083 Moan Dynamic Analysis Wind Turbine

11

Design and Analysis of Wind Turbine Support Structures

- with emphasis on integrated dynamic analysis

Torgeir Moan Centre of Ships and Ocean Structures (CeSOS)

Department of Marine Technology

Norwegian Centre for Offshore Wind Technology (NOWITECH)[email protected]

http://www.cesos.ntnu.no/http://www.marin.ntnu.no/~tormo

e

2 Outline Background

- Wind energy offshore and wind turbines Wind technology development and research

- technology challenges- design criteria- concept development (spar, semi, TLP hulls, rotor, drivetrain)- integrated dynamic analysis

Installation and Operation and Maintenance Demonstration projects (field testing) Utilization of the Ocean Space combined facilities

Concluding remarks

3

Offshore versus onshore wind energyPros Large areas available at a low price No noise and visual impacts

- larger rotor velocity (better efficiency) Higher wind velocities,

less turbulent wind- Power proportional with v3 (v- wind velocity)

Feasible transportation / installation

Cons

Wet & corrosive environment Difficult access for installation &

maintenanceHigher CapEx & OpEx

Background 4

Wind energy conversion into mechanicaltorque and finally into electrical power

Kinetic energy in wind: Power in the wind: Electrical power: P = CP Pmax Power generation depends on:

Air density Wind velocity (cubed) Swept area

Average annual produced power (kWh/h)- aerodynamic versus electricalpower

Rated power (instantaneous peak power) for design of power take off or drive train system

Background

SUPPORT STRUCTURE

Tower

5Control systems

Control system objectives: Ensure efficient and safe

operation- control torque at below rated speed and the power above rated, and limitthe structural loads.

Supervisory systems to control: Yaw control Rotor speed control (blade-pitch) Power control (generator torque)

Schematic illustration of power productionby a 5 MW bottom fixed wind turbine

Rotor condition

Maximizepower

Constantpower

Background 6

Development trends

- deeper waterfrom fixed to floating

- increasedrotor size (capacity):California 1980 : 55 kWto 3.6 MW and upwards

5 MW

Integratingknowledge

One of a kind OG install. versus mass produced WTs.

No hydro carbons and people on board wind turbines

The wind energy sector is a marginal business

Return are more sensitive to IMMR (O&M) costs (access)

Background

7 Background

Cost reduction is needed!

Costs of offshore wind turbines

Year (main contract signed)

P

r

o

j

e

j

C

a

p

E

x

(

U

S

$

M

/

M

W

)

8

6

4

2

0

Onshoreturbines

1990 1995 2000 2005 2010 2015

OpEx (30 50 % of CapExlifetime costs)- including downtime

due to bad weather

(Source: Douglas Westwood, 2009; Dong Energy, 2010 etc)

8

Costs of (bottom fixed) offshore wind turbinesContribution to total CapExComponent Onshore Offshore

Turbine 70-85 % 40-50 %

Supportstructure

1-10 % 15-30 &

Grid connection

2-10 % 15-20 %

Electricalinstall.

1-10 % 5-10 %

Engineering 2-10 % 5-10 %

Other 2-10 % 1-10 %

CapEx- more expensive substructure/soil foundations- more complex and expensive installationOpEx (20 30 % of CapEx lifetime costs)- bad weather can lead to downtime- seabed preparation; e.g. to protect against scour

Courtesy: Dong Energy

Reduce costs as done for land based turbines:- Increase turbine size- Improve manufacturing- Improve infrastructure

Offshore vs. Landbased:

Background

9Support structure and drive train

Rotor to generator (drive train)

Power system:Innovation in transmission, grid connection and system integration while maximizing power availability, quality, and stability

Marine operations:Improve efficiency of installation (transportation, site surveying,cable laying; etc ) and personnel access to facilities whileminimizing the risks and the cost of operation.

Technology developmentMinimize costwhilecomplying withsafety and durabilityrequirements. Larger unitsand reducedfailure rates

10

Life Cycle Planning ofOffshore RenewableEnergy Facilities

for

11

11

- Safety level: in view of failure consequences (fatalities, pollution, material loss)

- limit states- Ultimate failure (ULS)- Ultimate failure initiated by faults (ALS)- Degradation (fatigue, corrosion, wear)

Design criteria w.r.t. to functionality and safety

Concepts (a system of rotor, machinery, generator, support structure)- charact. behaviour- satisfy criteria- costs

- Guidelines and standards: Fixed wind turbines: IEC 61400-3, GL, 2005, DNV-OS-J101. 2010 , Floating turbines stability, station-keeping?

Design of wind turbines

Methods of analysis

Technology Development12

12

Design for Servicability (use)

Platform for supportingpayload, and risers

Limited motions Mobility of drilling vessels Access for IMMR

Wind turbines for production ofelectrical power

Provide support of payload

Limited motions Access for IMMR

Platforms for drilling for andproduction of oil and gas

Design criteria

Feasible/economicfabrication, transport and Installation

13

13

Design for Safety

to avoid Fatalities or injury Environmental damage Property damage

Regulatory regime (depends on economy; accident potential):

Regulatory principles- Goal-setting vs. prescriptive- Probabilistic vs. deterministic- First principles vs. purely experiential

Offshore oil and gas Wind energy

- ISO/IMO- National regulatory bodies; - Industry: API, NORSOK, - Classification/Certification

bodies

- IEC- National regulatory bodies- Certification or classification

bodies

Design criteria 14

Safety criteria Design criteria

Load effects

Collapseresistance

SN-curve/fracturemechanics

Ultimateglobalresistance

Extrememoment (M)andaxial force (N)

Localstressrangehistory

Extremeglobalforce

Designcheck

ULS:

FLS:

ALS:Damagedstructure

Source: NREL/Wind power today, 2010.

Defined probability level

Design criteria

15 Design criteria: Limit states- Ultimate failure (ULS)--- UltimateUltimateUltimate failurefailurefailure initiatedinitiatedinitiated by by by faultsfaultsfaults (ALS)(ALS)(ALS)--- DegradationDegradationDegradation (((fatiguefatiguefatigue, , , corrosioncorrosioncorrosion, , , wearwearwear) ) )

Stability structures supported on the seafloor tension-leg platforms articulated towers floating platforms/ships

Structural integrity structure mooring foundation

Criteria based on explicit measure of

implied safety level in terms of reliability or risk

fS(s)

fR(r)

PF=P[RS]

r,s

fS(s)

fR(r)

PF=P[RS]

r,s

16

ULS- stability The stabilizing (righting) moment, MR is expressed by

GZM R =

The metacentric height, 1 = + W wGM I i KB KG

= O windM F aa- distance from wind resultant, Fwind to the centre of submergedvolume

Overturning moment due to wind

Design criteria: Limit states

= ( )GZ GM sin

a

B

G

= sin( )GZ GM

17

Limit states- Ultimate failure (ULS)--- UltimateUltimateUltimate failurefailurefailure initiatedinitiatedinitiated by by by faultsfaultsfaults (ALS)(ALS)(ALS)--- DegradationDegradationDegradation (((fatiguefatiguefatigue, , , corrosioncorrosioncorrosion, , , wearwearwear) ) )

ULS- strength

e0

M0N

M0N

Global collapse of members

Local collapse of members

Design criteria 18

Limit states--- UltimateUltimateUltimate failurefailurefailure (ULS)(ULS)(ULS)--- UltimateUltimateUltimate failurefailurefailure initiatedinitiatedinitiated by by by faultsfaultsfaults (ALS)(ALS)(ALS)- Degradation (fatigue, corrosion, wear)

FLS- strength

Fatigue strength is described by SN-curves and the Miner Palmgren approach.

For stress cycles following a Weibull distribution the Miner-Palmgren damage may be expressed e.g. by

iall

i

nDN

=

= = + 00

( / 1)(ln )

mi

i

n SND mN K N

[ ] oo NssP /1=>K, m - constants in the SN-curve: N = KS-mN - number of stress ranges in the service period

- shape parameter of the Weibull distribution

Design criteria

19

Concepts of bottom supported turbines

Thorntonbank Alpha VentusBeatrice

- tower design, alternative structuralmaterial, downwind /upwind rotor)

- foundation technology

- design for mass production and easy installation

- transport and installation (complete installation in-shore and then float-out)

Minimal platforms

Technology Development 20

Analysis and Design of Bottom Fixed Wind Turbines

Wind-aerodynamics

Wave, Current-Oceanography-Hydrodynamics

Geotechnical engineeringincl. soil materials technology

Steel and concrete structures

Structuralmaterialstechnology

Fault conditions

Structuralengineering

Machinery/electrical control

-systems analysis:Risk and reliability analysisOptimization

21

Environmental data Extratropical regions joint distribution of significant wave

height Hs, spectral peak period Tp and mean wind speed Uw

Simplified:

Tropical regions

The Weibull probability density function ofthe significant wave height ( Hs ) given themean wind speed at nacelle (Uw ) for theStatfjord offshore site at 59.7oN and 4.0oE and 70 km from the shore.

( ) ( ) ( ) ( )hutfuhfufthuf HsUwTpUwHsUwTpHsUw ,,, ,,, =

(Source: Li Lin et al, OMAE 2013)

22

Location of 18 potentialEuropean offshore sites.

50year contour surface for Site 14 (top left: threedimensional contour surface; top right: thecondition on the contour surface with themaximum mean wind speed; and the followingtwo subfigures show contour lines of Hs and Tpfor different levels of Uw).

23

Design Load Cases Standards

Standards used for the definition of loads and load cases offshore and onshore:

IEC 61400-3, Design Requirements for Offshore Wind Turbines, edition 1.0, 2009

IEC 61400-1, Wind Turbines, Design Requirements, edition 3, 2005 New amendment accepted and now used for Type Certification

Wake turbulence changed taken into account CT New chapter on statistical extrapolation on loads

DNV, GL standards

24Design of bottom supported turbines

Important ULS Load Cases for Bottom fixed Offshore Wind Turbines (IEC 61400-3 & Classification society rules)

Blades:Flapwise: Extreme turbulence (DLC1.3)Edgewise: Extreme wind (DLC6.1/6.2)

- Tower top:- Tilt: Safety system fault (DLC2.2)- Yaw: Safety system fault (DLC2.2)

- Tower bottom:- Along wind: Gust & lost grid (DLC2.3)- Across wind: Extreme wind (DLC6.1/6.2)

- Seabed:- Along wind: NTM & Extreme wave (DLC1.6)- Across wind: Extr. wind & Wave (DLC6.X)

Source: E.Jrgensen, DNV


Examples faults during power production

DLC 2.1 Faults relating to control functions or loss of electrical network (N) Overspeed caused by malfunction generator torque Pitch set to 0 at high winds -> overspeed Operation at large yaw error

DLC 2.2 Rare events, including faults relating to protection functions (A) Blade pitching blocked on one blade -> stopping with to blades only Controller independent overspeed guard triggered

DLC 2.3 Extreme operating gust and loss of electrical connection (A)

2)Powerproductionand occurenceof faults

26 Modelling Bottom Fixed Wind Turbines(to estimate the response of a given subsystem)

Aerodynamics incl. wakes in farms- BEM method (Simplified: Thrust; Refined:CFD)

Hydrodynamics- Morison formula for slender bodies- Potential theory (panel method) for large volumebodies linear versus nonlinear effects

Structural model- blades (nonlinear geometry)- tower, structure,(quasi-static versus dynamic)

Mooring model (for floating WTs)- FE model vs nonlinear spring- Damping

Soil foundation (for fixed WT, anchor in floating WTs)(spring versus FE model, linear versus nonlinear)

Drive-train

Stochastic analysisof the response to irregualr waves and turbulent wind,.. to reduce statisticaluncertainty

27

Aerodynamic loads

CFD: Navier-Stokes (NS) equations for the global compressible flow in addition to the flow near the blades.

BEM: blade element momentum theory based onlift, drag, moment coefficients (engineering methods)

- relies on airfoil data

-Refined vs simplified methodsAerodynamicsHydrodynamics..

Pitch angle

28

Effect of wakes Benchmarking exercisefrom Offshore Wind Accelerator

Two or three turbineson a single floater

Row number downstream

Turbines are arranged in a regular grid

CFD

Measurements from Horns rev in Denmark

Multiple turbine conceptsrequire turret mooring system

29

Aerodynamics on VAWTDouble multiple Streamtubes model

29

Momentum theory applied onthe double disk multiple streamtubes

0 20 40 60 80 100 120 140 160 180

0

5

10

15

20

25

30

Sandia 17-diameter wind turbine-50.6rpm 2 blades

(deg)R

o

t

o

r

t

o

r

q

u

e

,

T

*

1

0

0

0

(

N

.

m

)

ExperimentalPresent modelReferenceBerg dystall

2 4 6 8 10

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Sandia 17-diameter wind turbine-50.6rpm 2blades

P

o

w

e

r

c

o

e

f

f

i

c

i

e

n

t

Cp-expCp

30

Waves -wind generated waves-swell

Surface elevation (Airy theory) regular wave stochasticShort-term vs. long-term variation

Kinematics pressure particle velocity acceleration

effect of large body diffraction, ( radiation, reflection )

Wave forces large volume structures ( potential

theory ) numerical methods slender bodies

tubular members: Morison Formulaship hull :Strip theory

( )( )

0

0

sin

sin

= = +

i i i i

i i i i i

t k x

t k x

Wave loads

The sea surface can be represented as a superposition of regularwaves with differentfrequency and direction ofpropagation

31Wave kinematics

- velocity potential

Nonlinear boundary condition

(x,t)

= 0n

z

2 = 0

v - particle velocitya - particle accelerationp - pressure

as obtained from the velocity potential

x

vx =x = vx(0) exp[ z]

2

A B

Ddiametervx

-/2 /2

deep water-d

z

The satisfaction of the surfaceboundary condition is the major challenge, because it is dependent upon the wave elevation, (x,t) itselfIf (x,t) = 0 linear (Airy) theory is obtained.

Airy theory

0=

+

+

zyyxxt

021 2

22

=

+

+

+zyxt

g

Note:- Kinematics at a depth z:

i.e. in-phase for different z.- Phase difference in two points with

different coordinate x

= xta 2sin

32

Particle velocity, v

Airy Wave profile

= xta 2sin

Particle acceleration, a

Airy theory is valid up to mean water level(MWL). Since important contribution to waveloads come from the fluid above MWL it needs to be corrected or refined

One simple approach is the Wheelermodification of the Airy theory

33

Higher order regular wave models

Second order Stokes wave = a sin (t-kx) - ( )a cos [2( t-kx)]a

Wave profiles with increasing order of Stokes wave theory

wave steepness k a=0.4w

a

v

e

p

r

o

f

i

l

e

/

a

AIRY STOKES IVSTOKES IIISTOKES II

2 32 Phase 2

1

0.5

0

-0.5

-1 - velocitypotential

= 0

Nonlinear boundary condition

(x,t)

n

z

0

02 =

0=

+

+

zyyxxt

021 2

22

=

+

+

+zyxt

g

34

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

2.22.01.81.61.41.21.00.80.60.40.2

0

CM

D/

q(pr.unitlength)

Cross-section area2

4DA =

Hydrodynamic loading. Morisons formula Steady state loading on

slender members

q= CM Aa+CD D v|v|CM, CD - coefficients - densitya - fluid accelerationv - fluid velocity

For slender structures v and aare the values in the incident(undisturbed) wave

For large volume structures- diffraction theory

Effective inertia coefficient for vertical cylinder

D = diameter

35

Character of wave loading on slender members

x

z

D = diameter

vertical member

- density of waterCD - drag coefficient

Wave forceq = qD + qI

Drag forceqD = CD D vx |vx|Drag force due two wave components and current (when drag force is positive):

( )( )

( )( ) ( )( )

1

21 1 2 2

2 2 21 2

21 122 2

1 2 2

1 2 1 2 1 2

1 1 1 2 2 2

sin sin0.5

0.5 cos20.5 cos22 sin sin

cos cos;

X

D x x c

c x x

x

x

c x

x x

q v v v

v v v

vv

v v v

v vwhere t t

+ += + ++ ++ + += + = +

36Hydrodynamic loading - general approach

The main force components on a cylinder are( Clauss et al., 1991, Faltinsen, 1990)

Froude-Krylov force:

Pressure effects due to undisturbed incident waves

Hydrostatic added mass and potential damping force:

Pressure effects due to relative acceleration and velocity between water particles and structural components in an ideal fluid

Viscous drag force: pressure effect due to relative velocity:between water particles and structural components

Damping (potential-, viscous-)

FKvF dt

= For a slender body

A AF C d a=

1/ 2 | |D D n nF C d v v= Nomenclature: - density

- volume- volume per unit length- added mass coefficient- drag coefficient

D

A

CCd

37

Modeling of the dynamic behaviour of bottom-fixed offshore wind turbine foundations- pile-soil models

Simple spring models Finite Element Models

38

Structural modelling of jacket support structure

(Source: TDA, Oslo)

39

Wind-industry based Bladed (Garrad Hassan) Flex5(S.ye,DTU) HAWC2(Ris), FAST(NREL)

Offshore industry based; e.g. Ansys/Aqua Simo/Riflex/Nirvana (Marintek) Orcaflex Others (e.g.FEDEM,USFOS/VpOne

Tools

Software for Analysis ofBottom Fixed Wind Turbines


Hydrodynamics- Morison formula vs Potential theory (panel method) - linear versus nonlinear effects

Structural model- hull; blades (nonlinear geometry)- damping

Soil foundation(spring versus FE model, linear versus nonlinear)

Drive-train



Examples faults during power production DLC 2.2 Rare events, including faults relating to protection

functions (A) Blade pitching blocked on one blade after 252 seconds ->

stopping with to blades only

Source: E.Jrgensen, DNV

41Fatigue analysis of an offshore wind turbine with a jacket support structure at an exposed North Sea site

Largest contributionto fatigue due to wind loads only: v=20 m/sandwave only:Hs = 5 m

Contribution to cumulative fatigue damage ofwind loads and wave loads

Normal power production- Turbulence in wind (wake operation)- Misalignment of wind and waves- Windrose

Fault, survival, start-up and shut down, standstill conditions

(Sorce: WB Dong et al)

WaterDepth70 m

42

Design for efficient installation

BeatriceMonopile turbine installed more or less complete

43

Time history of lowering and landing a monopile. (Hs =2.5 m, Tp=6.0 s, Dir=45 deg, Case: V2G1L1-seed 1

Time domain simulation of installation

(Source: Li Lin et al. , OMAE 23013)

44

44

- Safety level: in view of failure consequences (fatalities, pollution, material loss)

- limit states- Ultimate failure (ULS): buoyancy,stability, strength

- Ultimate failure initiated by faults (ALS)- Degradation (fatigue, corrosion, wear)

Design criteria w.r.t. to functionality and safety

Concepts (a system of rotor, machinery, generator, support structure)- charact. behaviour- satisfy criteria- costs

- Guidelines and standards: Floating turbines stability, station-keeping (IEC, DNV,GL)?

Floating Wind Turbines

Methods of analysis

Technology Development

45

45

Concepts of floating wind turbines vs offshore production systems for oil and gas

Variation of concepts Number of blades Support structure

HA Upwind / downwind turbine,VAT Tower construction Number of units on a floater

Floating turbinesespecially fordeep water areas inthe North Sea, USA, Medeterrainean sea, Japan, Korea

State-of-the-artdesign practice foroil and gas platformsprovides guidance


- Design for mass production and easy installation; i.e..cost reduction

- At which water depthwould floating windturbines be competitive ?

46

Rotors for floating systems

Smart blades

Larger blades Can active devices

- reduce loads ?

- improve energy capturein low wind conditions ?

- increase the area sweptwith the same blade ?

(for a 5 MW turbine the blade is 63 m long)

47

Drive train from rotor to generator

10 to 1500 rpm

47

Fixed or variable speed w/gear box

Direct drive variable speed

A main question:- Is the drive train, rotor. used on bottom-fixedturbines feasible for floating ones?

Hydraulic transfer(Chapdrive concept)

48N atu ra l perio ds

R igid b od y m od es

- 30 s

- 50 s 50-25 s

Jacke ts(G rav ity p la tfo rm )

(Jack ups)

A rticu la ted tow er

8-1s e las tic bend ing m odes

Te nsion-legp latfo rm

G uyed tow er(C om plian t

tow er)

S em i-subm ersib le (m o ored)

Low frequ ency fo rces

surge heave/p itch

- 30 s

- 40 s 6-2 s ax ia l te ther m odes



Less than 1-2 s

W ave frequ ency fo rces

H igh fre quency force s

10 6 2100 60 40 30 20

W ave lo ad perio d

Flexiblem od es

seconds

Natural periods of marine structures and wave excitation periods

49

Design of Semi-submersible or Spar Concepts

CriteriaStability The tilt angle should be limited (e.g. to 7 degrees)

under design overturning moment (800KN*90m)- implying pitch a minimum restoring stiffness

(C55)

Restoring by- water plane - KB>>KG


Minimum displacement, or increase added mass

1 = + W wGM I i KB KG

50

Design of Semi-submersible or Spar Concepts

Criteria

Dynamic performance (wave induced motions) Heave natural period (T33) should be above 20s Pitch natural period should (T55) be around 30s

Structure response ULS/ALS, FLS

+= 2 ii iiiii

M ATK


Minimum displacement, or increase added mass

51

Design of Semi-submersible wind turbines

Cost effectiveness Steel weight Displacement Fabrication complexity

Displacement of semi - submersible designs vary between 4500 14 000 tRef.: Spar: 7500 t


Mooring system for single turbine concepts:- catenary, spread mooring

Mooring system for multiple turbine concepts:- turret mooring, - single point mooring

52

Surge/Sway natural periods > 25 s

Heave/Roll/Pitch natural periods < 3.5 s Mean surge offset

< 5% water depth No tendon slack:

Tendons may not yield for 2 times initial tendon tension

Minimum displacement 2 000 tonnes

Challenge:-non-converging design spiral

Constraints

Design of Tension Leg Platform Wind Turbine -excessive buoyancy creates pretension and limited heave and pitch

(Bachynski and Moan, ISOPE Conf., 2012)

53

Analysis for design of offshore wind turbines Aerodynamic, hydrodynamics,. Integrated (aero-, hydro-, elastic-,

servo-) analysis- loads: irregular waves, turbulentwind, rotor rotation in agravitational field and a nonuniform wind field,

- conditions: operating, parked intact or with faults

- response extremes and histories -st.dev. (for fatigue, wear..) for different failure modes,

- time versus frequencydomain simulation

- refined versus simplifiedmethods

Laboratory or field testsDifferent failure modes

Source: NREL/Wind power today, 2010.

54

Integrated dynamic analysis of floating wind turbinesScope:Determination of load effects forthe designof

the supportstructure,tower,rotor,drivetrainThesystemmodelling includes:modelingofexcitationmechanisms(wind,wavesandcurrent)

rotoraerodynamics hydrodynamics structuraldynamics automaticcontroltheory powergeneration

Wind-industry based Bladed (Garrad Hassan) Flex5(S.ye,DTU) HAWC2(Ris), FAST(NREL)

Offshore industry based Simo/Riflex (Marintek) Orcaflex

Tools - Integrated analysis tightly coupled system

- Time domain simulation(time step, spectral repres.)

- Fault conditions- Drivetrain- Dynamic power cable

55

The resulting wind forces on the rotor consist of 3 force and 3 moment components. A simplified model is achieved by only considering the thrust force.

2 212 a T REL

T R C U=

Simplified aerodynamic load and response analysis

Further simplification is achieved by simulatingthe effect of control in the over rated responseup to cut-out wind speed by a filter.

Tower bending moment

Load cases for operational conditions

The simulation time for 1 hour real time:-15 min for SRT -24 hours for

the full method

(Karimirad and Moan, J. Marine Structures, 2012)

56

56

Integrated analysis of wind turbines

Hydrodynamic loads

57

(Source: O.Faltinsen/M.Greco)

Linear Wave-Induced Body Motions

58

Hydrodynamic loading-general approachThe main force components on a cylinder are( Clauss et al., 1991, Faltinsen, 1990)

Froude-Krylov force:

Pressure effects due to undisturbed incident waves

Hydrodynamic added mass and potential damping force:

Pressure effects due to relative acceleration and velocity between water particles and structural components in an ideal fluid

Viscous drag force: pressure effect due to relative velocity:

Between water particles and structural components

FKvF dt

= For a slender body

A AF C d a=

1/ 2 | |D D n nF C d v v= Nomenclature: - density

- volume- volume per unit length- added mass coefficient- drag coefficientD

A

CCd


59

2 [M+ A()] r() + i B()r() + Kr() = R(),

Formulation and Solution of Dynamic Eqs. of Motionfor Rigid Floating Structures

Frequency domain formulation for wave loading

M - mass; A - added massB - potential dampingK - restoring (stiffness)X - excitation (load)

Rr () = [ 2A()r() + iB()r()]

+ + + + = (M A )r B r k(t )r( )d Kr(t) R(t)

Time domain formulation for wave loading

= 0

2k(t) (B( ) B )cos( t)dwhere:

60

Wave induced motions of floating windturbinesDynamic equilibrium (SDOF; e.g. heave motion) under regular waves

( )( )1= +

= +

inertia a

a a

F a C a r

C C r

( )212

= drag DF C A v r

+ + = totmr cr kr Ftot inertia dragF F F= +

For a circular cylinder: = D2/4 , A = D per unit length;

where D is the diameter

RAOHeave

61

Courtesy of Principal Power

Simplified hydrodynamics for semi-submersibleWT?

Wave period

Can Morisons equation be applied to a semi-sub?Can Morisons equation be applied to a semi-sub?

(Kvittem and Moan, ISOPE 2012)

Integrated analysis of wind turbines 62

Regular wave analysis of a semi-submersibleResponse amplitudesResponse amplitudes

(Kvittem and Moan, ISOPE 2012)


63

63

Integrated dynamic analysis of wind turbines

Challenging hydrodynamics phenomena:

Impulsive loading

Lighthouse in large waves

Shallow water kinematics

Wave run up, deckimpact, ringing loads Dynamics excited by sum frequency

wave load components

( )( )

( )( ) ( )( )

1

21 1 2 2

2 2 21 2

21 122 2

1 2 2

1 2 1 2 1 2

1 1 1 2 2 2

sin sin0.5

0.5 cos20.5 cos22 sin sin

cos cos;

X

D x x c

c x x

x

x

c x

x x

q v v v

v v v

vv

v v v

v vwhere t t

+ += + ++ ++ + += + = +

64

Dynamic Equations of MotionsThe dynamic equilibrium of a spatial discretized FE model of a windturbine can be expressed as the following equation:

where the terms from left to right are:the inertia force vector; the damping forcevector; the internal structural reaction force vector; the external force vector;

Aerodynamic loads Hydrodynamic loads Gravitational loads Inertial loads Control loads Mooring system loads Current loads Ice loads Soil interaction loads.

The primary loadsfor an offshore wind turbine are

Frequencydependent properties,nonlinearities

65 Modelling of different subsystems(to estimate the response of a given subsystem)


Hydrodynamics- Morison formula for slender bodies- Potential theory (panel method) for large volumebodies linear versus nonlinear effects

Structural model- hull- blades (nonlinear geometry)(quasi-static versus dynamic)

Mooring model- FE model vs nonlinear spring- Damping

Soil-anchor

Drive-train


66

66

Dynamic performance: Spar type turbine of size 5 MW

If resonancecan not be avoideddamping becomescrucial

water depth:300 m

67

67

NREL 5-MW Wind Turbine mounted on a 120-m spar platform

CatenaryMooredSpar (CMS) (similar to HYWIND)

Tension Leg Spar (TLS) (similar to SWAY)

Integrated dynamic analysisExample: Spar type wind turbines

(M. Karimirad, T.Moan, various papers)

68

68

NREL 5-MW Wind Turbine mounted on a 120-m spar platform

Catenary Moored Spar (CMS) (HYWIND type) Tension Leg Spar (TLS) (similar to SWAY)

Example: Spar type wind turbines

(M. Karimirad, T.Moan, various papers)

Load cases

69

Time Domain Stochastic analysis- Extreme values for ultimate strength assessment- Fatigue load effects

- Long-term environmental data, involvingmany sea states and wind conditions

- Sampling time of irregular wave - and turbulent wind loads to reduce statistical uncertainty

Surge motion spectra and their averages at the MWL for different seed numbers. Each smoothed spectra is based on 1-h simulations using the HAWC2 code (HS = 15 m and TP= 16 s).

(Karimirad and Moan, J.Marine Structures, 2011

Surge spectra at the MWL, smoothed spectra basedon 1-h time domain simulations for five different time steps using the USFOS/vpOne code, identical waveelevation (HS = 15 m and TP = 16 s).

70

Time Domain Stochastic analysis- Extreme values for ultimate strength assessment

Simulations of bending moment (BM)- 20 2-hour samples-1 40 hours sample

Expected max in 3 hourscorresponds to about 10-4

(M. Karimirad, T.Moan, JOMAE, 2011)

71

Response spectrum blade root moment

Blade root bending moment spectrum for the below-rated wind speed case basedon a 1 h analysis (V= 8 m/s, I=0.18, HS = 2.5 m and TP = 9.8 s), tower shadowand turbulence effects on the wave-wind-induced dynamic response of a downwind TLS. (Karimirad and Moan, J.Wind Energy, 2012)

72

The mean and standard deviation of the blade root bending moment (BM), tower shadow and turbulence effects on the wave-wind-induceddynamic response of a downwind TLS. The wind turbine is shut down at windspeeds higher than 25 m/s. Significant wave heights refer to the load cases in Table IV and the corresponding wind speeds. (Karimirad and Moan, J.Wind Energy, 2012)

Statistics of blade root moment

73

73

Example: Structural dynamic response ofCatenary Moored Spar

(Karimirad and Moan, 2012)

74 Fault scenariosTransient wind loads may come from -abnormal events such as shutdown, loss of electrical network connection, faults in control system for blade pitch,activation of the mechanical/aerodynamic/generator brake system, faults in protection system and so forth

- critical environmental phenomena such as gusts, turbulence and shift in wind direction

The aerodynamic load is altered due to these transient events.

Probability of fault, wind and wave condition ?

Blade pitch fault

75

Blade pitch and control system faults

Blade seize: imbalance loads Shutdown loads: impulse from aerodynamic braking can lead to pitch vibrations What about sensor faults? Does changing the shutdown pitch rate help? Possible instability for TLPWTs (idling with one blade pitched Jonkman and

Matha, 2010)

Wilkinsonetal.,2011

P

i

t

c

h

s

y

s

t

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m

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

-1

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0

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4

T

o

w

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r

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TLP, EC 5

time - TF, s

BC

Shut down turbine quickly

Fault occurs

Continue operating with faulted blade

76Example: response analysis under faults during power production of a spar wind turbine

IEC code requires checking of nearly 40 cases with environmental loads for a system which is intact or fault.

One case is:

(Jiang et al, to appear)

Time history of tower bottom bending moment of a spar-typewind turbine under different fault conditions. Mean wind speed: 25m/s, Turbulence Intensity: 0.15, Hs=5.9m, Tp=11.3s

77

Dynamic Power Cable

Fix point

Draught = 20 m

d = 100 mSimplified lazy waveconfiguration

seabed

MSL

Cable + Buoyancy Rad. of

curv. 3 m

Node 110

9 m

85 m

Node 401

(Nasution and Svik, Marina project 2012)

78 Case Studies (Water Depth 100 m and Draught 20 m)A. Configuration of Dynamic Power Cable (Static configuration)

Near fieldconfig.

meanconfig.

Far fieldconfig.

Node 401

Node 1 (Fix point)

(Nasution and Svik, Marina project 2012)

79

Decoupled analysis to determineTooth contact forces, Bearing forces, Gear deflections.

- Global aero-hydro-servo-elasticsimulation

- Drivetrain multi-body simulationbased on main shaft loading andnacelle motions

Comparison of drivetrain responses in FWT and WT

GRC Drive train config.

(Xing and Moan, J.Wind Energy, 2012; Xing et al., submitted toJ.Wind Energy, 2012)

80

Input forces/moments and motions into drivetrain model

Forces/moments

Dummy body (in blue) where the nacelle motions are applied

Both mean and st.dev. of the low speed shaft BM (and hence bearing, toothcontact forces) increases significantly

Comparison of the standard deviationXing et al., submitted to J.Wind Energy, 2012)

81

Design for easy installation offloating wind turbines

Hywind installation

Analternativespar installation

Semisubmersible(is ready to install.Only anchor line deployment is required)

TLP(requires extensiveInstallation operations)

- requires a weather window- consideration of human factors- analysis of operations Offshore GE 3.6 MW 104 m

rotor diameter

Boeing 747-400

Perspective onmarine operations

82

Operations and MaintenanceAlternative access for inspectors and maintenance personnel

Human Inspectorspartly replacedrobots ?

An alternativeinspection/monitoringapproach

83

Field tests or demonstration projects

Blue H (Dutch), HyWind, Norway, in Italy 1 2.3-MW turbine

Beatrice, UK, 2 5-MW Alpha Ventus, Germany12 5-MW (during construction)

Other projects for floating wind turbines:- Noweri (NOWITECH-NORCOWE); Norway- Principle Power (American) at a site in Portugal- Japan, Spain, USA

Field tests to demonstrate- functionality- validate analysis tools

Laboratory tests- Rotor blades- Drive train- Support structure- Model basins/wind tunnels.

84

Demo

El.-gridconnection

Technology focus

Cost focus

Market focus

Floating wind turbines from idea to a commercial product (technology qualification)

Medium park

Model testing

Concept &theory

Technologyrating

TimeTime

85

Utilization of the wind farm space

FishfarmWEC array Aquaculture farm

86Combined wind turbine and wave energy converters

ID 100 TLP and 6 PAs linked to the pontoons of the

structure

ID 48 TLP WT with 3 point absorbers in the legs of the

structure

ID 2 Floating Spar Wind Turbine Foundation with torus point absorber

ID 85 Floating overtopping device (like Wave Dragon) with two WTs on it

ID 29 Triangular Semisubmersible Platform with 2WTand PAs between the

columns (similar to W2POWER)

87

Combined WT and WEC concepts

Spar

Torus

Wind turbinewith rated power:5 MW

Shared mooring system and cable Synergy in maintenance

Power production

0

1000

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4000

5000

6000

7 9 11 13 15 17 19

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Mean Wind Power - Spar FWT alone

Mean Wind Power - STC

Mean Wave Power - STC

(Muliawan et al., J. Renewable Energy, 2013)

Spar-Torus concept

Semi-sub-Flap concept(Luan, Michailides, et al, 2013)

88

Concluding remarks A huge untapped potential for offshore wind power exists. Technology is still at an early stage, especially for floating wind turbines

- Various concepts need to be pursued- possible influence on rotor and drive train design

Rules and standards for design of floating wind turbine is urgently needed.

Significant efforts are required to - increase robustness/reliability, - reduce costs (utilise mass production potential)

Concerted efforts in R & D are required by theindustry, research institutes and universities

integrated dynamic analysis consideration of faults

Implement relevantknowledge from-Oil and gas industry-Coastal engineering-Aquaculture technology

89

AcknowledgementThanks to researchers and PhD candidatesin CeSOS and Nowitech for excellent cooperation

Thank you!

Copyright: Faulkner, EWEA

90 General background informationBianchi DF, Battista HD, and Mantz RJ (2007) Wind Turbine Control Systems. Germany: Springer.Burton T, Sharpe D, Jenkins N, and Bossanyi E (2008) Wind Energy Handbook. Chichester, UK: John

Wiley and Sons Ltd.Det Norske Veritas/Ris National Laboratory (2002) Guidelines for Design of Wind Turbines, 2nd edn.,

Denmark: Jydsk Centraltrykkeri.DNV (2007) Design of offshore wind turbine structures. DNV-OS-J101. Oslo, Norway: Det Norske Veritas.EC (2009), International Standard 61400-3, Wind Turbines, Part 3: Design Requirements for Offshore

Wind Turbines. Geneva, Switzerland: IEC.Faltinsen OM (1995) Sea Loads on Ships and Offshore Structures. Cambridge, UK: Cambridge University

Press.Hansen MOL (2008) Aerodynamics of Wind Turbines. 2nd edn., London, UK: Earthscan.Hansen MOL, Sorensen JN, Voutsinas S, et al. (2006) State of the art in wind turbine aerodynamics and

aeroelasticity. Progress in Aerospace Sciences Journal 42: 285330.Henderson AR (2003) Hydrodynamic Loading on Offshore Wind Turbines. OWTES Task 4.2, The

Netherlands: TUDelft.Lysen EH (1983) Introduction to Wind Energy. The Netherlands: SWD Publications, SWD 82-1.Manwell JF, McGowan JG, and Rogers AL (2006) Wind Energy Explained, Theory, Design and

Application. Chichester, UK: John Wiley and Sons Ltd.Naess A and Moan T (2005) Probabilistic design of offshore structures. In: Chakrabarti S (ed.) Handbook

of Offshore Engineering, ch. 5, pp. 197277. Oxford, UK: Elsevier Ltd.Roddier D, Cermelli C, and Weinstein A (2009) WINDFLOAT: A floating foundation for offshore wind

turbine, Part I: Design basis and qualification process. Paper No. OMAE2009-79229. Proc.OMAEConf., Hawaii, USA, May 31June 5.

Twidell J and Gaudiosi G (2008) Offshore Wind Power. Essex, UK: Multi-Science Publishing Co Ltd.

Further references in the following

91

Karimirad M. and Moan T., Stochastic Dynamic Response Analysis of a Tension Leg Spar-Type Offshore Wind Turbine, Journal of Wind Energy (Wiley), Wind Energ. (2012) 2012 John Wiley & Sons

Karimirad M. and Moan T., A simplified method for coupled analysis of floating offshore wind turbines, Journal of Marine Structures 27 (2012), pp. 45-63 /

Moan, T. Z Gao, M Karimirad, E E Bachynski, M Etemaddar, Z Jiang, M I Kvittem, M. Muliawan, Y Xing, RECENT DEVELOPMENTS OF THE DESIGN AND ANALYSIS OF FLOATING WIND TURBINES, The Royal Institution of Naval Architects (RINA), ICSOT 2012: International Conference Ship & Offshore Technology 23-24 MAY 2012, Busan, South KOREA

Karimirad M. and Moan T., Comparative Study of Spar-Type Wind Turbines in Deep and Moderate Water Depths, the 31st International Conference on Ocean, Offshore and Arctic Engineering, OMAE2012 conference, OMAE2012-83559, Published by ASME, 1-6 July, Brazil

Karimirad M. and Moan T., Feasibility of the Application of a Spar-type Wind Turbine at a Moderate Water Depth, DeepWind conference, 19-20 January 2012, Trondheim, Norway, Journal of Energy Procedia, Elsevier, Energy Procedia 24 (2012 ) 340 350

Gao, Z. et al., Comparative Study of Wind- and Wave-Induced Dynamic Responses of Three Floating Wind Turbines Supported by Spar, Semi-Submersible and Tension-Leg Floaters, Proc.ICOWEOEConference, Beijing, 2011.

Wind turbine modelling and analysis92

Muliawan, M.J., Karimirad, M., Moan, T. and Gao, Z. STC (SPAR-TORUS COMBINATION): A COMBINED SPAR-TYPE FLOATING WIND TURBINE AND LARGE POINT ABSORBER FLOATING WAVE ENERGY CONVERTER PROMISING AND CHALLENGING, the 31st OMAE2012 conference, ASME, 1-6 July, Brazil

Muliawan, M.J., Karimirad, M. Moan, T.Dynamic Response and Power Performance of a Combined Spar-type Floating Wind Turbine and Coaxial Floating Wave Energy Converter, Journal of Renewable Energy; Volume 50, February 2013, Pages 47-57

Combined WT and WEC

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Xing, Y.H., Moan, T., Multi-body modelling and analysis of a planet carrier in a wind turbine gearbox, J. Wind Energy (accepted) (2012).

Xing, Y.H., Karimirad, M., Moan, T., Effect of spar-type floating wind turbine nacelle motion on drivetraindynamics, European Wind Energy Association annual event, Copenhagen, Denmark, April 2012.

LaCava, W., Xing, Y.H., Guo, Y., Moan, T., Determining wind turbine gearbox model complexity using measurement validation and cost comparison, European Wind Energy Association annual event, Copenhagen, Denmark, April 2012.

Link, H., LaCava, W., van Dam, J., McNiff, B., Sheng, S., Wallen, R., McDade, M., Lambert, S., Butterfield, S., Oyague, F., Gearbox Reliability Collborative Project Report: Findings from Phase 1 and Phase 2 testing, Technical Report, National Renewable Energy Laboratory, Colorado, USA, 2011.

Drive train modelling and analysis

083 Moan Dynamic Analysis Wind Turbine

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