Introduction to Corrosion in Offshore Wind Turbines.
The use of larger capacity wind turbines offshore is possible because of the more regular and higher velocity winds available of the coasts which are capable of driving the huge blades. But all this comes at a price; the marine offshore environment causes corrosion problems to most areas of the wind turbine.
This is another article in my series on corrosion protection of structures in the marine environment. Here we examine the anticorrosion methods used on offshore wind turbines which are similar in some ways to that of oil and gas installations.
We begin with an overview of offshore wind turbines, going onto the effects of corrosion on their operation.
Offshore Wind Turbines Overview.
Large capacity wind turbines are now being installed offshore ranging from 3MW to 5MW rated capacity and are a truly massive being of similar design as their land-based cousins (apart from the helipad positioned on top of the nacelle).
The regular and high velocity winds are capable of driving the large fibreglass blades and remember the bigger the swept area by the rotor, the greater the output and, the electrical output being the cube of the wind speed.
Due to the UK government’s latest allocation of offshore areas in January 2010, the power from these large capacity windfarms will be exported via sub-sea cabling to HVDC Converter Stations. These will be positioned above the water line, supported by subsea structures. From here the HVDC cables will run subsea again to the onshore landfall.
All these electrical components will require specialist and extensive protection against attack from the marine environment atmosphere and underwater corrosion. These will be the subject of a future article on corrosion.
Corrosion Protection of Wind Turbine Support Structures
There are three distinct areas of the support structure which usually consists of a monopod and tower that require protection,
- Atmospheric Area -The area above the Splash Zone which includes the turbine tower
- The submerged area
- The Splash Zone
This area is above the splash zone and includes the turbine tower.
It is normally protected by the following applications,
1. Zinc primer applied preferably as a thermal spray
2. A silicon epoxy resin sealant
3. A coating of two-part liquid epoxy coating
4. A final coat consisting of polyurethane which consist of one or two- part and applied by brush or spray and are normally moisture curing and drying if solvent free.
The submerged area (Monopod Type)
This can be protected by the application of a coating of two-part epoxy compound, being applied at the construction yard.
A method of cathodic protection using zinc anodes should also be used to compliment the epoxy coating.
The anodes can be either fitted at the construction phase or an underwater sledge arrangement used. Both methods have their merits, the onshore fixing of the anodes to the structure being quicker and more economical, but difficult to replace under water.
An underwater sledge consists of a frame which contains numerous anodes welded into it. The frame is then positioned close to the support on the seabed and connected to the structure by heavy copper wire cables. This method allows for easy replacement of anodes but is expensive due to its installation to the location and, connection to the structure which requires underwater welding by divers.
Splash Zone Protection
This area requires treatment due to its contact with the seawater combined with the salt-laden air of the marine environment.
The offshore oil and gas industry has historically used rubber coatings or aluminium spray as a corrosion protection method. However, because of the simple structure design of a monopod, compared to the intricate oil and gas platform steel jacket supports, it doesn’t really warrant these expensive applications.
It can be adequately protected by using an identical anticorrosion procedure as that of the atmospheric area. Alternately a relatively new concept in corrosion attack is to apply two layers of polyester based powder which should attain at least a 1000um thick coating.
Corrosion Protection of Wind Turbine Electrical System
The turbine nacelle is designed to inhibit the ingress of water. However due to the harsh marine environment of high velocity winds containing salt, other measures should be employed to protect the electrical and instrument control systems.
The components to be protected against condensation, dampness and subsequent corrosion are as follows;
- Power Generation Units
- Electrical Distribution Panel, Instrument and Electrical Isolation and Junction Boxes
- Hydraulic Pump Drive Motors
Power Generation Units.
This should be protected from condensation and dampness by the fitting of anti-condensation heaters. These use little power and have been proven to work in the offshore oil and gas installations.
The use of a Vapour Corrosion Inhibitor (VCI) such as supplied by Corroless, which I have used for over 20 years in the offshore oil and gas industry are very effective.
They are supplied in self-adhesive strips which emit a vapour which forms a microscopic layer of anticorrosion protection the inside of electrical and instrument enclosures, coil windings, contacts, and transformers etc protecting these components from corrosion. We used to replace the strips every 12 months during installation of equipment to the offshore platforms. (See my article on preservation of offshore equipment)
Electrical and Instrument Distribution Panels
These should be protected by fixing a strip of VCI tape inside the enclosures, this will protect the contacts and connections as well as the internal surfaces from corrosion. Sealing around the box lid with denso-tape will add to the enclosures protection against ingress of moisture.
These should be protected as per the enclosures the VCI tape being fixed inside the Junction Box.