Offshore Wind Technology and Status - Structural Integrity of Offshore Wind Turbines - page 20

foundations are not practical because higher stiffness is needed to avoid
sympathetic vibrations at turbine rotor blade-passing frequencies and
because the greater wall thickness makes the monopile impossible to drive
into the seabed. Fixed substructures have been developed for such depths
that use multiple driven piles of much smaller diameter to pin the struc-
ture to the seabed, an approach commonly used for offshore oil and gas
platforms. For offshore wind, transitional substructures include tripods
and four-legged jackets. Fewer than 10 of each type have been installed
worldwide (AlphaVentus 2010).
Generally, the project developer is responsible for ensuring that the sub-
structure design, fabrication, and installation are compatible with the tur-
bine and tower designs, which the turbine manufacturers usually specify
for a particular International Electrotechnical Commission wind regime.
Appropriate integration of design of the substructure with the turbine and
tower selected for a project is a primary concern for both developers and
regulators.
Offshore wind turbine power output is greater than that of average
land-based turbines. As noted earlier, this is because offshore winds are
stronger and steadier than those on land and because offshore turbines
can be larger. The size of onshore turbines is constrained in part by lim-
its on the size and the weight of loads—turbine blades and towers, con-
struction equipment, and erection equipment—that can be transported
over land. Offshore turbines can be larger because larger and heavier loads
can be transported over water.
Onshore turbines tend to be placed on taller towers to take advantage
of the higher wind speeds that exist at higher elevations, above the influ-
ence of trees and topographic obstacles that create drag on the wind and
slow it down. With vast stretches of open water offshore, higher wind
speeds can exist at lower elevations, so offshore wind turbine towers can
be shorter than their land-based counterparts for a given power output.
Infrastructure mobilization and logistical support for construction of
a large offshore wind plant are major portions of the total system cost.
The wind turbines are arranged in arrays that are oriented to minimize
losses due to turbine-to-turbine interference and to take advantage of the
prevailing wind conditions at the site. Turbine spacing is chosen to estab-
lish an economic balance between array losses and interior array turbu-
lence and the cost of cabling between turbines, which increases with