IEA Wind Task 23 OC3: Phase IV Results Regarding Floating Wind Turbine Modeling NREL – Jason Jonkman MARINTEK – Ivar Fylling Risø-DTU – Torben Larsen GH – James Nichols Anders Hansen LUH – Martin Kohlmeier IFE – Tor Anders Nygaard Acciona – Javier Pascual Vergara UMB – Karl Jacob Maus Daniel Merino NTNU – Madjid Karimirad POSTECH – Wei Shi Zhen Gao Hyunchul Park Torgeir Moan Operated for the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy by the Alliance for Sustainable Energy, LLC

Floating Challenges & Phase IV Model Low frequency modes: Influence aerodynamic damping & stability Large platform motions: Coupling with turbine Complicated shape: Radiation & diffraction Moorings Statoil supplied data for 5-MW Hywind conceptual design OC3 adapted spar to support the NREL 5-MW turbine: Rotor-nacelle assembly unchanged Tower & control system modified Challenges OC3-Hywind OC3-Hywind Model

Aero-Hydro-Servo-Elastic Capabilities

Phase IV Load Cases

Output Parameters & Results Legend Drivetrain & Generator Loads & Operation 7 Outputs Rotor Blade Loads & Deflections 13 Outputs Tower Loads & Deflections 15 Outputs Environment Wind & Waves 4 Outputs Mooring System Fairlead & Anchor Tensions & Angles 12 Outputs Platform Displacements 6 Outputs Output Parameters (57 Total) Results Legend

Full-System Eigenanalysis

Free Decay Free Decay in Platform Surge Free Decay in Platform Pitch

Hydro-Elastic Response with Regular Waves

Hydro-Elastic Response with Irregular Waves

Aero-Hydro-Servo-Elastic Response with Regular Waves

Aero-Hydro-Servo-Elastic Response with Irregular Waves

Aero-Hydro-Servo-Elastic “Effective RAOs”

Unresolved Issues of OC3 Phase IV Close agreement was not achieved by all codes: What was the reason? The ”effective RAO” load case was somewhat ”academic”: What response charateristic is more relevant? Alternative suggested by IF — RAOs could be derived from irregular time series & cross spectra between excitation & response The stochastic response statistics & spectra are sensitive to simulation length: What length would be more appropriate? How can we eliminate start-up transients from the comparisons?

Limitations of OC3 Phase IV OC3-Hywind platform was considered as a rigid body; no hydro-elastic effects OC3-Hywind platform is simple in shape; only a single member Hydrodynamic radiation & diffraction was negligible in the OC3-Hywind spar buoy Sea current was never considered Few sea states were tested; larger waves may be interesting The relative importance of 2nd versus 1st order hydrodynamics was never assessed The relative importance of dynamic versus quasi-static mooring models was never assessed The influence of platform motion on rotor aerodynamics was never looked at in detail

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 the Alliance for Sustainable Energy, LLC

Background OWTs are designed using aero-hydro-servo-elastic codes The codes must be verified to assess their accuracy

OC3 Activities & Objectives The IEA Offshore Code Comparison Collaboration (OC3) is an international forum for OWT dynamics code verification Discuss modeling strategies Develop suite of benchmark models & simulations Run simulations & process results Compare & discuss results Assess simulation accuracy & reliability Train new analysts how to run codes correctly Investigate capabilities of implemented theories Refine applied analysis methods Identify further R&D needs Activities Objectives

OC3 Approach & Phases Approach Phases All inputs are predefined: NREL 5-MW wind turbine, including control system Variety of support structures Wind & wave datasets A stepwise procedure is applied: Load cases selected to test different model features OC3 ran from 2005 to 2009: Phase I – Monopile + Rigid Foundation Phase II – Monopile + Flexible Found’tn Phase III – Tripod Phase IV – Floating Spar Buoy 3-year follow-on project recently initiated: Phase V – Jacket Phase VI – Floating semisubmersible Approach Phases

Semisubmersible Concept Summary OC3 aims to verify OWT dynamics codes Simulations tested a variety of OWT types & model features Code-to-code comparisons have agreed well Differences caused by variations in: Model fidelity Aero- & hydrodynamic theory Model discretization Numerical problems User error Future work will consider offshore jacket & semisubmersible Verification is critical to advance offshore wind Spar Concept by SWAY Semisubmersible Concept