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

UPWIND WP4: Offshore Support Structures and Foundations
• Multi-megawatt, offshore, 3-blade, monopile tower, downwind, variable speed, pitch regulated
• < 500kW, onshore, 3-blade, monopile tower, upwind, fixed speed, stall regulated
• ~1 Megawatt, onshore, 2-blade, tethered tower, upwind, variable speed, pitch regulated
Figure 7.5 and Figure 7.6 present comparisons of selected loads from one of the above load sets. In general
good agreement was found between the previous versions of Bladed and Bladed v4.0. The main difference is
in the edgewise fatigue loads, which have reduced a little. This is due to the use of individual blade modes
rather than rotor modes, which tends to increase the damping in the edgewise direction due to the additional
component of aerodynamic damping. The use of blade modes rather than rotor modes is one of the major advantages
of the new structural model as it allows for the correct multidirectional mode shapes as well as the
correct modelling of individual blade modes for transient load cases.
FlapwiseM [%]
100
80
60
40
20
0
DLC
1.0
1.0
1.1
1.1
1.3
1.3
1.5
1.5
1.6
1.6
1.7
1.7
1.8
1.8
1.9
1.9
88
2.1
2.1
2.2
2.2
3.2
3.2
3.3
3.3
4.1
4.1
4.2
4.2
5.1
5.1
6.0
6.0
6.1
6.1
Figure 7.5: Blade root flapwise bending moment: extreme loads [98]
Datum MB
Cumulative cycles [.]
Figure 7.6: Blade root edgewise bending moment: cumulative cycles [98]
The final level of testing involved a code-to-measurement comparison. The CART2 research turbine at NREL in
Colorado was used to obtain measurements, primarily for the purpose of testing advanced control features [99].
A Bladed model was set up based on data supplied by NREL, with a number of assumptions where data was
not available (e.g. shaft torsional damping, teeter brake friction, pitch actuator model, rotor imbalance). Four
measured datasets were used to compare against simulations. Initial simulations were carried out using the
above-mentioned assumptions, as a result of which some adjustments were made to the assumptions before
the final runs.
The measured and simulated results were compared by means of spectral analysis, and also using rainflow
cycle counting to give an indication of fatigue loading. Figure 7.7 presents an example of tower top load spectra
for the four datasets. In general a very good level of agreement is demonstrated, with the spectral peaks representing
structural resonances corresponding very closely with the predicted frequencies of the coupled system
modes. There are some discrepancies, but in most cases this is likely to be due to the uncertainties in modelling
the wind field. The fatigue loading is sensitive to the turbulence model used in the simulations, and since
the detailed structure and coherence of the actual wind field could not be known it was not possible to produce
an exact fit. Differences are also present due to noise on the measured signals, and imbalances in the real turbine.
6.2
6.2
6.3
6.3
Datum
Multibody
7.1
7.1
8.1
8.1