Cables and composites create lean offshore

WORLDWIDE: Achieving 150GW of offshore wind round Europe by 2030 would require installation of four 6MW wind turbines every day. At that number in the middle of the North Sea you need to be able to arrive, install, connect and go quickly, using highly automated machinery controlled remotely.

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Lean design uses shape, not brute force or weight. French company Vergnet manufactures 1MW wind turbines with guyed towers, for installation on land in remote locations with difficult access. This halves the amount of material in the tower and foundations and improves efficiency and installation.

As wind turbines move offshore, the turbine and nacelle remain essentially unchanged but, as the water depth increases, the weight of conventional towers and foundations can easily be double what they would have been on land - and installation becomes much harder.

For a guyed tower design, as the water deepens the geometry extends, but the components essentially stay the same and the weight does not significantly increase. Putting down the tower with a central pad "foot" for it to stand on, installing three screw anchors and attaching the guy cables is straightforward.

You deliver a fully assembled wind turbine, with tower and cabling included, stand it upright, lower it onto the seabed, couple up the guy cables and go. A remote-controlled seabed vehicle unrolls the cable across the sea floor to the central connection point, where it plugs it into the sub-sea quick-connect and then heads to the next turbine to do it all over again.

One assumption for offshore wind-energy work so far has been that towers and foundations will be steel and concrete. But these materials are intrinsically heavy, making transportation and lifting expensive and slow. Additionally, these materials are energy intensive in their manufacture, adversely affecting the energy investment/payback calculations for offshore turbines as the steel and concrete elements of the tower and the foundations increase in size. They become expensive not only in financial terms but also in energy terms.

A third downside is that steel and monopile foundations, which create high stresses in both the pile and the soil by loading the foundation in bending, have a poor fatigue capacity, giving them a 20-year design life. But infrastructural goods are usually designed to have a far longer service life than this.

The solution is already at hand, however, within the wind-energy industry itself. The whole development of technical composites, the middle ground between very expensive aerospace composites and low-cost fibreglass boat and automotive manufacturing, has transformed the composites industry globally in the last two decades. Developments in this sector, and its sheer size, at over ten times larger than the aerospace and automotive composites industry combined, are making composites applications economically viable for more and more demanding applications.

Fibre-reinforced composites have very good fatigue properties, excellent lifetimes, and low weight. Making the towers out of composites is just like making wind-turbine blades out of composites - easier, in fact - just bigger. Compared to a conventional tower and foundation in steel, a guyed tower is half the weight. In composites it is a quarter of the weight - and takes half the energy to produce - with a design life of 40 to 60 years. All the technology that created the zone now known as technical composites and that also transformed the global composites industry originated in the wind-energy industry. It is now time to extend this to wind-turbine towers.

Jim Platts is a lecturer at the University of Cambridge, Institute for Manufacturing. He has developed wood/composite and bamboo blade technology for Composite Technology (now Vestas Global Blade Technology).

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