Environmental Engineering Reference
In-Depth Information
Lightning protection is mandatory for both land-based and offshore systems. The major
portion of the turbine nacelle covers and towers are painted light blue or gray to minimize
their visual impact, especially at long distances.
The rated power of a typical offshore wind turbine ranges from 2 MW to 5 MW, and
its three-bladed horizontal-axis upwind rotors is typically 80 m to 130 m in diameter. Off-
shore machines are generally larger than land-based wind turbines because there are fewer
constraints on transportation of components and erection equipment, which limit the size of
land-based machines. Blade tip speeds of offshore wind turbines (80 m/s or greater) are also
typically higher than those of land-based turbines because aerodynamic noise is less of an
issue.
The basic drivetrain topology of an offshore wind turbine differs very little from that
illustrated in Figure 3-41 for land-based systems. Typical offshore drivetrains consist of a
modular, three-stage, hybrid planetary-helical gearbox that steps up the rotor speed to gen-
erator speeds of 1,000 to 1,800 revolutions per minute and generally runs with variable-
speed torque control. However, direct-drive generators may prove to be a viable alternative.
Offshore towers are shorter than land-based towers because wind shear proiles are more
gradual, diminishing the gains in energy capture with increased height (see Figure 2-22).
Offshore Foundations
Offshore foundation and substructure systems differ most substantially from those of
land-based turbines. The most common substructure is the monopole , which is a large steel
tube ranging in length from 35 to 50 m, with a diameter up to 6 m and a wall thickness up
to 60 mm. A monopole substructure extends from the foundation to above the water surface
where a transition piece is attached, which contains a lange to fasten the monopole to the
tower.
Foundation embedment depths vary with the soil type of the sea bottom, but typical
North Sea installations require pile embedments of 25 to 30 m below the mud line. This
type of monopile foundation requires special installation equipment for driving piles into the
seabed and lifting the monopole into place. In several projects, gravity-based foundations
(large concrete slabs) have been deployed as a viable alternative, which avoids the need for
large pile driving equipment. Gravity-based systems require a signiicant amount of bottom
preparation prior to installation and are only compatible with irm soil substrates in relatively
shallow waters. Newer multi-pile foundations ( i.e. multiple-legged substructures) may pro-
vide yet another alternative to the conventional monopiles.
The costs of infrastructure installation and logistical support are signiicant portions of
the total cost for a large offshore wind power station. As with land-based stations, offshore
wind turbines are arranged in arrays that take advantage of the measured prevailing wind
conditions at the site, and turbine spacing is chosen to minimize aggregate power plant power
losses, interior plant turbulence, and the cost of cabling between turbines [Elkinton et al.
2008].
Connections to the Electrical Grid
In a typical offshore plant, the output from each turbine at a voltage of 480 V to 690 V
is stepped up with individual turbine transformers (which can be dry air cooled) to a distri-
bution voltage within the station of about 34 kilovolts (kV). An electrical service platform
provides a common electrical interconnection in the distribution system and serves as a sub-
station where the collection cables are combined and brought into common phase. For small-
er arrays of wind turbines that are closer to shore, this function can be done entirely onshore.
For larger projects, the voltage would be stepped up at the offshore substation to about 138
kV for transmission to a land-based substation connected to the onshore grid.
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