1. Offshore Code Comparison Collaboration, Continued (IEA Task 30): Phase II Results of a Floating Semisubmersible Wind System EWEA Offshore Conference November 20, 2013 Frankfurt, Germany Amy Robertson Senior Engineer, NREL NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

2. EWEA Offshore Wind Conference 2 National Renewable Energy Laboratory Background Offshore wind systems are designed using simulation tools that predict their loads and performance for prescribed environmental conditions. These tools are adaptations of ones used for: –Land-based turbines –Offshore structures It is important these tools capture the coupled behavior of the complete offshore wind system (blades to soil).

3. EWEA Offshore Wind Conference 3 National Renewable Energy Laboratory Offshore Wind Modeling Tools Soil Conditions Wind-inflow Waves & Currents External Conditions Aerodynamics Applied Loads Hydrodynamics System Response Nacelle Dynamics/Loads Tower Dynamics/Loads Substructure Dynamics/Loads Rotor Dynamics/Loads Mooring Dynamics/Loads Drivetrain Dynamics/Loads Control System Power Generation Soil/Struct. Interaction Foundation Dynamics/Loads

4. EWEA Offshore Wind Conference 4 National Renewable Energy Laboratory Verification of Offshore Wind Turbine (OWT) Simulation Tools Simulation tools for OWT are fairly new. They need to be verified to assess their accuracy, and give confidence in their usefulness to users. Two research tasks were initiated under IEA Wind to address this issue, the OC3 and OC4 projects. OC3 = Offshore Code Comparison Collaboration OC4 = Offshore Code Comparison Collaboration, Continued OC5 = Offshore Code Comparison Collaboration, Continued, with Correlation

5. EWEA Offshore Wind Conference 5 National Renewable Energy Laboratory IEA Wind Tasks IEA = International Energy Agency IEA Wind Tasks are cooperative research on issues affecting wind energy The technical results of Tasks are shared among participants. Final reports are often made public to benefit the entire wind energy community

6. EWEA Offshore Wind Conference 6 National Renewable Energy Laboratory 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 analysis methods Identify further R&D needs Activities Objectives OC3/OC4 Activities & Objectives

7. EWEA Offshore Wind Conference 7 National Renewable Energy Laboratory The OC3/OC4 Systems Examined Fixed-Bottom Floating

8. EWEA Offshore Wind Conference 8 National Renewable Energy Laboratory OC4 Phase II – Floating Semi-submersible Mooring Layout Flooding Geometry

9. EWEA Offshore Wind Conference 9 National Renewable Energy Laboratory CompanySimulation Tool 4SubseaOrcaFlex ABSCHARM3D + FAST CENEROPASS + FAST CENTECFAST CeSOS (NTNU)Simo+Riflex+Aerodyn CGCBladed 4.3 DHIWAMSIM DTUHAWC2 GHBladed 4.4 /Bladed Advanced Hydro Beta GoldwindFAST IFE3DFLOAT ISTFAST LMS-IRECSWT MARINTEKRIFLEX-Coupled NTUAhydro-GAST NRELFAST POSTECHGH Bladed PRINCIPIADeepLinesWT SWESIMPACK +HydroDyn Univ. of TokyoCAST Univ. of UlsanUOU + FAST WaveECWavec2Wire OC4 Phase II Participants & Tools

10. EWEA Offshore Wind Conference 10 National Renewable Energy Laboratory Load Cases 1.X – System Identification Eigenanalysis Static equilibrium Free decay 2.X – Inverted Pendulum Flexible support structure Rigid tower top Hydro-elastics without aero: –Regular & irregular waves –Current –RAOs 3.X – Full-System Analysis Full-system flexibility Full aero-hydro-servo-elastics: –Steady winds with regular waves –Turbulent winds with irregular waves –RAOs with wind –Damage scenarios o Sudden mooring loss o Flooded compartment

11. EWEA Offshore Wind Conference 11 National Renewable Energy Laboratory Hydrodynamic load model impacts global response behavior –Potential-flow terms Panel method (e.g., WAMIT) or Morison’s Equation (ME) –Viscous terms ME or quadratic drag (QD) matrix –Corrections Wave stretching Instantaneous position Dynamic pressure Mooring model impacts mooring loads –Quasi-static or dynamic OC4 Phase II – Key Findings

12. EWEA Offshore Wind Conference 12 National Renewable Energy Laboratory Results – LC 2.1 – Regular Waves For both ME and PF approaches, mean drift loads need to be considered. Platform Surge Motion (m)

13. EWEA Offshore Wind Conference 13 National Renewable Energy Laboratory Results – LC 3.1 – Regular Wind/Waves Tower Base Bending Moment (kN-m)

14. EWEA Offshore Wind Conference 14 National Renewable Energy Laboratory Results – LC 3.8 – Mooring Line Loss Trajectory of system after mooring loss After mooring loss – floats in opposite direction of anchor

15. EWEA Offshore Wind Conference 15 National Renewable Energy Laboratory Extension of the OC4 project – additional 4 years Three phases – examining three different systems Compare simulation results to physical response data OC5 – Simulation Tool Validation Monopile - Tank Testing Semi - Tank Testing Jacket/Tripod – Open Ocean

16. EWEA Offshore Wind Conference 16 National Renewable Energy Laboratory OC3/OC4 aims to verify OWT dynamics codes Benchmark models & simulations established Simulations test a variety of OWT types & model features Code-to-code comparisons have agreed well Differences caused by variations in: –Model fidelity –Aero-, hydro-, & structural-dynamic theories –Model discretization –Numerical problems –User error Many code errors have been resolved Engineers equipped with modeling experience Verification is critical to advance offshore wind Spar Concept by SWAY Summary

17. Operated for the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy by Midwest Research Institute Battelle NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC. Amy Robertson Amy.Robertson@nrel.gov Thank You for Your Attention

18. Operated for the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy by Midwest Research Institute Battelle Additional Slides Available

19. EWEA Offshore Wind Conference 19 National Renewable Energy Laboratory OC4 Phase II – Key Findings ME under-predicts heave response if no dynamic pressure term is included.

20. EWEA Offshore Wind Conference 20 National Renewable Energy Laboratory OC4 Phase II – Key Findings Viscous drag loads approximated using a quadratic drag matrix (for PF solutions) need to include diagonal terms to capture coupling between DOFs.

21. EWEA Offshore Wind Conference 21 National Renewable Energy Laboratory OC4 Phase II – Key Findings For both ME and PF approaches, mean drift loads need to be considered.

22. EWEA Offshore Wind Conference 22 National Renewable Energy Laboratory OC4 Phase II – Key Findings Dynamic mooring models show more frequencies in the mooring loads, different phase, and higher magnitude as compared to a quasi-static model.