Aug 22, 2011
Although still only a tiny fraction of the world's installed wind capacity, offshore wind is about to explode. BTM Consult predicts that more than 16 GW of new offshore will be installed by the end of 2014, with a global total of 75 GW by 2020.
As turbine foundations comprise a quarter to a third of the overall cost of an installed offshore turbine, depending on water depth, turbine size and who you talk to, today's foundation designers have a tough remit: develop new designs and installation procedures to cope with deeper water challenges - while simultaneously reducing the cost of manufacture, deployment and operation.
Fatigue load, installed strength, resistance to dynamic loadings and cost per installed-MW are four important metrics, but there are many others such as resistance to scour and corrosion plus ease of maintenance and even decommissioning. Water depth, the turbine size and the nature of the seabed - rock, sand or clay, for instance - all inform which type of substructure and footing is used.
Seabed fixings or footings can include piles, drilled and grouted holes, and innovations like in-line and pre-piling which help improve strength and reduce installation time. There's also the suction bucket or suction caisson where a vacuum is used to suck a footing into the seabed, dispensing with the need to pile or drill.
At the start of 2011, some 889 of the world's 1,318 offshore wind turbines used monopiles. This reflects the preponderance of shallow water locations and the trust placed in this proven design, though there have been some recent problems. Almost the default choice in up to 25 metres of water with a firm seabed, the monopile's shape makes for simple calculations and many monopiles can be packed tightly onto a barge for deployment. They typically weigh around 500 tonnes, though on deeper sites they can weigh up to 810 tonnes and are up to 69 metres long.
Gravity Base Foundations are another proven shallow-water design choice used at early developments like Vindeby in Denmark. The weight of the base and the rest of the structure holds the wind turbine in place with no need to pile or drill into the seabed. Concrete is the normal material, though steel is also an option.
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