Floating wind turbine
A floating wind turbine is an offshore wind turbine mounted on a floating structure that allows the turbine to generate electricity in water depths where bottom-mounted towers are not feasible. Locating wind farms out at sea can reduce visual pollution while providing better accommodation for fishing and shipping lanes. In addition, the wind is typically more consistent and stronger over the sea, due to the absence of topographic features that disrupt wind flow.
- 1 History
- 2 Topologies
- 3 Engineering considerations
- 4 Economics
- 5 Floating design concepts
- 6 Proposals
- 7 Research
- 8 Other applications
- 9 See also
- 10 References
- 11 Bibliography
- 12 External links
The concept for large-scale offshore floating wind turbines was introduced by Professor William E. Heronemus at the University of Massachusetts Amherst in 1972. It was not until the mid 1990s, after the commercial wind industry was well established, that the topic was taken up again by the mainstream research community. As of 2003, existing offshore fixed-bottom wind turbine technology deployments had been limited to water depths of 30 metres. Worldwide deep-water wind resources are extremely abundant in subsea areas with depths up to 600 metres, which are thought to best facilitate transmission of the generated electric power to shore communities. Two-thirds of the North Sea is between 50 and 220 meters deep.
Operational deep-water platforms
In 2011 three floating wind turbine support structures were installed.
Blue H deployed the first 80-kW floating wind turbine 21.3 kilometres (13.2 mi) off the coast of Italy in December 2007. It was then decommissioned at the end of 2008 after completing a planned test year of gathering operational data.
The first large-capacity, 2.3-megawatt floating wind turbine is Hywind, which became operational in the North Sea near Norway in September 2009, and is still operational as of October 2010[update].
In October 2011, Principle Power's WindFloat Prototype was installed 4 km offshore of Aguçadoura, Portugal in approximately 45 m of water (previously the Aguçadoura Wave Farm site). The WindFloat was fitted with a Vestas V80 2.0-MW offshore wind turbine and grid connected. The installation was the first offshore wind turbine to be deployed without the use of any offshore heavy lift vessels as the turbine was fully commissioned onshore prior to the unit's being towed offshore. This is the first offshore wind turbine installed in open Atlantic waters, and the first to employ a semi-submersible type floating foundation.[better source needed]
SeaTwirl deployed its first floating grid connected wind turbine off the coast of Sweden in August 2011. It was tested and de-commissioned. This design intends to store energy in a flywheel. Thus, energy could be produced even after the wind has stopped blowing.
Blue H Technologies
Blue H Technologies of the Netherlands operated the first floating wind turbine, a prototype deep-water platform with an 80-kilowatt turbine off the coast of Apulia, southeast Italy, in 2008. Installed 21 km off the coast in waters 113 metres deep in order to gather test data on wind and sea conditions, the small prototype unit was decommissioned at the end of 2008.
The Blue H technology utilized a tension-leg platform design and a two-bladed turbine. The two-bladed design can have a "much larger chord, which allows a higher tip speed than those of three-bladers.
As of 2009[update], Blue H was building a full-scale commercial 2.4-MWe unit in Brindisi, Italy which it expected to deploy at the same site of the prototype in the southern Adriatic Sea in 2010.[dated info] This is the first unit in the planned 90-MW Tricase offshore wind farm, located more than 20 km off the Puglia coast line.
The world's first operational deep-water floating large-capacity wind turbine is the Hywind, in the North Sea off Norway. The Hywind was towed out to sea in early June 2009. The 2.3-megawatt turbine was constructed by Siemens Wind Power and mounted on a floating tower with a 100-metre deep draft. The float tower was constructed by Technip. Statoil says that floating wind turbines are still immature and commercialization is distant.
The installation is owned by Statoil and will be tested for two years. After assembly in the calmer waters of Åmøy Fjord near Stavanger, Norway, the 120-meter-tall tower with a 2.3-MW turbine was towed 10 km offshore into 220-metre-deep water, 10 km southwest of Karmøy, on 6 June 2009 for a two-year test deployment." Alexandra Beck Gjorv of Statoil said, "[The experiment] should help move offshore wind farms out of sight … The global market for such turbines is potentially enormous, depending on how low we can press costs." The unit became operational in the summer of 2009. Hywind was inaugurated on 8 September 2009. As of October 2010[update], after a full year of operation, the Hywind turbine is still operating and generating electricity for the Norwegian grid, and still is as of December 2014.
The turbine cost 400 million kroner (around US$62 million) to build and deploy. The 13-kilometre (8.1 mi) long submarine power transmission cable was installed in July, 2009 and system test including rotor blades and initial power transmission was conducted shortly thereafter. The installation is expected to generate about 9 GW·h of electricity annually. The SWATH (Small Waterplane Area Twin Hull), a new class of offshore wind turbine service boat, will be tested at Hywind.
Hywind delivered 7.3 GWh in 2010, and survived 11 meter waves with seemingly no wear. As of June 2011[update], additional pilot Hywind installations were planned in the US and in the North Sea off the coast of Scotland. In 2013, Statoil pulled out of the $120 million project of four 3-MW turbines floating in 460 feet of water near Boothbay Harbor, Maine citing change in legislation, and focused on their five 6-MW turbines in Scotland instead. The UMaine Aqua Ventus project continues.
WindFloat is a floating foundation for offshore wind turbines designed and patented by Principle Power. A full-scale prototype was constructed in 2011 by Windplus, a joint-venture between EDP, Repsol, Principle Power, A. Silva Matos, Inovcapital, and FAI. The complete system was assembled and commissioned onshore including the turbine. The entire structure was then wet-towed some 400 kilometres (250 mi) (from southern to northern Portugal) to its final installed location 5 kilometres (3.1 mi) offshore of Aguçadoura, Portugal, previously the Aguçadoura Wave Farm. The WindFloat was equipped with a Vestas v80 2.0-megawatt turbine and installation was completed on 22 October 2011. A year later, the turbine had produced 3 GWh.
The subsea metal structure is reported to improve dynamic stability, whilst still maintaining shallow draft, by dampening wave– and turbine–induced motion utilizing a tri-column triangular platform with the wind turbine positioned on one of the three columns. The triangular platform is then "moored" using a conventional catenary mooring consisting of four lines, two of which are connected to the column supporting the turbine, thus creating an "asymmetric mooring."
As the wind shifts direction and changes the loads on the turbine and foundation, a secondary hull-trim system shifts ballast water between each of the three columns. This permits the platform to maintain even keel while producing the maximum amount of energy. This is in contrast to other floating concepts which have implemented control strategies that de-power the turbine to compensate for changes in turbine thrust-induced overturning moment.
This technology could allow wind turbines to be sited in offshore areas that were previously considered inaccessible, areas having water depth exceeding 40 metres and more powerful wind resources than shallow-water offshore wind farms typically encounter.
The cost of this project is around €20 million (about US $26 million). This single wind turbine can produce energy to power 1300 homes.
Platform topologies can be classified into:
- single-turbine-floater (one wind turbine mounted on a floating structure)
- multiple turbine floaters (multiple wind turbines mounted on a floating structure)
Undersea mooring of floating wind turbines is accomplished with three principal mooring systems. Two common types of engineered design for anchoring floating structures include tension-leg and catenary loose mooring systems. Tension leg mooring systems have vertical tethers under tension providing large restoring moments in pitch and roll. Catenary mooring systems provide station–keeping for an offshore structure yet provide little stiffness at low tensions." A third form of mooring system is the ballasted catenary configuration, created by adding multiple-tonne weights hanging from the midsection of each anchor cable in order to provide additional cable tension and therefore increase stiffness of the above-water floating structure.
The IEC 61400–3 design standard requires that a loads analysis be based on site-specific external conditions such as wind, wave and currents. The IEC 61400–3-2 standard applies specifically to floating wind turbines.
Technically, the [theoretical] feasibility of deepwater [floating] wind turbines is not questioned as long-term survivability of floating structures has already been successfully demonstrated by the marine and offshore oil industries over many decades. However, the economics that allowed the deployment of thousands of offshore oil rigs have yet to be demonstrated for floating wind turbine platforms. For deepwater wind turbines, a floating structure will replace pile-driven monopoles or conventional concrete bases that are commonly used as foundations for shallow water and land-based turbines. The floating structure must provide enough buoyancy to support the weight of the turbine and to restrain pitch, roll and heave motions within acceptable limits. The capital costs for the wind turbine itself will not be significantly higher than current marine-proofed turbine costs in shallow water. Therefore, the economics of deepwater wind turbines will be determined primarily by the additional costs of the floating structure and power distribution system, which are offset by higher offshore winds and close proximity to large load centres (e.g. shorter transmission runs).
As of 2009[update] however, the economic feasibility of shallow-water offshore wind technologies is more completely understood. With empirical data obtained from fixed-bottom installations off many countries since the late 1990s, representative costs are well understood. Shallow-water turbines cost between 2.4 and 3 million United States dollars per megawatt to install, according to the World Energy Council.
As of 2009[update], the practical feasibility and per-unit economics of deep-water, floating-turbine offshore wind is yet to be established. Initial deployment of single full-capacity turbines in deep-water locations began only in 2009.
As of October 2010[update], new feasibility studies are supporting that floating turbines are becoming both technically and economically viable in the UK and global energy markets. "The higher up-front costs associated with developing floating wind turbines would be offset by the fact that they would be able to access areas of deep water off the coastline of the UK where winds are stronger and reliable."
The recent Offshore Valuation study conducted in the UK has confirmed that using just one third of the UK's wind, wave and tidal resource could generate energy equivalent to 1 billion barrels of oil per year; the same as North Sea oil and gas production. A significant challenge when using this approach is the coordination needed to develop transmission lines.
When oil fields become depleted, the operator injects water to keep pressure high for further extraction. This requires power, but installing gas turbines means shutting down the extraction process, losing valuable income. The classification society DNV GL has calculated that in some cases a floating wind turbine can economically provide power for injection, as the oil platform can keep on producing, avoiding a costly pause.
Floating design concepts
Ideol is a French company that has patented a new floating platform concept specifically designed for offshore wind.
While the floater concept and patents are not yet publicly disclosed, the company is communicating on its web site its mobility solutions to reduce wake losses in an offshore wind farm by repositioning the floating turbines depending on the wind direction. The company has patented a mechanical solution to move the floater along its mooring lines and has developed a software to optimize in real-time the farm layout. Eliminating wake losses allows significant increase in the power production as well as reduction of long–term component failures.
According to publicly released information, Ideol has a construction and installation cost of around 1M Euro per MW. As such, the company intends to offer an alternative to fixed foundations starting from 40 m water depth.
Nautica Windpower has proposed a technique for potentially reducing system weight, complexity and costs for deepwater sites. Scale model tests in open water have been conducted (September 2007) in Lake Erie and structural dynamics modeling was done in 2010 for larger designs. Nautica Windpower's Advanced Floating Turbine (AFT) uses a single mooring line and a downwind two-bladed rotor configuration that is deflection tolerant and aligns itself with the wind without an active yaw system. Two-bladed, downwind turbine designs that can accommodate flexibility in the blades will potentially prolong blade lifetime, diminish structural system loads and reduce offshore maintenance needs, yielding lower lifecycle costs.
The International Energy Agency (IEA), under the auspices of their Offshore Code Comparison Collaboration (OC3) initiative, has completed high-level design and simulation modeling of the OC-3 Hywind system, a 5-MW wind turbine installed on a floating spar buoy, moored with catenary mooring lines, in water depth of 320 metres. The spar buoy platform would extend 120 meters below the surface and the mass of such a system, including ballast would exceed 7.4 million kg.
Risø and 11 international partners started a 4-year program called DeepWind in October 2010 to create and test economical floating Vertical Axis Wind Turbines up to 20 MW. The program is supported with €3m through EUs Seventh Framework Programme. Partners include TUDelft, Aalborg University, SINTEF, Statoil and United States National Renewable Energy Laboratory.
VertiWind is a floating Vertical Axis Wind Turbine design created by Nenuphar [full citation needed] whose mooring system and floater are designed by Technip.[full citation needed][non-primary source needed].
An open source project was proposed by former Siemens director Henrik Stiesdal in 2015 to be assessed by DNV GL. It suggests using tension leg platforms with replaceable pressurized tanks anchored to sheet walls.
Floating wind farms
As of September 2011[update], Japan plans to build a pilot floating wind farm, with six 2-megawatt turbines, off the Fukushima coast of northeast Japan where the recent disaster has created a scarcity of electric power. After the evaluation phase is complete in 2016, "Japan plans to build as many as 80 floating wind turbines off Fukushima by 2020." The cost is expected to be in the range of 10–20 billion Yen over five years to build the first six floating wind turbines. Some foreign companies also plan to bid on the 1-GW large floating wind farm that Japan hopes to build by 2020. In March 2012, Japan’s Ministry of Economy, Trade and Industry approved a 12.5bn yen ($154m) project to float a 2-MW Fuji in March 2013 and two 7-MW Mitsubishi hydraulic "SeaAngel" later about 20–40 km offshore in 100–150 meters of water depth. The Japanese Wind Power Association claims a potential of 519 GW of floating offshore wind capacity in Japan. The first turbine became operational in November 2013. 
The US State of Maine solicited proposals in September 2010 to build the world's first floating, commercial wind farm. The RFP is seeking proposals for 25 MW of deep-water offshore wind capacity to supply power for 20-year long-term contract period via grid-connected floating wind turbines in the Gulf of Maine. Successful bidders must enter into long-term power supply contracts with either Central Maine Power Company (CMP), Bangor Hydro-Electric Company (BHE), or Maine Public Service Company (MPS). Proposals were due by May 2011. [dated info]
In April 2012 Statoil received state regulatory approval to build a large four-unit demonstration wind farm off the coast of Maine. As of April 2013[update], the Hywind 2 4-tower, 12–15 MW wind farm was being developed by Statoil North America for placement 20 kilometres (12 mi) off the east coast of Maine in 140–158 metres (459–518 ft)-deep water of the Atlantic Ocean. Like the first Hywind installation off Norway, the turbine foundation will be a spar floater. The State of Maine Public Utility Commission voted to approve the construction and fund the US$120 million project by adding approximately 75 cents/month to the average retail electricity consumer. Power could be flowing into the grid no earlier than 2016.
As a result of legislation in 2013 (LD 1472) by the State of Maine, Statoil placed the planned Hywind Maine floating wind turbine development project on hold in July 2013. The legislation required the Maine Public Utilities Commission to undertake a second round of bidding for the offshore wind sites with a different set of ground rules, which subsequently led Statoil to suspend due to increased uncertainty and risk in the project. Statoil is considering other locations for its initial US demonstration project.
Some vendors who could bid on the proposed project in Maine expressed concerns in 2010 about dealing with the United States regulatory environment. Since the proposed site is in Federal waters, developers would need a permit from the US Minerals Management Service, "which took more than seven years to approve a yet-to-be-built, shallow-water wind project off Cape Cod", and is also the agency under fire in June 2010 for lax oversight of deepwater oil drilling in Federal waters. "Uncertainty over regulatory hurdles in the United States … is 'the Achilles heel' for Maine's ambitions for deepwater wind."
Scale modeling and computer modeling attempt to predict the behavior of large–scale wind turbines in order to avoid costly failures and to expand the use of offshore wind power from fixed to floating foundations. Topics for research in this field include:
- Overview of integrated dynamic calculations for floating offshore wind turbines
- Fully coupled aerohydro-servo-elastic response; a basic research tool to validate new designs
- Water tank studies on 1:100 scale Tension-leg Platform and Spar Buoy platforms
- Dynamic response dependency on the mooring configuration[full citation needed]
As they are suitable for towing, floating wind turbine units can be relocated to any location on the sea without much additional cost. So they can be used as prototype test units to practically assess the design adequacy and windpower potential of prospective sites.
Floating wind turbines can be used to provide motive power for achieving artificial upwelling of nutrient-rich deep ocean water to the surface for enhancing fisheries growth in areas with tropical and temperate weather. Though deep seawater (below 50 meters depth) is rich in nutrients such as nitrogen and phosphorus, the phytoplankton growth is poor due to the absence of sunlight. The most productive ocean fishing grounds are located in cold water seas at high latitudes where natural upwelling of deep sea water occurs due to inverse thermocline temperatures. The electricity generated by the floating wind turbine would be used to drive high–flow and low–head water pumps to draw cold water from below 50 meters water depth and mixed with warm surface water by eductors before releasing into the sea. Mediterranean sea, Black sea, Caspian sea, Red sea, Persian gulf, deep water lakes/reservoirs are suitable for artificial upwelling for enhancing fish catch economically. These units can also be mobile type to utilise the seasonal favourable winds all around the year.
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|Wikimedia Commons has media related to Floating wind turbines.|
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- Nancy Stauffer (MIT): Giant wind turbines, floating out of sight. 2006 preliminary design with 5 MWe turbine units mounted 90 metres above the sea with massive 140-metre-diameter blades; MIT-NREL design.
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