Steel or concrete, companies that today want to take up the challenge of floating wind turbines are still hesitating about the material to use. If it is made of steel, the float weighs, with the turbine, between 2,500 and 4,000 tonnes. If it’s concrete, it’s more like 10,000 tonnes. A real issue for ports, where the load-bearing capacities of quays, lifting and towing equipment, as well as storage space are insufficient.
But weight is not the only criterion. The draft is also decisive. A concrete float will need a deeper port for launching than a steel model. This is one of the reasons why the EolMed pilot farm, off Gruissan (Aude), finally preferred to choose the metal version.
However, concrete has its advantages. “In Europe, it is difficult to obtain competitive steel, while concrete can be prepared with local cement, sand and aggregates. Its price is also less volatile, which is an important element in the current inflationary context.”emphasizes Paul de la Guérivière, CEO of the BW Ideol company, which developed the EolMed floats.
The “milk bottle” family
This float is a type of barge 40 to 60 meters square and 10 meters high, the size of a four-storey building, attached to the bottom by chains and marine anchors. The center of the parallelepiped, empty, absorbs the energy of the swell and the currents. The turbine rests on one of the four sides.
This solution was chosen for the wind turbine that Ideol commissioned in 2018 in Japan, and for a Scottish floating farm awarded to the French start-up at the beginning of 2022 for a power of 1 gigawatt, double the wind farm installed by Saint Nazaire.
Other designs compete with the ring float. All are made of steel, either cylindrical or prismatic (plates welded to form parallelepipeds). There is the so-called “milk bottle” family, a single cylinder weighted with several hundred tons of concrete, held vertical by Archimedes’ thrust and stabilized, again, by traditional anchors. The family of semi-submersibles consists of giant barrels connected by tendons. The turbine rests on one of them and the other two are ballasted. Or the three are surmounted by a tripod in the middle of which sits the wind turbine.
This is the choice of EDF Renouvelables in Port-Saint-Louis-du-Rhône, based on a concept developed by the Dutch company SBM Offshore. Except that the anchor lines will be stretched to increase general stability. And that the anchors, bells 9 meters in diameter inside which the vacuum will be made, will never move.