Design of support structures for offshore wind turbines - Wind Power Generation and Wind Turbine Design - page 570

structure will be mainly in axial direction. Therefore, scour protection is generally
not required.

3.2.3 Jacket
Jackets, also known as space-frames or truss-towers, are relatively complex steel
structures. Despite a reduction in construction materials (and hence weight), jack-
ets are relatively expensive.
At the water depth where monopiles become uneconomical, jackets take over.
However, due to ongoing developments, the monopile concept is used for increas-
ingly deeper waters and the application of jackets is shifted to even deeper waters.
For installation, a method similar to the one for tripods is applied (see Fig. 17).

3.2.4 Gravity-based structures
As the name implies, a gravity-based structure (GBS) utilizes the earth's gravita-
tional force to stabilize its position.
GBSs usually have reinforced concrete foundations, referred to as caissons, in
which a tower is placed. An example of GBSs for offshore wind turbines is
shown in Fig. 18. Such GBSs are a proven technology for shallow waters. Occa-
sionally they are used in deeper waters. The offshore wind farm Thornton Bank
off the coast of Belgium applied reinforced concrete GBSs for a water depth of
approximately 28 m.
Deeper waters require constructions with larger footprints in order to absorb
greater moments. The increase of mass with water depth follows an approximately

Lifting and
landing of jacket

Foundation piles Turbine Tower Nacelle Rotor blades

Figure 17: Installation sequence of main components for a jacket foundation.

Figure 18: GBSs for Thornton Bank, off the coast of Belgium.