Stuttgart Wind Energy

@ Institute of Aircraft Design

Load Simulation of Offshore Wind Turbines -

Modeling Techniques and Validation by

Measurements

SIMPACK Wind and Drivetrain Conference 2015

Hamburg, Germany

October 7th, 2015

Dipl.-Ing. Friedemann Beyer

Dipl.-Ing. Matthias Arnold, Dipl.-Ing. Birger Luhmann,

Dipl.-Ing. Matthias Kretschmer, Prof. Dr. Po Wen Cheng

Tradition

Ulrich Hütter: pioneer work on wind turbine design and GRP (1950s)

F.X. Wortmann: airfoil design, LWT (IAG)

Test site Schnittlingen: UNIWEX (ICA)

Endowed chair of wind energy (SWE, since 2004)

Current Research Fields • Testing and Measurement

• Conceptual Design and System

Simulation

• Control, Optimization and Monitoring

• Aeroelasticity (IAG & SWE)

• Automated fibre

composite

manufacturing

techniques

• Aerodynamics and

aeroacoustics with CFD

• Airfoil design, wind tunnel tests

IFB

2

Wind Energy Research at University of Stuttgart

ITM • Multibody Dynamics

• Particle simulation IST

• Control Theory

• System Theory

• Applications

3

Development of numerical design tools Conceptual design and analysis

Integrated coupled system simulation

Aerodynamic modelling:

BEM

Free Wake

CFD

Aeroelasticity of rotor blades:

Structural modelling

Aeroelastic interaction

Hydrodynamics of floating structures:

Potential flow vs. CFD

Linear vs. non-linear wave theories

Code2Code verification

Validation of simulation tools (scaled models,

full-scale measurements)

Load calculation & analysis according to

standards

Identification of critical operational conditions

and components

Load hypotheses of

onshore/offshore sub-structures

Design of floating offshore foundations

Hydroelasticity of tidal current turbines

Load reduction mechanism of

2-bladed wind turbines

Overview of Group

Conceptual Design and System Simulation

[IDEOL]

[ID

EO

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Content

6

I. Hydrodynamic Modeling

I. Floating Offshore Wind Turbine

II. Monopile Foundation

II. Aerodynamic Modeling

I. Wind Farm Simulation

III. Summary and Conclusions

Morison Equation:

simple, fast

semi-empiric

slender cylindrical

bodies, D/λ<0.2

inertia, drag dominated

flow separation

HydroDyn

Potential Flow:

more advanced,

medium comp time

potential flow theory,

viscous drag from ME

non-transparent structures

diffraction dominated

HydroDyn, AQWA (Pre), WAMIT (Pre)

Range of hydro dynamic models

7

Computational Fluid

Dynamics:

very advanced,

very high comp time

finite volume, RANS

equation, structured

and unstructured grids

VOF approach for free

surface

fully implicit, transients

ANSYS CFX

Motivation

Fluid-Structure-Interaction:

flow physics often

nonlinear and highly

complex

simple methods not

capable of including all

effects

need for detailed loads

distribution for design

purposes

Objectives:

focus on fidelity rather than quantity

investigate floater motion and flow

separation at large sea states

investigate wave run-up and green water

evaluate potential of CFD as substitute or

addition to model tank tests

8

[IDEOL]

colormap: velocity

Project: FLOATGEN

Overview:

The FLOATGEN demo project

will see the deployment of a 2

MW floating turbine in the

Atlantic Ocean, at SEM-REV

test site located 12 nautical

miles from the city of Le Croisic

on the French Atlantic coast.

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Content:

demonstrate the technical and

economic feasibility of fitting a 2

MW turbine model on a ring-

shaped surface-floating platform

monitor and test the operating

systems in real open sea

conditions 9

SEMREV

Scale Testing

10

System Identification Tests / RAOs

- hammer

- static offset

- free decay

- waves (regular, irregular)

- waves + wind

- Model / load validation

- Parameter tuning

- Calibration and input to numerical models

for design optimization

- Design selection

numerical

model tuning

Load Cases

- regular waves + wind

- irregular waves + wind

- wind-wave-misalignment

Scaled Model

Data Analysis

Numerical Model

Test matrix

Wave Tank Model Test

11

Test Campaign:

mooring system and dynamic

behaviour in extreme wave

conditions and shallow depth

Froude similitude at 1/32th scale

RNA represented by lumped

mass

free decay tests, regular and

irregular waves combined with

current without wind

measurements: 6 DOF floater

motion, wave elevations, green

water forces, axial tension of the

mooring lines

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Tasks within the FMBI Coupling Code

translator: coordinate transformations & interpolation

sender: collecting local data & transfer to common storage

receiver: distributing coupling data & synchronization of time

moderator: communication procedure & convergence control

CF

D:

CF

X

MB

S:

SIM

PA

CK

Tra

nsla

tor

Sender

(loads)

Receiver

(motion)

Tra

nsla

tor

Sender

(motion)

Receiver

(loads)

Transfer memory

Moderator

12

Advantages:

integrated analysis: aero-, hydro, structural dynamics, control system

inclusion of flexible bodies with reasonable computational effort

Damping Characteristics

16

Heave Damping:

entrapped water in the pool

piston like in- and outflow during floater motion

shedding of large 3D vortices at the skirt and inner hull

colormap: velocity colormap: velocity

floater bow floater stern

Floater Motion: Surge and Heave

Surge:

drift loads acting on the

floater

mean surge position after

transients

good correlation (period,

mean, extreme values)

Heave:

little transients

high heave damping

very good correlation

(period, mean, extreme

values)

19

Ongoing Scale Testing and Validation

20

Froude-scaled rotor thrust

Redesigned blades

Low wind speeds

Ducted fan

Real-time controlled (HIL)

No wind generator necessary

Figs. INNWIND.EU; Politecnico di Milano; University of Stuttgart

Computational Fluid

Dynamics:

very advanced,

very high comp time

finite volume, RANS

equation, structured

and unstructured grids

VOF approach for free

surface

fully implicit, transients

ANSYS CFX

Potential Flow:

more advanced,

medium comp time

potential flow theory,

viscous drag from ME

non-transparent structures

diffraction dominated

HydroDyn, AQWA (Pre), WAMIT (Pre)

Range of hydro dynamic models

21

Morison Equation:

simple, fast

semi-empiric

slender cylindrical

bodies, D/λ<0.2

inertia, drag dominated

flow separation

HydroDyn

Project: IEA Wind Task 30 Extension - OC5

Overview:

OC5 = Offshore Code Comparison Collaboration,

Continued, with Correlation

validation of design codes through code‐to‐data comparisons

Phase II:

semi, tank testing

Jun 2015 – May 2016

Phase III:

jacket/tripod, open ocean

Jun 2016 – May 2017

[IN

RE

L]

[DO

TI]

22

Phase I:

monopile, tank testing

Jan 2014 – Nov 2015

[ID

TU

, D

HI]

OC5 Phase Ib: Model Properties, Measurements

23

[ID

TU

, D

HI]

Property model scale (1:80) full scale

cylinder diameter 0.075 m 6.0 m

cylinder height 2.0 m 160.0 m

wall thickness 1.8 mm 144.0 mm

density 0.64 kg/m 4200.0 kg/m

natural frequency, f1 2.5 Hz 0.28 Hz

natural frequency, f2 18.0 Hz 2.0 Hz

Measurements:

wave elevation at cylinder

total wave force on cylinder at bottom

cylinder acceleration along length

Load Cases:

regular wave case:

H = 0.118 m, T = 1.5655 s

irregular wave case:

Hs = 0.14 m, Tp = 1.55 s

water depth d = 0.51 m

OC5 Phase Ib: 2nd Order Loads

26

non-linear waves:

higher peaks, smaller

troughs

2nd order wave

theory required to

capture higher order

wave loads

recommendation:

check validity of

applied wave theory,

use Simpack 9.9

(HydroDyn 2.02.02)

for large waves/small

depths

Range of aerodynamic models

32

Computational Fluid

Dynamics:

very advanced,

very high comp time

finite volume, RANS,

LES, structured and

unstructured grids

use for calculation of

airfoil tables possible

ANSYS CFX,

FLOWER, SOWFA

Blade Element

Momentum:

simple, fast

momentum balance

industry standard

airfoil table required

ECN Aeromodule,

AeroDyn

v1

v2

v3

S

[Univ

ers

ity o

f S

tutt

ga

rt,

IAG

]

Free Vortex Wake:

more advanced, medium

comp time

potential flow, viscous

vortex core models

rotor-wake-interaction

wind farm simulation

airfoil table required

ECN AeroModule, WInDS

Applied Simulation Method

33

[DO

TI]

Wind turbine model:

NREL 5MW reference wind turbine

structural: MBS (Simpack)

aerodynamics: Free vortex (WInDS)

control: Pitch and torque

hydrodynamics/foundation: none

Load case:

wind: steady, w/ and w/o shear at 12 m/s

half wake condition (50% shadowing)

AV4 AV5

tower: rigid flexible

blade: rigid flexible

control: fixed pitch, speed enabled AV

4/5

mo

de

lled

by N

RE

L 5

MW

,

dis

tan

ce

ba

se

d o

n A

V p

ark

la

yo

ut

Jonkm

an

, J. M

., B

utt

erf

ield

, S

., M

usia

l, W

., S

cott

, G

. (2

009).

Defin

itio

n o

f a 5

-MW

Refe

rence W

ind T

urb

ine f

or

Off

shore

Syste

m D

evelo

pm

ent.

Gold

en,

CO

.

Basics of Free Vortex Methods

34

Free Vortex Methods:

potential flow approach

velocity induction via Biot-Savart law

viscosity via vortex core models

vortex filaments convect and deform

freely, account for flow unsteadiness

and spanwise variation in lift

lifting-line model: lift distribution related

to strengths of vortex filaments

[Katz

, P

lotk

in]

Wake Induced Dynamic Simulator

(WInDS):

Matlab® based

GPU accelerated

implicit and explicit coupling to

Simpack

[DO

TI]

Sebastian, T. (2010). Understanding the Unsteady Aerodynamics and Near Wake of an Offshore Floating Horizontal Axis Wind Turbine. Dissertation. Amherst, MA.

Lenz, D., Beyer, F., Luhmann, B., Cheng, P. W. (2014). Untersuchung instationärer aerodynamischer Effekte an Windenergieanlagen mittels Free Vortex Methoden, Bachelor

Thesis. Universität Stuttgart.

Results: System Behaviour, Blade Loads

36

vhub = 12 m/s

steady, uniform

Summary and Conclusions

Simpack and CFX:

successfully applied for wave impact on floating offshore wind

turbine foundation

good correlation for global floater motion

save model tests to assess impact loads, design

optimisation

Simpack and HydroDyn:

ongoing validation study within OC5 and INNWIND.EU

project

good correlation for wave kinematics, structural loads and

motion

use 2nd order wave theory (Simpack 9.9) for extreme conditions

Simpack and WInds:

effects of wakes on structural loads and system behavior in a

wind farm

validation by Lidar and load measurements within 2016/2017

37

Acknowledgements

38

The presented work is funded partially by the European Community’s

Seventh Framework Programme (FP7) under grant agreement number

295977 (FLOATGEN) and Voith Hydro Ocean Current Technologies

GmbH & Co. KG. The presented work is supported by Simpack AG and

Ansys Germany GmbH.

Stuttgart Wind Energy

@ Institute of Aircraft Design

Thank you for your attention!

Contact:

Dipl.-Ing. Friedemann Beyer

Team Leader Conceptual Design and System Simulation

Stuttgarter Lehrstuhl für Windenergie (SWE)

Universität Stuttgart

Allmandring 5B - D-70569 Stuttgart, Germany

T: +49 (0) 711 / 685 - 60338

F: +49 (0) 711 / 685 - 68293

E: beyer@ifb.uni-stuttgart.de

http://www.uni-stuttgart.de/windenergie

http://www.windfors.de