Offshore Wind Farm: Loads, dynamics and structures

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Transcript of Offshore Wind Farm: Loads, dynamics and structures

  • 1. Loads, dynamics and structural designOffshore Wind Farm DesignMichiel Zaaijer2007-20081DUWIND

2. OverviewIntroductionModelling offshore wind turbinesTypes of analysis and toolsLoads and dynamics in design2007-2008 2 3. 2007-2008 3 4. IntroductionLoads, dynamics and structural design2007-20084 5. Harmonic loadingSimple Harmonic Loading1F (t ) = F sin ( t + ) Loading010 1 23 45 6 Time- Gravity loads on blades- Mass imbalance rotor (1P)- Aerodynamic imbalance (1P)27 RPM = 0.45 Hz- Small regular waves2007-20085 6. Non-harmonic periodic loadingF (t ) =Complex Cyclic Loading 0.5 1st Signal (0.5 Hz)1 Loading0 0.5 0 123 4 5 6 a0 + ak F sin ( kt + k ) Time Loading 2nd Signal (1 Hz) 0.50 Loading0 0.5 k 0 123 4 5 6 Time 3rd Signal (1.5 Hz)10.5 Loading0 12 3 4 5 60Time 0.5 0 123 4 5 6 Time- Wind-shear- Yaw misalignment- Tower shadow3P- Rotational samplingof turbulence (all 2P or 3P and multiples)2007-20086 7. Non-periodic random loading Random Load 1 Loading 0 1 0 1 2 34 5 6 Time- Turbulence (small scale)- Random waves2007-2008 7 8. Other (non-periodic) loadingTransientsShort eventsStep Load Impulsive Load1 1LoadingLoading0 01 10 1 2 3 4 5 6 0 1 23 4 5 6Time Time- Start/stop- Extreme gust- Turbine failures- Extreme waves- Storm front2007-20088 9. IntroductionLoads, dynamics and structural design2007-20089 10. The effect of dynamics2 Force [N]10F -1 1 k-2m21 Response [m]k = 1 [N/m] x 0-1-2 1 ?2007-200810 11. The effect of dynamics2 21 10 011 -1 -1Force [N]m = 0.0005 [kg] -2 -22 21 (Static) 10 011 -1 -1m = 0 [kg] m = 0.1 [kg] -2 -22007-200811 12. The effect of dynamics 2 SystemF Force [N] 1k0m1-1-2Fintern = k x 2Response [m]Internal forces external forces 1due to dynamics 0 1-1Internal forces drive the design,m = 0.0005 [kg]-2not external forces!2007-200812 13. Dynamic amplification factor 5Dynamic Amplification Factor (DAF) 4 3 ResonanceDAF = Dynamic amplitude2Static deformation 1 0 0 1 2 34 5Frequency Ratio fexcitation damping ratio = 0.1 damping ratio = 0.2 damping ratio = 0.5fnaturalNote: the DAF is defined for harmonic excitation2007-200813 14. Character of resonanceExcitation frequency natural frequencyLarge oscillationsFatigue damage (due to severe cyclic loading)Generally not destructive (anticipated in design)Natural frequencies of wind turbine (-components)are close to several excitation frequencies2007-200814 15. Classification for wind turbines soft-soft soft-stiffstiff-stiffExcitationDAF1Response1P3P f02007-200815 16. Soft-stiff example 1P = 0.45 Hz fnatural = 0.55 Hz 3P = 1.35 Hz2007-200816 17. Reduced response to loadingAlleviation of (wind) loading byDAF shedding loads through motionTypical rotor loadingfrequencies1 soft-softstructureTypical wave loadingfrequencies1P3P f02007-2008 17 18. Increased response to loading Quasi-static or amplified response to wave loading 1-P *-P2007-200818 19. Single degree of freedom system x k m c FF = m && + k x + c xx&2007-200819 20. Wind turbine characteristics Stiffness Material properties / soil properties Buoyancy of a floating structure Damping Material properties / soil properties Aerodynamic loading Control (Viscosity of water / radiation in soil) Inertia Material properties Hydrodynamic loading (water added mass) Entrained water mass2007-2008 20 21. Linear / non-linear systemsLinear system: x(t)y(t)ax1(t) + bx2(t) ay1(t) + by2(t)Initial condition x0: x(t) + x0 y(t) + y0Non-linear system: No superposition possible Possible dependency on initial conditions Possible variation in output statistics for the sameinput (statistics) 2007-2008 21 22. Non-linearities for wind turbines Aerodynamic loading Hydrodynamic loading extreme waves waves and currents Speed and pitch control some algorithms settings for various wind speeds Extreme deformations (2nd order effects)2007-200822 23. IntroductionLoads, dynamics and structural design2007-200823 24. Lifelong response signalResponse(loading + dynamics)Extreme events Time Lifelong variations2007-2008 24 25. Effects of loads and dynamics Ultimate limit state (ULS)(maximum load carrying resistance) Yield and buckling Loss of bearing / overturning Failure of critical components Fatigue limit state (FLS)(effect of cyclic loading) Repeated wind and wave loading Repeated gravity loading on blade2007-200825 26. Effects of loads and dynamics Accidental limit state (ALS)(accidental event or operational failure, local damageor large displacements allowed) Ship impact Serviceability limit state (SLS)(deformations/motion, tolerance for normal use) Blade tip tower clearance Vibrations that may damage equipment Tilt of turbine due to differential settlement2007-200826 27. Design drivers of wind turbinesComponent Design drivers Ultimate FatigueTower toptop mass-Tower- wind/waveSubmergedwind/wave/current wind/wavetowerFoundation wind/wave/current -2007-2008 27 28. Importance of dynamics in design Increase or decrease of maximum load Affects Ultimate Limit State conditions Increase or decrease of number of load cycles andtheir amplitudes Affects Fatigue Limit State / Lifetime2007-2008 28 29. Effect on structural design Support structure cost Soft-stiff monopile 100 % Soft-soft monopile 80 % Energy cost Soft-stiff monopile 100 %Monopile MonopileSoft-soft monopile 95 %soft-stiff soft-soft2007-200829 30. Use of dynamic models 1.2.3.Analyse systemReduce internal Assess lifelongpropertiesloads and match loadingresistanceAvoid resonance Validate reliabilityand instabilities Make lightest and technicaland cheapestlifetimestructural design 2007-200830 31. Modelling of offshore wind turbinesStructural models of rotor, nacelle and support structure2007-200831 32. Flexibility of wind turbines Rotor - Rotation Drive train - Torsion Blades- Flapwise bending - Edgewise bending - Torsion Tower - Bending - Torsion Foundation - Rotation - Horizontal - Vertical2007-2008 32 33. Integrated dynamic modelOffshore wind turbineControllersWindRotor Drive train Generator GridWaveSupport structure2007-200833 34. Rotor model Distributed mass-stiffnessAerodynamic properties Beam theory FEMTables, EIx,y,pCl Cd 2007-2008 34 35. Drive train modelTransmission ratio IgeneratorIhub+low speed shaftStiffness torsion in Damping transmissiontransmission and main shaft; suspension and generatormain shaft bending torque control2007-2008 35 36. Generator modelTk,gsynchronous generator:generator0 0 n0 nmotorTk,g induction generator:Slip generator0 0n0 2n0 nmotor2007-2008 36 37. Tower modelDistributed mass-stiffness Modal representationBeam theoryFEM+ =, EIx,y,p Deflection Deflection Total1st mode 2nd mode deformationEffective reduction of DOFs2007-2008 37 38. Foundation model H kx x k x u M = kk x 2007-2008 38 39. Modelling of offshore wind turbinesDeriving parameters for foundation models2007-200839 40. Importance of foundation model 3.5m Rotor/nacelle mass 130,000 kg 3.0mFirst natural frequency (Hz)30 x 4.6m without foundation 0.34627 2800 x 20 6.0m2800 x 25 with foundation0.29055 40.0m 12.0m with scour 0.28219 2800 x 32 12.0m2800 x 60 5.0m Second natural frequency (Hz) 12.0m Flange without foundation 2.2006 15.0m2800 x 60 Tower 15.4mBoat Landing MSL with foundation1.3328 2500 x 1005.0m 6.0m with scour 1.2508 21.0m 100x 3.0mPile3500 x 75 J-Tube 37.0m Scour Protection (25.0m Penetration)2007-2008 40 41. Enhanced foundation model External shaft friction Lateral resistance(t-z curves) (p-y curves)Use:Standards (API/DNV)Internal shaft frictionExisting software (t-z curves)(In exercise: ANSYS Macros)Pile plug resistence Pile point resistance(Q-z curves) (Q-z curves) 2007-200841 42. ScourPile0 Vertical effective soil pressureSeabedGeneral scour depthNo scour conditionLocal scour depth Typically 1-1.5 timesGeneral scour only pile diameterLocal scour condition Overburden reduction depth Typically 6 times pile diameter2007-2008 42 43. Effective fixity length ConfigurationEffective fixitySeabedlengthEffectiveStiff clay 3.5 D 4.5 Dfixity Very soft silt 7D8D length General calculations 6D Experience with3.3 D 3.7 D offshore turbines2007-200843 44. Uncoupled springsForced displacement/rotationuIn exercise: Use MF ANSYS Macrosand method B forTower a monopile Method AIgnore M Ignore F Applied force/momentFMRotationu Seabed Method BTranslationsIgnore Ignore u2007-2008 44 45. Stiffness matrixRun two load cases with FEMTower model with py-curves(See next slide)Seabed H kx x k x u M = kk x Stiffness matrix2007-2008 45 46. FEM-based pile-head stiffness1. Solve FEM for F1, M1F1 = k xx u1 + k x 1 (F1, M1 near loading situation of interest) M 1 = kx u1 + k 12. Solve FEM for F2, M2F2 = k xx u 2 + k x 2 (F2, M2 near loading situation of interest) M 2 = kx u 2 + k 23. Scratch one equation and solve kxx, kx, k (kx = kx, assume matrix equal for both loads)4. Check assumption with another FEM solution2007-200846 47. Selection of pile foundation models Foundation flexibility significant enough to requireclose consideration of modelling Effective fixity length model dissuaded Stiffness matrix much more favourable than uncoupledsprings For exercise: Monopile in Bladed modeled with uncoupled springs (unfortunately)2007-200847 48. Documented GBS model parameters Spring ViscousInertiaLumped springsGBSstiffnessdamping and dashpots for:- Horizontal G D3D4 G D5 - VerticalRocking 3 (1 )0.65 32 (1 ) 0.6432 (1 ) - Rocking 16G D (1 ) D2 G0.76 D3Horizontal7 84.6 4 (2 )8 (2 )2G DD2 G D33.41.08Vertical 1 4 (1 ) 8 (1 ) 2007-2008 48 49. Types of analysis and toolsNatural frequency and mode analysis2007-200849 50. FEM modal analysis1FEM analysis provides: Natural frequencies Mode shapes (Pre-processed)Y Z Xmatrices of structuralproperties:Parametric support structure model generation Mass Stiffness Damping2007-2008 50 51. Natural freque