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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 priceNo 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 environmentDifficult 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
5
Control 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)
Pro
jejC
apE
x(U
S$M
/MW
)
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
9
Support 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 risersLimited motionsMobility of drilling vesselsAccess for IMMR
Wind turbines for production ofelectrical power
Provide support of payloadLimited motionsAccess 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[R≤S]
r,s
fS(s)
fR(r)
PF=P[R≤S]
r,s
16
ULS- stability The stabilizing (righting) moment,
MR is expressed by
GZM R ⋅= Δ
The metacentric height, 1 ⎡ ⎤= − + −⎣ ⎦∇ ∑W wGM I i KB KG
= ⋅O windM F a
a- distance from wind resultant, Fwind to the centre of submergedvolume
Overturning moment due to wind
Design criteria: Limit states
= ⋅ θ( )GZ GM sin
a
B
G
= ⋅ θsin( )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
M0
NM0
N
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
= ≤ Δ∑
= = Γ +∑ β0
0( / 1)
(ln )m
i
i
n SND mN K N
[ ] oo NssP /1=>
K, m - constants in the SN-curve: N = KS-m
N - number of stress ranges in the service period
- shape parameter of the Weibull distributionβ
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.Jørgensen, DNV
25Design of bottom supported turbines
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
θ(deg)R
otor
tor
que,
T*1
000(
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
λ
Pow
er c
oeffi
cien
t
Cp-expCp
30
• Waves -wind generated waves-swell
• Surface elevation (Airy theory)− regular wave− stochastic
Short-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
ς ς ω
ς ς ω ε
Wave loads
The sea surface can be represented as a superposition of regularwaves with differentfrequency and direction ofpropagation
31Wave kinematics
Φ - velocity potential
Nonlinear boundary condition
ζ(x,t)
= 0∂Φ∂n
z
∇2Φ = 0
v - particle velocitya - particle accelerationp - pressure
as obtained from the velocity potential
x
vx = ∂Φ∂x
= vx(0) exp[ z]2πλ
A B
Ddiametervx
-λ/2λ/2
deep water-d
z
The satisfaction of the surfaceboundary condition is the major challenge, because it is dependent upon the wave elevation, ζ(x,t) itself
If ζ(x,t) = 0 linear (Airy) theory is obtained.
Airy theory
0=∂Φ∂
−∂∂
∂Φ∂
+∂∂
∂Φ∂
+∂∂
zyyxxtζζζ
021 222
=⎟⎟
⎠
⎞
⎜⎜
⎝
⎛⎟⎠⎞
⎜⎝⎛
∂Φ∂
⎟⎟⎠
⎞⎜⎜⎝
⎛∂Φ∂
+⎟⎠⎞
⎜⎝⎛
∂Φ∂
+∂Φ∂
+zyxt
gζ
Note:- Kinematics at a depth z:
i.e. in-phase for different z.- Phase difference in two points with
different coordinate x
⎟⎠⎞
⎜⎝⎛ −= xta λ
πωζζ 2sin32
Particle velocity, v
Airy Wave profile
⎟⎠⎞
⎜⎝⎛ −= xta λ
πωζζ 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 Stoke’s waveζ = ζa sin (ωt-kx) - π ( )ζa cos [2(ω t-kx)]ζa
λ
Wave profiles with increasing order of Stokes wave theory
wave steepness k ζa=0.4w
ave
p ro
file
ζ/ζ a
AIRY STOKES IVSTOKES IIISTOKES II
π2 π
32 πPhase 2π
1
0.5
0
-0.5
-1Φ - velocitypotential
= 0
Nonlinear boundary condition
ζ(x,t)
∂Φ∂n
z
0
02 =Φ∇
0=∂Φ∂
−∂∂
∂Φ∂
+∂∂
∂Φ∂
+∂∂
zyyxxtζζζ
021 222
=⎟⎟
⎠
⎞
⎜⎜
⎝
⎛⎟⎠⎞
⎜⎝⎛
∂Φ∂
⎟⎟⎠
⎞⎜⎜⎝
⎛∂Φ∂
+⎟⎠⎞
⎜⎝⎛
∂Φ∂
+∂Φ∂
+zyxt
gζ
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/λ
q(pr.unitlength)
Cross-section area2
4DA π
=
Hydrodynamic loading. Morisons formula• Steady state loading on
slender members
q= CM · ρ·A·a·+½CD · ρ · D · v|v|
CM, CD - coefficientsρ - density
a - 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
ρ - density of waterCD - drag coefficient
Wave forceq = qD + qI
Drag forceqD = ½ CD ρ 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
β β
β
β
β β
β β β β
β ω ε β ω ε
∝ + +
= + +
−
−
+ +
+ + − +
= + = +
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
ρ ∂= ⋅ ∀⋅
∂
For a slender body
A AF C d aρ= ⋅ ⋅ ∀⋅
1/ 2 | |D D n nF C d v vρ= ∀⋅Nomenclature: - density
- volume- volume per unit length- added mass coefficient- drag coefficient
D
A
CCd∀∀ρ
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.Øye, DTU)• HAWC2 (Risø), • 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
• Aerodynamics – incl. wakes in farms- BEM method (Simplified: Thrust; Refined:CFD)
• 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
Stochastic analysisof the response to irregualr waves and turbulent wind,.. to reduce statisticaluncertainty
40Design of bottom supported turbines
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.Jørgensen, 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
Technology Development
- 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
8-1s e las tic bend ing m odes
8-1s e las tic bend ing 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
Criteria“Stability”• 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
Technology Development
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
+= π2 ii ii
iii
M ATK
Technology Development
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
Technology Development
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 turbinesAerodynamic, 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 for the design of
the support structure, tower, rotor, drive trainThe system modelling includes: ■modeling of excitation mechanisms (wind, waves and current)
■ rotor aerodynamics
■ hydrodynamics
■ structural dynamics
■ automatic control theory
■ power generation
Wind-industry based• Bladed (Garrad Hassan)• Flex5 (S.Øye, DTU)• HAWC2 (Risø), • 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 RELT R C Uπρ=
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
ρ ∂= ⋅ ∀⋅
∂
For a slender body
A AF C d aρ= ⋅ ⋅ ∀⋅
1/ 2 | |D D n nF C d v vρ= ∀⋅
Nomenclature: - density- volume- volume per unit length- added mass coefficient- drag coefficientD
A
CCd∀∀ρ
Integrated analysis of wind turbines
59
− ω2 [M+ A(ω)] r(ω) + i ω B(ω)r(ω) + Kr(ω) = R(ω),
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 (ω) = −[− ω 2A()r(ω) + iB(ω)r(ω)]
∞∞ ∞
−∞
+ + + − τ τ τ + =∫(M A )r B r k(t )r( )d Kr(t) R(t)
Time domain formulation for wave loading
∞∞= ω − ω ω
π ∫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
ρ ρ
ρ ρ
( )212
= −drag DF C A v rρ
+ + = totmr cr kr F
tot inertia dragF F F= +
For a circular cylinder: = πD2/4 , A = D per unit length;
where D is the diameter∀
RAOHeave
61
Courtesy of Principal Power
Simplified hydrodynamics for semi-submersibleWT?
Wave period
Can Morison’s equation be applied to a semi-sub?Can Morison’s 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)
Integrated analysis of wind turbines
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
β β
β
β
β β
β β β β
β ω ε β ω ε
∝ + +
= + +
−
−
+ +
+ + − +
= + = +
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)
• 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- hull- blades (nonlinear geometry)(quasi-static versus dynamic)
• Mooring model- FE model vs nonlinear spring- Damping
• Soil-anchor
• Drive-train
Stochastic analysisof the response to irregualr waves and turbulent wind,.. to reduce statisticaluncertainty
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)
Wilkinson et al., 2011
Pitch system
-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
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 Sævik, 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 Sævik, 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
2000
3000
4000
5000
6000
7 9 11 13 15 17 19
Ele
ctri
cal
Pow
er P
rodu
ctio
n (k
W)
Vmean (m/s)
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 remarksA 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 analysisconsideration 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/Risø 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: 285–330.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. 197–277. 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 31–June 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
93
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