Design of support structures for offshore wind turbines - Wind Power Generation and Wind Turbine Design - page 562

occurrence of wave frequency per year

60%

50%

40%

30%

20%

10%

0%

0 0.2 0.4 0.6 0.8 1 1.2

1 2

P P

Support structure's
natural frequency

Frequency [Hz]

Figure 4: Rotation (1P) and blade passing frequency (2P) of the Opti-OWECS
turbine with the structure's natural frequency and a histogram of the
occurring wave frequencies.

45 superposition

40

35

offshore technology
design tool

30 integrated

design tool

wind turbine
design tool

25

20

15

10

5

0

pure wind wave no aero.

damp.

wind + waves wind & waves

Figure 5: Comparison of fatigue calculations for wind only, wave only, wind and
wave combines from separate and simultaneous analyses.

The fact that the structure's natural frequency coincided with a large portion of
wave frequencies was further investigated. The aerodynamic damping of the tur-
bine was found to reduce fatigue signifi cantly, doubling the structures fatigue life
when taken into account. To enable the analysis of this feature, full non-linear time
domain simulations were found to be necessary of simultaneous wind and wave
loading. Should wind and wave loads be analysed separately, the effect will not
become visible by just adding the separate analyses as can be seen in Fig. 5.
Next to the detailed investigation of the dynamic behaviour in the design, a large
number of practical issues were addressed in an integrated way. For installation
it was found that onshore pre-installation would cause large cost reductions.
For the correction of misalignment of the driven foundation pile, a transition piece
was proposed. Installation of fully operational turbines and the misalignment
correction are shown in Fig. 6. It was concluded that large-scale offshore wind