Environmental Engineering Reference
In-Depth Information
quadratic relation. So even though building with concrete appears an economic
choice, the large amounts required will make the structure relatively expensive.
An advantage with respect to installation is that a GBS can be fl oated out to its
offshore location. A disadvantage is the necessary preparation of the seabed, which
is done to provide the structure with a stable, horizontal fl oor. Another drawback is
that, due to the great weight of the GBS, heavy lifting vessels are needed to perform
the installation. The conventional installation sequence is shown in Fig. 19.
3.2.5 Floaters
The maximum water depth of wind farms has been steadily increasing over the last
decade. Although monopiles will likely continue to be the most applied support
structure for years to come, deeper waters appear to favour jacket structures.
Floating structures are seen by many as the solution to place wind farms in
deeper waters (>70
). To keep it in place, the fl oating substructure is attached to
the seabed through cables. In terms of installation costs, the question is whether
such a system will require new installation procedures and dedicated vessels, or if
it can simply be pre-assembled and transported by standard tugs (see Fig. 20).
μ
4 E nvironmental loads
4.1 Waves
When calculating wave loads different wave categories can be distinguished, regular
waves and irregular waves. Regular waves are periodic in nature and are usually asso-
ciated with extreme load events. Irregular waves have a random appearance and are
related to normal sea conditions and as such are to be adopted for fatigue evaluations.
For both regular and irregular waves several wave theories exist that allow the
calculation of wave particle kinematics: the orbital motion, velocity and accelera-
tion of infi nitesimal quantities of water beneath the surface of the waves. Linear
wave theory is valid for waves with infi nitely small amplitudes, whereas non-linear
wave theories are required for fi nite amplitude waves. Non-linear waves have a
different surface profi le compared to linear waves, with sharper, higher crests and
longer and shallower troughs. Figure 21 shows which wave theory applies under
Seabed
preparation
Lifting and
landing of GBS
Turbine Tower
Nacelle
Rotor blades
Figure 19: Installation sequence of main components for a GBS foundation.
Assemble
completely in
harbor
Attach cables to
seabed
Tug to location
Figure 20 : Proposed installation sequence for fl oating turbines.
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