1. Aero-Hydro-Servo-Elastic Analysis of Floating Wind Turbines with Tension Leg Moorings
Erin Bachynski, PhD candidate at CeSOS
May 15, 2013
CeSOS – Centre for Ships and Ocean Structures

2. Floating wind turbine concepts studied at CeSOS
We need to understand floating wind turbine behavior so that we can bring the cost down
Make use of offshore wind resource
Spar
Semi-submersible
TLP

3. Tension Leg Platform (TLP)
Stability from tension legs,
implying motions as an inverted pendulum
Small motions (+)
Flexible w.r.t. water depth (+)
Smaller steel weight (+)
Small footprint area on seabed (+)
Challenging installation (-)
Don’t spend too much time on the analysis challenges now

4. TLPWT Design Displacement Pontoon radius Tendons Shimada, 2011
MIT-NREL TLPWT (Matha, 2009)
Botta, 2009
Shimada, 2011
Moon, 2010
Displacement
Increases cost
Decreases risk of slack
Pontoon radius
Increases stability
Increases hull loads
Tendons
Inkluder et lysark som viser alternative design
Kanskje også et kombinert WT+WEC konsept
på samme lysark, samt kanskje resultater senere

5. Integrated aero-hydro-servo-elastic analysis
control
aerodynamics
structural dynamics
Challenges:
-complexity
-tight coupling
-nonlinear
-time domain
-long term periods
-transient (faults)
hydrodynamics
Jeg vil også vise denne og kommentere
kompleksiteten i dynamisk analyse –
samtidig som vindturbiner representerer den ultimate
målsetting for CeSOS – på kombinere hydro., konstr. dyn. og
reguleringsteknikk (”control”)
Mention
Software ?
Source: NREL/Wind power today, 2010.

6. Aerodynamics J. de Vaal, 2012 Our approach: BEM
Challenges: yawed flow, engineering approximations, wind input
Mention Jabus’ work with more sophisticated models, Mahmoud with icing
J. de Vaal, 2012

7. Control system Serves to Actions regulate rotor rotation speed
regulate power output
protect structure
Actions
Change generator torque
Change blade pitch

8. Blade pitch mechanism failures
Contribution to failure rate (failures/turbine/yr) (%)
Jiang, 2012
PhD candidates at CeSOS
studying the effects of control system failures on
different platforms :
Z. Jiang, M. Etemaddar,
E. Bachynski, M. Kvittem,
C. Luan, A. R. Nejad
– many faults occur with an annual rate of per year
Pitch system
Wilkinson et al., 2011

9. What happens if one blade stops pitching?
TLP, U=20m/s, Hs = 4.8m, Tp = 10.8s
Continue operating with faulted blade
Fault occurs
Shut down turbine quickly

10. Comparison of controller fault effects on different platforms
Spar
TLP
Semi-Sub 1
Semi-Sub 2

11. Environmental/ Fault Conditions
Definition A
No fault B
Blade seize C
Blade seize + shutdown D
Grid loss + shutdown
Environmental/ Fault Conditions EC
U (m/s)
Hs (m)
Tp (s)
Turb. Model
Faults
# Sims.
Sim. length* (s) 1
8.0
2.5
9.8
NTM
A, B, C, D 30
16 min. 2
11.4
3.1
10.1 3
14.0
3.6
10.3 4
17.0
4.2
10.5 5
20.0
4.8
10.8 6
49.0
14.1
13.3
A (idling)
3 hours 7
11.2
ETM A
Max. thrust
50 yr. storm
Ext. turb.
* Simulation length after 200s initial constant wind period

12. No fault
Blade seize
Blade seize + shutdown
Grid loss + shutdown
Storm condition
Extreme turbulence at rated speed
Tower Top FA Bending Moment

13. Hydrodynamics Large volume structures: potential flow
aerodynamics
control
Large volume structures: potential flow
First order
Second order sum-frequency
Slender structures: Morison’s equation
Tension-moored structures: ringing forces (3rd order)

14. structural dynamics
hydrodynamics
aerodynamics
control
Structural Modeling
Flexible beam elements (tower, blades, mooring system)
Rigid hull
Global model – simplified generator

15. TLPWT Parametric Design Study:
Pitch
Tower Base Bending
Diameter
Water Depth
Pontoon Radius
Ballast Fraction
45 resulting designs
7 environmental conditions
Responses at wave, wind, natural frequency, turbine frequencies
Aerodynamic forces must be transferred to tower bending, inertia, or line tension
Changes in diameter, pontoon radius, and ballast affect both stiffness and mass – complex results!
Tendon tension variation decreases with pretension, but tower bending decreases with increased ballast
Cost increases with displacement
A design with three pontoons, large pontoon radius, and mid-range (4000 to 7000 tonnes) displacement may be reasonable
Line Tension

16. Concluding remarks
TLP wind turbines present complex, unanswered design and analysis challenges
Numerical simulations require coupled aero-hydro-servo-elastic tools and expertise
A wide variety of environmental and operational conditions must be considered
In our studies of floating wind turbines at CeSOS we hope to provide insights that can help inform designers and regulatory bodies
Time-domain simulations
Modeling choices may affect results
Storm conditions tend to drive hull, mooring system design
Pitch fault conditions may drive blade, tower, gearbox designs

17. Thank you !
Thank you!

19. TLPWT + 3 Point Absorbers
Preliminary results indicate no significant change in power output for WEC or WT by combining
Reduced tendon tension variation (5-10%) and motions
% difference calculated as
[(TLPWTWEC) – TLPWT]/TLPWT

20. Simo-Riflex-AeroDyn
Java: control
AeroDyn: aerodynamic forces
Nonlinear time domain coupled code (Riflex: MARINTEK)
Single structural solver
Aerodynamic forces via DLL
Advanced hydrodynamics (Morison, 1st and 2nd order potential, ringing) (SIMO: MARINTEK)
Control code (java) for normal operation and fault conditions
Good agreement with HAWC2 (land-based and spar, including fault)
SIMO: wave forces
Riflex: structural deflections, time stepping