Environmental Engineering Reference
In-Depth Information
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Figure 38: Wind farm layout with the wind rose on site.
Because the wind fl ow offshore is less turbulent, the mixing of the air behind
the turbine (which has been slowed down due to the extraction of energy) takes
longer to regain strength from the undisturbed wind around it. This means that
turbines need to be placed further apart in offshore wind farms compared to
onshore where turbulence intensity is higher. The rule of thumb is to place tur-
bines at least seven times the rotor diameter apart. Smart layout confi gurations
help increase the distance between the turbine in the governing wind direction
to 10D or more, while maintaining the 7D minimum distance to make sure the
cable length does not increase too much. Figure 38 shows such a layout with the
wind rose on site.
6.2.2 Electrical infrastructure
The offshore wind power needs to be transported to the electricity grid. Within
the offshore wind farm, in-fi eld cables connect the turbines to each other. For
most larger offshore wind farms (>100 MW) a transformer station is normally
used. Here all in-fi eld string cables come together. The total power is gathered
and the voltage is increased to reduce losses to shore along the “shore connec-
tion cable”. Typical voltages in fi elds are order of magnitude 33 kV, to shore
50 kV.
The in-fi eld cable routing has infl uence on the design of the support struc-
tures: to most turbines two J-tubes will guide the incoming and outgoing cable,
requiring secondary steel attachments that can add signifi cant hydrodynamic
loads. For installation purposes it can be very benefi cial to choose a simple grid
type of layout. Should a “star grid” or other, non-regular pattern be chosen,
maintenance vessels may damage them in later years when there is confusion
about the exact location.
6.2.3 The support structure in the offshore wind farm
Offshore wind farms are complex systems placed in a harsh environment, designed
to operate on their own for large periods of time. The support structure is an integral
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