Figure 40 Corroded monopile structure, courtesy of Materialenkennis.
Function
Corrosion protection protects the foundation from corrosion to the extent that is required.
What it costs*
About £28 million for the foundation corrosion protection for a 1 GW wind farm using monopiles.
Who supplies them
Suppliers of corrosion protection coatings suitable for use on offshore foundations include CRC Evans, Gorilla Corrosion, Hempel, International Paint and Jotun.
Suppliers of cathodic protection systems include CORROSION, Cathelco, Imenco Corrosion Technology and Impalloy.
Key facts
In offshore wind, corrosion predominantly occurs when sea water interacts with metallic surfaces. This can lead to oxidation (or rusting) of metallic surfaces, which can compromise the strength and performance of metal structures such as substructures. Corrosion protection mitigates general and localised wall loss in steel substructures and is a prerequisite for attaining the fatigue of the structure. Corrosion can also occur from microbiological activity.
Methods for corrosion protection include cathodic protection and corrosion protective coatings. Other methods for corrosion control, such as corrosion allowance and use of corrosion resistant materials, are important considerations in the design of foundations but are not covered in this section. Corrosion protection must also account for fabrication, transport and installation to avoid damage even before the wind turbine is operational. The corrosion protection system mitigates general and localised wall loss and is a prerequisite for attaining the fatigue of the structure.
The external surfaces of the atmospheric and splash zones are normally coated with high performance marine coatings. Although the atmospheric zone coating can be accessible for repair, it is costly to repair any coating offshore, more so in the splash zone. For this reason, the coatings are combined with a design corrosion allowance to give maintenance-free service at least for the foreseen lifetime of the wind turbine.
Cathodic protection systems are typically used to provide corrosion protection to the part of the foundation in the submerged zone. The application of a negative current to the steel structure reduces the voltage on the structure to a level at which oxidation, and hence corrosion, is suppressed.
There are two types of cathodic protection systems:
- Galvanic anode cathodic protection systems (GACP) which comprise several sacrificial anodes made of aluminium or zinc-based alloys that are fixed to the steel structure below the waterline. The more active alloy in the anodes is consumed in preference to the structural steel. This galvanic action provides a self-regulating current source that protects the structural steel and other metal components of the foundation. The zinc or aluminium bars can be designed to be replaced periodically to extend the useful lifetime of the corrosion protection.
- Impressed current cathodic protection system (ICCP) which uses an external power source and rectifier to supply a negative current to the steel structure and a corresponding positive current to non-consumed anodes mounted adjacent to the structure. An ICCP is substantially lighter than GACP and causes less drag in the water than the numerous sacrificial anodes required by a traditional cathodic protection system. A reliable power supply and associated instrumentation is required, and the design needs to be robust enough to withstand possible damage from sea conditions such as waves and currents as well as from maintenance and operational activities.
- Unprotected structures, particularly embedded in the upper layers of the sea bed, may be subject to microbiological influenced corrosion related to naturally occurring bacteria. The resultant damage can reduce fatigue life of these areas.
Reactions between the foundation, the sea and the sea bed material, together with reactions from the cathodic protection system can generate noxious gases, which will accumulate inside a monopile. The lower deck of monopiles will be sealed for the safety of maintenance technicians working above, whilst gas detection and ventilation systems may be used to monitor and safely vent the concentrations of the noxious gases.
In closed internal compartments, such as in jacket tubes, which have been welded shut, corrosion may be mitigated by control of humidity or depletion of oxygen. The inside of a monopile is not considered to be a closed internal compartment.
What’s in it
- Paints and thermal metal spray coatings
- Zinc or aluminium based sacrificial anodes
- Impressed current cathodic protection systems