A Quantitative Comparison of Three Floating Wind Turbines

1. A Quantitative Comparisonof Three Floating Wind Turbines NOWITECH Deep Sea Offshore Wind Power Seminar January 21-22, 2009 Jason Jonkman, Ph.D. Operated for the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy by Midwest Research Institute • Battelle

2. Offshore Wind Technology Onshore ShallowWater 0m-30m Transitional Depth 30m-60m Deepwater60m+

3. Floating Wind Turbine Pioneers

4. Floating Wind Turbine Concepts • Design Challenges • Low frequency modes: • Influence on aerodynamic damping & stability • Large platform motions: • Coupling with turbine • Complicated shape: • Radiation & diffraction • Moorings, cables, & anchors • Construction, installation & O&M

5. Modeling Requirements • Coupled aero-hydro-servo-elastic interaction • Wind-inflow: • Discrete events • Turbulence • Waves: • Regular • Irregular • Aerodynamics: • Induction • Rotational augmentation • Skewed wake • Dynamic stall • Hydrodynamics: • Diffraction • Radiation • Hydrostatics • Structural dynamics: • Gravity / inertia • Elasticity • Foundations / moorings • Control system: • Yaw, torque, pitch

6. Coupled Aero-Hydro-Servo-Elastics

7. Floating Concept Analysis Process • Run IEC-style load cases: • Identify ultimate loads • Identify fatigue loads • Identify instabilities • Compare concepts against each other & to onshore • Iterate on design: • Limit-state analysis • MIMO state-space control • Evaluate system economics • Identify hybrid features that will potentially provide the best overall characteristics • Use same NREL 5-MW turbine & environmental conditions for all • Design floater: • Platform • Mooring system • Modify tower (if needed) • Modify baseline controller(if needed) • Create FAST / AeroDyn / HydroDyn model • Check model by comparing frequency & time domain: • RAOs • PDFs

8. Three Concepts Analyzed NREL 5-MW on OC3-Hywind Spar NREL 5-MW on ITI Energy Barge NREL 5-MW on MIT/NREL TLP

9. Sample MIT/NREL TLP Response

10. Design Load Case Table Summary of Selected Design Load Cases from IEC61400-1 & -3

11. Normal Operation: DLC 1.1-1.5 Ultimate Loads Blade Root Bending Moment Low-Speed Shaft Bending Moment Yaw Bearing Bending Moment Tower Base Bending Moment

12. Floating Platform Analysis Summary • MIT/NREL TLP • Behaves essentially like a land-based turbine • Only slight increase in ultimate & fatigue loads • Expensive anchor system • OC3-Hywind Spar Buoy • Only slight increase in blade loads • Moderate increase in tower loads; needs strengthening • Difficult manufacturing & installation at many sites • ITI Enery Barge • High increase in loads; needs strengthening • Likely applicable only at sheltered sites • Simple & inexpensive installation

13. Ongoing Work & Future Plans • Assess role of advanced control • Resolve system instabilities • Optimize system designs • Evaluate system economics • Analyze other floating concepts: • Platform configuration • Vary turbine size, weight, & configuration • Verify under IEA OC3 • Validate simulations with test data • Improve simulation capabilities • Develop design guidelines / standards Spar Concept by SWAY Semi-Submersible Concept

14. Model Verification through IEA OC3 • The IEA “Offshore Code Comparison Collaboration” (OC3) is as an international forum for OWT dynamics model verification • OC3 ran from 2005 to 2009: • Phase I – Monopile + Rigid Foundation • Phase II – Monopile + Flexible Foundation • Phase III – Tripod • Phase IV – Floating Spar Buoy • Follow-on project to be started in April, 2010: • Phase V – Jacket • Phase VI – Floating semi submersible

15. OC3 Activities & Objectives • Discussing modeling strategies • Developing a suite of benchmark models & simulations • Running the simulations & processing the results • Comparing & discussing the results • Assessing the accuracy & reliability of simulations to establish confidence in their predictive capabilities • Training new analysts how to run & apply codes correctly • Investigating the capabilities / limitations of implemented theories • Refining applied analysis methodologies • Identifying further R&D needs Activities Objectives

16. Thank You for Your Attention Jason Jonkman, Ph.D. +1 (303) 384 – 7026 jason.jonkman@nrel.gov Operated for the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy by Midwest Research Institute • Battelle

17. Normal Operation: DLC 1.2 Fatigue Loads Side-to-Side Fore-Aft Side-to-Side Fore-Aft Out-of-Plane In-Plane 0° 90° Blade Root Bending Moments Yaw Bearing Bending Moments Tower Base Bending Moments Low-Speed Shaft Bending Moments

18. Idling: DLC 6.2a Side-to-Side Instability • Aero-elastic interaction causes negative damping in a coupled blade-edge, tower-S-S, & platform-roll & -yaw mode • Conditions: • 50-yr wind event for TLP, spar, & land-based turbine • Idling + loss of grid; all blades = 90º; nacelle yaw error = ±(20º to 40º) • Instability diminished in barge by wave radiation • Possible solutions: • Modify airfoils to reduce energy absorption • Allow slip of yaw drive • Apply brake to keep rotor away from critical azimuths

19. Idling: DLC 2.1 & 7.1a Yaw Instability • Aero-elastic interaction causes negative damping in a mode that couples rotor azimuth with platform yaw • Conditions: • Normal or 1-yr wind & wave events • Idling + fault; blade pitch = 0º (seized), 90º, 90º • Instability in TLP & barge, not in spar or land-based turbine • Possible solutions: • Reduce fully feathered pitch to allow slow roll while idling • Apply brake to stop rotor