Of the two types of offshore cranes used for wind turbines -- floating vessels and jack up barges -- the barge is the current preference. A jack up barge has legs, retracted during towing, which are lowered to the seabed to the point where the hull is jacked up above the water, providing a secure base above the waves from which to work. They are used in the offshore oil industry as a drilling unit not permanently fixed to the seabed. Floating vessels, which are by far the cheapest, are out of the question, Ruer says. "You can't install one huge structure on top of another one at sea using a floating vessel. It's too imprecise. The crane just releases its load from a metre above the target -- this may be a shock for some to hear, but this is the way it's done."
The company, eager to enter the offshore wind business, investigated options such as modifying existing jack up barges with longer cranes, floating barges and building new crane vessels, but none of these seemed to make sense on the bottom line. Through a database survey of nearly 150 offshore construction vessels around the world, the company determined what would be required of each unit to install wind turbines.
"One floating barge could handle 8000 tonnes at 50 metres," Ruer says. "It could also handle 400 tonnes at 80 metres -- which is more like a wind turbine -- but now you've downgraded the capacity of this barge, and rental costs of barges are according to their full capacity. This doesn't make economic sense. This is why we looked at a self-installing turbine."
For the SeaFlower, no crane is needed at sea. "We just need a crane at the harbour to erect the turbine on its base moored at quayside -- the kind of land crane used to erect any onshore turbine," Ruer says.
The base begins as a hollow, concrete caisson with a flat bottom which can float like a boat, he adds -- the same type of foundation used at the Tunø Knob offshore wind plant in Denmark; Tunø was different because those turbines then needed to be installed on top of the towers at sea, he points out.
Since the whole SeaFlower turbine is in its retracted position when assembled in the harbour, the centre of gravity is low, and it can float upright without toppling over. One or two tugboats can then pull the complete unit out to its installation site, where the caisson is then filled with sand or gravel, planting it firmly on the seabed. The mechanics required to extend the tower to full height are installed inside the tower.
For a 2 MW turbine in 30 metres of water, the hollow base will weigh some 2000 tonnes, where the final, filled foundation weighs 9700 tonnes, the company estimates. "It has to be the same weight as solid concrete after installation in order to secure the turbine in place," Ruer explains. The seabed must also be able to withstand this heavy structure, he adds, so it is not a universal solution.