1. Application of two passive strategies on the load mitigation of large offshore wind turbines
Rasoul Shirzadeh and Martin Kühn
ForWind – Center for Wind Energy Research, Oldenburg, Germany
Wind Europe Summit 2016
September 29, 2016
Good morning everybody, Novel 10MW offshore wind turbines need to have larger tower and blades to capture maximum energy from the wind.
This means that the support structure experiences larger bending forces and moments resulted from the wind and wave actions. We should find out a way to mitigate external loads applied on the structure.
This is the topic of this presentation. The title as you can see is the Application …

2. Outline Introduction Description of performed studies
Numerical modeling of INNWIND.EU 10 MW RWT
Load mitigation concepts
Implementation of two passive strategies
Results
Conclusions
During this presentation, I am going to introduce the current challenges we face for the design of the support structure in large offshore wind turbine.
Then I describe the performed studies by explaining the numerical simulations
Later on, two mitigation approaches will be discussed and the procedure to implement them in the tower structure is explained.
Afterwards, the results will be shown and finally some conclusions will be drawn. 2

3. Introduction
The rotor diameter and the tower height sizes are pushing the engineering limits!
Direct upscaling of support structure from 5 MW reference wind turbine  rotor-tower resonance problem
For large offshore wind turbines, the rotor diameter and tower height are exceeding 100 m which is towards the engineering limits.
A proper design is required to avoid resonances between the first eigenfrequency of the support structure and excitations from waves or rotor-blades harmonic frequencies.
- However, the resonance problem is the consequence of direct upscaling of both support and rotor design.
As mentioned before, larger blades and supporting structure means higher bending forces and moments we see at the tower base.
- To have a better view on these challenges, design trends of the first eigenfrequency of wind turbines are plotted.
As you can see, the first eigenfrequency of WTs mounted on monopile foundations are far away from the blade passing frequency range. However, the application of monopile foundations is limited up to 6-8 MW class. For larger WTs in deep water locations, the most economic alternative is the jacket structure. For these structures as you can see from this figure, the fundamental eigenfrequency of the support structure overlaps with the blade passing 3P frequency. This causes the dynamic excitation of the structure. Therefore, the OWT dynamics need to be reconsidered when upscaling the structure.
Monopile foundations are limited up to 6-8 MW class
Jacket structure is the most economic option for large wind turbines
A strong and severe 3P resonance is expected for WTs with jacket foundation 3

4. Campbell diagram INNWIND.EU 10 MW Reference Wind Turbine
Coincidence of the 3P mode and the first fundamental mode at 5.7 rpm  dynamic excitation
Solution: mitigation via control strategy using an exclusion zone between 5.2 and 6.3 rpm
At early stage of the support structure design, the dynamics of wind turbines can be estimated by the Campbell diagram.
The Campbell diagram plots the rotor rotational speed frequency (1P) and blades harmonic frequencies (3P, etc.) as well as the eigenfrequencies of the entire structure are plotted over the rotor speed.
This plot shows the Campbell diagram of the 10MW INNWIND RWT with a jacket structure. This is in agreement with what I explained previously.
At rotor speed of 5.7 rpm a resonance between the blade passing frequency and 1st fundamental mode of the support structure is observed. To reduce the dynamic excitations, a rotor speed exclusion zone between rpm is considered.
speed exclusion zone 4

5. Overall objectives and contributions
Two passive strategies are developed for large offshore wind turbines.
 Passive concepts: More simple manufacturing, lower maintenance and repair costs
Assessment of the lifetime extension for integrated damping devices
 Total weight and fatigue loads can be reduced by load mitigation concepts
Numerical modeling of a viscous fluid damper for wind turbine application
To summarize, the overall adjectives and contributions of this work can be expressed as below:
Passive = less money, maintenance and repair costs 5

6. Numerical simulations
OWT type: INNWIND.EU 10MW
Aeroelastic simulations: DNV.GL Bladed
Foundation: 4-leged jacket structure
DLC 1.2 according to IEC standard for operational condition
Wind-wave misalignment: 0°
10 min simulations with 6 random seeds
Post-processing: Fatigue Limit State (FLS)
tower base 6

7. Load mitigation strategies
Jacket specific control
Structural control and regulation
e.g.: active tower damping (collective pitch control,
individual pitch control, generator torque, active idling)
Damping devices, e.g. passive or (semi)-active
collective pitch
individual pitch
generator torque
Structural control
In principle, there are two different approaches for the load mitigation. The first one is based on the structural control and regulation concepts while the second one is mainly focused on different damping systems. For the first concept, several well known approaches are tested and approved in the industry. For example, the collective pitch control can be used to control vibrations in the fore-aft direction. The individual pitch regulation is used for controlling the structure in both FA and SS directions and the generator torque for the excitations in SS direction.
During the second category, a damping system is designed and inserted in the structure to reduce the external applied forces. Depending on the requirements, this damper system can be passive, semi active or active.
Rambøll
Interface loads
[O. Altay et al, RWTH Aachen, EURODYN 2014]
Rambøll 7

8. Load mitigation concepts
1. Tuned Mass Damper (TMD):
2-4% of modal mass is common for the mass of damper
Effective only at one frequency
Already implemented and tested in wind turbines
[Source:
2. Viscous Fluid Damper (VFD):
Tested and used in civil structures
Effective at wide frequency bandwidth
Strong potential to be used in large wind turbines
[S. Y. Chu, Active, Hybrid and Semi-active structural control] 8

9. Implementation of passive strategies
Tuned Mass Damper (TMD)
Tower base damage equivalent loads
fore-aft
side-to-side
TMD is more effective in sideway direction: low aerodynamic damping in SS direction 9

10. Implementation of passive strategies
Viscous Fluid Damper (VFD)
with
C0: damping coefficient
u: relative displacement at attachement points of the damper
FD: damper force, α: damper nonlinearity
The mechanism to calculate the viscous damper force:
1- The simulated time series of accelerations at the ending points of the damper where it is attached to the tower are recorded.
2- Accelerations are transferred to a Simulink model of a VFD via an external Dynamic-Link Library (DLL).
3- The damper force, FD, is then obtained in Simulink using both velocities and the effective damping coefficient.
4- This force is divided in two and returned to the Bladed whereas it will be applied as reaction forces at the damper
DNV.GL Bladed
Simulink
Viscous Fluid Damper (VFD) model
No. of dampers
Dynamic Link Library (DLL)
Tower accelerations
Effective damper coefficients
Damper forces 10

11. Results: Viscous Fluid Damper
Nacelle displacement at wind speed of 12 m/s
The tower top vibration, particularly in the sideways direction, is considerably dissipated compared to the reference configuration.
fore-aft
side-to-side
Three viscous dampers in 0o, 120o and 240o
Tower top vibrations are dissipated mainly in the sideways direction 11

12. Results: Viscous Fluid Damper
Tower base damage equivalent loads
fore-aft
side-to-side
Although the exclusion zone is activated, resonances are observed at wind speed range of 6-8 m/s
Maximum load reduction = 69% in sideway direction at wind speed of 6 m/s
Minimum load reduction = near the rated wind speed in the fore-aft direction 12

13. Results: comparison of damping strategies
Normalised lifetime weighted DELs calculated for different damping strategies
The lifetime weighted equivalent loads of the tower base moments for the reference turbine are plotted here.
The values are normalised with respect to the DEL of the reference turbine in the fore-aft direction.
Best improvement observed for the viscous fluid damper.
TMD with 2% damping ratio: DEL reduces by 4% and 24.3% in fore-aft and side-side directions, respectively.
VFD: DEL reduces by 17.4% and 31% in fore-aft and side-to-side directions, respectively. 13

14. Conclusions
DELs in the sideways direction can be lowered up to 29% and 69% compared to the reference design using respectively a TMD and VFD.
The impact of the TMD in the fore-aft direction is not significant.
The VFD mitigates loads in both directions.
The integration of the innovative concepts could have a positive impact on the lifetime of the system.
The integration of the innovative concepts could result in an optimized jacket design with reduced entire mass and applied loads.
Two damping concepts for the 10MW INNWIND.EU reference wind turbine are considered: a passive tuned mass damper (TMD) and a toggle brace viscous fluid damper (VFD). 14

15. Acknowledgment
The research leading to these results has received funding from the European Community’s Seventh Framework Programme FP7-ENERGY STAGE under grant agreement No (INNWIND.EU).
This research was supported by European Community under the INWWIND project. We are thankful for their financial support. 15

16. Thanks for your attention!