Traditionally, offshore wind farms are installed in relatively shallow waters. Nominally, Europe has very large areas of seabed with a suitable water depth and sea floor. However, shipping lanes, fishing banks, bird migration zones, defence testing grounds and recreational interests all tend to limit the area potentially available for offshore wind farms. Taking these limitations into account, a number of European countries including Norway, Portugal and Spain simply do not have sufficient shallow water areas for large-scale offshore wind farms using traditional turbine foundations.
Furthermore, in the United States, China, and Japan, most of the offshore wind resource potential is available in water deeper than 30m. All of the existing European offshore wind turbines are fixed-bottom substructures, mostly installed in water shallower than 20m by driving monopiles into the seabed or by relying on conventional concrete gravity bases.
These technologies are not economically or technically feasible in deeper water. Many alternative solutions have been suggested, including tripods and jacket platforms, amongst others. Some have been tested, but the general perception is that the foundation costs become prohibitive at water depths of 50m or more.
A solution to this is to replace the traditional, fixed foundation with a floating platform, tethered with mooring lines to the seabed. Since the 1970s, various concepts for floating wind turbines have been investigated. The Spar-buoy concept uses ballast to lower the centre of gravity below the centre of buoyancy to make the structure stable. It can be moored by catenary or taut lines.
A Tension Leg Platform uses mooring line tension and excess buoyancy in the tank to make the structure stable. The barge concept is stabilised through its water plane area and is generally moored by catenary lines. Hybrid concepts, which use features from all three classes, have also been considered.
In 2007 StatoilHydro approached Siemens with a proposal to jointly develop the concept for StatoilHydro’s Hywind floating wind turbine into a full-scale, proof-of-concept turbine. For several years StatoilHydro investigated floating wind turbines in order to develop offshore wind energy, drawing on its offshore expertise gained from the oil and gas industry.
At an early stage, a slender cylinder concept was selected, mainly due to its simplicity. This solution is similar to production platforms and offshore loading buoys. A 3m scale model was successfully tested in SINTEF Marintek’s wave simulator in Trondheim, Norway, to qualify the basic technology.
StatoilHydro and Siemens entered a technology development agreement, and over the next two years the project was taken from a concept stage to a full-blown proof-of-concept installation using a modern MW-class wind turbine, the Siemens SWT-2.3-82. This turbine type has a long track record offshore, being used in the Samsoe (2002) and Nysted (2003) projects.
The larger variant of this type – with a rotor diameter of 93 m – is employed at the Lillgrund (2007) and Horns Rev 2 (2009) projects. The attractive simplicity of the slender cylinder concept comes at a price. Even though it is tethered, and sits on a large floating foundation, the wind causes the tower to sway. Such swaying adds to the structure’s fatigue load.
Finding solutions to the slender cylinder concept’s disadvantage has been one of the core elements of the Hywind technology development. An advanced adaptive regulation has been developed, using the pitch system of the rotor blades to stabilise the movements.
This improves both power production and minimises the loads on the blades and the tower. The software controlling this process is able to measure the success of previous changes to the rotor angle and use that information to fine-tune future attempts to dampen wave-induced movement.
The proof-of-concept turbine was assembled in Åmøyfjorden near Stavanger during the summer of 2009. It was then towed to its final location, 12km out to sea off Karmøy on the western coast of Norway. The wind turbine itself was supplied and installed on a floater built by Tecnip of Finland. The cylinder is a 117m long steel cylinder, weighing 3,000 tonnes in ballasted condition.
The anchoring system enables Hywind technology to be used at depths from about 120m to 700m or maybe even more. The proof-of-concept turbine was formally inaugurated on 8 September 2009, and at the time of writing it is passing the commissioning tests.
Ahead waits two years of operational testing. Both Siemens and StatoilHydro are well aware that floating wind power is in its infancy, and that the road to commercialisation and large-scale development is long. In addition to the need for reduction in infrastructure costs, challenges include establishing an efficient arrangement for service operations.
Service operations on floating offshore wind turbines are likely to require new technologies. Not only are conditions by definition likely to be more severe, because floating turbines will be installed further offshore and in deeper waters, but the floater’s movements also brings its own challenges. For major service operations on fixed-foundation offshore wind turbines, jack-up vessels are used, creating a situation where both the crane and the target are fixed.
This will not be possible with floating wind turbines; neither the crane nor the target is fixed. Unless one accepts that crane operations can only be carried out during very calm conditions, which may not occur during the entire winter season, new technologies will need to be developed for the replacement of main components.
Notwithstanding these challenges, both StatoilHydro and Siemens are hopeful that floating wind power will become a genuine commercial alternative. For fixed offshore wind turbines, it took nine years to move from the world’s first offshore wind turbine demonstration project built at Vindeby in 1991 to the first large-scale project with multi-megawatt turbines at Middelgrunden in 2000. But then the expansion really took off.
The prospects of commercial floating wind energy will benefit not only from the experience already gained with offshore wind power, but also from the development of much larger wind turbines.
Indications are that the infrastructure costs do not increase proportionally to the energy generation potential of large turbines. Depending on the outcome of the Hywind demonstration project, Siemens and StatoilHydro expect that floating wind turbines may be commercially viable for large wind turbines within a time frame of 5-10 years.
By Henrik Stiesdal, Chief Technology Officer, Siemens Wind Power