A Quantitative Comparison of Three Floating Wind Turbines AWEA Offshore Wind Project Workshop December 2-3, 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

Offshore Wind Technology Onshore Shallow Water 0m-30m Transitional Depth 30m-60m Deepwater 60m+

Floating Wind Turbine Pioneers Developer StatoilHydro, Norway Blue H, Netherlands Principle Power, USA SWAY, Norway Platform “Hywind” spar buoy with catenary moorings Tension-leg concept with gravity anchor “WindFloat” semi-submersible with catenary moorings Spar buoy with single taut tether Wind Turbine Siemens 2.3-MW upwind, 3-bladed Gamma 2-bladed, teetering, yaw-regulated Coordinating with suppliers for 5-MW+ units Swivels downwind Partnering with Multibrid Status $78M demonstration project in North Sea First PoC installed in Summer 2009 Plans to license technology Deployed PoC system with 80-kW turbine in Italy in summer 2007 Receiving funding from ETI for UK-based projects Extensive numerical modeling Tested in wave tank Planning demonstration projects

Floating Wind Turbine Concepts + relative advantage 0 neutral – relative disvantage TLP Spar Barge Pitch Stability Mooring Ballast Buoyancy Natural Periods + – Coupled Motion Wave Sensitivity Turbine Weight Moorings Anchors Construction & Installation O&M 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

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

Coupled Aero-Hydro-Servo-Elastics

Floating Concept Analysis Process 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 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

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

Sample MIT/NREL TLP Response

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

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

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

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

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

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

DLC 6.2a Side-to-Side Instability 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

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