Future of Wind Energy - Wind Energy Systems for Electric Power Generation - page 165

Gravity base Monopile Tripod

Fig. 8.1 Foundation structures

- Monopile foundation
This is the most common type for water depths up to 25 m. It has minimal foot-
print, but on the other hand features low stiffness. Important for the avoidance
of resonance-induced dynamic oscillations is the knowledge of the structural fre-
quency by design which may be a difficult task when there are uncertainties about
the ground condition.

- Tripod
For deeper waters, tripod support structures are being considered for offshore
and coastal regions. Use of tripod foundation has been made down to 450 m wa-
ter depth in the oil & gas industry, but with wind turbine systems there is little
experience. Tripod constructions of which there are variants use a larger foot-
print.

- Floating
Floating offshore wind turbines, e.g. on basis of a buoy concept, are a candi-
date mainly in shallow water. Currently they have the drawback of high cost, but
future solutions may increase their potential.

Figure 8.1 [OWE07] gives sketches of the foundation structure types.

8.1.3 Connection

Selection of the connection techniques between offshore wind parks and onshore
stations is an important issue. Available solutions are HVAC connection and HVDC
connection. Given the power rating, the choice of system has to take into account
the wind park distance from shore defining the sea-cable length, the water depth and
ground properties to assess investment cost, transmission loss and cable temperature
rise, besides further concerns discussed in 8.1.3.