French company IDEOL has introduced a floating foundation for 2-10MW class turbines, which it claims is competitive with jacket foundations for water depths from 35 metres. IDEOL co-founder and chief executive officer Paul de la Guérivière speaks to Windpower Offshore about the product’s main innovative features. He also expands on his company’s strategy for the offshore wind market.
Novel floating offshore foundation designs have been introduced in the past few years. Some solutions come as a spar buoy type (eg Hywind/Siemens), but most often these designs comprise a multiple tubular steel structure - semi-submersible or Tension Leg Platform. The turbine is then either mounted in the structure centre (eg Blue H), or atop one of the legs, as in Principle Power’s design.
Concrete floater
IDEOL floating foundation consists of a square-ring shaped concrete floater, with a compartmented hull of about 45x45m breadth and only 6.4m draught. The latter design feature is considered an important added benefit when the structures are being built inside and/or towed to construction sites from shallow ports such as those found in the UK. Mooring is applied to anchor individual turbine systems to the seabed.
The turbine tower is mounted atop a steel interface that sticks out above the water surface and is incorporated into one floater side centre. The floater central open area is called Damping Pool®. The water trapped inside this ‘pool’ acts as a motion absorber that dampens dynamic movements of the combined floater-turbine system.
This patent-protected technology feature acts entirely as a passive system not requiring any external intervention or control, explained De la Guérivière. He added that the dynamic dampening behaviour is highly effective and makes commercial offshore-certified wind turbines combined with IDEOL’s floating foundations fully compatible with similar turbines but put atop a fixed foundation.
Power curve
De la Guérivière: "Only the tower must be upgraded and the blade pitch control software adapted to fit the system changes caused by our floating foundation behaviour. Most important for owners/operators, a given wind turbine atop an IDEOL foundation delivers the same power curve compared with fixed foundations and essentially without operational restrictions due to floater movements."
Some ideas for the IDEOL foundation construction methods originate from port floating dam technologies used for decades along the Mediterranean. Here seafloor depth often increases steeply even close to shore, which seriously hampers the application of seabed-fixed foundations, De la Guérivière further elaborated.
Built in concrete, IDEOL’s design is claimed to be well suited for large-scale mass production and big wind farms. Floaters can be built onshore or in on-site docks, while prefab construction at separate locations is another option. This significantly reduces delivery time, costs and risks. Such advantages are reinforced by the turbine installation and pre-commissioning at quay side under sheltered circumstances.
Generic turbine
IDEOL’s product strategy focuses on 5-7MW class turbines. For the initial design process, the company used a generic 5MW turbine known as NREL offshore 5-MW baseline wind turbine. This is a conventional three-bladed upwind pitch-controlled variable-speed design with a heavy emphasis on Repower’s 5M technical specifications, including a 126m rotor diameter.
Based upon these generic turbine parameters, IDEOL has completed system designs for three distinct geographical areas: the North Sea, Mediterranean coasts, and Norway. De la Guérivière: "Our plan is to achieve a prototype installation with a commercially available 5-6MW turbine model by the middle of 2014, and a second turbine about a year later. We have further decided to initially focus on the North Sea, and are currently engaged in discussions for both German and UK Round 3 projects."
Including construction and offshore installation, a typical 500MW North Sea wind farm with IDEOL foundation technology should require less than 14 months to fully complete, with minor weather-related risks and restrictions. De la Guérivière confidentially quotes over 90% annual availability. He mentions the use of standard vessels for towing and central North Sea installation activities among the key contributing factors to such high availability.
