The idea is certainly appealing. Instead of installing offshore windturbines on huge, complex and expensive tripods, simple steel cylinders could be used in the future. These foundations are called monopiles – and they are far cheaper than jacket or tripod foundations. But why have turbine manufacturers only just come up with this apparently obvious idea?
The answer lies in drilling technology. So far, it has only been possible to drive monopiles to a depth that allows them to support turbines with a power of two or three megawatts (MW) at most. Sheet steel is bent and soldered to make the large cylinders, which are then held by a claw from a ship or hoisting platform and driven into the seabed with great force using hydraulic or diesel piling hammers. The latter are heavy steel constructions that hang from a crane and lift heavy weights by igniting a compound of diesel and air. When the weight falls down again, it hits the monopile, thus driving it a little further into the seabed each time.
However, diesel piling hammers can only cope with foundations that have a diameter of up to six metres. Too much power is needed for anything bigger than this. But larger turbines need foundations with a larger diameter. Another problem is the volume of sound produced when metal hits metal. As noise carries a long distance underwater, marine fauna may suffer permanent damage. As a result, the authorities have set a noise limit of 160 decibels at a distance of 750 metres from the building site. But even this limit can prove tricky when the monopiles are rammed into the seabed.
This is an abridged version of the article – the full text is available in new energy issue 03/2013.
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