Final Report 4.1

UPWIND
5. Load mitigation concept analysis at operational
control level
In the following Chapter, several concepts for load mitigation in the operational control level are
introduced. These concepts include already available turbine operations in order to reduce
overall loading. The shown concepts just give an overview of possible options and could be
extended.
5.1 Rotational speed window
As explained in Chapter 2, the design ranges for support structures are important from a
dynamic point of view. In general, most bottom-mounted support structure concepts are
designed for the soft-stiff design region, which is between the 1P and 3P of the rotor speed
range. An example of such a design is shown in Figure 5.1, where a support structure is
designed for a first eigenfrequency of 0.22 Hz (here named as old design). In many cases the
support structure‟s eigenfrequency does not coincide with the prediction as illustrated in Figure
5.1 as new design. This can happen due to changes in the foundation properties, such as scour
holes, or simply due to errors in the soil measurements performed prior to the support structure
erection, on which the design was based. This means first of all that the design moves into the
high energy range of the wave spectrum, as illustrated in Figure 5.1 for a typical wave spectrum.
This will cause higher excitation from the hydrodynamics. But beside that, the eigenfrequency
falls within the 1P rotational speed range, which means that at some operational points the rotor
will operate at the same frequency as the first eigenfrequency of the support structure. The
result is that the support structure can vibrate at an unacceptable level and the loading in the
structure will increase.
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Figure 5.1: Frequency ranges for different support structure designs
Such a resonance can also be shown using a Campbell diagram, see Figure 5.2. The Figure
shows that for the first design (here named as old design) the first support structure