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Introduction to Offshore Wind Turbines
Offshore wind turbines have a much higher capacity than the conventional turbines employed in land wind farms, mainly due to the enhanced wind patterns experienced offshore.
Offshore turbines previously were mounted on structures fixed to the seabed which restricted them to relatively shallow water- a maximum depth of 150ft.
However, offshore wind turbines can be mounted on floating structures, allowing them to operate in deeper waters further out to sea.
This article considers renewable energy from offshore wind turbines and in particular from turbines mounted on a floating structure.
We begin with an examination of current methods of supporting floating offshore structures.
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Offshore Floating Structures Suitable for Mounting Wind Turbines
Over the last forty years or so, the offshore oil and gas industry has designed and developed various floating structures to support hydrocarbon production platforms. Most of these designs are suitable as support structures for offshore wind turbines; the more popular types are listed below;
These structures are comprised of hulls fabricated from large horizontal pontoons onto which vertical steel columns are welded. The columns and horizontal pontoons are interconnected and braced by a lattice of tubular steel supports.
The structures are held to the seabed by anchors, whose chains are maintained in a catenary mooring mode by winches situated on the main deck.
Various structures have been examined for potential use as floating supports for offshore wind turbines, including a multi wind turbine support. However, this type of semi-submersible would require automatic weathervaning, controlled by satellite navigation that would turn the structure into the wind without interfering with the mooring system.
Semi-submersibles can be self-propelled using their own marine diesel engines; the non-propulsion type is towed to the required location.
Semi Submersible Production Platform for Offshore Oil and Gas
The offshore oil and gas industry has used semi-submersible floating structures for many years both as drilling rigs and production platforms, operating in depths of water of up to 6,000 feet.
- Tension Leg Platform (TLP)
This is a floating platform fixed by high-tensile solid steel round tie rods, attached to a specially designed template piled and grouted onto the subsea bedrock. The tie rods are kept in tension by overhead winches located under the main deck. TLPs can operate in depths of 6000 feet.
- Spar Floating Structure
The spar is just a large floating round hull that can be fabricated from steel or concrete. It has several sections built inside the hull which serve as ballast and oil storage tanks, being tethered to the seabed with conventional chains and winches. Spars can also operate in water depths of 6000 feet.
The Petronius Spar hydrocarbons production platform, weighing 45000 tons, is currently operating at a depth of 1750 feet in the Gulf of Mexico.
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Offshore Semi-submersible Wind Turbines in Production
Hywind Statoil was the first company to produce electricity from a wind turbine mounted on a floating structure. In this case the structure was a steel fabricated spar type which was fabricated and towed in the horizontal position to an inshore assembly fiord.
Here it was ballasted with seawater and brought to the vertical position, and further permanently ballasted with concrete.
The tower was fitted onto the top of the spar, followed by the nacelle and rotor, and then the structure was towed to the offshore location where it was further ballasted and moored to the seabed. The wind turbine has a capacity of 2.6MW, and there are plans to add more floating turbines, eventually constructing an offshore floating wind-farm.
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Transmitting Power Ashore Through Subsea Cables in the UK
The third round of offshore windfarm allocation areas within UK waters by the Crown Estate has been awarded to various companies. It has been estimated that when the wind farms are operational, the total output could be as much as 25GW.
This will undoubtedly lead to some of the wind turbines being mounted on floating structures, as they are suitable in supporting wind turbines 5MW and above.
It is proposed to transmit this power ashore through conventional sub-sea cabling, but also utilising an innovative process known as High Voltage Direct Current (HVDC). The units will be mounted on conventional bottom-fixed structures such as steel jackets or monopods.
The AC current produced by the wind turbine is converted to DC current before being transmitted ashore. This has several advantages over transmitting high voltage AC current;
- Less power loss giving greater efficiency.
- Better control of distribution, less effects of land network faults.
- Instant determination of power being generated and distributed ashore.
There are other offshore wind farms companies such as Wetfeet Offshore, who are constructing a 400MW offshore windfarm in the German Sector of the North Sea. They are not only proposing to use HVDC substations in subsea power transmission, but mount them on a floating platform.