According to early versions of the DNV(1) guidelines for the corrosion protection of WTGs (Wind Turbine Generators) stated that “If the inside of piles such as monopiles is ensured to be airtight, i.e. there is no or very low content of oxygen, corrosion protection inside of the piles is not required.” The LID monopiles followed this design philosophy, i.e. no CP was installed internally nor was any coating applied on the monopile or inside of the TP below the airtight deck.
Later versions of the standard(2) recognizing the concerns from operators that there was a potential risk of corrosion inside the closed compartments, the DNV standard on guidance for Wind turbine designers, revised this view with the statement “should have cathodic protection with coating around the “splash zone”.” A visit was carried out in October 2011 to two of the Wind Turbines and the results concluded corrosion, particularly in the submerged zone, was occurring within the supposed airtight section below the TP.
Corrosion rates measured from retrieved coupons in the submerged sections, varied in corrosion rate. As expected the corrosion rates in the sealed monopile were generally less than in the freely flooded condition. Although the corrosion rate was not a concern, fatigue curves used to calculate the life expectancy of the monopile were based on a corrosion free state. This was not the situation and leaving the monopile without any corrosion prevention system would heavily reduce the theoretical life.
Rather than risk not having a corrosion prevention system, a fast-tracked strategy was implemented. In addition to J-tube repairs (re-sealing), the project chose cathodic protection as the most viable method of re-instating a protected structure fatigue curve rather than a freely corroding fatigue, which was now the case.
Papers already published also found this issue on other WTG structures in Europe(3), however, this project is thought to be one of the, if not the first, to have CP installed inside the submerged section of the monopile. Due to the conditions and environment of the monopiles, any retrofit strategy would involve huge costs and require careful planning.
In 2011 there were no specific standards or papers relating to guidance in applying CP inside monopile structures offshore. Specifically, there is little guidance on CP retrofit strategies offshore generally.
CP design is relatively straight forward. Calculate the surface area to protect, apply coating factors (where coating exists, here they do not), apply a current density value (guideline from standards) and one can easily calculate the total current requirement. A few other calculations can then determine anode geometry and quantity to meet the current requirement and use a given anode capacity to determine weight requirements to meet the life.
However, the design had several constraints;
- Weight and handle ability of the anodes offshore,
- How are the anodes going to be mounted?
- Concern over hydrogen evolution in a sealed environment
Surface area is based on a submerged section (primary structure and J-tubes) but also a buried section, up to 30 m seabed depth. How much should be applied (or considered to be lost in seabed)? Biggest question then is what current density to use in the sealed monopile submerged section given that low oxygen diffusion in the water was expected because of the “sealed” airtight hatch on entry into the submerged section of the monopile?
With so many unknown questions and so much riding on the installation of 54 No. structures, optimizing the design was required and a CP trial was implemented in early 2012 with a view to roll out the design in the summer of 2012 with the monopile remedial repairs.
Although much easier and likely more economical to install, the risks from chlorine gas and mixture with damp conditions (hydro-chlorides) and even risky likelihood of hydrogen, the option of ICCP was strongly dismissed as an alternative. Trials have been ongoing and designs are now appearing since this trial was carried out, however based on events this project had neither the inclination or time to consider this option.
The galvanic anodes were to be mounted at several hangoff locations. Anode strings were therefore to be used and to save on wiring, anodes cast on galvanized steel wire were to be used, to be re-assembled immediately prior to installation.
Weight is a major concern from a manhandling perspective. The anodes would need to be loaded on a vessel, taken to the field, lifted (50 kg weight limited) on to the TP platform by a Davit crane, hauled inside, lowered through two decks, assembled, lifted and lowered into position on the lower deck hangoff location. This is not easy.
The operator/ construction crew requested that the choice of anode was either 1 No. 50 kg anode or 2 No. 25 kg anodes connected by the rope, aluminum alloy (Al-Zn-In) or zinc. The electrochemical capacity of aluminum is up to 2500 Ah/kg (although DNV-RP-B401 20104 recommend no more than 2000 Ah/kg in a CP design). This still has 2-3 times greater capacity, i.e. 2-3 less weight, than that required for a zinc alloy. Therefore, on the manhandling and cost of installation alone, zinc anodes were dismissed.
However, how much total weight is required to meet protection, in this case a minimum potential of -800 vs Ag/AgCl (Silver Silver/Chloride) reference electrode was sought. Previous findings from the site initial visit to the field, did not give the project team any cause for concern regarding MIC (Microbial Induced Corrosion).
The monitoring system for the offshore trial consisted of;
- 6 No Dual reference electrodes (Ag/AgCl) measuring the structure
- 1 No Dual reference electrode (Ag/AgCl) measuring one of the anode strings
- pH probe
- Dissolved Oxygen probe
- 3 No. Hydrogen monitors
- 1 No. 1m2 Current Density plate
- 1 No. Remotely accessed (by cell phone data) datalogger
- 1 No. monitor of the ventilation system current input
A total of four strings of Al-Zn-In alloy anodes were lowered into the confined space of one monopile (The Trial Position - TTP) and hung-off the lower platform. However, of these four anode strings, only two were initially connected to the structure. The trial commenced (connecting two anode strings to the structure) in March 2012 and 4 weeks later, for reasons seen further down, a third anode string was connected to provide additional current. To make life easier and prevention of entering the airtight column, which requires a full rescue team and is very costly, for the trial the anodes were isolated from the structure at the hang-off and 95 mm2 cables were run above the airtight deck into the monitoring panel. This allowed relatively easy access to “switch” the anode strings to and from the structure. The latter being a major consideration should dangerous levels of hydrogen be detected.
Six silver/silver chloride dual reference electrodes were located in two positions at three elevations to monitor the structure to seawater potential and an additional reference electrode was placed next to an anode string – identified as Ref. No. 7.
A ventilation system was implemented with a fan, which was monitored to confirm operation, however, this was switched off after a short period of time to see if there was any hydrogen build up, which was one the major concerns over installing a CP system in an airtight facility.
In addition, a current density probe, and hydrogen sensors were installed at the top of the sealed column and one above the airtight hatch.
As seen in the first set of results below, unexpected potentials were detected. The first plan of action was to connect another anode string to the structure, however, this had no effect and in fact seemed to make matters worse. An intervention into the monopile below the airtight deck was established and calibration checks were undertaken. After water samples were taken it was found that the water had to be replenished and the trial was reactivated with a flushing system implemented.
As of compiling this paper, we are still receiving good data.
A further four monopiles were installed with anode current and potential measurement equipment including a monopile that had been drilled as part of the long-term monitoring strategy.
The first set of results can be seen in the following figure. As can be seen, the structure started to polarize as expected, with initial high current but a reduction in output as the structure achieved more protection. The current density plate also can be seen to polarize very quickly starting at 150 mA/m2 reducing to 20 mA/m2. It should be noted the current density plate was relatively clean sheet steel, as opposed to the corroded uncoated monopile structure.