Computer controlled tower stiffness

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A new tower concept could cut the cost of offshore wind by 15% claims the Dutch company, Smart Tower BV, behind the idea. Making its first public appearance at Sicily's offshore conference in April, Smart Tower is a joint venture between the consultancy branch of electric power specialists KEMA, and Volker Stevin Offshore.

"As a specialist in the production of towers for offshore use, Smart Tower is ideally placed to draw on KEMA's experience in the construction of towers for overhead power lines and Volker Stevin's expertise in the design, construction and installation of offshore oil and gas platforms," says director Henk Hutting. Although the company will eventually offer a range of offshore towers suitable for all turbine sizes, it is looking to take the North European market by storm with its "flexible" or "smart tower," which has been designed to accommodate the harmonics of turbines in the 3 MW plus range.

Big turbines pose a particular problem for tower design, explains Hutting. Rotational speeds vary between 8 or 9 rpm to 20 rpm, giving rise to harmonic frequencies of 1/6 to 1/3 Hz. "Unfortunately this coincides with wave frequencies, meaning that soft-soft towers need a harmonic frequency below 1/6 Hz which is extremely low, too low," he says. "We've solved that problem by designing a lattice tower with adjustable diagonal elements that will allow the tower to alter its harmonic frequency," he explains. "Depending on wave frequency and the turbine's rotational speed, the harmonics of the tower can be adjusted to either soft-soft for high wind speeds or soft-stiff for low wind speeds to maximise output and reduce fatigue."

Constructed from pre-stressed concrete poles linked by a lattice of stainless steel rods, the towers are virtually maintenance free and considerably cheaper than traditional monopile designs, says Hutting. The control system which alters the stiffness of the tower by adjusting tension in the lattice work is directly driven by the wind turbine's existing on-board computer and hydraulic systems -- and its cost is more than offset by the savings in materials and maintenance.

Project costs can also be cut by the use of a caterpillar truck capable of driving a fully-assembled turbine out to sea and erecting, says Hutting. The truck -- adapted from ocean-bottom survey vehicles in use off the Dutch coast -- can operate to a depth of 30 metres in all but the most severe weather conditions. "This will allow turbines to move straight from production into operation and also means that maintenance can be carried out as and when necessary without the costly down-time of ship-serviced turbines."

Hutting estimates the overall savings of the innovations on the price of offshore wind power will be around 15%. Although the truck's use is restricted to areas with flat sea-bottoms, these include the North Sea coasts of England, Belgium, the Netherlands, Germany and Denmark, "which is a big enough market for the time being" says Hutting.

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