Environmental Engineering Reference
In-Depth Information
Fig. 1.2 Typical onshore tower designs for wind turbines
wind farms and by the repowering of land-based installations, leading to a doubling in
the annual installed output by the year 2020.
The towers supporting onshore wind turbines are mainly of steel or prestressed
concrete with internal or external prestressing. Steel lattice masts are also used in
isolated instances. The prestressed concrete towers make use of both
in situ
and precast
concrete. In recent years, the use of hybrid towers, consisting of a prestressed concrete
shaft and a steel top section, has proved to be a very economical solution, especially for
wind turbines in the multi-megawatt category. The choice of a suitable tower design is
governed by the conditions at the site (fabrication, transport, erection, etc.). Figure 1.2
illustrates typical towers for onshore wind turbines.
Both shallow and deep foundations can be used for onshore wind turbines. Soil
improvement measures can be employed to upgrade subsoil properties to those
required for shallow foundations [4, 5]. Driven piles of steel or concrete with appro-
priate toe forms are frequently used as deep foundations.
So far, about 25 wind farms have been approved for construction off the German coast
in the North Sea and Baltic Sea within the 12-mile zone and the exclusive economic
zone (EEZ) for water depths of up to 45m. But the better wind conditions at sea call for
a greater technical input for the loadbearing structure and the fabrication and erection
of the wind turbines [6]. Besides the depth of the water, the choice of a suitable offshore
structure is especially dependent on the wave and current conditions plus the subsoil
beneath the seabed. Concrete structures in the form of gravity bases are economic
propositions for nearshore sites and for greater depths of water, see [7]. Such
foundations are built in a dock, for example, then floated out to their final position
and sunk. Resolved designs with individual members made from prestressed high-
strength concrete are also feasible. An overview of the offshore foundation concepts
currently under discussion can be found in Section 5.
