Simulating OC3 phase II coupled springs in FAST v7

Wind & Water Structural Analysis
1

Hello,

For context, I am using FAST v7 to simulate OWT monitoring data with rigid, distributed springs and coupled springs foundation types, with the aim of trying to identify the foundation properties from the acceleration data with system identification techniques.

I am currently recreating the OC3 phase II results to validate that I am using the software correctly. For the rigid foundation and distributed springs I used the NRELOffshrBsline5MW_Monopile_RF and DS and supporting files uploaded by Jonkman and have achieved similar results.

For the coupled springs, I could not find the files NRELOffshrBsline5MW_Monopile_CS for the OC3 study uploaded anywhere, would you be able to upload them if you still have them?

I have tried creating the files myself, replicating the load case 1.2 with Eigenanalysis. The natural frequencies I am getting seem quite different from those reported in Figure 20 of ‘Offshore Code Comparison Collaboration (OC3) for IEA Task 23 Offshore Wind Technology and Deployment’.

I used BModes to find the modeshapes (using CS_Monopile.bmi and CS_monopile_tower_secs.dat), the natural frequencies were as follows:
Mode 1 - 1st SS: 0.242 Hz
Mode 2 - 1st FA: 0.244 Hz
Mode 4 - 2nd SS: 1.369 Hz
Mode 5 - 2nd FA: 1.523 Hz

After adjusting the input files for a CS foundation with the mode shapes found (and recompiling with UserPtfmLd_CS), I ran the periodic linearized model, and got the following natural frequencies:
1st SS and FA: 0.176Hz and 0.275Hz
2nd SS and FA: 1.590Hz and 1.856Hz

In figure 20 from the study it looks at though the 1st are approx 0.19 Hz and 0.2Hz, and the 2nd are 1.6Hz and 1.75 Hz.

I understand the you found the modeshapes using ADAMS (with a flexible RNA) rather than BModes (rigid RNA), but do you think that could account for this large discrepancy? Or maybe I have an error somewhere else in my work.

Any help would be greatly appreciated, and if you could upload the files NRELOffshrBsline5MW_Monopile_CS, NRELOffshrBsline5MW_Tower_Monopile_CS and NRELOffshrBsline5MW_Platform_Monopile_CS files that would be really helpful.

I’ve uploaded my files for reference, thanks very much,
Emily
NRELOffshrBsline5MW_Platform_Monopile_CS.zip (8.97 KB)

2

Dear Emily,

I haven’t reviewed your files, but I’ve attached the tower input file from our FAST v7 model of the NREL 5-MW baseline wind turbine atop the OC3-monopile with coupled springs (CS) foundation. I’m not sure why the forum won’t allow me to attach the associated platform input file, but I’ve pasted this below:

NRELOffshrBsline5MW_Tower_Monopile_CS.dat.txt (4.32 KB)

-------------------------------------------------------------------------------- ---------------------- FAST PLATFORM FILE -------------------------------------- NREL 5.0 MW offshore baseline monopile platform with discrete coupled springs foundation input properties. ---------------------- FEATURE FLAGS (CONT) ------------------------------------ True PtfmSgDOF - Platform horizontal surge translation DOF (flag) True PtfmSwDOF - Platform horizontal sway translation DOF (flag) False PtfmHvDOF - Platform vertical heave translation DOF (flag) True PtfmRDOF - Platform roll tilt rotation DOF (flag) True PtfmPDOF - Platform pitch tilt rotation DOF (flag) False !JASON:True !JASON:False PtfmYDOF - Platform yaw rotation DOF (flag) ---------------------- INITIAL CONDITIONS (CONT) ------------------------------- 0.0 PtfmSurge - Initial or fixed horizontal surge translational displacement of platform (meters) 0.0 PtfmSway - Initial or fixed horizontal sway translational displacement of platform (meters) 0.0 PtfmHeave - Initial or fixed vertical heave translational displacement of platform (meters) 0.0 PtfmRoll - Initial or fixed roll tilt rotational displacement of platform (degrees) 0.0 PtfmPitch - Initial or fixed pitch tilt rotational displacement of platform (degrees) 0.0 PtfmYaw - Initial or fixed yaw rotational displacement of platform (degrees) ---------------------- TURBINE CONFIGURATION (CONT) ---------------------------- 20.0 TwrDraft - Downward distance from the ground level [onshore] or MSL [offshore] to the tower base platform connection (meters) 20.0 PtfmCM - Downward distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters) 20.0 PtfmRef - Downward distance from the ground level [onshore] or MSL [offshore] to the platform reference point (meters) ---------------------- MASS AND INERTIA (CONT) --------------------------------- 0.0 PtfmMass - Platform mass (kg) 0.0 PtfmRIner - Platform inertia for roll tilt rotation about the platform CM (kg m^2) 0.0 PtfmPIner - Platform inertia for pitch tilt rotation about the platform CM (kg m^2) 0.0 PtfmYIner - Platfrom inertia for yaw rotation about the platform CM (kg m^2) ---------------------- PLATFORM (CONT) ----------------------------------------- 1 PtfmLdMod - Platform loading model {0: none, 1: user-defined from routine UserPtfmLd} (switch) ---------------------- TOWER (CONT) -------------------------------------------- 1 TwrLdMod - Tower loading model {0: none, 1: Morison's equation, 2: user-defined from routine UserTwrLd} (switch) 6.0 TwrDiam - Tower diameter in Morison's equation (meters) [used only when TwrLdMod=1] 1.0 TwrCA - Normalized hydrodynamic added mass coefficient in Morison's equation (-) [used only when TwrLdMod=1] [determines TwrCM=1+TwrCA] 1.0 TwrCD - Normalized hydrodynamic viscous drag coefficient in Morison's equation (-) [used only when TwrLdMod=1] ---------------------- WAVES --------------------------------------------------- 1027.0 WtrDens - Water density (kg/m^3) 20.0 WtrDpth - Water depth (meters) 2 WaveMod - Incident wave kinematics model {0: none=still water, 1: plane progressive (regular) with optional phase specification (i.e., specify 1P#, where # is the phase in degrees to override WaveSeed), 2: JONSWAP/Pierson-Moskowitz spectrum (irregular), 3: user-defind spectrum from routine UserWaveSpctrm (irregular)} (switch) 3 WaveStMod - Model for stretching incident wave kinematics to instantaneous free surface {0: none=no stretching, 1: vertical stretching, 2: extrapolation stretching, 3: Wheeler stretching} (switch) [unused when WaveMod=0] 3630.0 WaveTMax - Analysis time for incident wave calculations (sec) [unused when WaveMod=0] [determines WaveDOmega=2Pi/WaveTMax in the IFFT] 0.25 WaveDT - Time step for incident wave calculations (sec) [unused when WaveMod=0] [0.1<=WaveDT<=1.0 recommended] [determines WaveOmegaMax=Pi/WaveDT in the IFFT] 5.0 WaveHs - Significant wave height of incident waves (meters) [unused when WaveMod=0, 3, or 4] 12.4 WaveTp - Peak spectral period of incident waves (sec) [unused when WaveMod=0, 3, or 4] DEFAULT WavePkShp - Peak shape parameter of incident wave spectrum (-) or DEFAULT (unquoted string) [used only when WaveMod=2] [use 1.0 for Pierson-Moskowitz] 0.0 WaveDir - Incident wave propagation heading direction (degrees) [unused when WaveMod=0 or 4] 123456789 WaveSeed(1) - First random seed of incident waves [-2147483648 to 2147483647] (-) [unused when WaveMod=0, 1P#, or 4] 1011121314 WaveSeed(2) - Second random seed of incident waves [-2147483648 to 2147483647] (-) [unused when WaveMod=0, 1P#, or 4] True WaveNDAmp - Normally-distributed amplitudes in incident waves spectrum (flag) [used only when WaveMod=2 or 3] "Dummy" GHWvFile - Root name of GH Bladed files containing wave data (quoted string) [used only when WaveMod=4] ---------------------- CURRENT ------------------------------------------------- 0 CurrMod - Current profile model {0: none=no current, 1: standard, 2: user-defined from routine UserCurrent} (switch) 0.0 CurrSSV0 - Sub-surface current velocity at still water level (m/s) [used only when CurrMod=1] DEFAULT CurrSSDir - Sub-surface current heading direction (degrees) or DEFAULT (unquoted string) [used only when CurrMod=1] 20.0 CurrNSRef - Near-surface current reference depth (meters) [used only when CurrMod=1] 0.0 CurrNSV0 - Near-surface current velocity at still water level (m/s) [used only when CurrMod=1] 0.0 CurrNSDir - Near-surface current heading direction (degrees) [used only when CurrMod=1] 0.0 CurrDIV - Depth-independent current velocity (m/s) [used only when CurrMod=1] 0.0 CurrDIDir - Depth-independent current heading direction (degrees) [used only when CurrMod=1] ---------------------- OUTPUT (CONT) ------------------------------------------- 0 NWaveKin - Number of points where the wave kinematics can be output [0 to 9] (-) 0 WaveKinNd - List of tower nodes that have wave kinematics sensors [1 to TwrNodes] (-) [unused if NWaveKin=0]
The user-platform load routine (UserPtfmLd) for this model is available in the following forum topic: Turbine-soil interaction. Influence on the mode shapes. - #13 by Jason.Jonkman.

I hope that helps.

Best regards,

3

Hi,

Thanks for uploading those files last year, it was really helpful.

I am still having some issues with FAST v7 with the coupled springs foundation.

I am working with the NREL 5MW baseline OWT and analysing the dynamics under standstill conditions so I therefore run both the time marching and the linearisation simulations.

I take the motions of various points on the tower (both FA and SS) and view them in the frequency domain (by computing the power spectral density).

My problem is that the frequency results from the linearisation for the first two modes (dominated by FA and SS first tower bending) never match the first peaks seen in the PSD.
e.g. the first peak in the PSD is very clearly 0.24 Hz for both the FA and SS directions, however the results from the linearisation give 0.22Hz and 0.34Hz.

I do not have this problem when using a distributed springs case or a rigid foundation. I have checked and rechecked my files for the coupled springs, and there is nothing that I change between running the two simulation types except AnalMode.

Can you think of any reason for this error? Do you know of any error with the linearisation in FAST v7 with a coupled springs foundation?

Thank you very much,
Emily

4

Dear Emily,

My guess is the the precision of your linearization output file is too coarse (too few digits of precision). Our experience is that Eigensolvers are sensitive to the precision of the state matrix “A”. To increase the precision, change input parameter OutFileFmt from the FAST primary (*.fst) input file to add more digits of precision and rerun the linearization analysis.

I hope that helps.

Best regards,

5

Sir,

I was trying to conduct a linearization analysis to get natural frequencies for platform model 2(i.e, Fixed bottom offshore) coupled spring model in seismic. But while performing analysis I was getting an error message like an error occurred while trying to read CurrMod. This was the echo file I got and from the Current section, echo file was not reading. I Can’t figure out what’s wrong and another doubt was which Matlab scripts should I use to process the linearization output files. Can I use the old Matlab scripts mentioned in the Matlab toolbox for Openfast to this model? Can someone help me through these doubts?

This file of echoed input was generated by FAST (v7.02.00d-bjj, 20-Feb-2013) on 21-Jun-2020 at 23:29:23.

Turbine input data from file “E:\Seismic\linear\NRELOffshrBsline5MW_Onshore.fst”:

NREL 5.0 MW Baseline Wind Turbine for Use in Offshore Analysis.

---------------------- SIMULATION CONTROL --------------------------------------
T Echo - Echo input to “echo.out”
1 ADAMSPrep - ADAMS preprocessor mode
2 AnalMode - Analysis mode
3 NumBl - Number of blades
6.3000E+02 TMax - Total run time
5.0000E-03 DT - Integration time step
---------------------- TURBINE CONTROL -----------------------------------------
0 YCMode - Yaw control mode
9.9999E+03 TYCOn - Time to enable yaw control
0 PCMode - Pitch control mode
0.0000E+00 TPCOn - Time to enable pitch control
2 VSContrl - Variable-speed-generator control switch
9.9999E+03 VS_RtGnSp - Rated generator speed for simple variable-speed generator control
9.9999E+03 VS_RtTq - Rated generator torque/constant generator torque in Region 3 for simple variable-speed generator control
9.9999E+03 VS_Rgn2K - Generator torque constant in Region 2 for simple variable-speed generator control
9.9999E+03 VS_SlPc - Rated generator slip percentage in Region 2 1/2 for simple variable-speed generator control
2 GenModel - Generator model
T GenTiStr - Start generator based upon T: time or F: generator speed
T GenTiStp - Stop generator based upon T: time or F: generator power = 0
9.9999E+03 SpdGenOn - Generator speed to turn on the generator
0.0000E+00 TimGenOn - Time to turn on generator
9.9999E+03 TimGenOf - Time to turn off generator
1 HSSBrMode - HSS brake model
9.9999E+03 THSSBrDp - Time to initiate deployment of the HSS brake
9999.9 TiDynBrk - Time to initiate deployment of the dynamic generator brake [CURRENTLY IGNORED] (s)
9.9999E+03 TTpBrDp(1) - Time to initiate deployment of tip brakes
9.9999E+03 TTpBrDp(2) - Time to initiate deployment of tip brakes
9.9999E+03 TTpBrDp(3) - Time to initiate deployment of tip brakes
9.9999E+03 TBDepISp(1) - Deployment-initiation speed for the tip brakes
9.9999E+03 TBDepISp(2) - Deployment-initiation speed for the tip brakes
9.9999E+03 TBDepISp(3) - Deployment-initiation speed for the tip brakes
9.9999E+03 TYawManS - Time to start yaw maneuver
9.9999E+03 TYawManE - Time to end yaw maneuver
0.0000E+00 NacYawF - Final nacelle-yaw angle for maneuvers
9.9999E+03 TPitManS(1) - Time to start pitch maneuvers
9.9999E+03 TPitManS(2) - Time to start pitch maneuvers
9.9999E+03 TPitManS(3) - Time to start pitch maneuvers
9.9999E+03 TPitManE(1) - Time to end pitch maneuvers
9.9999E+03 TPitManE(2) - Time to end pitch maneuvers
9.9999E+03 TPitManE(3) - Time to end pitch maneuvers
0.0000E+00 BlPitch(1) - Initial pitch angle
0.0000E+00 BlPitch(2) - Initial pitch angle
0.0000E+00 BlPitch(3) - Initial pitch angle
0.0000E+00 BlPitchF(1) - Final pitch angle for maneuvers
0.0000E+00 BlPitchF(2) - Final pitch angle for maneuvers
0.0000E+00 BlPitchF(3) - Final pitch angle for maneuvers
---------------------- ENVIRONMENTAL CONDITIONS --------------------------------
9.8067E+00 Gravity - Gravitational acceleration
---------------------- FEATURE FLAGS -------------------------------------------
T FlapDOF1 - First flapwise blade mode DOF
T FlapDOF2 - Second flapwise blade mode DOF
T EdgeDOF - First edgewise blade mode DOF
False TeetDOF - Rotor-teeter DOF (flag) [unused for 3 blades]
T DrTrDOF - Drivetrain rotational-flexibility DOF
F GenDOF - Generator DOF
T YawDOF - Yaw DOF
T TwFADOF1 - First tower fore-aft bending-mode DOF
T TwFADOF2 - Second tower fore-aft bending-mode DOF
T TwSSDOF1 - First tower side-to-side bending-mode DOF
T TwSSDOF2 - Second tower side-to-side bending-mode DOF
F CompAero - Compute aerodynamic forces
F CompNoise - Compute aerodynamic noise
---------------------- INITIAL CONDITIONS --------------------------------------
0.0000E+00 OoPDefl - Initial out-of-plane blade-tip deflection
0.0000E+00 IPDefl - Initial in-plane blade-tip deflection
0.0 TeetDefl - Initial or fixed teeter angle (degrees) [unused for 3 blades]
0.0000E+00 Azimuth - Initial azimuth position for blade 1
0.0000E+00 RotSpeed - Initial or fixed rotor speed
0.0000E+00 NacYaw - Initial or fixed nacelle-yaw angle
0.0000E+00 TTDspFA - Initial fore-aft tower-top displacement
0.0000E+00 TTDspSS - Initial side-to-side tower-top displacement
---------------------- TURBINE CONFIGURATION -----------------------------------
6.3000E+01 TipRad - Preconed blade-tip radius
1.5000E+00 HubRad - Preconed hub radius
1 PSpnElN - Number of the innermost blade element which is still part of the pitchable portion of the blade for partial-span pitch control [1 to BldNodes] [CURRENTLY IGNORED] (-)
0.0 UndSling - Undersling length [distance from teeter pin to the rotor apex] (meters) [unused for 3 blades]
0.0000E+00 HubCM - Distance from rotor apex to hub mass
-5.0191E+00 OverHang - Distance from yaw axis to rotor apex or teeter pin
1.9000E+00 NacCMxn - Downwind distance from tower-top to nacelle CM
0.0000E+00 NacCMyn - Lateral distance from tower-top to nacelle CM
1.7500E+00 NacCMzn - Vertical distance from tower-top to nacelle CM
8.7600E+01 TowerHt - Tower height
1.9626E+00 Twr2Shft - Vertical distance from tower-top to rotor shaft
0.0000E+00 TwrRBHt - Tower rigid base height
-5.0000E+00 ShftTilt - Rotor shaft tilt angle
0.0 Delta3 - Delta-3 angle for teetering rotors (degrees) [unused for 3 blades]
-2.5000E+00 PreCone(1) - Blade coning angle
-2.5000E+00 PreCone(2) - Blade coning angle
-2.5000E+00 PreCone(3) - Blade coning angle
0.0000E+00 AzimB1Up - Azimuth value to use for I/O when blade 1 points up
---------------------- MASS AND INERTIA ----------------------------------------
0.0000E+00 YawBrMass - Yaw bearing mass
2.4000E+05 NacMass - Nacelle mass
5.6780E+04 HubMass - Hub mass
0.0000E+00 TipMass(1) - Tip-brake mass
0.0000E+00 TipMass(2) - Tip-brake mass
0.0000E+00 TipMass(3) - Tip-brake mass
2.6079E+06 NacYIner - Nacelle yaw inertia
5.3412E+02 GenIner - Generator inertia about HSS
1.1593E+05 HubIner - Hub inertia about rotor axis
---------------------- DRIVETRAIN ----------------------------------------------
1.0000E+02 GBoxEff - Gearbox efficiency
9.4400E+01 GenEff - Generator efficiency
9.7000E+01 GBRatio - Gearbox ratio
F GBRevers - Gearbox reversal flag
2.8116E+04 HSSBrTqF - Fully deployed HSS brake torque
6.0000E-01 HSSBrDT - Time for HSS-brake to reach full deployment once initiated
DynBrkFi - File containing a mech-gen-torque vs HSS-speed curve for a dynamic brake [CURRENTLY IGNORED] (quoted string)
8.6764E+08 DTTorSpr - Drivetrain torsional spring
6.2150E+06 DTTorDmp - Drivetrain torsional damper
---------------------- SIMPLE INDUCTION GENERATOR ------------------------------
9.9999E+03 SIG_SlPc - Rated generator slip percentage
9.9999E+03 SIG_SySp - Synchronous (zero-torque) generator speed
9.9999E+03 SIG_RtTq - Rated torque
9.9999E+03 SIG_PORt - Pull-out ratio
---------------------- THEVENIN-EQUIVALENT INDUCTION GENERATOR -----------------
9.9999E+03 TEC_Freq - Line frequency
9998 TEC_NPol - Number of poles
9.9999E+03 TEC_SRes - Stator resistance
9.9999E+03 TEC_RRes - Rotor resistance
9.9999E+03 TEC_VLL - Line-to-line RMS voltage
9.9999E+03 TEC_SLR - Stator leakage reactance
9.9999E+03 TEC_RLR - Rotor leakage reactance
9.9999E+03 TEC_MR - Magnetizing reactance
---------------------- PLATFORM ------------------------------------------------
2 PtfmModel - Platform model switch
PtfmFile - Name of file containing platform properties
“E:/Seismic/linear/NRELOffshrBsline5MW_Onshore_Ptfm.dat”
---------------------- TOWER ---------------------------------------------------
20 TwrNodes - Number of tower nodes used for analysis
TwrFile - Name of file containing tower properties
“E:/Seismic/linear/NRELOffshrBsline5MW_Tower_Onshore.dat”
---------------------- NACELLE-YAW ---------------------------------------------
9.0283E+09 YawSpr - Nacelle-yaw spring constant
1.9160E+07 YawDamp - Nacelle-yaw damping constant
0.0000E+00 YawNeut - Neutral yaw position
---------------------- FURLING -------------------------------------------------
F Furling - Read in additional model properties for furling turbine
FurlFile - Name of file containing furling properties
“FurlFile”
---------------------- ROTOR-TEETER --------------------------------------------
0 TeetMod - Rotor-teeter spring/damper model {0: none, 1: standard, 2: user-defined from routine UserTeet} (switch) [unused for 3 blades]
0.0 TeetDmpP - Rotor-teeter damper position (degrees) [used only for 2 blades and when TeetMod=1]
0.0 TeetDmp - Rotor-teeter damping constant (N-m/(rad/s)) [used only for 2 blades and when TeetMod=1]
0.0 TeetCDmp - Rotor-teeter rate-independent Coulomb-damping moment (N-m) [used only for 2 blades and when TeetMod=1]
0.0 TeetSStP - Rotor-teeter soft-stop position (degrees) [used only for 2 blades and when TeetMod=1]
0.0 TeetHStP - Rotor-teeter hard-stop position (degrees) [used only for 2 blades and when TeetMod=1]
0.0 TeetSSSp - Rotor-teeter soft-stop linear-spring constant (N-m/rad) [used only for 2 blades and when TeetMod=1]
0.0 TeetHSSp - Rotor-teeter hard-stop linear-spring constant (N-m/rad) [used only for 2 blades and when TeetMod=1]
---------------------- TIP-BRAKE -----------------------------------------------
0.0000E+00 TBDrConN - Tip-brake drag constant during normal operation
0.0000E+00 TBDrConD - Tip-brake drag constant during fully-deployed operation
0.0000E+00 TpBrDT - Time for tip-brake to reach full deployment once released
---------------------- BLADE ---------------------------------------------------
BldFile(1) - Names of files containing blade properties
E:/Seismic/linear/NRELOffshrBsline5MW_Blade.dat
BldFile(2) - Names of files containing blade properties
E:/Seismic/linear/NRELOffshrBsline5MW_Blade.dat
BldFile(3) - Names of files containing blade properties
E:/Seismic/linear/NRELOffshrBsline5MW_Blade.dat
---------------------- AERODYN -------------------------------------------------
ADFile - Name of file containing AeroDyn parameters
“E:/Seismic/linear/NRELOffshrBsline5MW_AeroDyn.ipt”
---------------------- NOISE ---------------------------------------------------
NoiseFile - Name of file containing aerodynamic noise parameters
“NoiseFile”
---------------------- ADAMS ---------------------------------------------------
ADAMSFile - Name of file containing ADAMS-specific properties
“E:/Seismic/linear/NRELOffshrBsline5MW_ADAMSSpecific.dat”
---------------------- LINEARIZATION CONTROL -----------------------------------
LinFile - Name of file containing FAST linearization parameters
“E:/Seismic/linear/NRELOffshrBsline5MW_Linear.dat”
---------------------- OUTPUT --------------------------------------------------
T SumPrint - Print summary data to “*.fsm”
1 OutFileFmt - Format for output file(s)
T TabDelim - Use tab delimiters in text output file
OutFmt - Output format for text tabular data
“ES10.3E2”
0.0000E+00 TStart - Time to begin tabular output
1 DecFact - Decimation factor for tabular output
1.0000E+00 SttsTime - Amount of time between screen status messages
-3.0953E+00 NcIMUxn - Downwind distance from the tower-top to the nacelle IMU
0.0000E+00 NcIMUyn - Lateral distance from the tower-top to the nacelle IMU
2.2334E+00 NcIMUzn - Vertical distance from the tower-top to the nacelle IMU
1.9120E+00 ShftGagL - Distance from hub or teeter pin to shaft strain gages
0 NTwGages - Number of tower “strain-gage” output stations
TwrGagNd - List of tower nodes that have strain gages

        3  NBlGages       - Number of blade "strain-gage" output stations
           BldGagNd    - List of blade nodes that have strain gages

5 9 13
OutList - The next line(s) contains a list of output parameters. See OutList.txt for a listing of available output channels, (-)
OutList - Output list
“PtfmTDxi, PtfmTDyi, PtfmTDzi”
OutList - Output list
“PtfmTVxi, PtfmTVyi, PtfmTVzi”
OutList - Output list
“PtfmTAxi, PtfmTAyi, PtfmTAzi”
OutList - Output list
“PtfmTAxt , PtfmTAyt , PtfmTAzt”
OutList - Output list
“YawBrFxp , YawBrFyp , YawBrFzp”
OutList - Output list
“YawBrMxp , YawBrMyp , YawBrMzp”
OutList - Output list
“TwrBsFxt , TwrBsFyt , TwrBsFzt”
OutList - Output list
“TwrBsMxt , TwrBsMyt , TwrBsMzt”
OutList - Output list
“END”

Platform input data from file “E:/Seismic/linear/NRELOffshrBsline5MW_Onshore_Ptfm.dat”:

---------------------- FEATURE FLAGS (CONT) ------------------------------------
T PtfmSgDOF - Platform surge DOF
F PtfmSwDOF - Platform sway DOF
F PtfmHvDOF - Platform heave DOF
F PtfmRDOF - Platform roll DOF
F PtfmPDOF - Platform pitch DOF
F PtfmYDOF - Platform yaw DOF
---------------------- INITIAL CONDITIONS (CONT) -------------------------------
0.0000E+00 PtfmSurge - Initial or fixed platform surge
0.0000E+00 PtfmSway - Initial or fixed platform sway
0.0000E+00 PtfmHeave - Initial or fixed platform heave
0.0000E+00 PtfmRoll - Initial or fixed platform roll
0.0000E+00 PtfmPitch - Initial or fixed platform pitch
0.0000E+00 PtfmYaw - Initial or fixed platform yaw
---------------------- TURBINE CONFIGURATION (CONT) ----------------------------
2.0000E+01 TwrDraft - Downward distance from ground [onshore] or MSL [offshore] to tower base platform connection
2.0000E+01 PtfmCM - Downward distance from ground [onshore] or MSL [offshore] to platform CM
2.0000E+01 PtfmRef - Downward distance from ground [onshore] or MSL [offshore] to platform reference point
---------------------- MASS AND INERTIA (CONT) ---------------------------------
1.6000E+06 PtfmMass - Platform mass
0.0000E+00 PtfmRIner - Platform inertia for roll tilt rotation about the platform CM
0.0000E+00 PtfmPIner - Platform inertia for pitch tilt rotation about the platform CM
0.0000E+00 PtfmYIner - Platform inertia for yaw rotation about the platform CM
---------------------- PLATFORM (CONT) -----------------------------------------
0 PtfmLdMod - Platform loading model switch
---------------------- TOWER (CONT) --------------------------------------------
0 TwrLdMod - Tower loading model switch
6.0 TwrDiam - Tower diameter in Morison’s equation (meters) [used only when TwrLdMod=1]
1.0 TwrCA - Normalized hydrodynamic added mass coefficient in Morison’s equation (-) [used only when TwrLdMod=1] [determines TwrCM=1+TwrCA]
1.0 TwrCD - Normalized hydrodynamic viscous drag coefficient in Morison’s equation (-) [used only when TwrLdMod=1]
---------------------- WAVES ---------------------------------------------------
1.0270E+03 WtrDens - Water density
2.0000E+01 WtrDpth - Water depth
2 WaveMod - Wave kinematics model switch
0 WaveStMod - Model switch for stretching incident wave kinematics to instantaneous free surface
3.6300E+03 WaveTMax - Analysis time for incident wave calculations
2.5000E-01 WaveDT - Time step for incident wave calculations
5.0000E+00 WaveHs - Significant wave height
1.2400E+01 WaveTp - Peak spectral period
WavePkShp - Peak shape parameter
“DEFAULT”
0.0000E+00 WaveDir - Wave heading direction
123456789 WaveSeed(1) - First random seed
1011121314 WaveSeed(2) - Second random seed
True WaveNDAmp - Normally-distributed amplitudes in incident waves spectrum (flag) [used only when WaveMod=2 or 3]
“Dummy” GHWvFile - Root name of GH Bladed files containing wave data (quoted string) [used only when WaveMod=4]

6

Dear Dhaneesh,

Are you missing the “CURRENT” section at the bottom of the FAST v7 platform input file, i.e.:

---------------------- CURRENT ------------------------------------------------- 0 CurrMod - Current profile model {0: none=no current, 1: standard, 2: user-defined from routine UserCurrent} (switch) 0.0 CurrSSV0 - Sub-surface current velocity at still water level (m/s) [used only when CurrMod=1] DEFAULT CurrSSDir - Sub-surface current heading direction (degrees) or DEFAULT (unquoted string) [used only when CurrMod=1] 20.0 CurrNSRef - Near-surface current reference depth (meters) [used only when CurrMod=1] 0.0 CurrNSV0 - Near-surface current velocity at still water level (m/s) [used only when CurrMod=1] 0.0 CurrNSDir - Near-surface current heading direction (degrees) [used only when CurrMod=1] 0.0 CurrDIV - Depth-independent current velocity (m/s) [used only when CurrMod=1] 0.0 CurrDIDir - Depth-independent current heading direction (degrees) [used only when CurrMod=1] ---------------------- OUTPUT (CONT) ------------------------------------------- 0 NWaveKin - Number of points where the wave kinematics can be output [0 to 9] (-) 0 WaveKinNd - List of tower nodes that have wave kinematics sensors [1 to TwrNodes] (-) [unused if NWaveKin=0]
Yes, the “old” directory in the current MATLAB Toolbox MBC code, includes MATLAB scripts, e.g., GetMats.m, to process old FAST v7-style linearization output files.

Best regards,

7

Sir,

Yes. I was missing the Current section in the echo file with an error displaying “error occurred while reading CurrMod” while doing linearization analysis for the fixed bottom offshore case with a coupled spring model.Can I get some help regarding this?

Thank you.

8

Sir,

I was getting an error while conducting linearization analysis for a bottom fixed offshore wind turbine with foundation modeled as a coupled spring model in fastv7. The error message I got was “error occurred while reading CurrMod”. I also tried conducting linearization analysis with the ADAMS model to see whether it was executing without error. And Even ADAMS was aborting with the same error. And I can’t figure out the reason for this error. Can anyone help me how to solve this issue?

Thank you.

9

Dear Dhaneesh,

This sounds like an error with the input file formatting of your FAST v7 platform input file. As with any input file processing error, I suggest enabling “Echo” in the FAST v7 primary input file to debug.

Best regards,

10

Sir,

  1. I have enabled the echo option in the fastv7 input file for debugging errors. In the echo file I was missing the “CURRENT” section at the bottom of the FAST v7 platform input file, i.e.: Was it meant there was something wrong with my input files?

---------------------- CURRENT -------------------------------------------------
0 CurrMod - Current profile model {0: none=no current, 1: standard, 2: user-defined from routine UserCurrent} (switch)
0.0 CurrSSV0 - Sub-surface current velocity at still water level (m/s) [used only when CurrMod=1]
DEFAULT CurrSSDir - Sub-surface current heading direction (degrees) or DEFAULT (unquoted string) [used only when CurrMod=1]
20.0 CurrNSRef - Near-surface current reference depth (meters) [used only when CurrMod=1]
0.0 CurrNSV0 - Near-surface current velocity at still water level (m/s) [used only when CurrMod=1]
0.0 CurrNSDir - Near-surface current heading direction (degrees) [used only when CurrMod=1]
0.0 CurrDIV - Depth-independent current velocity (m/s) [used only when CurrMod=1]
0.0 CurrDIDir - Depth-independent current heading direction (degrees) [used only when CurrMod=1]
---------------------- OUTPUT (CONT) -------------------------------------------
0 NWaveKin - Number of points where the wave kinematics can be output [0 to 9] (-)
0 WaveKinNd - List of tower nodes that have wave kinematics sensors [1 to TwrNodes] (-) [unused if NWaveKin=0]

  1. For a monopile offshore wind turbine with foundation modeled as coupled springs, I Considered offshore wind turbine was operating at a turbulent wind speed of 11.4 m/s and irregular waves of Hs=5m and Tp=12.4sec when a ground motion of 0.64g strikes the offshore wind turbine after 400 seconds. The total run time was 630 seconds for the simulation. In this case, the tower-top displacement and overturning base moment seem to behave in a similar way but the tower base shear behavior is in a different pattern. And also initially the time series was not starting from zero. Was something wrong with my simulation results? I have given the TTDspFA, TwrBsFxt, TwrBsMyt results and the input files I was using. Am I using the proper input files?

Thank you.
NREL OffshrBsline5MW_CS_files.zip (48 KB)

11

Dear Dhaneesh,

You need to include the CURRENT and OUTPUT sections in the FAST v7 platform input file; it now looks like you’ve done this correctly.

I’m not sure I understand what your concern with the tower response is. From my quick look, I don’t see anything wrong in your input files. The FAST outputs will rarely start at zero. Regardless, I would always neglect the start-up transients (first 30 s or so) from subsequent analysis.

Best regards,

12

Sir,

  1. Regarding tower response when an earthquake of 0.64g strikes the OWT after 400 seconds, both the TTDspFA and TwrBsMyt have shown a similar graph but the TwrBsFxt graph was different. Can you think of any reason for this? And I have done 44 such simulations for earthquake ground motions ranging from 0.02g to 1g, for all simulations the TTDspFA, TwrBsFxt, TwrBsMyt response was not oscillating w.r.t zero. Does it mean that the tower responses are oscillating w.r.t to the initial values?

  2. After running the mbc3 for the monopile offshore wind turbine CS model, this was the order of natural frequencies(Hz) I obtained. Can You know the reason why my 1st mode(1st tower fore-aft mode) is showing such a big value of 3.618Hz? or Should the obtained frequencies be read in ascending order?

3.61898612782713
2.40705362471741
2.34313257218863
1.85419556886847
2.02016048829373
1.99642941714785
1.07930292246344
1.09112058544908
0.284536782875767
0.287632095314915
0.621653076102780
0.698908649515926
0.666578057561514
0.675842514902850

Row/column 1 = 1st tower fore-aft bending mode DOF (internal DOF index = DOF_TFA1)’
‘Row/column 2 = 1st tower side-to-side bending mode DOF (internal DOF index = DOF_TSS1)’
‘Row/column 3 = 2nd tower fore-aft bending mode DOF (internal DOF index = DOF_TFA2)’
‘Row/column 4 = 2nd tower side-to-side bending mode DOF (internal DOF index = DOF_TSS2)’
‘Row/column 5 = Drivetrain rotational-flexibility DOF (internal DOF index = DOF_DrTr)’
‘Row/column 6 = 1st flapwise bending-mode DOF of blade 1 (internal DOF index = DOF_BF(1,1))’
‘Row/column 7 = 1st flapwise bending-mode DOF of blade 2 (internal DOF index = DOF_BF(2,1))’
‘Row/column 8 = 1st flapwise bending-mode DOF of blade 3 (internal DOF index = DOF_BF(3,1))’
‘Row/column 9 = 1st edgewise bending-mode DOF of blade 1 (internal DOF index = DOF_BE(1,1))’
‘Row/column 10 = 1st edgewise bending-mode DOF of blade 2 (internal DOF index = DOF_BE(2,1))’
‘Row/column 11 = 1st edgewise bending-mode DOF of blade 3 (internal DOF index = DOF_BE(3,1))’
‘Row/column 12 = 2nd flapwise bending-mode DOF of blade 1 (internal DOF index = DOF_BF(1,2))’
‘Row/column 13 = 2nd flapwise bending-mode DOF of blade 2 (internal DOF index = DOF_BF(2,2))’
‘Row/column 14 = 2nd flapwise bending-mode DOF of blade 3 (internal DOF index = DOF_BF(3,2))’

Thank you.

13

Dear Dhaneesh,

I’m not sure what you mean by “looks different”. From the sample results you shared earlier, it looks like the start-up transients have dissipated within the first 10-20 s. I would expect that TTDspFA, TwrBsFxt, and TwrBsMyt to all have nonzero means as result of the aerodynamic thrust load on the rotor.

The MBC3 output is not sorted in any specific way. To interpret the MBC3 output, you’ll have to review the eigenvectors, e.g., to identify which modes correspond to which frequency and to generate a Campbell diagram, as has been discussed many times on this forum.

Best regards,

14

Sir,

Thank you for clearing my doubts about my sample results and information regarding the Campbell diagram. But one thing which concerns me is my tower base shear response. From my sample results, for the TwrBsFxt(Tower base shear) response when the ground motion of 0.64g strikes after 400 seconds a large peak was observed initially. But for the other two i.e, the TTDspFA and TwrBsMyt response parameters there was no such large peak initially as of TwrBsFxt response. Can you think of what was the reason for that?

And can you please share platform and tower input files of bottom fixed monopile OWT and MLife files, Because I was not able to find them on NREL website.

Thank you.

15

Dear Dhaneesh,

I would expect some phase shift between the tower-top shear force and tower-base force and moment. The displacements, of course, are twice integrated from the acceleration. Forces are impacted by acceleration (not displacement) and moments are impact by both acceleration and displacement (the later because of the moment-arm effects).

I’ve attached the FAST v7 tower and platform input files associated with the NREL 5-MW turbine atop the OC3-monopile with rigid foundation.
NRELOffshrBsline5MW_Monopile_RF.zip (3.72 KB)
Regarding MLife, similar question was asked and answered in my post dated Jun 17, 2020 in the following forum topic: viewtopic.php?f=4&t=805&start=30.

Best regards,

16

Hi,

Could I know where I can edit the foundation properties and soil model in Openfast for 5MW_OC3Mnpl_DLL_WTurb_WavesIrr.fst?

Thanks,
Satish J

17

Dear Satish,

This OpenFAST model of the NREL 5-MW turbine atop the OC3-monopile assumes a fixed (clamped) connection at the seabed. In the context of the IEA Wind Task 30 OC6 Phase II project, NREL has developed new functionality in OpenFAST for modeling soil-structure interaction, including the introduction of coupled springs in SubDyn, as well as a new module (SoilDyn) that supports both couple springs and an interface to the REDWIN soil-structure interaction superelement from NGI (and a placeholder for distributed springs in the future). This functionality has been made available to OpenFAST users in the OC6 Phase II project, but has not yet been merged into the Dev or Master branches of OpenFAST. Until then, OpenFAST only supports the apparent fixity model or the user-defined external platform.

Best regards,

18

Dear Jason,

Thanks for your rely.

Okay, could I know where can I find apparent fixity model in the input files?

Could you please provide insights on user defined platform as how to consider in the main model?

Thanks,
Satish J

19

Dear Satish,

You can define the apparent fixity in whichever structural module you are using to model the substructure (this would be SubDyn for the OC3 monopile, or ElastoDyn for a land-based system). Basically, define joints and members that extend below the seabed. This has been discussed multiple times on this forum.

The user-defined external platform is available by setting CompSub = 2 in the OpenFAST primary input file. More information on this model is provided in a recent discussion on OpenFAST github issues: github.com/OpenFAST/openfast/issues/549.

Best regards,

20

Dear Jason,

Thanks for your reply.

Right now, I am concentrating on monopile structure. If I want to do apparent fixity, just changing the joints and members in subdyn is sufficient?
Where can I change the stiffness and mass properties?

Could you please elaborate on this or could you please provide any reference?

Thanks,
Satish J