Environmental Engineering Reference
In-Depth Information
Figure 11: Response spectrum for mudline bending stress for idling (left) and
operating (right) turbine.
near the turbines breaking waves are a common phenomenon. Although their infl u-
ence did not affect the design dramatically in this particular case, they prove the
importance of taking all details of a site into account.
Although the natural frequency of the structure is rather high at 0.48 Hz, the
effect of both wind and wave loading on resonance is signifi cant, as is the aerody-
namic damping. Figure 11 shows the response spectrum for the mudline bending
stress for equal environmental condition with an idling rotor (left) and a turbine in
operation (right). The signifi cant resonance peak in the wave-only case is damped
dramatically when the turbine is operating.
From the measurements at Blyth it can be concluded that current modelling
techniques are able to represent the critical features of offshore wind turbines
properly, especially when on hindsight all structural and environmental param-
eters are known. It has also been shown that monopile structures are very dynam-
ically sensitive, even in this case with relatively high natural frequency and that
therefore proper analysis of resonant behaviour and aerodynamic damping
deserve special attention.
3 S upport structure concepts
3.1 Basic functions
The basic function of the support structure is to keep the wind turbine in place. This
means that it has to be built to withstand loads originating from sea currents, waves
and wind - acting on both the support structure and the turbine in operation.
A variety of wind turbines is available on the market, designed by different
turbine manufacturers, in a range of power ratings. Each wind turbine has different
characteristics. The offshore environmental conditions may also vary from site to
site. Therefore, support structures are designed specifi cally for each case. It is not
uncommon for one offshore site to have several variations of one type of support
structure for one type of turbine.
The cost of the support structure on average amounts to around 25% of the total
offshore wind turbine cost [1].
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