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Floating systems

The US DoE has hosted conferences on ‘deepwater’ solutions in recent years. In both the EU and the USfor over 10 years, there has been exploratory research of floating offshore systems and preliminary development of design tools for modelling a wind turbine system on a dynamically active support that is affected by wave climate.  Until recently, such technology, even at the level of a first demonstration, was considered rather far in the future.  However interest has accelerated and demonstration projects have been announced.

The main drivers for floating technology are:

• Access to useful resource areas that are in deep water yet often near shore
• Potential for standard equipment that is relatively independent of water depth and sea bed conditions
• Easier installation and decommissioning
• The possibility of system retrieval as a maintenance option

The main obstacle to the realisation of such technology is:

• Development of effective design concepts and demonstration of cost effective technology, especially in respect of the floater and its mooring system

StatoilHydro-Siemens

StatoilHydro and Siemens Power Generation entered into an agreement to cooperate on technology to develop floating wind turbines, based on StatoilHydro's Hywind concept.  StatoilHydro is aiming to build the world’s first full scale floating wind turbine and test it over a two-year period offshore near Karmøy, an island to the southwest of Norway.  The company has announced an investment of approximately NOK 400 million for a planned start up in Autumn 2009.

A Siemens 2.3 MW wind turbine (80 m rotor diameter) is set on a floating column of the spar-buoy type, a solution long established in oil and gas production platforms and other offshore floating systems.

The flotation element is proposed to have a draft of 100 m below the sea surface, and to be moored to the seabed using three anchor points.  The system can be employed in waters depths ranging from 120 to 700 m.

Figure 5.10 The SWAY® concept

The combination of two established technology solutions in the wind turbine and spar buoy may be considered a prudent approach to the development of offshore floating systems.

StatoilHydro has also acquired a substantial share in the technology company Sway, which is developing a highly innovative solution for system support.  The SWAY® system is a floating foundation capable of supporting a 5 MW wind turbine in water depths ranging from 80 m to more than 300 m in challenging offshore locations.

The tower is stabilised by elongation of the floating tower to approximately 100 m under the water surface and by around 2,000 tons of ballast in the bottom.  A wire bar gives sufficient strength to avoid tower fatigue.  Anchoring is secured with a single tension leg between the tower and the anchor.

The tower takes up an equilibrium tilt angle (typically around 5 to 10°) due to the wind thrust on the rotor.  During power production in storm conditions, there is expected to be a further variation of only +/-0.5 to 1.0° from the equilibrium tilt angle due to wave action.

The concept exploits active control of rotor thrust, and claims to achieve substantial cost savings over competing technology for deep water applications.  The first full-scale wind turbine is expected to be built and installed in 2010-2012.

Blue H

A prototype installation using a concept similar to the tension-leg platform developed in the oil and gas industry was launched in late 2007 by Blue H Technologies of the Netherlands.  The installation carries a two-bladed wind turbine, and is due for full installation and testing in 2008 in a water depth of over 100 m, approximately 17 km offshore from Puglia, Italy.