1. Verification of Offshore Wind Modeling Tools through IEA Wind Tasks 23 and 30
ICOWEOE
October 31, 2011
Beijing China
Amy Robertson, Ph.D.
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. Outline What are IEA Tasks Overview of IEA Wind Tasks 23 and 30
Status and results of these tasks

3. 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

4. IEA Wind Tasks Country Commitments
To be a participant of an IEA Wind Task, your country must first have general membership in IEA Wind.
Then, interested countries must join an individual task. This requires a fee to support the operating agents of that task.
Spar Concept by SWAY

5. IEA Wind Tasks Tasks 23 and 30
IEA Wind Task 23 : Subtask 2 of this task was the Offshore Code Comparison Collaboration (OC3)
Ran from 2005 to 2009
IEA Wind Task 30: The Offshore Code Comparison Collaboration Continuation (OC4)
Continues work of OC3 project
Will run from 2010 to 2012
Focus is on offshore wind turbine (OWT) code verification & benchmarking, with emphasis on the support structure
Spar Concept by SWAY

6. The OC3 & OC4 Projects Background
Offshore Wind Turbines (OWTs) are designed using aero-hydro-servo-elastic codes
The codes must be verified to assess their accuracy
OC3 and OC4 approach verification through code-to-code comparisons

7. The OC3 & OC4 Projects Background
IEA Task Wind 30 (OC4) is a follow-on task to IEA Wind Task 23 (OC3)
OC4 will continue the examination of more OWT configurations
Floating Spar Buoy
Monopile
Tripod

8. OC3/OC4 Activities & Objectives
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

9. OC3 Participants & Codes
3Dfloat
ADAMS-AeroDyn-HydroDyn
ADAMS-AeroDyn-WaveLoads
ADCoS-Offshore
ADCoS-Offshore-ASAS
ANSYS-WaveLoads
BHawC
Bladed
Bladed Multibody
DeepC
FAST-AeroDyn-HydroDyn
FAST-AeroDyn-NASTRAN
FLEX5
FLEX5-Poseidon
HAWC
HAWC2
SESAM
SIMPACK-AeroDyn
Simo

10. Aero-Hydro-Servo-Elastic Capabilities

11. OC3/OC4 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
OC4 will run from 2010 to 2012:
Phase I: Jacket
Phase II: Floating semisubmersible
Approach
Phases

12. NREL 5-MW Baseline Wind Turbine
Specifications developed for a representative multi-megawatt wind turbine
Heavily influenced by the REpower 5M prototype & DOWEC project turbines
Properties developed:
blade structural & aerodynamic properties
nacelle & hub
drivetrain
tower
control system:
Also used as UpWind reference model
REpower 5M Wind Turbine
Gross Properties Chosen for the Baseline Turbine

13. Load Cases 1.X – Full-System Eigenanalysis 2.X – Rigid
Full-system flexibility
Elastic response only
Compared natural frequencies & damping ratios
2.X – Rigid
Rigid turbine
Aerodynamics without hydro:
Steady & turbulent winds
Hydrodynamics without aero:
Regular & irregular waves
3.X – Onshore Wind Turbine
Flexible tower, drivetrain, & rotor
Rigid substructure
Aero-servo-elastics without hydro:
4.X – Inverted Pendulum
Flexible support structure
Rigid tower top
Hydro-elastics without aero:
Regular & irregular waves
5.X – Full-System Dynamics
Full-system flexibility
Full aero-hydro-servo-elastics:
Steady winds with regular waves
Turbulent winds with irregular waves

14. Output Parameters

15. Overview of OC3 Project Phases & Status
Phase I – Monopile + Rigid Foundation, 20 m water depth
Winter 2005 – Summer 2006
Paper presented at Science of Making Torque from Wind, 2007
Phase II – Monopile + Flexible Foundation, 20 m water depth
Summer 2006 – Fall 2007
Paper presented at European Offshore Wind, 2007
Phase III – Tripod, 45 m water depth
Winter 2007 – Fall 2008
Paper presented at AIAA, 2009
Phase IV – Floating Spar, 320 m depth
Summer 2008 – Winter 2009
Paper presented at EWEC, 2010
Final Report – Spring 2010 – NREL Report #TP
Summary journal article under review for Wind Energy
Monopile
Tripod
Spar

16. OC3 Phase I Results Monopile
Modal-based codes predict different 2nd (& higher) eigenmodes than the higher fidelity multibody- & FEM- based codes
Codes using full-field wind in polar coordinates predict smoother aerodynamic loads than codes using rectangular coordinates:
Because of the method used to generate the wind datasets
Differences in implementation of aerodynamic models attributed to variations among the codes in mean turbine loads
Differing model discretizations lead to differing code predictions:
Most apparent in substructure loads that depend highly on the discretization of hydrodynamic loads near the free surface
User error happens

17. Review of Phase I (Monopile) Results Full-System Eigenanalysis

18. Review of Phase I Results Onshore Turbine with Turbulent Winds
Flexible tower, drivetrain, & rotor
Aerodynamics with control system
Full-field turbulent wind with: Vhub = 11.4m/s, TI1 = 17.4%

19. Tripod (Image: J. Nichols, GH)
OC3 Phase III Results
Tripod
Jump in complexity from monopile to tripod:
Multiple members
Statically indeterminate (loads influenced by relative deflection of members)
Nonaxisymmetric
Key findings:
A fine discretization of hydrodynamic loads is required near the free surface
Overlapping regions where structural members join at nodes have a large effect
Despite having thin members, shear deflection through Timoshenko beam theory has a large effect on the tripod load distribution
Tripod (Image: J. Nichols, GH)
Spar Concept by SIWAY

20. OC3 Phase III Results Eigenanalysis

21. OC3 Phase IV Results Spar Buoy
Jump in complexity from fixed to floating:
Low-frequency modes (influence on aerodynamic damping & stability)
Large platform motions (coupling with turbine)
Complicated shapes (radiation & diffraction)
Moorings (new component)
Key findings (may only apply to this spar):
Radiation damping is negligible; so, codes that apply Morison’s equation are adequate
Quasi-static mooring models provide adequate reactions for global response analysis; dynamic mooring models, however, result in more line excitation at higher frequencies
Turbine structural flexibilities had an effect on turbine loads, but little effect on spar motions
Coupled dynamics issues not fully resolved
OC3-Hywind Spar Buoy
Spar Concept by SIWAY

22. Overview of OC4 Status of Phases
Phase I: Jacket
Nearing completion of analysis
Technical paper to be presented at International Offshore and Polar Engineering Conference (ISOPE) in Rhodes, Greece, June17- 23, 2012
Phase II: Semi-submersible
DeepCwind semi design will be used
Design specification document will be presented at next face-to-face meeting on December 2, 2011

23. UpWind WP4 Reference Jacket (Images: F. Vorpahl,
OC4 Phase I
Jacket
Title: Verification of simulation codes for a jacket-supported fixed-bottom WT
Coordinator: Fraunhofer-IWES
Rambøll has kindly agreed to make the UpWind WP4 reference jacket available to OC4 participants:
Jacket supports the NREL 5-MW turbine
4 legs, 4 levels of X braces, & mud braces
Concrete Transition Piece (TP) & 4 central piles
50-m water depth
UpWind WP4 Reference Jacket (Images: F. Vorpahl,
Fraunhofer-IWES)
Spar Concept by SWAY

24. Output Locations on Jacket for Response Comparison
Transition piece and grout connection to tower

25. UpWind WP4 Reference Jacket (Images: F. Vorpahl,
OC4 Phase I
Status
Nearing completion of analysis
Technical paper to be presented at International Offshore and Polar Engineering Conference (ISOPE) in Rhodes, Greece, June17- 23, 2012
UpWind WP4 Reference Jacket (Images: F. Vorpahl,
Fraunhofer-IWES)
Spar Concept by SWAY

26. OC4 Phase II Plans Semi-submersible
Title: Verification of simulation codes for a WT on a floating semi-submersible
Coordinator: NREL
OC4 participants have chosen to use the DeepCwind semi-submersible design
A generic, publically available design
Wave-tank tested at 1/50th scale in May 2011
Code-to-code comparison results will be published in a conference paper in 2012/2013
Spar Concept by SWAY
DeepCwind
Semi-submersible

27. OC4 Coordination & Meetings
coordination
Meetings
Net-meetings held every 1-2 months
Physical meetings held 1-2 times
per year
3rd physical meeting held at ISOPE
Conference in Hawaii, USA, June 2011
Next meeting at EWEA Offshore 2011
in Amsterdam, Netherlands, November
IEA Wind Task 23 and 30
SharePoint website:
Hosted by NREL
Includes meeting presentations, minutes, model and load case descriptions, and simulation results
Password required to access site, and certain directories open only to IEA Wind Task 30 members

28. OC4 Code Validation Experts Meeting
OC4 focuses on code-to-code verification; code-to-experiment validation also needed
OC4 participants don’t comprise all experts needed to develop field validation plan
Separate meeting proposed for 2011
NREL (Walt Musial) will organize the meeting
Meeting objectives:
Catalogue existing datasets for selected configurations available for code validation
Establish methods for collecting new data
Make recommendations for R&D agencies that may want to support the effort as a follow-on task
Stand-alone report will be published
Image: J. ‘t Hooft,
SenterNovem

29. Summary 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
Semi-submersible
Concept

30. Thank You for Your Attention
Amy Robertson
+1 (303) 384 – 7157
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.

31. OC3 Phase II Results Foundation Modeling
Foundation designed by applying realistic soil properties & typical design procedures
Design made to have a noticeable effect on the system’s dynamic response
Needed for facilitation of model testing
Nonlinear + depth-dependent p-y model
Apparent Fixity
(AF) Model
Coupled Springs
(CS) Model
Distributed Springs
(DS) Model
Soil Profile
The p-y model not necessarily valid for transient analysis
Simplified models derived to give equivalent response under particular conditions
Simplified Models of a Monopile with Flexible Foundation

32. OC3 Phases II Results Overview
All of the results of Phase I also apply to results of Phase II
The simplified foundation models can be implemented so as to ensure that the overall response of the system above the mudline is the same under a given set of loading conditions:
At least for the lowest system eignemodes
Model discretization problems result in higher excitation in the 2nd eigenmodes of the support structure:
This is only so when the turbine is not operating, because of the aerodynamic damping while operating
Differences in aerodynamic models have more effect on the mean values of loads than on power spectra

33. IEA Task 23 Organizational Structure

34. OC4 Project Schedule
Spar Concept by SWAY

35. OC4 Organizational Structure

36. 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

37. 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

38. Phase IV Load Cases

39. 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

40. 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?

41. 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