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United Kingdom

United Kingdom

Windtech: Lightweight design for upwind floating turbine

UK: Floating turbine technology is one of the fastest developing areas in the wind industry, and TetraFloat is just one of an increasing number of solutions being created across the world.

Buoys… Conical heave-plates “grip” the water and reduce vertical motion
Buoys… Conical heave-plates “grip” the water and reduce vertical motion
This patented, integrated, lightweight, floating offshore wind solution for turbines up to at least 10MW has already been basin tested with scaled prototypes, with more to come.

TetraFloat was conceived in 2008 to offer a reduction in the substantial material, weight and cost associated with large-scale future offshore wind turbines, says Seamus Garvey, mechanical engineering professor and TetraFloat inventor. "Today's prevailing view for floating turbines is that they are an expensive necessity when the water depth is too great for seabed-fixed foundations," he adds. "TetraFloat opens up a new perspective because its capex (capital expenditure) can be significantly lower compared to monopile or other foundation designs wherever water depths are 25 metres or more."

The lightweight design is a key benefit, enabled by the downwind force always acting in the same direction relative to the structure, explains Garvey. "This force therefore does not cause large bending stresses in a slender-design tower."

TetraFloat comprises a wide base triangular shape that rotates around and is held in place by a slack seabed anchored cable or chain. This anchor element in turn is attached to a harness chain interlinking the two legs parallel to the upwind rotor. TetraFloat's entire structure yaws bodily over the water when the wind direction changes in order to keep it facing directly into the wind, eliminating the need for a built-in turbine yaw system.

"Despite the fact that it would be convenient to fit a downwind machine atop the structure, our project focuses on incorporating 'conventional' horizontal-axis three-bladed upwind turbine solutions as these are today's semi-industry standard," Garvey says.

Aligning

One specific patented element is that the linkage point on the harness chain can be adjusted to achieve perfect alignment even in the presence of a prevailing sea current acting to push the entire structure sideways. The structure floats on heave-plate buoys fitted at each corner of the base triangle.

Structural loading is reduced because the slack cable arrangement acts as a pivoting point and keeps the rotor assembly always facing the prevailing wind. This in turn allows a strong, stiff but still lightweight structure, Garvey says. "Calculations have shown that a Tetrafloat platform for a 10MW turbine will utilise less than 3,000 tonnes of steel, which includes the turbine (650-750 tonnes head mass). Because of its low total structural mass, one might expect that the structure would respond heavily to severe wind and wave events, but we have introduced three distinct measures that prevent this," he says.

The first measure was to spread the buoys over a wide area, reducing pitching accelerations. The second measure was to reduce the amount of water surface displayed by the buoys — the water plane area — to a level where it reduces the natural frequencies for heave, pitch and roll without making the platform unstable.

Wide grip

The third measure was the conical heave-plate design, another patent owned by the TetraFloat company. These provide a wide "grip" on the water so that the seawater itself is deployed to add both mass and damping, thereby limiting vertical (heaving) motions.

Electric power is transmitted via the central mooring point through a rotating transformer that also provides the mechanical anchorage or stay.

TetraFloat, like competing floating turbine concepts, can be constructed, assembled and pre-commissioned in a sheltered harbour.

To enable easier marine transportation, on-site repair, and to protect against heavy-storms, the installation can be equipped with a facility to lower and fold the turbine assembly over the floater structure, for virtually no added cost, the company claims.

TetraFloat received a grant from the UK government's Department of Environment and Climate Change (Decc) last year for testing in a small wave-tank and subsequently at sea. It also received funding from the Marine Renewables Infrastructure Network programme for larger-scale tank testing. Garvey said that these tests have shown that the design principles are sound and that the structure is quite "transparent" to the waves. "Our next planned step after the Decc project is to build a full-scale prototype with a medium-size offshore turbine, for which wind industry negotiations are ongoing," Garvey says.

TetraFloat: The whole body yaws to always face the prevailing wind and the structure can be delivered collapsable as an option

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