Speaking exclusively to OWJ in November 2014, Mr Siegfriedsen provided an update on the 8-megawatt (MW) floating offshore turbine the company first unveiled at Wind Energy Hamburg earlier this year and about the way that his views on floating offshore wind had changed.
“Until quite recently, I used to think that there was so much shallow water. Why did we need floating offshore wind turbines,” he told OWJ. “But more and more people wanted to talk to us about floating solutions, so we began a detailed evaluation and looked at some ideas. We did a lot of calculations and looked at how we might design the tower and the foundations for a floating turbine and at how we might want to modify the turbine.”
Aerodyn already has a two-bladed 6MW offshore turbine, the first example of which was being commissioned by its long-time partner Yang Ming as we spoke. The 8MW floating turbine will see some modifications compared with this unit, but the most important considerations were the type of foundation Aerodyn might use and the tower itself.
“After a while, I came to realise that floating offshore turbines weren’t quite such a crazy idea,” said Mr Siegfriedsen. “We think the concept that we have developed will be a very cost-effective one, and we are now at the point where we are actively looking for partners to produce it.” However, Yang Ming will probably not be that partner, because as Mr Siegfriedsen noted, the company has plenty of shallow-water sites available to it in which conventional fixed foundations are a suitable solution. This being the case, a partner in Europe, Japan or the US seems on the cards.
The Super Compact Drive (SCD) ‘Nezzy’ 8MW floating turbine (the name comes from Nessie, as in the Loch Ness monster) uses a foundation built up from three inexpensive pre-stressed concrete elements arranged in a Y formation. A profiled leaning steel tower sits on this concrete floating body – the base of the tower is also constructed from concrete – and is fixed by pre-loaded steel rods. Aerodyn is collaborating with another German company, Max Bögl, on the design of the concrete structures. “Steel would be too light,” said Mr Siegfriedsen of the design of the base of the tower structure.
The nacelle, hub and blades give the 8MW turbine a top head mass of 360 tonnes, which is relatively light for such a powerful unit, but concrete is needed in the lower part of the tower to provide sufficient mass and stability. Interestingly, the tower itself is not round, as might be expected, but is droplet or “lens shaped” as Mr Siegfriedsen put it. “It is shaped to reduce drag and provide some lifting force,” he explained. “It also helps to ensure that bending forces from the rotor are not transmitted to the tower, so fatigue is not an issue.” He explained that, in operation, the rounded part of the tower faces the wind, which reduces the drag coefficient from around Cd = 1.3 (for a tube-shaped tower) to Cd = 0.3–0.4. Overall, he explained, the shape adopted for the tower enhances overall stability and limits nacelle and rotor movement. The leg itself is manufactured in two pieces, with the outer ends sealed by a metal bracket. The bracket on the lower leg incorporates a rotating mechanism to enable the turbine to turn in the direction of the prevailing wind.
The turbine itself is designed with a downwind rotor for self-adjustment. It has a rotor diameter of 168m and hub height of 110m. Steel ropes on the turbine head transfer and distribute the loads directly into the anchor chains. Because the turbine is a downwind machine, no yaw bearing, yaw drives or yaw brakes are required. The wind turns the turbine into the incoming wind direction.
Aerodyn already has plenty of experience with its offshore turbines, including the 6MW unit mentioned above and the 3MW unit that preceded it, and has decided to make a few changes to the SCD design. The company produced its first two-bladed turbine back in the 1980s, when it made a conscious effort to avoid anything superfluous in the design. It had to be as compact as possible and “reduced to the max”. The 3MW and 6MW machines have permanent magnet generators, but concern about the cost of the materials used in them has prompted Aerodyn to opt for an electrically excited machine. This will add a little weight to the generator – Mr Siegfriedsen estimates about 5 tonnes – but not enough to adversely affect the project, especially bearing in mind that, because the turbine turns about its mooring point, a yaw system is not required.
The company has also made some changes to the cooling system and pitch system on the 8MW turbine. Instead of a permanent magnet generator, the Nezzy has a synchronous generator with brushless electrical field excitation. One or two other design modifications have also been made – for the new, more powerful turbine, a few modifications were needed to take account of wave-induced forces, and Aerodyn has opted for taper-roller bearings for the planetary gearbox stages.
Overall, said Mr Siegfriedsen, fewer parts will result in fewer problems and less maintenance, which will reduce the lifetime cost of the 8MW turbine, but it is in the manufacturing and installation of the turbine and foundations that the greatest cost savings will be made. Intended for operation in water depths from 35m and deeper, the SCD Nezzy can be installed on any type of seabed, whatever the geological conditions. Because it doesn’t require the use of piles, it will be quicker and easier to install and more environmentally friendly without any of the noise issues associated with piling. The anchors will be pre-installed before the foundation and turbine arrive, and the anchor chains are connected to the floating hub unit, which serves as chain connection, pivot point, grid connection and power slip ring. When the tug-towed turbine/foundation arrives, it is attached to this unit, and operation can start without any time-consuming commissioning work. Should major maintenance work be required, the turbine can be replaced with a new one, or the entire unit can be towed back to shore to be worked on. When the time comes to decommission the turbine, it can be towed ashore and scrapped under safe, environmentally controlled conditions, and the anchors, power slip ring and chains can be removed and do not leave a footprint on the seabed.
Detailed design of the 8MW turbine should be completed by mid-2105, said Mr Siegfriedsen, by which time he is confident that Aerodyn will have found an industry partner to produce the innovative floating offshore unit. OWJ
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