Final report for WP4.3: Enhancement of design methods ... - Upwind

The following results are found: Firstly, the frequencies associated with the first global bending modes of
the support structure are shifted by approximately 5% towards lower frequencies as mentioned before.
Those frequencies are among the most important design parameters for OWT support structures as it is
common practice to design structures with a first natural frequency between the 1P and the 3P range
(soft-stiff design) of the turbine. The allowable frequency gap is relatively small, especially as a distance of
about 10% between natural frequency and excitation range has usually to be provided for safety reasons
(not shown in Figure 4.4). For the NREL turbine, the more detailed modeling would lead to higher theoretical
safety margins, as the frequencies are shifted towards the dead center between the 1P and the 3P
range.
Secondly, the third full system natural frequency is shifted into the upper 3P frequency range due to the
more realistic support structure modeling. The difference between the third natural frequency calculated
with the beam model and the super-element model is about 7%. The excitations mainly due to disturbed
wind flow around the tower in the 3P range are considered to bring more energy into the system than its
harmonics and even more than the excitations in the 1P range (e.g. see [65]). Furthermore, the natural
frequency is found near the upper bound of the 3P range which is associated with rated rotor speed. It is
obvious that the turbine operates much more often at rated rotor speed than at lower speeds, as this is the
operational speed at all wind speeds above rated wind speed. The maximum rotor thrust occurring at
rated wind speed makes a strong dynamic excitation at this speed even more probable.
Thirdly, the ninth natural frequency is shifted to the upper 6P range. This is not too critical, other natural
frequencies are found in the 6P range as well.
All in all, the support structure modeling with super-elements leads to changes in comparison to the basic
beam model concerning the full system eigenstates that are not negligible.
Load case definition for load comparison
The next step is the study of the impact of super-element modeling of joints on the results of aero-elastic
time domain simulations of realistic load cases as described in the respective standards. A heavily reduced
set of sub-cases is to be used to get a reasonable balance between simulation effort and proximity
to loads simulation as done in OWT certification.
The IEC 61400-3 standard [69] is used to define the load cases, since it is commonly used internationally.
All assumptions for external conditions are based on the UpWind design basis [55]. Only DLC 1.2 (normal
operation) is taken into account, which is a major fatigue load case covering a significant part of the whole
turbine lifetime. This is done for the following reason: the mean wind speeds over the turbine lifetime are
well described with a Weibull distribution with the highest probability at approximately 10 m/s for the described
offshore site. Very high and very low wind speeds are therefore relatively rare. This means that a
turbine with a realistic availability passes a large fraction of the total life time operating under conditions
covered by this load case. Furthermore, the summed occurrence of all average wind speeds (10 min
average) over Vave = 24m/s is only 73 hrs/yr, therefore these speeds are not taken into account. With the
bin size of 2 m/s, this leads to 11 wind bins as shown in Table 4.1. The average wind speed Vave, turbulence
intensity TI, significant wave height Hs and peak period Tp of the Pierson-Moskowitz spectrum and
summed occurrence of this wind and wave combination are also shown.
Table 4.1: Lumped scatter diagram used in super-element study (modified from [55])
Vave TI Hs Tp Hrs/yr
4 0.2042 1.1 5.88 780.6
6 0.175 1.18 5.76 1230.6
8 0.1604 1.31 5.67 1219.7
10 0.1517 1.48 5.74 1264.9
12 0.1458 1.7 5.88 1121.8
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