Wind Turbine Offshore Foundations
The major challenge for offshore wind energy is cutting costs: Undersea cabling and foundations have until recently made offshore wind energy an expensive option.
New studies of foundation technology, however, plus megawatt-sized wind turbines are now on the point of making offshore wind energy competitive with onshore sites, at least for shallow water depths up to 15 metres (50 ft.).
Since offshore wind turbines generally yield 50 per cent higher output than turbines on nearby onshore sites (on flat land), offshore siting may be quite attractive, cf. the page on offshore wind conditions.
Steel is Cheaper Than Concrete
Two Danish power company groups and three engineering firms made a pioneering study on the design and costing of offshore wind turbine foundations in 1996-1997. The report concluded that steel is far more competitive than concrete for larger offshore wind farms.
It appears that all of the new technologies will be economic until at least 15 metres water depth, and possibly beyond such depths. In any case, the marginal cost of moving into deeper waters is far smaller than what was previously estimated.
With these concepts foundation and grid connection costs for large 1.5 megawatt turbines are only 10 to 20 per cent higher than the corresponding costs for the 450-500 kW turbines used at Vindeby and Tunø Knob offshore wind parks in Denmark.
50 Year Design Lifetime
Contrary to popular belief, corrosion is not a major concern with offshore steel structures. Experience from offshore oil rigs has shown that they can be adequately protected using cathodic (electrical) corrosion protection.
Surface protection (paint) on offshore wind turbines will routinely be delivered with a higher protection class than for onshore turbines.
Oil rig foundations are normally built to last 50 years. This is also the design lifetime for the steel foundations used in these studies.
Reference Turbine
The reference turbine for the study is a modern 1.5 MW three-bladed upwind turbine with a hub height of about 55 metres (180 ft.) and a rotor diameter of some 64 metres (210 ft.).
The hub height of the reference turbine is low compared with the typical onshore turbine of that size. In Northern Germany the typical hub height of a 1.5 MW turbine varies from 60 to 80 m (200 to 260 ft.). Because of the very smooth surface (low roughness ) of water surfaces it is cost-efficient to use lower towers. You may verify these conclusions using the Wind Turbine Power Calculator which already has a built in example of a 1.5 MW offshore wind turbine.
© Copyright 1997-2003 Danish Wind Industry Association
Updated 23 July 2003
http://www.windpower.org/en/tour/rd/foundat.htm
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