dynamic analysis of floating wind turbines - cesos - … bachynski.pdf · journal of fluid...

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www.cesos.ntnu.no Erin Bachynski – Centre for Ships and Ocean Structures

Dynamic Analysis of Floating Wind Turbines Erin Bachynski, PhD candidate at CeSOS [email protected] May 28, 2013

www.cesos.ntnu.no CeSOS – Centre for Ships and Ocean Structures

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TLP Semi-submersible Spar

We need to understand floating wind turbine behavior so that we can bring the cost down

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Spar • Deep draft, heavy ballast at the

bottom • Small diameter at the water

level • Small, slow motions (+) • Straightforward installation (+) • Requires large water depth (-)

• Analysis challenge: mooring

system

Image: Statoil

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Semi-Submersible • Stability from large

waterplane moment of inertia

• Relatively large motions (-) • Straightforward installation

(+) • More flexible w.r.t. water

depth (+)

• Analysis challenge: large columns and heave plates, structural response of the braces

Image: Principal Power

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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 (-)

• Analysis challenges: elastic coupling

between tower/platform/tendons, high-frequency forcing and response

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Source: NREL/Wind power today, 2010.

structural dynamics

hydrodynamics

aerodynamics control

-complex -tightly coupled -nonlinear -time domain -long term periods -transient (faults)

Integrated aero-hydro-servo-elastic analysis

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Simo-Riflex-AeroDyn • 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 DTU Wind’s HAWC2 (land-based and spar, including fault)

SIMO: wave forces

Java: control AeroDyn:

aerodynamic forces

RIFLEX: structural deflections, time stepping Ormberg, H. & Bachynski, E. E. Global analysis of floating wind turbines: Code development,

model sensitivity and benchmark study. 22nd International Ocean and Polar Engineering Conference, 2012, 1, 366-373

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Aerodynamics

Image: J. de Vaal, 2012

Image: J. de Vaal, 2012

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Ormberg, H. & Bachynski, E. E. Global analysis of floating wind turbines: Code development, model sensitivity and benchmark study. 22nd International Ocean and Polar Engineering Conference, 2012, 1, 366-373

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Control system

• Serves to – regulate rotor rotation

speed – regulate power output – protect structure

• Actions – Change generator torque – Change blade pitch

0

100

200

300

400

500

600

700

800

900

0

5

10

15

20

25

0 5 10 15 20 25

Thru

st

Rot.

Spee

d, B

l.Pitc

h, P

ower

Wind Speed (m/s)

Blade Pitch (deg) Power (MW) Rotor Speed (RPM) Thrust (kN)

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Blade pitch mechanism failures

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

Wilkinson et al., 2011

Jiang, 2012

Con

trib

utio

n to

failu

re ra

te (f

ailu

res/

turb

ine/

yr) (

%)

Pitc

h sy

stem

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-200 -150 -100 -50 0 50 100 150 200-1.5

-1

-0.5

0

0.5

1

1.5x 10

4

Tow

er T

op B

MY

, kN

m

TLP, EC 5

time - TF, s

BC

What happens if one blade stops pitching?

Shut down turbine quickly

Fault occurs

Continue operating with faulted blade

TLP, U=20m/s, Hs = 4.8m, Tp = 10.8s

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Comparison of controller fault effects on different platforms

Spar TLP

Semi-Sub 1 Semi-Sub 2

Bachynski, E. E.; Etemaddar, M.; Kvittem, M. I.; Luan, C. & Moan, T. Dynamic analysis of floating wind turbines during pitch actuator fault, grid loss, and shutdown Energy Procedia, 2013 . (Accepted for publication)

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Environmental/ Fault Conditions

Fault Definition

A No fault

B Blade seize

C Blade seize + shutdown

D Grid loss + shutdown

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 NTM A, B, C, D 30 16 min.

3 14.0 3.6 10.3 NTM A, B, C, D 30 16 min.

4 17.0 4.2 10.5 NTM A, B, C, D 30 16 min.

5 20.0 4.8 10.8 NTM A, B, C, D 30 16 min.

6 49.0 14.1 13.3 NTM A (idling) 6 3 hours

7 11.2 3.1 10.1 ETM A 6 3 hours

* Simulation length after 200s initial constant wind period

Max. thrust

50 yr. storm

Ext. turb.

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No fault Blade seize Blade seize + shutdown Grid loss + shutdown Storm condition Extreme turbulence at rated speed

Tow

er T

op F

A Be

ndin

g M

omen

t

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Hydrodynamics

• Large volume structures: potential flow – First order – Second order (sum- and

difference-frequency) – Third order (ringing)

• Slender structures: Morison’s equation

hydrodynamics


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Hydrodynamics: Morison vs. Potential Flow for FWTs • Semi-submersible:

– Morison’s equation compares well to potential flow if one chooses the coefficients carefully and integrates to the instantaneous free surface

• TLPWTs: – For medium and large diameters,

Morison’s equation gives larger forcing at high frequencies compared to potential flow

– Small diameters: Morison’s Equation works well

0 0.5 1 1.5 2 2.5 30

1

2

3

4

5

6

7x 10

4

F 5/ ζ, k

Nm

/m

ω, rad/s

P1+V (sim)M (sim)P1 (theory)M inertia (theory)

Large difference at pitch/bend natural frequency

Kvittem, M. I.; Bachynski, E. E. & Moan, T. Effects of hydrodynamic modelling in fully coupled simulations of a semi-submersible wind turbine. Energy Procedia, 2012, 24, 351-362

Bachynski, E. E. & Moan, T. Hydrodynamic Modeling of Tension Leg Platform Wind Turbines 32nd International Conference on Ocean, Offshore and Arctic Engineering, 2013

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Hydrodynamics: Ringing forces on TLPWTs • Ringing: transient response at

frequencies higher than the wave frequency

• Third order diffraction loads • Wave loads by FNV formulation1

(long-wave approximation, sum-frequency components only, bandwidth limitation in interacting waves)2

520 530 540 550 560 570-2

-1

0

1

2

3x 10

F x, kN

520 530 540 550 560 570-2

-1

0

1

2x 10

5

MFA

, kN

m

Time, s

520 530 540 550 560 5704000

6000

8000

10000

12000

14000

T 1, kN

P1+V, Turbine OnP1+V, Turbine OffP2+V+FNV, Turbine OnP2+V+FNV, Turbine Off

W

ave

Forc

e To

wer

Bas

e B

endi

ng

Dow

nwin

d Te

nsio

n D=14 m, Hs = 8.71 m, Tp =10 s

1) Faltinsen, O. M.; Newman, J. N. & Vinje, T. Nonlinear wave loads on a slender vertical cylinder. Journal of Fluid Mechanics, 1995, 289, 179-198

2) Johannessen, T. B. Nonlinear Superposition Methods Applied to Continuous Ocean Wave Spectra.Journal of Offshore Mechanics and Arctic Engineering, 2012, 134, 011302-1-011302-14

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Structural Modeling

• Flexible beam elements (tower, blades, mooring system)

• Rigid hull • Global model – simplified generator • Blades: complex cross-section!

structural dynamics hydrodynamics


Aerodynamic axes

Principal bending axes Center of mass

Shear center

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Design and Analysis of TLPWTs

• Design comparisons – Parametric design study of TLPWTs – Combined wind and wave energy platforms

• Analysis alternatives – Can a frequency-domain analysis be used? – Can Morison’s equation be used? – Are second order potential flow effects important? – How do different ringing force models affect the results?

• Extreme conditions – How likely are we to lose tension in a storm? – What happens if the wind turbine controller fails? – What happens when the wind and waves come from different

directions?

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TLPWT Parametric Design Study:

• Variations in: – Diameter – Water Depth – Pontoon Radius – Ballast Fraction

5 baseline designs:

45 resulting designs 7 environmental conditions

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0 5000 10000 15000

Ft (k

N)

Displacement (m3)

TLPWT 1 TLPWT 2 TLPWT 3 TLPWT 4 TLPWT 5 MIT/NREL UMaine GLGH IDEAS Crozier

Bachynski, E. E. & Moan, T. Design Considerations for Tension Leg Platform Wind Turbines Marine Structures, 2012, 29, 89-114

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Parametric Design Study Results • 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

Res

pons

es

Parameters

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Concluding remarks

• Floating 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

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Thank you !

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References • Bachynski, E. E.; Etemaddar, M.; Kvittem, M. I.; Luan, C. & Moan, T.

Dynamic analysis of floating wind turbines during pitch actuator fault, grid loss, and shutdown. Energy Procedia, 2013. (Accepted for publication)

• Bachynski, E. E. & Moan, T. Design Considerations for Tension Leg Platform Wind Turbines Marine Structures, 2012, 29, 89-114

• Bachynski, E. E. & Moan, T. Hydrodynamic Modeling of Tension Leg Platform Wind Turbines. 32nd International Conference on Ocean, Offshore and Arctic Engineering, 2013

• Bachynski, E. E. & Moan, T. Linear and Nonlinear Analysis of Tension Leg Platform Wind Turbines.The 22nd International Ocean and Polar Engineering Conference, 2012

• Bachynski, E. E. & Moan, T. Point Absorber Design for a Combined Wind and Wave Energy Converter on a Tension-Leg Support Structure. 32nd International Conference on Ocean, Offshore and Arctic Engineering, 2013

• Faltinsen, O. M.; Newman, J. N. & Vinje, T. Nonlinear wave loads on a slender vertical cylinder. Journal of Fluid Mechanics, 1995, 289, 179-198

• Jiang, Z.; Karimirad, M. & Moan, T. Steady State Response of a Parked Spar-type Wind Turbine Considering Blade Pitch Mechanism Fault. The 22nd International Ocean and Polar Engineering Conference, 2012

• Johannessen, T. B. Nonlinear Superposition Methods Applied to Continuous Ocean Wave Spectra.Journal of Offshore Mechanics and Arctic Engineering, 2012, 134, 011302-1-011302-14

• Jonkman, J.; Butterfield, S.; Musial, W. & Scott, G. Definition of a 5-MW Reference Wind Turbine for Offshore System Development. NREL/TP-500-38060, National Renewable Energy Laboratory, 2009

• Jonkman, J. & Matha, D. A Quantitative Comparison of the Responses of Three Floating Platforms European Offshore Wind 2009 Conference and Exhibition, 2009

• Kvittem, M. I.; Bachynski, E. E. & Moan, T. Effects of hydrodynamic modelling in fully coupled simulations of a semi-submersible wind turbine. Energy Procedia, 2012, 24, 351-362

• Ormberg, H. & Bachynski, E. E. Global analysis of floating wind turbines: Code development, model sensitivity and benchmark study. 22nd International Ocean and Polar Engineering Conference, 2012, 1, 366-373

• Wilkinson, M.; Harmann, K.; Spinato, F.; Hendriks, B. & van Delft, T. Measuring Wind Turbine Reliability - Results of the Reliawind Project. European Wind Energy Conference (EWEA 2011), 2011

dynamic analysis of floating wind turbines - cesos - … bachynski.pdf · journal of fluid...

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