Offshore wind power

OFFSHORE WIND POWER

An offshore wind turbine works on the same principle as an onshore model.
The main difference between a marine wind turbine and a terrestrial model is the nature of foundations that allow them to be fixed in the ground or anchored to the seabed.
Offshore wind turbines must also be very robust to withstand the difficult marine conditions.
The sea is flat, the winds encounter fewer obstacles and are, therefore, more sustained, more regular and less turbulent than on land. At equal power, an offshore wind turbine can produce up to 2 times more electricity than an onshore wind turbine.
An offshore wind begins to operate by a wind of 2, on the Beaufort scale, (approximately 10 km/h or 3 m/s), reaches its cruising speed to 6 Beaufort (45 km/h or 12 m/s), and must be stopped at 11 Beaufort (110 km/h, 30 m/s). 
The sea offers great obstacle-free spaces where the installation of machines is possible, subject to consultation with other users of the sea.
Offshore wind turbines are most often mustered in a "wind park" or "wind farm" usually containing between 20 and 50 wind turbines from 2 to 5 MW. The traditional offshore parks are not usually installed in areas where the depth exceeds 40 metres.

Offshore wind farm "Alpha Ventus", Germany (© 2011)

See the movie: “Alpha Ventus offshore Wind Farm”












The development of offshore wind turbines:

Evolution of the size of offshore wind turbines (© 2011)

The current technological developments, in particular the expected evolutions of the machines in size and power are characteristic of offshore wind turbines.
Offshore wind turbines differentiate themselves more and more from onshore turbines by their technical design adapted to the marine environment. If they have the same look, their operating conditions are different.
The marine foundations are the most remarkable aspect of their particularities because they must be anchored or embedded in the seabed. They must also resist corrosion, storms and efforts created by the surrounding water masses.
Their nacelle is equipped with two hydraulic cranes enabling the handling of tools and spare parts at any point of the wind turbine.

In the year 2015, the parks might well muster from 50 to 100 wind turbines with a unit capacity of 5 and 10 MW and with a total installed capacity of 500 MW.

Limits of the offshore wind farms:
The installation of offshore wind farms near the coast (10-15 km) is a factor of anxiety for coast residents fearing that they are too visible.
Working out the constraint of the water depth is an interesting line, especially in countries like France where depths exceed 40 m quickly when moving away from the coast.

Technological breaking expected with the new floating wind-turbines (far-shore):
To install these turbines further offshore, some companies, like Principle Power or Ideol, have developed a system of semi-floating wind turbines to overcome this technical and economic constraint.
Floating foundations, on which are fixed wind turbines, allow installing wind farms more than 30 km from the coast, where the sea winds are stronger.
Foundations of offshore wind turbines are no longer embedded in the seabed but they are only anchored by cables (up to 50 m deep).
That is why the installation is simplified and material requirements greatly decreased.

Far shore Wind turbine, WindFloat structures (© Principle Power)

Comparative advantages of wind turbines "far-shore":
The "far-shore" technology has lot of interests:

• Technical interest: wind turbines can be assembled onshore with standard turbines, and then be towed before being anchored in depth. The mooring system uses simple and inexpensive components. This results in savings compared to wind turbines directly planted in the seabed or requiring heavy steel structures;
• Energy interest: wind turbines can be deployed off the coast where winds are stronger. They have a ballast system ensuring the stability of the structure, consisting of three columns. The shallow draft of structures allows them to withstand a rough seas;
• Social interest: wind turbines are far from coasts and therefore less visible by the coast residents. Their acceptability is enhanced.

 See the movie: “WindFloat”











See the movie: "IDEOL, floating wind energy "




 Limitations encountered in their exploitation:

• At the moment, an offshore wind costs about 30 to 50% more than onshore wind.
• Although the winds are more constant at sea than on land, offshore wind power is also intermittent.
• The wind turbine is mechanically subject, not only, to the wind efforts on the blades and structure, but also to the efforts created by the currents.
• The installation of offshore wind is more complicated than on land. Adapted boats must be used. The maintenance of wind turbines is also more complicated and more expensive than on land. If a failure occurs, it may take several days before the repair, which leads to a loss of production. 
• The electrical connection requires the installation of submarine cables, from the wind farm up to the coast, which can be to several kilometres distant. For large distances, it is necessary to use a DC voltage delivery (HVDC) well as power electronic converters to transform alternating current in DC (and the inverse) and filters to reduce electrical perturbations.

See the movie: "Dolwin 3 HVDC"











  (Click to go towards "Energy transport in HVDC").

Return to "Wind power"