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Floating foundations for offshore wind

As developments in offshore wind look set to take turbines into deeper waters, the difficulty of deployment is growing. But new foundation and platform designs aims to reduce the costly installation challenges to make way for the next generation of machines.

Floating technology will see turbines installed without needing monopiles
Floating wind technology will see offshore turbines installed without needing traditional monopile structures

Future wind projects are leading to the development of bigger, weightier turbines, with installations happening further out to sea where there are greater wind speeds.

Foundation structures, particularly, need to cope with heavier loads than the monopiles used for most of the world’s operating offshore fleet today, while installation in deeper waters and tougher seabed conditions requires costly methods of deployment.

With most of today’s existing offshore wind farms located in shallow waters, monopile foundations have been the structure of choice for most project developers. Gravity-based foundations, which provide an option where pile-driving is not viable, have also been used for projects, such as Thornton Bank in Belgium. Meanwhile, a suction bucket concept is attracting interest too. This consists of a wide base that is driven into the seabed by drawing a vacuum and using hydrostatic pressure to seat the bucket.

 

However, monopile foundations have their limits. Generally, they are only suitable for installations at water depths of between 25m and 30m. So with around 70 per cent of the UK’s Round Three offshore projects set for water depths of more than 30m, as with a host of others across Europe, they can be ruled out of the equation for most future projects.

 

In the meantime there are concerns about grouted connections in monopiles. They have become an issue after 600 of the 998 wind turbines installed in the North Sea suffered grout-cracking problems.

 

The UK’s Carbon Trust is now funding research into four potentially suitable future foundation designs, as part of its Offshore Wind Accelerator programme: a gravity structure from Gifford-BMT-Freyssinet (suitable for water depths of 30 to 45m); MBD’s suction bucket monopile (30 to 60m); Keystone’s twisted jacket design (30 to 60m), which has already been proven in Hurricane Katrina; and SPT Offshore & Wood Group’s self-installing tribucket (30 to 60m). With this design, the entire foundation and turbine structure is assembled in port and then transported to the wind farm.

 

Floating solutions
The Energy Technologies Institute (ETI) has announced it is investing up to GBP 25m in a demonstration project exploring platform systems for these floating turbines.

 

The Institute said: “It is estimated that the UK has over a third of the total European potential offshore wind resource, enough to power the country nearly three times over. Exploiting this natural resource economically, particularly in deeper waters off the west of the country, will require significant technology developments to build, operate and support large offshore wind arrays.”

 

Although large floating turbines are expected to have high capital costs, David Clarke, ETI chief executive, said “they can access near-to-shore, high wind speed sites off the west coast of the UK, which overall brings down the cost of electricity generation for the long term”.

The project will see the design, construction and installation of a floating system demonstrator by 2016. It will be located at a relatively near-to-shore site, with high wind speeds of around 10m/s, in water depths of between 60m and 100m.

 

“It will be operated for at least two years to show it can generate high levels of electricity, be maintained without using specially designed vessels and to verify the predicted technical and economic performance,” said the ETI. “The intention is that it would be operated for another eight years to allow further developments to take place.”

 

There are engineering companies making significant developments in floating turbine platforms. Arguably, at the forefront of these designs is American engineering firm Principle Power, which is developing Windfloat, a semi-submersible platform structure. Designed for water depths greater than 50m, the company hopes the structure will be capable of supporting 10MW turbines.

 

The company signed a project agreement and turnkey contract for the deployment of a full-scale 2MW WindFloat platform off the coast of Portugal. Project partners include companies EDP, InovCapital, Vestas Wind Systems A/S, A. Silva Matos (ASM), and Fundo de Apoio à Inovação (FAI).

The dry-dock job of installing the turbine on the prototype foundation was completed. A full-scale WindFloat, with a Vestas V80-2.0MW offshore wind turbine, will soon be tugged 350km out to sea and deployed. According to Vestas, WindFloat is the “world’s first floating platform facing the Atlantic and the first offshore turbine to be installed without the use of heavy lift vessels.”

 

The system will be tested at an EDP grid connected site in Aguçadoura for a minimum of 12 months, focusing on performance validation of the WindFloat and turbine integration. In addition, commissioning, decommissioning and O&M studies will be conducted. It is effectively the first phase of a planned 150MW offshore wind farm for the site. If all goes to plan, phase two will see the project expand to 25MW by 2016, before ramping up to the 150MW target.

 

According to António Mexia, EDP’s CEO, “WindFloat is one of the most promising technologies in this area of offshore wind energy.” The WindFloat is fitted with patented water entrapment (heave) plates at the base of three columns. The plates improve the motion performance of the system significantly due to damping and entrained water effects.

 

Principle Power says: “This stability performance allows for the use of existing commercial wind turbine technology. In addition, WindFloat's closed-loop hull trim system mitigates mean wind-induced thrust forces. This secondary system ensures optimal energy conversion efficiency following changes in wind velocity and direction.”

 

The company says WindFloat’s design and size also allow for onshore assembly, while its mooring system employs conventional components such as chain and polyester lines to reduce cost and complexity. Through the use of pre-laid drag embedded anchors, site preparation and impact on the seabed is also minimized.

 

Quick installation
According to IQPC, another potential “economically viable solution for deepwater installations due to low manufacturing and installation costs” comes from Norwegian firm Seatower.

 

The company says: “Our foundations are designed to be installed by buoyancy and low-cost towing vessels.” This means they are “optimized for the entire logistics of offshore wind construction, from efficient production to easy decommissioning”.

 

Already certified by DNV, the self-floating foundations are economical to depths of 50m and can also be fully manufactured at quayside, before being towed to the offshore site using standard tugs. Significantly, the entire installation process takes just 12 hours, which reduces the risk of downtime due to bad weather conditions.

 

Seatower says this technology and installation method “avoids the expensive and weather-sensitive crane, dredging and piling vessels that do installation today, and of which there are too few to meet the expected needs”.

 

Meanwhile, Dutch company, Blue H, is working on a floating platform option suitable for water depths of 60m and above, which it hopes to deploy with large-scale turbines in 2014. A prototype of the platform, with an 80kW turbine has already been tested off the coast of southern Italy in depths of 113m. Blue H’s Submerged Deepwater Platform (SDP) can also be manufactured onshore and towed to site. Each SDP consists of a hollow body, which provides the buoyancy.

 

“This is held semi-submerged under water by chains or tethers, which connect the buoyant body to a counterweight which lies on the sea bottom,” says the company. “With the buoyant body held semi-submerged in the water, the necessary uplifting force is created, which keeps the chains constantly tensioned.”

 

Blue H is currently working on developing a tension legged platform for a 2MW floating wind turbine. The company intends to complete the platform in 2012 and install it in its Tricase wind farm, off the Puglia coast.The 2MW unit will be followed by the deployment of a larger pre-production floating turbine in 2014, which will combine Blue H’s platform with a 3rd party offshore turbine.

 

At even greater depths, the Hywind floating wind turbine foundation, which Statoil and Siemens Wind Energy installed off the coast of Karmøy island, Norway, in 2009, is also gaining much attention. A prototype of the foundation is currently being tested which supports a 2.3MW turbine.

 

Hywind is designed to be installed at depths of between 120m and 700m. The floating structure, which extends 100m below the surface, is a steel cylinder, filled with ballast consisting of water and rocks. Three mooring lines are used to tether it to the seabed.