104 CHAPTER THREE
In the United States, land-based wind turbines (also called onshore turbines)
have been the most popular type because there is sufficient land area for, single
or multiple, wind turbine installations. In Europe, land scarcity led to offshore
wind farms where the wind strength and dependability are also a direct benefit.
Designing columns for wind turbines involves two steps: (1) the column
foundation for either land-based or sea-based wind turbines, and (2) the column
itself and the loads it must carry.
Most land-based commercial wind turbines in the United States are sup-
ported on a tubular steel column manufactured specifically for the site and the
expected wind velocities at the site. A concrete foundation for the column is
generally used, depending on the soil conditions. In northern Europe, precast
concrete piles are popular for onshore wind-turbine bases, with the column
being tubular steel. Overturning moments are produced by the wind load on the
turbine blades. Groundwater levels can be a consideration when designing the
column foundation. Dynamic loads also occur during wind gusts on the pro-
peller blades.
For sea-based commercial wind turbines, six types of support structures are
available: (1) monopile driven into the sea bed; (2) gravity base which can be a
steel or concrete caisson with suitable ballasting to resist the overturning moment
caused by the wind; (3) tripod with piles driven into the seabed; (4) suction
bucket in which an inverted type caisson is sunk into the sea bed using suction;
(5) tension legs in which the vertical wind turbine column is supported by an
underwater float anchored to the bottom by vertical anchors; and (6) floating
support—a concept still being worked on in various parts of the design world.
Designers of sea-based wind turbine farms and individual units must be
aware of the dangers to, and from, local shipping routes posed by the turbine
structure. Large European wind turbines have a total height of 650 ft (198 m),
and diameter of 413 ft (126 m).