Information about 5 - Dynamic Analysis of an Offshore Wind Turbine: Wind-Waves Nonlinear...

ASCE Earth & Space 2010 OWT Symposium

http://content.asce.org/files/pdf/EarthSpace2010Prelim-FINAL.pdf

http://ascelibrary.org/doi/book/10.1061/9780784410967

http://content.asce.org/files/pdf/EarthSpace2010Prelim-FINAL.pdf

http://ascelibrary.org/doi/book/10.1061/9780784410967

EARTH & SPACE 2010 – March 14-17 Honolulu HI 2/16 INTRODUCTION MODEL DESCRIPTION RESULTS CONCLUSIONS PURPOSE AND CONTENTS OF THE WORK In this work the dynamic analysis of a monopile supported offshore wind turbine forced by a random wind a wave excitation in the frequency and time domain is carried out by means of the ANSYS finite element model. The effects of non-linear interaction is investigated for possible reduction of vibration peaks in the structural response. In the following: 1. an introduction to the problem and the analysis methodology adopted is given; 2. the main features of the finite element model and the analytical model for simulating wind-wave random forcing are illustrated; 3. the results of the analyses carried out are discussed by pointing out the nonlinear effect induced by wind-waves interaction; 4. final conclusions concerning the study are then illustrated.

EARTH & SPACE 2010 – March 14-17 Honolulu HI 3/16 INTRODUCTION MODEL DESCRIPTION RESULTS CONCLUSIONS INTRODUCTION Offshore wind turbines represent rather complex structural systems. ENVIRONMENT STRUCTURE STOCHASTIC INTERACTING TIME-VARYING Though the major regularity and power of the offshore wind forcing, they could become competitive if a proper design approach is established by taking into account the above factors and assuring a good compromise between safety and costs related aspects. STRUCTURAL BEHAVIOR LOADS (wind, wave, current etc.) CONSTRAINTS (soil etc.) PROPERTIES (mechanical, geometrical etc.) MULTI-SCALE (support, junctions etc.)

EARTH & SPACE 2010 – March 14-17 Honolulu HI 4/16 INTRODUCTION MODEL DESCRIPTION RESULTS CONCLUSIONS INTRODUCTION To obtain such a goal, design method of an offshore wind turbine requires a critical revision according to the systemic approach. Systemic Decomposition sub-problem sub-problem sub-problem ENVIRONMENT LOADS (wind, wave, current etc.) CONSTRAINTS (soil etc.) STRUCTURE PROPERTIES (mechanical, geometrical etc.) MULTI-SCALE (support, junctions etc.) STOCHASTIC INTERACTING TIME-VARYING STRUCTURAL BEHAVIOR sub-problem sub-problem complexity

EARTH & SPACE 2010 – March 14-17 Honolulu HI 5/16 INTRODUCTION MODEL DESCRIPTION RESULTS CONCLUSIONS INTRODUCTION The natural frequency of a typical offshore wind turbine operating in intermediate-depth water is wedged between the wind and wave excitation frequency; in this context simulation of wind-wave nonlinear interaction become a crucial aspect as it can led to a beneficial damping by selecting proper structural stiffness of the turbine’s support: this would lead to an increase fatigue life and reduce the cost of the support. WIND excit. STRUCTURE WAVE excit. frequency Nonlinear Interaction INTERNATIONAL CODES AND STANDARDS

EARTH & SPACE 2010 – March 14-17 Honolulu HI 6/16 INTRODUCTION MODEL DESCRIPTION RESULTS CONCLUSIONS F.E. MODEL DESCRIPTION The In the present work a 5MW 3-bladed offshore wind turbine with monopile-type support is considered as economically convenient for intermediate water depth purposes: it represents a structure of interest for possible planning of an offshore wind farm in the Mediterranean Sea near the south-eastern cost of Italy. Monopile type support Z Y X Aerodynamic Fluid- dynamic Geotechnical Foundation Submerged Emergent d lfound H mud line Z Y X Z Y X Aerodynamic Fluid- dynamic Geotechnical Foundation Submerged Emergent d lfound H mud line H = 100m d=35m lfound=40m D =5m tw=0.05m Dfound=6m D = diameter of the tubular tower; tw = thickness of the tower tubular member; FIXED effects of foundation are neglected (the lower node is fixed at the sea bottom) beam elements (BEAM4) for simulating the tower blades and nacelle replaced by a concentrated mass (MASS21)

EARTH & SPACE 2010 – March 14-17 Honolulu HI 7/16 INTRODUCTION MODEL DESCRIPTION RESULTS CONCLUSIONS WIND-WAVE SPECTRA A typical wind-wave forcing with relatively small recurrence period is assumed in the following calculations (exercise load): this could be crucial for fatigue-induced long term damage. 4 5.19 4 4 5 2 74.0 , 4 5 ,0081.0 exp2)( === −⋅ ⋅ = mPM pPMPM p PM PM V g g S β ωβα ω ω β ω α πωηη α = hub hubmm z z VzV )( [ ] 6522 )(8701 )( )( 4 / i m i i ijij zf. zV zLf σ (f,z)Sf + = ( ) + − −= )()(2 )( exp)()()( 22 kmjm kjz ikikijijijik zVzV zzCf fSfSfS π wvui ,,= ( )[ ] 2 0 2 751)log(arctan116 *i u.zg.-σ += Pierson-Moskowitz wave spectrum Wind velocity: mean and turbulent spectrum z y x,x’ z’ y’ M ean water level Mud line Waves Mean wind Current P (t)vP (t)w P (t)uP Turbulent wind Vm(zP) P Mean water level Mud line Hub level R H h vw(z’) Vcur(z’) z y z y x,x’ z’ y’ x,x’ z’ y’ M ean water level Mud line Waves Mean wind Current P (t)vP (t)w P (t)uP P (t)vP (t)w P (t)uP Turbulent wind Vm(zP) P Mean water level Mud line Hub level R H h vw(z’) Vcur(z’) normalized half-side von Karman

EARTH & SPACE 2010 – March 14-17 Honolulu HI 8/16 INTRODUCTION MODEL DESCRIPTION RESULTS CONCLUSIONS Assuming linear wave theory and performing the Fourier transform of the elementary force experienced by a structural member, force spectra are obtained for both wind and wave. WIND-WAVE FORCE SPECTRA d |z| z x d+z dF(z,t)dz A A Sect. A-A D tw d z x dF(z,t)dz A A Sect AA D tw ),(),( 8 ),(),( tzxtzCtzxCtzdF xDI &&& &σ π += Linearized Morison equation [ ]2 ),( 2 1 ),( tzxdACtzdF DD &ρ= Aerodynamic drag force [ ] )( )()cosh( 8 )cosh( )sinh( ),( 2 2 2 ω σ π ω ω ω ηηS zkzC kzC kd zS ixiD iI iFF + = & ( ) ∫∫= A ikijDmkjFiFi dAdASCVzzS 2 ),,( ρω Wind force spectrum Wave force spectrum

EARTH & SPACE 2010 – March 14-17 Honolulu HI 9/16 INTRODUCTION MODEL DESCRIPTION RESULTS CONCLUSIONS Assuming linear wave theory and performing the Fourier transform of the elementary force experienced by a structural member, force spectra are obtained for both wind and wave. WIND-WAVE FORCE SPECTRA ),(),( 8 ),(),( tzxtzCtzxCtzdF xDI &&& &σ π += Linearized Morison equation [ ]2 ),( 2 1 ),( tzxdACtzdF DD &ρ= Aerodynamic drag force [ ] )( )()cosh( 8 )cosh( )sinh( ),( 2 2 2 ω σ π ω ω ω ηηS zkzC kzC kd zS ixiD iI iFF + = & ( ) ∫∫= A ikijDmkjFiFi dAdASCVzzS 2 ),,( ρω Wind force spectrum Wave force spectrum Vm hub = 20m/s 1.E-01 1.E+01 1.E+03 1.E+05 1.E+07 1.E+09 1.E+11 1.E-04 1.E-02 1.E+00 1.E+02 1.E+04 freq [Hz] Forcespectra[N2 /Hz] Wind Wave

EARTH & SPACE 2010 – March 14-17 Honolulu HI 10/16 INTRODUCTION MODEL DESCRIPTION RESULTS CONCLUSIONS RESPONSE SPECTRA: WAVE ONLY The frequency of the first relative maximum corresponds to the peak frequency of the wave force spectrum (about 0.1Hz); the absolute maximum of the structural response occurs however at about 0.2Hz which is very close to the first vibration mode of the structure. 1.E-01 1.E+01 1.E+03 1.E+05 1.E+07 1.E+09 1.E+11 1.E-04 1.E-02 1.E+00 1.E+02 1.E+04 freq [Hz] Forcespectra[N2 /Hz] Wind Wave fp = 0.1 Hz 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 freq [Hz] Responsespectra[m 2 /Hz] X direction fp = 0.1 Hz fn = 0.2 Hz

EARTH & SPACE 2010 – March 14-17 Honolulu HI 11/16 INTRODUCTION MODEL DESCRIPTION RESULTS CONCLUSIONS A spectrum in the direction y orthogonal to the mean wind speed appears due to component correlation. Two maxima occur for the peak frequency of the wind spectrum and close to the first mode frequency of the structure. RESPONSE SPECTRA: WIND ONLY 1.E-01 1.E+01 1.E+03 1.E+05 1.E+07 1.E+09 1.E+11 1.E-04 1.E-02 1.E+00 1.E+02 1.E+04 freq [Hz] Forcespectra[N2 /Hz] Wind Wave fp = 0.1 Hz 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 freq [Hz] Responsespectra[m 2 /Hz] X direction Y direction fn = 0.2 Hz

EARTH & SPACE 2010 – March 14-17 Honolulu HI 12/16 INTRODUCTION MODEL DESCRIPTION RESULTS CONCLUSIONS The increasing roughness length of the sea surface owing to the presence of the wave field has been modeled (iterative procedure). No contribution is present in y-axis due to the absence of wave directional spreading. RESPONSE SPECTRA: COMBINED WIND-WAVE The in x-direction wind-wave combination produces the appearance of a relative maximum at the wave peak frequency. The resultant response spectrum appears to be the superposition of the wind-only and wave-only response. 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 freq [Hz] Responsespectra[m2/Hz] X direction Y direction fp = 0.1 Hz fn = 0.2 Hz

EARTH & SPACE 2010 – March 14-17 Honolulu HI 13/16 INTRODUCTION MODEL DESCRIPTION RESULTS CONCLUSIONS The increasing roughness length of the sea surface owing to the presence of propagating waves has been modeled (iterative procedure). RESPONSE SPECTRA: COMBINED WIND-WAVE 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 freq [Hz] Responsespectra[m2/Hz] X direction Y direction 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 freq [Hz] Responsespectra[m 2 /Hz] X direction 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 freq [Hz] Responsespectra[m 2 /Hz] X direction Y direction WIND ONLY WAVE ONLY WIND + WAVE

EARTH & SPACE 2010 – March 14-17 Honolulu HI 14/16 INTRODUCTION MODEL DESCRIPTION RESULTS CONCLUSIONS A load time history is generated in time domain with Montecarlo method; both wind and wave actions associated with 4 different wind mean speeds are considered; corresponding peak displacements at the hub height are evaluated. TIME DOMAIN ANALYSIS: COMBINED WIND-WAVE -1.00 -0.80 -0.60 -0.40 -0.20 0.00 0.20 0.40 0.60 200 700 1200 1700 2200 2700 3200 time [s] dalong hub [m] 0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 15 20 25 30 35 40 45 50 55 Vm hub [m/s ] dpeak along hub [m] Time domain (* =samples) Frequency domain Comparison with results from spectral analysis shows that nonlinear interaction can be reasonably neglected for wind speed lower than 20m/s; Vm hub=20 [m/s]

EARTH & SPACE 2010 – March 14-17 Honolulu HI 15/16 INTRODUCTION MODEL DESCRIPTION RESULTS CONCLUSIONS A load time history is generated in time domain with Montecarlo method; both wind and wave actions associated with 4 different wind mean speeds are considered; corresponding peak displacements at the hub height are evaluated. TIME DOMAIN ANALYSIS: COMBINED WIND-WAVE Comparison with results from spectral analysis shows that nonlinear interaction can be neglected for wind speed lower than 40m/s; -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 200 700 1200 1700 2200 2700 3200 time [s] dacross hub [m] Vm hub=20 [m/s] 0.000 0.010 0.020 0.030 0.040 0.050 0.060 0.070 15 20 25 30 35 40 45 50 55 Vm hub [m/s ] dpeak across hub [m] Time domain (* =samples) Frequency domain

EARTH & SPACE 2010 – March 14-17 Honolulu HI 16/16 INTRODUCTION MODEL DESCRIPTION RESULTS CONCLUSIONS CONCLUSIONS In this work a finite element model for the dynamic analysis in both time and frequency domain of a monopile-type support structure for offshore wind turbine has been presented. Excitation wind and wave spectra are calculated for typical exercise conditions and nonlinear interaction is evaluated concerning the structural response spectrum. The obtained results have shown that wind-wave nonlinear interaction becomes important for elevated wind speed and should be considered in the design phase of a safe and cost- effective offshore wind turbine. This can be done performing a time-domain analysis which is however computationally cumbersome: in order to obtain analogous results from the frequency-domain analysis, which is intrinsically linear, the wind-wave spectra correlation and geometrical nonlinearity should be introduced; this is currently under development.

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