Environmental Engineering Reference
In-Depth Information
At equal power yield offshore installation of wind energy converters allows for
a reduction of hub heights by approximately 25 %, when compared to onshore in-
stallation, due to different wind conditions at increased heights above ground (i.e.
the mean wind speed at sea increases faster at enhanced heights above ground
than on the mainland). Decreased hub heights correspondingly reduce tower costs.
Foundations. The type of foundation used to anchor towers, and thus wind en-
ergy converters, into the ground depends on the plant size, meteorological and op-
erational stress and local soil conditions. On principle, support structures are sub-
divided into shallow and deep foundations. Both are state-of-the-art technologies
but differ considerably with regard to costs. The optimum foundations design is
determined by appropriate soil investigations.
Anchoring wind energy converters on the coastline is much more costly (see
also /7-7/, /7-8/). There are various technologies that ensure stability. Foundation
technologies include the support structures (i.e. foundation structure plus tower)
and the technology required for anchoring the converters on the ocean floor. In-
stallation is generally aimed at low manufacturing costs (e.g. by series production,
material selection), low assembly costs (with regard to logistics, fast installation)
and a long service-life (considering factors such as corrosion and fatigue).
Currently, floor-mounted support structures are preferred for water depths be-
low 50 m. Floating support structures are also technically feasible, but will most
probably be applied in areas with water depths exceeding by far 50 m. The exact
dimensions of such floor-mounted support structures depend, among other factors,
on the expected wind, wave and ice charges and geographical site conditions (e.g.
water depth, soil conditions).
Floor-mounted support structures (Fig. 7.13) are subdivided further into grav-
ity, monopole, and tripod foundations, outlined as follows. A forth type, currently
of less importance, consists of a four-pile trelliswork construction, similar to the
lattice towers applied for onshore wind energy converters.
Gravity foundation (Fig. 7.13, left). The gravity foundation principle is based on
gravity force utilisation. Gravity foundations consist of concrete or steel frames
which are filled with ballast on site. Foundations are put on the seabed on a lev-
elled surface provided with a compensating layer to prevent transmittance of ten-
sile forces onto the seabed and to make the system sensitive for extreme hydrody-
namic loads. Since the maximum wave height, which partly influences the forces
effective on the foundation, depends on the water depth, among other factors, lar-
ger foundations are required with increasing depths. According to current eco-
nomic knowledge, this technology should only be applied up to a sea depth of
10 m. From a physical point of view, it should only be applied up to 20 m. Gravity
foundations have, for instance, been applied for the offshore wind parks of Vin-
deby and Middelgrunden (both located in Denmark); the mass of such a founda-
tion amounts to approximately 1,500 t at a water depth of about 5 m and an in-
stalled wind energy converter capacity of approximately 1.5 MW.
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