- More than 100 meters water depth
- Assembly of turbine in harbor or near shore
- Large structures
- Receptive to roll and sway
Among support structures for wind turbines, the least used and proven are the floating designs. This is an area where the number of blueprints, ideas, and plans stands in contrast to the actual number of floating turbines.
To this day, only a few wind turbines in the world stand on a floating support structure. One is the Hywind in Norway, fitted with a turbine from Siemens. Another is the Windfloat, installed off the coast of Portugal, with a Vestas turbine. Moreover, a couple of scale models float in the oceans: the Blue H, near Italy, and Sway, a prototype in the waters of Norway.
A floating support structure is recognized by the fact that the support comes from the water and not from the ground. Generally, the contact to the seabed is through anchor lines, also called mooring cables. All the different types of floating structures have their origin in the oil and gas industry, but modifications and hybrids are beginning to emerge in their use for wind turbines.
Within offshore wind structures, three different types are present:
– in that order.
The spar floater
The basic structure of the spar floater is cylindrical. It is a large tube that floats due to large amounts of air in the top of the structure, and stays upright due to a large amount of ballast at the bottom.
The size of the Hywind is impressive. For comparison, the most famous WW2 U-boat, the U-48, is drawn next to the structure. Illustration: LORC
The Hywind is constructed in this way. The steel tube reaches 100 meters down below the waterline, it has a diameter of 8.3 meters (6 meters at the water line), and is constructed from 1500 tonnes of steel. With ballast and turbine, the total weight is 5300 tonnes. Hywind carries a Siemens 2.3 MW turbine.
The spar floater is secured to the seabed with mooring lines. It tilts slightly as the water and wind affects the structure. This is the main disadvantage for all floating concepts, as wind turbines are designed for a stable base and an angle no more than 0.5 degrees out of vertical.
The solution for this in the case of the spar floater is weight. The larger the ballast, the calmer the movements. For Hywind, Statoil says the tilt is down to 3 degrees out of vertical, and the oscillations – the swaying from side to side – last between 20 and 30 seconds. This is set out in this article.
The advantage of the spar floater in comparison with other floaters is the small cross-section at the surface. This way, the spar floater is not as sensitive to wave motions. For the Hywind, the diameter at the water line is 6 meters.
Installation of the spar floater is done in two or three steps. First, the support structure is sailed horizontally out of the harbor. The floater is up-ended in deep water (100 meters) – not necessarily at the final site. After up-ending, the tower, turbine, and rotor are installed. The complete support structure and turbine can now be sailed to the final position and secured with mooring lines and anchors.
The owner Statoil and manufacturer Technip show the installation in this video.
The tension leg platform
Also in actual use is the tension leg platform or TLP for short. The Dutch company Blue H has so far been the only one to produce a tension leg platform: a 3/4 scale prototype off the coast of Italy. At full scale it will be suited for waters with depth over 60 meters.
The principle of the tension leg platform is to create an underwater platform with buoyancy instead of the large amount of ballast to keep the structure stable. The buoyancy exceeds the weight of the platform and hence causes a pretension in the vertical cables which keep the platform on location.
The legs can either be secured to a template (i.e. a large concrete ring) at the seabed, by individual piles or by suction anchors. The platform is kept underwater to create a small cross-section at the waterline. This limits the amount of hydrodynamic loads from waves.
The first oil-producing tension leg platform was installed in the Hutton oilfield in the summer of 1984.
The barge floater
This design is widely known as semi-submersible in the oil and gas industry, but not used in the wind industry so far. The main advantage of this design would be the installation. A barge could be sailed into any harbor at shallow waters, less than 10 meters depth. This allows for complete installation in port, without any up-ending, lowering or other maneuvers that are needed for the other floating designs.
However, when installed, the barge floater is seriously challenged in terms of sway, pitch, and rolling. The large surface makes it very receptive to hydrodynamic loads.
Hybrids and improvements
The spar floater has been the baseline for another Norwegian design: Sway. The cylinder has been strengthened by adding tension wires to the structure, possibly inspired by a ship’s mast. The wires add stiffness to the structure, allowing the manufacturer to save on steel and weight.
One could say that instead of competing against nature’s forces, Sway goes with them. The turbine is placed downwind, not upwind – thus allowing the wires to attach at the top.
At the bottom, only one anchor secures the support structure. This, combined with the downwind turbine principle, makes Sway adopt the wind direction much like a vane. Thus, no yaw is needed in the turbine.
So far, only a prototype exists, but a new full-scale structure is planned to be placed off Karmøy in Norway. This is also where the Hywind is located, and it is no coincidence: Statoil also owns a huge part of Sway.
Three spars in one
Instead of operating with one large cylinder, the WindFloat design uses three cylinders. They are only partly submerged, thus like the barge floater, the structure is susceptible to hydrodynamic loads from waves at the surface.
The concept is subject to a joint operation between Principle Power Inc., who owns the patent, Vestas Wind Power and Energias de Portugal. A full-scale test of the WindFloat has been undertaken off the coast of Portugal, where it carries a Vestas V80 2MW wind turbine. Prior to the contract on the prototype, this feasibility study was carried out.
To compensate for the hydrodynamic impact from waves, the WindFloat has a system of dynamic ballast. It can move the ballast around, in and out of the three cylinders and this way keep the turbine stable. Details of the technology are not available, for instance the energy consumed by the ballast pumps, which would presumably make this system more expensive to run than a passive one like Sway.
Hybrid between wind and wave
The floating support structures have inspired the Danish company Floating Power Plant to make a hybrid between a wind turbine and a wave energy device. This concept is going into its second prototype phase.
The design is covered in a separate article at LORC Knowledge. Click to read more about the hybrid between wind and wave energy.