OC3 Phase II load case 1.2
We have separated this load case from the rest of the OC3 Phase II benchmarks because this load case compares eigenfrequencies, which we compare differently than the load cases that compare time series.
1 Test description
In this load case, we compare the eigenfrequencies of the model between aeroelastic software. The results are given in figure 20 in Jonkman et al. (2010g).
2 Model
The model is the Offhsore NREL 5-MW reference wind turbine, which corresponds to the offshore template in Ashes. The monopile has been modified according to the specification document to model the Apparent Fixity (AF) and the Distribted Springs (DS) behaviours.
3 Benchmarks
In the following sections, the different mode shapes are illustrated.
3.1 First Support Structure fore-aft
3.2 First Support Structure side-to-side
3.3 First Drivetrain torsion
3.4 First Blade Collective Flap
3.5 First Blade Asymmetric Flapwise Pitch
3.6 First Blade Asymmetric Flapwise Yaw
3.7 First Blade Asymmetric Edgewise Pitch
3.8 First Blade Asymmetric Edgewise Yaw
3.9 Second Support Structure Fore-Aft
3.10 Second Support Structure Side-to-Side
3.11 Second Blade Collective Flap
3.12 Second Blade Asymmetric Flapwise Pitch
3.13 Second Blade Asymmetric Flapwise Yaw
4 Results
The eigenfrequences of the model are solved and compared to those presented in the OC3 Phase II project. Note that the numbering of the eigenfrequencies in Ashes is from the lowest frequency to the highest, which is not the same as in the OC3 Phase II project. The table below summarises the numbering correspondance.
| Mode number in Ashes | Mode name | Mode number in OC3 Phase II |
| 1 | 1st Support Structure Side-to-Side | 2 |
| 2 | 1st Support Structure Fore-Aft | 1 |
| 3 | 1st Drivetrain torsion | 3 |
| 4 | 1st Blade Asymmetric Flapwise Yaw | 6 |
| 5 | 1st Blade Asymmetric Flapwise Pitch | 5 |
| 6 | 1st Blade Collective Flap | 4 |
| 7 | 1st Blade Asymmetric Edgewise Pitch | 7 |
| 8 | 1st Blade Asymmetric Edgewise Yaw | 8 |
| 9 | 2nd Support Structure Fore-Aft | 9 |
| 10 | 2nd Support Structure Side-to-side | 10 |
| 11 | 2nd Blade Asymmetric Flapwise Yaw | 13 |
| 12 | 2nd Blade Asymmetric Flapwise Pitch | 12 |
| 13 | 2nd Blade Collective Flap | 11 |
A bar plor with the eigenfrequencies from the different participants in the OC3 Phase II project and from Ashes is available on the following link.
Note that only a visual check is performed here.