Final Report 4.1

UPWIND
Another fact is that the dynamically critical and costly transition piece, as for jacket-tubular tower
configurations, can be saved for truss-type support structures. Therefore the transition piece is
moved to the connection between truss tower top and nacelle, which can also be difficult to
design but in general lighter due to the lower bending moments from the aerodynamic loading
from the rotor. Besides, the truss tower offers a geometrical flexibility. They can be designed as
three leg towers but also as four leg solution. They can have different types of bracings, for
example x-braces or z-braces.
The complex structure of a truss tower also imposes much higher cost for manufacturing and
maintenance, as the number of welds is increased significantly as well as the amount of joints to
maintain and to secure against corrosion. The transparent tower cannot be used anymore for
storage of power electronics or spare parts like heavy converters, as done for some offshore
turbines in order to reduce the mass of the nacelle.
Beside all the disadvantages in fabrication and maintenance, truss towers experience also
different loading phenomena compared to designs with tubular towers. In general, the tubular
joints with their stress concentrations are sensitive to high cycle fatigue introduced by the
aerodynamic tower top loading and the reduced torsional stiffness. This can potentially lead to
dynamic problems. At the bottom of the structure (close to seabed) the bending or buckling of
the elements is critical and closer to the tower top (close to the nacelle) the torsional modes are
also critical. The torsion at the tower top is induced by unbalanced loadings on the rotor from
wind shear or skewed inflow. This also includes a much higher sensitivity to certain extreme
events such as extreme directional changes. Therefore truss towers would benefit from
particular aerodynamic load mitigation concepts like individual pitch control in order to reduce
the torsional response.
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Figure 4.5: Torsional loading at truss-tower top with and without IPC
Figure 4.4 illustrates an exemplary truss tower design for a 5 MW reference turbine (see
Appendix A). In the here shown case the support structure consists of a 3-leg truss tower and a
z-type bracing (see Appendix A and [24]). As stated before, for such structures the torsional
loading at the tower top can become a critical design driver. However, an industry-standard
individual pitch controller without additional tuning for the tower loading can mitigate these loads
already. As an example an advanced power controller designed for the UpWind project [25] is
applied. The controller includes 1P individual pitch control to reduce asymmetric rotor loads, and
here especially 1P loads on rotating components and lower frequency loads on non-rotating