1. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of
Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND No. SAND2015-9794 PE
Challenges and Opportunities for
Large-scale Floating Offshore
Vertical Axis Wind Turbines
D. Todd Griffith, PhD
Sandia National Laboratories
2016 Wind Turbine Blade Workshop
Albuquerque, NM
August 31, 2016
Document Number: SAND2015-9794 PE

2. Characteristics of Offshore Wind
 Opportunities
 Proximity to population centers
 Better winds
 Vast resource
 Scale-up opportunity
 Challenges
 High LCOE
 High BOS costs
 Accessibility
 Inexperience, Immaturity
2

3. Large reduction in Deepwater Offshore
COE may require non-incremental system
solutions
Floating VAWTs
Available at
www.sandia.gov/wind
Partners

4. Sandia Team: Roles and Responsibilities
4
T. Griffith
Project Coordinator
System Design, Dynamics & Loads, Cost Analysis
B. Owens VAWT Codes & VAWT Design
D. Bull
K. Ruehl
Carlos Michelen
B. Gunawan
S. Bredin
V. Neary
Floating Systems (platform and mooring)
Met ocean conditions
Wave characterization & analysis
Hydrodynamics codes
B. Ennis
K. Moore
VAWT aero-hydro-elastic simulations
Modal Analysis
Drivetrain Modeling
G. Bacelli Controls; Design Optimization; Dynamic Stability
M. Barone VAWT Aerodynamics
J. Paquette VAWT Structural Analysis

5. Sandia Wind Program: From 1970s to 1990s
5

6. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of
Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND No. SAND2015-9794 PE
Standards Applicable to Large-
scale Floating VAWTs

7. Standards and Load Cases
 IEC 61400-3 addresses offshore wind design conditions
 Gust with Direction Change
 VAWTS are less sensitive as tower clearance would not be an
issue and they aren’t as affected by direction changes
 Vertical Shear
 VAWTs are less sensitive to vertical wind shear as they
assimilate those changes along the height of the rotor.
 Extreme Gusts
 VAWTs may be more impacted as the total blades solidity will
likely be higher.
 Cut-Out
 VAWTs will likely be much more sensitive to cut-out load cases
because of stall control
7

8. Project Objectives
8
 Improve knowledge of the technical and economic feasibility of a floating
offshore VAWT system.
The most critical barrier to offshore wind, high Cost of Energy (COE), is
specifically targeted through a series of studies:
1. Technical Barriers & Standards
2. Rotor Design Studies
3. Platform and Mooring Design Studies
4. VAWT Design Codes
5. Innovations & Opportunities
6. LCOE Analysis

9. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of
Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND No. SAND2015-9794 PE
Rotor Design Studies

10. VAWT Structural Design Study
 Reduction of parameter space was essential at the early
stages of the design process:
 Architecture of the rotor
 Number of blades
 Chord length
 Material
 Blade tapering scheme
10

11. Structural Design Study: Configurations
Parameter
Values
Considered
Architecture Darrieus, V
Number of Blades 2, 3
Tip Chord Length 2m, 3m
Composite Material: Glass/Epoxy,
Carbon/Epoxy
Tapering Scheme
(Darrieus only, V-
VAWTS used Single
Taper)
No Taper, Single
Taper, Double
Taper
Curvature or Power
Law Exponent (V-
VAWT)
n=1, n=3, n=5
0
20
40
60
80
100
120
140
160
180
0 20 40 60
Height(m)
Radius (m)
Darrieus
V, n=1
V, n=2
V, n=3
V, n=4
V, n=5
ANSYS Beam
Models of D and V
VAWTS
D and V VAWT
Shapes

12. Rotor Aero Design Population
 24 Candidate Rotor Design External Shapes
 12 Darrieus :
 large/small chord
 single/double/no blade taper
 two/three blades
 12 “V”-Rotors :
 large/small chord
 power law shape exponent = 1/3/5
 two/three blades
 Constraints
 Max radius = 54 m
 Same capture area
 NACA 0021 airfoil section
12

13. VAWT Aerodynamic Design Analysis
13
AEP
Platform
Fatigue
Power curves
Parked Loads
Operating Loads
1 2
3 4
XY
Z
X Y
Z
X
Y
Z
X
Y
Z
JUL 13 2012
14:35:33
JUL 13 2012
14:35:33
JUL 13 2012
14:35:33
JUL 13 2012
14:35:33
DISPLACEMENT DISPLACEMENT
DISPLACEMENT DISPLACEMENT
STEP=1
SUB =5
FREQ=2.0265
DMX =.021155
STEP=1
SUB =5
FREQ=2.0265
DMX =.021155
STEP=1
SUB =5
FREQ=2.0265
DMX =.021155
STEP=1
SUB =5
FREQ=2.0265
DMX =.021155

14. CACTUS to OWENS Code Coupling Results
14
Configuration
Max Tower
Top Disp.
(m)
Max Blade
Total Disp.
(m)
% Tower
Top Disp.
% Blade
Disp. at
Mid-Span
DC2LCDT 2.14 2.05 1.6 3.1
DC3LCDT 2.68 2.35 2.0 3.6
DG2LCDT 7.53 4.41 5.7 6.7
DG3LCDT 1.26 0.95 1.0 1.4
VC2N5LC 0.23 1.92 0.4 1.9
VC3N5LC 0.23 1.66 0.4 1.6
 Ultimate strain allowable:
6000 micro-strain
 Fatigue strain allowable:
3500 micro-strain
Displacements:

15. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of
Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND No. SAND2015-9794 PE
Platform & Mooring Design
Studies

16. Platform Options
 Tailored solution to a floating VAWT
16

17. Spar
17
Semi-Submersible
Platform Sizing Study for
Range of Topside Masses

18. Dynamic Loading on Platform is Important for VAWTs
RPM 2P (seconds) 4P (seconds)
4 7.500 3.750
7.2 4.167 2.083
RPM 3P (seconds) 6P (seconds)
4 5.000 2.500
6.3 3.175 1.587
Two sources: Wave and VAWT Dynamics
VAWT periods: Due to oscillating VAWT loads (torque, pitch, and roll)
VAWT Periods
for 2 and 3
bladed
Darrieus
machines:
The lower per rev is dominant.
3 bladed VAWT has lower period of
primary loading (close to 3 seconds).

19. Comparison: Spar vs. Semisubmersible
19
HAWT
VAWT (with
equal mass of
600 mt)
VAWT (with
mass of 973
mt)
Spar-
buoy
Semi-
Sub
Spar-
buoy
Semi-
Sub
Spar-
buoy
Semi-
Sub
Topside Mass
(mt)
600 600 600 600 973 973
Platform Steel
Mass (mt)
2,000 2,900 1045 1708 1,500 2,370
Percent mass
reduction
versus HAWT
-- -- 48% 41% 25% 18%
Comparison of platform steel mass for two platform/floater
types and two topside types of equal power rating (5MW)
1. Fowler, M., Bull, D., and Goupee, A.: A Comparison of Platform Options for Deep-water Floating Offshore Vertical Axis
Wind Turbines: An Initial Study, Sandia National Laboratories Technical Report, SAND2014-16800, August 2014.
2. Griffith, D. T., Paquette, J., Barone, M., Goupee, A., Fowler, M., Bull, D., and Owens, B.: A study of rotor and platform
design trade-offs for large-scale floating vertical axis wind turbines, Science of making torque from wind conference,
Munich, Germany, 2016.

20. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of
Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND No. SAND2015-9794 PE
VAWT Design Codes
@ Sandia

21. Sandia VAWT Codes List
 Geometry/Modeling & Post-processing
 VAWTGen Code
 Aerodynamics
 CACTUS code
 Structural Dynamics
 OWENS code
 Features: Modal, Transient, Static
 Hydrodynamics
 WaveEC2Wire code
 Notes: Coupled with OWENS
21

22. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of
Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND No. 2012-5130P
Innovations and Analysis to Mitigate
Barriers and Design Challenges

23. Summary of Investigated Topics
1. Novel VAWT Airfoils (TU-Delft)
2. Aero-elastic Stability Analysis
3. Rotor-Platform Coupled Dynamic Stability
4. Manufacturing (ISU/Sandia)
5. Balance of Station Cost Reduction
6. Storm Survival and Load Alleviation
23

24. Is Aeroelastic Stability an issue for large-scale VAWTs?
0 5 10 15 20 25 30
-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
0.1
Rotor Speed (RPM)
DampingRatio
DG2LCDT
DG3LCDT
DC2LCDT
DC3LCDT
VC2N5LC
VC3N5LC
Owens B.C. and Griffith, D.T. “Aeroelastic Stability
Investigations for Large-scale Vertical Axis Wind
Turbines,” Science of Making Torque from Wind
Conference, Copenhagen, Denmark, June 2014
5 MW Design Studies
Trend with Increasing Blade Length:
Approaching Flutter Speed for
Horizontal Axis Wind Turbines (HAWTs

25. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of
Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND No. SAND2015-9794 PE
Deep-water
Offshore VAWT
LCOE Analysis

26. System LCOE Trends: AEP, RPM, Rotor vs Platform
26

27. Website and References (selected)
27
http://www.sandia.gov/wind
http://energy.sandia.gov/energy/renewable-energy/wind-power/offshore-
wind/innovative-offshore-vertical-axis-wind-turbine-rotors/
Publications
1. Sutherland, H.J., Berg, D.E., and Ashwill, T.D., “A Retrospective of VAWT Technology,” Sandia National Laboratories
Technical Report, SAND2012-0304, January 2012.
2. Owens, B., Hurtado, J., Barone, M., and Paquette, J., “An Energy Preserving Time Integration Method for Gyric Systems:
Development of the Offshore Wind Energy Simulation Toolkit” Proceedings of the European Wind Energy Association
Conference & Exhibition. Vienna, Austria, 2013.
3. Owens, B.C., Hurtado, J.E., Paquette, J., Griffith, D.T., and Barone, M., “Aeroelastic Modeling of Large Offshore Vertical-axis
Wind Turbines: Development of the Offshore Wind Energy Simulation Toolkit,” 54th AIAA/ASME/ASCE/AHS/ASC Structures,
Structural Dynamics, and Materials Conference, April 8–11, 2013, Boston, MA, USA, AIAA-2013-1552.
4. Owens, B.C., Griffith, D.T., and Hurtado, J.E., “Modal Dynamics and Stability of Large Multi-megawatt Deepwater Offshore
Vertical-axis Wind Turbines: Initial Support Structure and Rotor Design Impact Studies,” 32nd ASME Wind Energy
Symposium, National Harbor, MD, USA, January 2014.
5. Ragni, D., Simao-Ferreira, C., and Barone, M., “Experimental and Numerical Investigation of an Optimized Airfoil for Vertical
Axis Wind Turbines,” 32nd ASME Wind Energy Symposium, National Harbor, MD, USA, January 2014.
6. Fowler, M.J., Owens, B.C., Goupee, A.J., Hurtado, J.E., Griffith, D.T., and Alves, M., “Hydrodynamic Module Coupling in the
Offshore Wind Energy Simulation (OWENS) Toolkit,” Proceedings of the 33rd ASME International Conference on Ocean,
Offshore and Arctic Engineering (OMAE2014), June 8–13, 2014, San Francisco, California, USA, Paper OMAE2014-24175.
7. Owens, B.C., and Griffith, D.T., “Aeroelastic Stability Investigations of Large-scale Vertical Axis Wind Turbines,” Journal of
Physics Conference Series, Science of Making Torque from Wind Conference, June 18–20, 2014, Lyngby, Denmark.
8. Fowler, M., Bull, D., and Goupee, A, “A Comparison of Platform Options for Deep-water Floating Offshore Vertical Axis Wind
Turbines: An Initial Study,” Sandia National Laboratories Technical Report, SAND2014-16800, August 2014.
9. Griffith, D. T., Paquette, J., Barone, M., Goupee, A., Fowler, M., Bull, D., and Owens, B.: A study of rotor and platform design
trade-offs for large-scale floating vertical axis wind turbines, Science of making torque from wind conference, Munich,
Germany, 2016.

28. High-resolution
Offshore Wind
Farm Modeling
Offshore Wind @ Sandia
Deepwater
Offshore
VAWT
Offshore Siting Analysis
Large Offshore Rotors
Sensing,
Structural
Health, and
Prognostics
• Vision: Promote & accelerate the
commercial OW industry and reduce
costs through technical
innovation:
• Siting/Permitting: Sediment Transport & Radar
• Large offshore HAWT rotors
• Deepwater VAWT system
• Structural health and prognostics management
• Offshore wind farm modeling