In recent years there have been two significant trends in the wind industry: developers seeking higher quality wind resources and turbines growing in size. In response to these developments, the idea of 'floating' offshore wind turbines is becoming increasingly popular. Because they float, these turbines can provide developers with better access to offshore wind resources, unconstrained by water depth. Done right, they include a support structure able to accommodate today's large, and tomorrow's even larger turbines.
Instead of relying on physical foundations like bottom-fixed turbines, floating platforms are attached to the seabed by mooring lines. This means that they can be viably deployed in much deeper water – good news for countries with deepwater coastlines like Norway, Portugal and Greece.
There are three main types of floating wind platforms. One type, known as ‘ballast stabilized,’ uses spar buoy platforms with catenary mooring drag-enabled anchors. A second, called ‘Tension Leg Platforms’ or ‘Mooring Line Stabilized Platforms,’ is attached to the seabed with suction pile anchors. The third type is the ‘Buoyancy Stabilized’ platform, which employs a ‘barge’ type device with catenary mooring lines.
A key advantage of using floating wind platforms is that they allow developers access to previously inaccessible waters where there is stronger yet less turbulent winds – helping to reduce the overall cost of wind energy.
Another benefit is that floating platforms can generally be commissioned and assembled at the quayside, without the need for heavy-lift jackup or dynamic positioning (DP) vessels, further reducing the cost and risk of deployment activities.
“Eliminating offshore lifting operations also provides for decreased weather window restrictions on installation,” says Craig Andrus, Senior VP – Europe at Principle Power.
The fact that foundations are not necessary with floating technology also means that piling activities and sea life disturbance can be minimized - greatly reducing negative environmental impacts. Moreover, reduced geotechnical requirements mean that core sampling is only needed to test the seabed ahead of appropriate anchor selection, as opposed to the necessity of core sampling at every pile site.
In order to test the viability of floating wind turbines and support structures a number of pilot initiatives are currently underway at various locations around the world.
Developed by Norwegian energy company Statoil ASA, Hywind is the world’s first full-scale floating wind turbine. Located around ten kilometers off the southwest coast of Norway, the structure itself is a steel cylinder, similar to a spar buoy, filled with a ballast of water and rocks, which extends 100 meters beneath the sea’s surface. Attached to the seabed by a three-point mooring spread, it can be employed at ocean depths of 120 to 700 meters.
The turbine itself comes from Siemens, but the ‘floater’ was built by engineering specialists Technip – which also undertook all the offshore installation work. A submarine power line, laid by Nexans Norway, reaches the mainland near Skudeneshavn at the southern end of Karmøy, where local grid operator Haugaland Kraft operates a receiving station.
Although the Hywind turbine has been generating electricity to the Norwegian grid since late September 2009, its main objective is to test the impact of wind and waves on the structure over a two-year period. The results have so far been promising and indicate the long-term viability of this type of floating turbine technology.
“There is no reason to believe it shouldn’t eventually be cost competitive with ‘bottom-fixed’ approaches,” says Hywind Project Manager, Sjur Bratland.
Following the initial test period, Statoil intends to start work on commercialising the concept – with the ultimate goal of reducing costs so that floating wind power can compete in the global energy market. According to Bratland, the company has already identified a few potential sites outside Norway.
“We are looking for feasibility in Scotland and [the] U.S. in the time frame of 2015-2017,” he says.
Dutch company Blue H Technologies has devised a ‘Submerged Deepwater Platform’ (SDP). Essentially a modified form of a Tension Leg Platform, SDP’s are made of a buoyant hollow body that is ‘semi-submerged’ in water by chains or tethers, which are in turn connected to a counterweight on the sea bed – thus creating the necessary uplifting force to keeps the chains constantly tensioned.
In 2008, the company installed a 75% scale prototype SDP with a small wind turbine in 113-metre deep water some 11 nautical miles off the coast of Southern Italy, near the site of the future offshore Tricase project. After 6 months at sea, the unit was decommissioned early in 2009.
In 2008, Blue H started engineering a second proof of concept, a tension-legged platform for a 2-MW floating wind turbine. The concept is slated for completion next year, when the company intends to install it in its Tricase wind farm. This will be followed by the deployment of a larger pre-production floating turbine in 2014, combining Blue H’s platform with a 3rd party offshore turbine.
In the UK, Blue H also led a consortium of companies involved in Project Deepwater, a two-year project that ran from 2009-2010 and looked at the feasibility and costs of generating electricity using offshore wind turbines mounted on a floating, tension legged platform in water depths of 70 to 300 meters.
In addition, the company is currently undertaking extensive research work with partners Timolor Leroux & Lotz in what it calls Project DIWET (Deepwater Innovative Wind Energy Technology). The project, located off the coast of Brittany, France, consists of a floating platform concept that is anchored using rigid taut lines.
In the U.S., technology company Principle Power has devised WindFloat, an integrated system, consisting of a semi-submersible floating platform capable of supporting commercial offshore horizontal axis wind turbines. The system utilizes drag embedment anchors and a conventional catenary mooring and is designed to accommodate any multi-megawatt offshore turbine.
Principle Power, alongside partners EDP, InovCapital, Vestas and others, has recently signed an agreement for the deployment of the first full-scale WindFloat, with a Vestas V80 – 2.0 MW turbine, off the coast of Portugal later this year.
Testing at the grid-connected site in Aguçadoura will focus specifically on performance validation of the WindFloat and turbine integration, as well as commissioning, decommissioning and O&M studies.
“From a project perspective, Portugal has a rich maritime culture, a history of embracing marine renewables and unfortunately, or fortunately, a dearth of commercially viable shallow water sites for offshore wind deployment,” says Andrus.
“WindFloat can be regarded as an economically viable competitor against conventional concepts at sites with water depths ranging from 40 to 50 m,” he adds.
For the near future, Principle Power is focused on deploying WindFloat systems in all ‘primary markets’ - namely Western Europe, the UK and the U.S. on a commercial basis.
“The Portuguese prototype serves as validation of the technology, proof of commercial viability and a test bed for optimization of the integrated system,” says Andrus.
HiPRwind is the world’s largest publicly-funded research project to develop deep-water offshore wind technology. Led by the German Fraunhofer research institute, the five-year €20-million initiative combines the expertise of no less than nineteen companies, including Acciona Energy, ABB Schweiz, Bureau Veritas, Angewandten Forschung and Norges Teknisk.
A central objective of HiPRwind is to deliver a fully functional floating wind turbine installation at approximately one tenth of the scale of future commercial systems, deployed in real sea conditions.
“The idea behind the project is to analyze current approaches for floating turbines, pick the best one, build a downscaled model and test it,” says project director Andreas Reuter, Professor of Wind Energy at Fraunhofer IWES.
“The turbine used is a modern pitch-regulated variable-speed turbine. The main question will be the type of floating structure to be used - here we have a couple of options,” he adds.
Key focus areas will include reliability, remote maintenance and grid integration, with a particular emphasis on how floating wind technology can help to overcome the financial and technological limitations of current wind turbines and support structures. In doing so, the project team will research improvements in rotor blade designs, structural health monitoring systems, reliable power electronics and control systems.
Ultimately, the aim is to bridge the gap in technology development between small-scale tank testing and full-scale offshore deployment and reduce the risks and costs of commercialising deep-water wind technology.
Since the project has only just begun, Reuter explains that there are no results yet. However, he is keen to stress that the cost structure of floating turbines is the key challenge.
“The floating body is extremely expensive compared to a classic onshore foundation. On the advantage side we have the higher winds offshore, no visual impact and complaining neighbours and the possibility to ship the turbines back into the harbour for maintenance,” he says.
A French consortium, led by wind power specialist Nass & Wind, has confirmed it will be installing a full-scale demonstration model of its WinFlo floating turbine off the coast of southern Brittany, near Lorient port. The Winflo is an integrated floating wind turbine on a semi-submersible platform with an innovative anchoring system ‘suitable for all seabed types’ in excess of 50 meters. Nass & Wind plan to manufacture a pre-series version of the machine and market it from 2015 onwards
Also in France, engineering company Technip and wind-power startup Nenuphar have recently announced plans to launch the Vertiwind project to test a pre-industrial prototype of a vertical-axis offshore floating wind turbine. Land-based testing of a 0.5-scale prototype is currently underway at the ‘Carrieres’ site, in Boulonnais. Once this phase is complete, testing will commence at sea.
In addition there are two other interesting projects being managed by Norwegian companies. The WindSea concept is based on a semi-submersible platform with three turbines and mooring lines connected to a detachable turret. Following the completion of a two-year design phase, there are plans to build a WindSea prototype in 2012. Meanwhile, Sway is a wind power concept that is fixed using a Spar-type float and anchored to the seabed by a taut anchorage system - allowing it to face in different directions depending on the direction of the wind. Last month, the company deployed the prototype off the coast of Norway, with transport to site and a full test program scheduled to start later this spring.
So, how long might it be before floating wind turbines are commercialized – and what challenges must be overcome before this happens?
According to Andrus, the commercialization of floating wind turbines will certainly come much sooner than previously anticipated only a few years ago. This is because the challenges, disadvantages and hidden long-term costs of fixed structures are becoming more evident and better understood.
“We believe that the sizing and design flexibility, turbine ‘agnostitcity,’ logistical ease of deployment and decreased risk of the WindFloat is an economically viable alternative and solution for offshore wind industry,” says Andrus.
“That being said, more demonstrations, like ours later this year, need to take place in an effort to prove bankability and track record to the wind industry as a whole,” he adds.
Reuter, however, predicts that full commercialization will take a little longer, perhaps up to a decade.
“[We] will be busy improving the systems for quite a while - including some tests with demonstrators. Going commercial is probably realistic for the years after 2020 - cost is the key issue,” he says.
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