by the Materials and Coatings Laboratory (LMR) of LNEG (National Laboratory for Energy and Geology) in Portugal with the overall aim of testing and evaluating different corrosion protection systems for offshore steel structures. The performance of the anticorrosive coatings will be evaluated by exposure at the Aguçadoura test site for a two-year period.
Mr Clausen said most of the coatings that are applied to offshore wind turbines are based on formulations originally developed for the offshore oil and gas market, but said that Hempel is also working on industry-specific coatings for offshore wind. “Oil and gas structures are very large and are not built in large numbers in the way that wind turbines are,” he told OWJ. “Productivity isn’t quite so important, but with series construction of offshore wind turbines, issues such as curing time are more important.”
James Morton, a power product manager at International in the UK, part of the Akzo Nobel Group, recommended high build epoxies of the type in which the company specialises and a polyurethane top coat. The performance of products such as these has long been proven in the offshore oil and gas industry. “The transition piece is the really critical area,” Mr Morton told OWJ, “but for all coatings it is important to keep the cost down whilst providing the required level of corrosion protection and reduced maintenance. It is very much a case of prevention is better than cure.
“The owner of the asset needs confidence that a protective coating system is going to perform, but it is not feasible to run 15- to 20-year product trials in the offshore wind market, so the industry has relied on accelerated testing,” he explained. “Having been involved in offshore corrosion protection for over 35 years we can correlate this accelerated testing with our in-field performance testing.” The company’s track record in the market includes many well-known projects such as the transition pieces for the Belwind offshore windfarm in Belgium, where Interzone 954 and Interthane 990 were used, and the Beatrice demonstrator, where the splash zone and foundations were coated with Interzone 505. More recently, Interzone 954 has also been selected for use on the London Array project.
Should a turbine have a problem with corrosion, it can be particularly expensive to remedy, as experts from UK-based coatings specialist Safinah confirm. Sending a crew out to a turbine is expensive in its own right, and erecting staging in order to carry out work can be difficult. “If the build specification fails to perform, failure repetition can multiply by the number of towers in the field,” said the company, which estimates that a repair to just 3 per cent of the area coated could cost more than the total initial cost of painting.
Safinah says that coating performance failures can occur because of one or more of the following: design, product specification/selection, product quality, management processes, preparation/application, maintenance, climate/environmental control, and worker skill. “Product quality is rarely the cause of failure,” said Safinah, noting that coating breakdown may occur because of an incorrect specification, poor application, or because of other issues such as sharp edges on structures where coatings may not work, or stress cracking in dry coating films.
Safinah says the application contractor controls about 75-85 per cent of the cost of the process and good application is fundamental for success. Specified standards of secondary surface preparation for steel and appropriate quality control are essential, as is the use of trained and skilful spray-painting personnel. “Applicators need to conduct their own quality control work for multilayer coating systems, and have control over environmental conditions during the application process,” the company concluded.