Design of support structures for offshore wind turbines - Wind Power Generation and Wind Turbine Design - page 561

> 10 m/s
8.5-10 m/s

7.5-8.5 m/s

6.0-7.5 m/s
<6.0 m/s

Figure 2: Yearly average wind speed at 100 m height for the European Seas.

Figure 3: Subjects covered in the integrated design approach of the Opti-OWECS
study.

In 1995 the Joule I “Study of Offshore Wind Energy in the EC” was published [3].
The study gave an overview of the wind potential offshore as shown in Fig. 2. The
study described the design of offshore wind turbines in a more generic way with
example designs for different types of offshore wind turbines. It was found that for
one turbine wave loads could be dominant while for the other wind was the domi-
nant load source. One of the main issues found was the benefi t of aerodynamic
damping on the dynamic behaviour of the structure when the turbine is in opera-
tion. It was also stated that a softer support structure would further enhance the
aerodynamic damping effect, but at the cost of increased tower motion.
The Joule III Opti-OWECS [1] report fi nally made a complete design focusing
on the integrated dynamic features of fl exible offshore wind turbines. The design
incorporated the entire offshore wind farm with all its features from turbines to oper-
ation and maintenance philosophy to cost modelling. Figure 3 gives an overview
of all subjects covered in this integrated design scheme.
The Opti-OWECS study further explored the possibilities of fl exible dynamic
design. Although several types of support structures were reviewed, it was decided
to make a full design of a soft monopile structure to benefi t in full from the aero-
dynamic damping and assess the potential negative consequences of large struc-
tural motion. It was found that a structure could be designed with a natural
frequency below both the rotation and the blade passing frequency of the turbine,
a so-called soft-soft structure. The frequency distributions are shown in Fig. 4.