The cost of corrosion damage in offshore wind monopile foundations can be crippling, yet industry guidance is falling short. Andrew Williams reports.
The provision of detailed guidance relating to the corrosion control design of monopile foundations for offshore wind turbines is a vital requirement for manufacturers, given the risk of serious financial implications of ‘getting it wrong’.
Independent risk management firm, Det Norske Veritas (DNV), provides much of the corrosion advice currently offered to the industry, but is their most up to date advice adequate to ensure long-term prevention of corrosion damage? If not, how can the industry ensure access to state-of-the-art guidance?
The main source of DNV corrosion guidance for offshore monopiles is DNV-OS-J101, which is applied as the primary code of practice at many offshore sites. Two items relating to corrosion have been updated in the latest version of the document. The first relates to the outer surface of monopiles, which is typically protected by a cathodic protection system.
In cathodic protection, metal ‘sacrificial’ anodes that are more reactive to the corrosive environment of the system to be protected, (in this case the monopile foundation), are linked to the protected system. The sacrificial anode partially corrodes or dissolves and, in doing so, helps to protect the metal of the system it is connected to.
However, according to Claus Christensen, Head of DNV’s Wind Energy Certification Department, clustering of anodes on the lower part of the transition piece of monopiles can result in interference between the anodes. This means that they may not be able to provide the required potential (where the steel surface is uniformly polarized to negative, removing the driving force for the corrosion reaction) resulting in insufficient protection, especially at larger water depths.
Adding more sacrificial anodes, more uniformly, would help because it would further divert corrosive processes away from the monopile, thus increasing the protection. "The updated standard therefore requires that the amount of anodes shall be increased [by] 50% compared to the previous requirement,” confirms Christensen.
Manufacturers and installers will need to be more aware of the potentially detrimental effects of anode clustering. To this end he adds, “the possible interference of clustering will also be considered”.
No airtight solution
The other issue relates to internal corrosion in the monopiles themselves. Previously, this was typically handled by having an airtight deck inside the monopile, preventing further corrosion once the available oxygen was consumed. The latest update suggests that manufacturers should consider internal painting or corrosion allowance.
"Experience has indicated that obtaining an airtight compartment is difficult and that more internal corrosion has been observed than expected," explains Christensen.
"Whether this is because ... the airtight deck has been opened more often than expected, [perhaps during inspections] or if it is due to the change of water through the soil, or a possible leak in the sealing around the J-tube entrance in the monopile is unknown," he adds.
Christensen's view is that, where internal corrosion is concerned, it is not necessarily the management of corrosion itself that is the main issue, but rather that limited progression of corrosion on the monopile inner surface has an impact on the fatigue assessment.
"Handling corrosion in fatigue assessments has some specific requirements, for which very slow progression is not necessarily clearly considered at this time," he adds.
The problem is that S-N curves only exist for 'free corrosion' (unprotected steel in sea water), 'in seawater with cathodic protection' or 'in air-curve' (steel in air with no seawater contact possible) - and more research is needed to determine when the different S-N curves need to be used.
"When this information is available the DNV-OS-J101 will be updated - if required," says Christensen.
Increased anode material is also recommended to address uncertainty regarding the possible effects anode clustering could have on attaining sufficient electrical potential. However, it is already a design requirement that actual potential should be measured, post-installation, to ensure that the cathodic protection system is working as designed - and that, if not, corrective actions shall be taken along with re-measurement.
"As we gain more information, DNV-OS-J101 may be updated again if the current requirement is either overly-conservative or non-conservative," says Christensen.
Short on feedback
Søren Viborg Kristensen, Design Manager - Offshore Wind at Aarsleff, believes the DNV guidance on corrosion control design to be generally adequate, and the DNV-RP-B401, which provides guidance for designs using galvanic anodes for cathodic protection, to be satisfactory. But he says additional guidance is needed with respect to the spatial distribution of anodes and interference.
He notes, however, that the latest revision DNV-OS-J101 partly addresses this by introducing a requirement for numerical analysis if interference or insufficient distribution of anodes is expected.
He also points out that impressed current cathode protection (ICCP) systems are still considered somewhat novel designs carried out by specialist companies - meaning that DNV can only provide limited design guidance.
"Several offshore wind farms ... have now been equipped with ICCP systems. It would be nice to see experience from these installations feeding back into updated guidelines," says Kristensen.
"We always try to channel experiences from previous projects into our new designs. All projects have some important lessons-learned and these must be communicated to ensure that new projects are really 'state-of-the-art,'" he adds.
In a rapidly developing industry like offshore wind, a high and increasing number of new entrants makes updating design standards vital. In this light, it is crucial that the industry continues to cooperate with guidance organisations like DNV to maintain the highest possible level of corrosion protection.
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