Final Report 4.1

UPWIND
� Classical soft-stiff design range, proven to be suffering from significant wave excitation
� Blade resonance range with excessive excitation from cyclic aerodynamic loading,
design impossible without a large exclusion window of the rotor speed
� Stiff-stiff, design is considered uneconomical due to the high consumption of material
required for the stiffness
Figure 3.1 illustrates these options applied for the Upwind 5 MW reference turbine (see
Appendix A).
At state of the art offshore wind farms, mostly the second and third design ranges shown in
Figure 3.1 are found. The reason is that most of the structures are supported by monopiles. For
such structures it is difficult to achieve the stiff-stiff region due to economical constraints. The
very soft region is critical due to high wave loading. Therefore most of the structures are placed
into the soft-stiff region, where the structures are out of any rotational-dependent resonance,
economic in material consumption and where the wave impacts are lower. For future larger
turbine types with 5 MW rated power and larger water depths, monopile structures in the softstiff
design region are difficult to design, as certain limits in pile diameter and wall thicknesses
are reached. Therefore the soft-soft design region, in the 1P rotor frequency range, could be an
option. To avoid resonances, different operational control concepts like a rotational speed
window can be used as described later in Chapter 4.
0
0
(1)
(2) (3)
(4)
(5)
Rotor frequency ± 10%
Rotor freq.
Blade passing frequency ± 10%
Blade passing frequency
6,9 12 20,7 36 [rpm]
0,10 0,22 0,31 0,66
rated rotor speed (i.e. maximum stationary rotor speed)
Figure 3.1: Design ranges for the fundamental eigenfrequency of the support structure of a variable-speed wind turbine
at the example of the Upwind 5 MW design
3.2 Critical load effects for certain support structure types
Depending on the type of the support structure, different loading events can be critical. An
important difference can be found for bottom-mounted and floating structures, but also for
single-piled and braced ones. The Section below deals with steel-type structures only and will
point out certain aspects of some exemplary structures.
For state of the art offshore wind farms, monopiles are by far the most widely used support
structure types. Monopiles consist of a single tubular pipe that transfers the loads mainly
laterally into the sea bed. This layout makes the structure relatively sensitive to the uncertainties
of the soil conditions. On the other hand, monopiles might be applied in a range from soft to
relatively stiff soil conditions. However, monopiles are not the best suited concept for very soft
or very stiff soil conditions or when boulders occur in the sea bed. In the presence of bedrock,
drilled and subsequently grouted monopiles can be applied, or a combination of drill and drive.
[Hz]
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