Fixed-foundation performance on a floating platform

WORLDWIDE: The Japanese could well be closest to commercialising floating offshore wind technology, but the Dutch and Germans are also making rapid progress.

EDP’s 2MW WindFloat floating wind turbine 5km offshore of Agucadoura, Portugal (photo credit: Untrakdrover)
EDP’s 2MW WindFloat floating wind turbine 5km offshore of Agucadoura, Portugal (photo credit: Untrakdrover)
German engineering consultancy Gicon has developed and fully tested a scale model of a modular-design floating foundation for deployment in water depths from 20-700 metres.

The company claims that overall performance and stability characteristics of its Gicon-SOF technology, a floating offshore foundation developed with financial support of the German science and technology ministry, are comparable to monopile-type fixed foundations, including behaviour under the most demanding high-wave and high-wind conditions.

Scale model
Early in 2012, Gicon successfully tested a 1:25 scale model of its tension leg platform (TLP) floating foundation with a mounted turbine at the Hamburg Ship Model Basin HSVA. The programme tested various operational and transportation conditions, including offshore towing stability.

The anchored structure was also exposed to various wave heights and directions, including simulated rogue waves equivalent to 20-metre waves in natural conditions.

"The structure’s response and performance under such extreme and harsh marine conditions demonstrated in Hamburg surpassed all our expectations," Gicon CEO Jochen Grossmann said at the Maritime Research Institute Netherlands (Marin) in June, when the institute was conducting final scale-model tests. 

"The Marin tests have provided further proof that Gicon-SOF marine behaviour fully matches our earlier simulations and calculations results while expanding upon last year’s Hamburg basin testing outcomes," Grossman added.

"These latest Marin tests also mark an essential last step prior to moving towards a full-scale pilot plant in the German Baltic Sea, off the Mecklenburg-Western Pomerania coast with testing planned to commence from 2014." Grossmann at this stage would not elaborate on what turbine make or size would be employed at the pilot plant.

Challenge 
Perhaps the single-most important challenge for any floating turbine design is sufficient operating stability with minimised nacelle movement. Acceleration forces and motion of the nacelle should be comparable to that common for conventional seabed-fixed foundations.

A key consideration is that continuous, but variable, forces from waves and wind introduce both linear and rotational movements impacting a floating structure. With floating offshore turbines, motions of the lower foundation are transferred and amplified at the higher elevations upward to the nacelle.

Dynamic nacelle movements introduce acceleration forces into the system, which increase turbine loads at crucial component levels, for example in the rotor blade foot, pitch bearings and the drivetrain.

Potential negative overall impact includes accelerated materials fatigue and risks of premature turbine failure, while negatively affecting turbine reliability and availability as well as lifetime expectancy.

Grossmann claims that Gicon-SOF is the only design offering operational stability comparable to conventional fixed monopile foundations. "This has been achieved through the elimination of pitching and toss effects," he says.

Pitching is the backward and forward motion of the rotor around its transverse axis, while toss effects include wave amplification through overlapping waves.

"Conventional" TLP-based floating turbines are typically connected to the seabed by multiple vertical steel cables. These cables extend to suction anchors — also called suction buckets — that are semi-fixed into the seabed or attached to a weighed-down structure resting on the seabed.

The platform is able to float by using either a tubular steel lattice structure incorporating multiple buoyancy elements or a hollow ring-shaped structure in concrete, steel or other construction materials offering comparable functionality. In current designs, buoyancy elements in operation are usually part or partly or totally submerged. 

Diagonal bracings
A key design difference between Gicon-SOF and conventional TLPs is the addition of diagonal cable bracings that extend at an angle from the foundation structure to the seabed anchoring. The diagonal bracings form an essential element of Gicon-SOFs operating stability.

The design calculations required are highly complex, explains Grossmann. "In engineering mechanics, this combination of multiple vertical and diagonal bracings is known as a statistically undetermined, hard-to-calculate condition and, therefore, represented a major design challenge for our engineering team," he says. The firm has teamed up with experts from German technology institute Fraunhofer to further develop and optimise the method.

A secondary Gicon-SOF design feature is that the buoyancy elements are fully submerged in operating mode, which minimises wave impact and thus reduces wave-induced motion.

The initial design comprised a four-legged cross-shape lattice structure with buoyancy elements integrated into the outer end of each individual arm, with the turbine tower mounted in the centre. "Our latest foundation design has evolved into a square-shaped structure with integrated buoyancy elements and optimised central turbine mounting.

The scale model is being tested at Marin. It combines lower demand for structural materials with enhanced suitability for industrialisation, thus substantially reduceding manufacturing costs," says Grossmann.

No size and water depth limit
The foundation has currently been calculated for power ratings up to 7MW but, according to Grossmann there are no real limits to size. He adds that turbine head mass is not a critical variable because the integrated modular design offers multiple alternative single and combined options for adaptation and fine-tuning to diverse turbine sizes and environmental circumstances.

For higher power ratings and bigger rotor diameters, the dimensions of the floating ring will increase, as will the size and number of buoyancy elements. Differences in water depth can be dealt with by varying the filling percentage in these buoyancy elements and their positioning below the water surface. 

The company’s development plan aims to install a 5MW floating model in the North Sea by 2015. Gicon also has an eye on the US. "The US is a late starter with huge offshore wind potential," says Grossman. "A key constraint is that there are currently no installation vessels available and the Jones Act prevents these being brought in from abroad."

The Jones Act is a US federal law stipulating that all goods transported by water between US ports be carried in US-constructed ships, owned and crewed by US citizens and permanent residents. Bringing in specialised floating-turbine installation equipment and staff for building offshore wind farms is thereby blocked in practice.

"Our technology could solve this issue, while at the same time offering an industrial stimulus by creating a new industry with many manufacturing and other jobs," Grossmann says.

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