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
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.
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.
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.