15MW towerbase pitching moment discrepancy

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Peter.Veldman
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15MW towerbase pitching moment discrepancy

Postby Peter.Veldman » Tue Jan 26, 2021 12:41 pm

Dear NREL team,

I am running openFAST simulations for the 15-MW reference wind turbine including wind turbine control (for floating applications) with a fully constrained floater and ignoring structural deformations of the wind turbine tower and wind turbine blades. Also, the tower drag loads are not taken into account. Now, for verification purposes, I am interested in the loads at the tower base of the turbine (located at the water line). Therefore, I changed the value of TowerBsHt from 15.0m to 0.0m in the "Turbine configuration" menu of ElastoDyn.dat, such that the height of the tower base is located at the waterline. This results in a distance equal to the hub height of 150.0 meter from MSL. The ElastoDyn.dat-file is shown below:

Code: Select all

------- ELASTODYN v1.03.* INPUT FILE -------------------------------------------
IEA 15 MW offshore reference model on UMaine VolturnUS-S semi-submersible floating platform
---------------------- SIMULATION CONTROL --------------------------------------
False         Echo        - Echo input data to "<RootName>.ech" (flag)
          3   Method      - Integration method: {1: RK4, 2: AB4, or 3: ABM4} (-)
"default"     DT          Integration time step (s)
---------------------- ENVIRONMENTAL CONDITION ---------------------------------
    9.80665   Gravity     - Gravitational acceleration (m/s^2)
---------------------- DEGREES OF FREEDOM --------------------------------------
False          FlapDOF1    - First flapwise blade mode DOF (flag)
False          FlapDOF2    - Second flapwise blade mode DOF (flag)
False          EdgeDOF     - First edgewise blade mode DOF (flag)
False         TeetDOF     - Rotor-teeter DOF (flag) [unused for 3 blades]
False         DrTrDOF     - Drivetrain rotational-flexibility DOF (flag)
True          GenDOF      - Generator DOF (flag)
False         YawDOF      - Yaw DOF (flag)
False          TwFADOF1    - First fore-aft tower bending-mode DOF (flag)
False          TwFADOF2    - Second fore-aft tower bending-mode DOF (flag)
False          TwSSDOF1    - First side-to-side tower bending-mode DOF (flag)
False          TwSSDOF2    - Second side-to-side tower bending-mode DOF (flag)
False          PtfmSgDOF   - Platform horizontal surge translation DOF (flag)
False          PtfmSwDOF   - Platform horizontal sway translation DOF (flag)
False          PtfmHvDOF   - Platform vertical heave translation DOF (flag)
False          PtfmRDOF    - Platform roll tilt rotation DOF (flag)
False          PtfmPDOF    - Platform pitch tilt rotation DOF (flag)
False          PtfmYDOF    - Platform yaw rotation DOF (flag)
---------------------- INITIAL CONDITIONS --------------------------------------
          0   OoPDefl     - Initial out-of-plane blade-tip displacement (meters)
          0   IPDefl      - Initial in-plane blade-tip deflection (meters)
         1.   BlPitch(1)  - Blade 1 initial pitch (degrees)
         1.   BlPitch(2)  - Blade 2 initial pitch (degrees)
         1.   BlPitch(3)  - Blade 3 initial pitch (degrees) [unused for 2 blades]
          0   TeetDefl    - Initial or fixed teeter angle (degrees) [unused for 3 blades]
          0   Azimuth     - Initial azimuth angle for blade 1 (degrees)
         7.56   RotSpeed    - Initial or fixed rotor speed (rpm)
          0   NacYaw      - Initial or fixed nacelle-yaw angle (degrees)
          0   TTDspFA     - Initial fore-aft tower-top displacement (meters)
          0   TTDspSS     - Initial side-to-side tower-top displacement (meters)
          0   PtfmSurge   - Initial or fixed horizontal surge translational displacement of platform (meters)
          0   PtfmSway    - Initial or fixed horizontal sway translational displacement of platform (meters)
          0   PtfmHeave   - Initial or fixed vertical heave translational displacement of platform (meters)
          0   PtfmRoll    - Initial or fixed roll tilt rotational displacement of platform (degrees)
          0   PtfmPitch   - Initial or fixed pitch tilt rotational displacement of platform (degrees)
          0   PtfmYaw     - Initial or fixed yaw rotational displacement of platform (degrees)
---------------------- TURBINE CONFIGURATION -----------------------------------
          3   NumBl       - Number of blades (-)
        120   TipRad      - The distance from the rotor apex to the blade tip (meters)
          3   HubRad      - The distance from the rotor apex to the blade root (meters)
         -4   PreCone(1)  - Blade 1 cone angle (degrees)
         -4   PreCone(2)  - Blade 2 cone angle (degrees)
         -4   PreCone(3)  - Blade 3 cone angle (degrees) [unused for 2 blades]
          0   HubCM       - Distance from rotor apex to hub mass [positive downwind] (meters)
          0   UndSling    - Undersling length [distance from teeter pin to the rotor apex] (meters) [unused for 3 blades]
          0   Delta3      - Delta-3 angle for teetering rotors (degrees) [unused for 3 blades]
          0   AzimB1Up    - Azimuth value to use for I/O when blade 1 points up (degrees)
    -11.075   OverHang    - Distance from yaw axis to rotor apex [3 blades] or teeter pin [2 blades] (meters)
          0   ShftGagL    - Distance from rotor apex [3 blades] or teeter pin [2 blades] to shaft strain gages [positive for upwind rotors] (meters)
         -6   ShftTilt    - Rotor shaft tilt angle (degrees)
     -4.985   NacCMxn     - Downwind distance from the tower-top to the nacelle CM (meters)
          0   NacCMyn     - Lateral  distance from the tower-top to the nacelle CM (meters)
      4.358   NacCMzn     - Vertical distance from the tower-top to the nacelle CM (meters)
     -4.985   NcIMUxn     - Downwind distance from the tower-top to the nacelle IMU (meters)
          0   NcIMUyn     - Lateral  distance from the tower-top to the nacelle IMU (meters)
      4.358   NcIMUzn     - Vertical distance from the tower-top to the nacelle IMU (meters)
     4.3478   Twr2Shft    - Vertical distance from the tower-top to the rotor shaft (meters)
    144.495   TowerHt     - Height of tower above ground level [onshore] or MSL [offshore] (meters)
         0   TowerBsHt   - Height of tower base above ground level [onshore] or MSL [offshore] (meters)
          0   PtfmCMxt    - Downwind distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters)
          0   PtfmCMyt    - Lateral distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters)
     -14.94   PtfmCMzt    - Vertical distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters)
          0   PtfmRefzt   - Vertical distance from the ground level [onshore] or MSL [offshore] to the platform reference point (meters)
---------------------- MASS AND INERTIA ----------------------------------------
          0   TipMass(1)  - Tip-brake mass, blade 1 (kg)
          0   TipMass(2)  - Tip-brake mass, blade 2 (kg)
          0   TipMass(3)  - Tip-brake mass, blade 3 (kg) [unused for 2 blades]
     190000   HubMass     - Hub mass (kg)
1.37347E+06   HubIner     - Hub inertia about rotor axis [3 blades] or teeter axis [2 blades] (kg m^2)
8.00865E+06   GenIner     - Generator inertia about HSS (kg m^2)
     507275   NacMass     - Nacelle mass (kg)
1.96179E+07   NacYIner    - Nacelle inertia about yaw axis (kg m^2)
     100000   YawBrMass   - Yaw bearing mass (kg)
1.7838E+07    PtfmMass    - Platform mass (kg)
1.2507E+10     PtfmRIner   - Platform inertia for roll tilt rotation about the platform CM (kg m^2)
1.2507E+10     PtfmPIner   - Platform inertia for pitch tilt rotation about the platform CM (kg m^2)
2.3667E+10     PtfmYIner   - Platform inertia for yaw rotation about the platform CM (kg m^2)
---------------------- BLADE ---------------------------------------------------
         20   BldNodes    - Number of blade nodes (per blade) used for analysis (-)
"ElastoDyn_blade.dat"    BldFile1    - Name of file containing properties for blade 1 (quoted string)
"ElastoDyn_blade.dat"    BldFile2    - Name of file containing properties for blade 2 (quoted string)
"ElastoDyn_blade.dat"    BldFile3    - Name of file containing properties for blade 3 (quoted string) [unused for 2 blades]
---------------------- 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   TeetDmpP    - Rotor-teeter damper position (degrees) [used only for 2 blades and when TeetMod=1]
          0   TeetDmp     - Rotor-teeter damping constant (N-m/(rad/s)) [used only for 2 blades and when TeetMod=1]
          0   TeetCDmp    - Rotor-teeter rate-independent Coulomb-damping moment (N-m) [used only for 2 blades and when TeetMod=1]
          0   TeetSStP    - Rotor-teeter soft-stop position (degrees) [used only for 2 blades and when TeetMod=1]
          0   TeetHStP    - Rotor-teeter hard-stop position (degrees) [used only for 2 blades and when TeetMod=1]
          0   TeetSSSp    - Rotor-teeter soft-stop linear-spring constant (N-m/rad) [used only for 2 blades and when TeetMod=1]
          0   TeetHSSp    - Rotor-teeter hard-stop linear-spring constant (N-m/rad) [used only for 2 blades and when TeetMod=1]
---------------------- DRIVETRAIN ----------------------------------------------
        100   GBoxEff     - Gearbox efficiency (%)
          1   GBRatio     - Gearbox ratio (-)
  8.235E+11   DTTorSpr    - Drivetrain torsional spring (N-m/rad)
  6.215E+06   DTTorDmp    - Drivetrain torsional damper (N-m/(rad/s))
---------------------- FURLING -------------------------------------------------
False         Furling     - Read in additional model properties for furling turbine (flag) [must currently be FALSE)
"unused"      FurlFile    - Name of file containing furling properties (quoted string) [unused when Furling=False]
---------------------- TOWER ---------------------------------------------------
         50   TwrNodes    - Number of tower nodes used for analysis (-)
"ElastoDyn_tower.dat"    TwrFile     - Name of file containing tower properties (quoted string)
---------------------- OUTPUT --------------------------------------------------
True         SumPrint    - Print summary data to "<RootName>.sum" (flag)
          1   OutFile     - Switch to determine where output will be placed: {1: in module output file only; 2: in glue code output file only; 3: both} (currently unused)
True          TabDelim    - Use tab delimiters in text tabular output file? (flag) (currently unused)
"ES10.3E2"    OutFmt      - Format used for text tabular output (except time).  Resulting field should be 10 characters. (quoted string) (currently unused)
          0   TStart      - Time to begin tabular output (s) (currently unused)
          1   DecFact     - Decimation factor for tabular output {1: output every time step} (-) (currently unused)
          1   NTwGages    - Number of tower nodes that have strain gages for output [0 to 9] (-)
         20   TwrGagNd    - List of tower nodes that have strain gages [1 to TwrNodes] (-) [unused if NTwGages=0] TwrGagNd    - List of tower nodes that have strain gages [1 to TwrNodes] (-) [unused if NTwGages=0] TwrGagNd    - List of tower nodes that have strain gages [1 to TwrNodes] (-) [unused if NTwGages=0]
          3   NBlGages    - Number of blade nodes that have strain gages for output [0 to 9] (-)
          5,          9,         13    BldGagNd    - List of blade nodes that have strain gages [1 to BldNodes] (-) [unused if NBlGages=0] BldGagNd    - List of blade nodes that have strain gages [1 to BldNodes] (-) [unused if NBlGages=0] BldGagNd    - List of blade nodes that have strain gages [1 to BldNodes] (-) [unused if NBlGages=0]
                   OutList             - The next line(s) contains a list of output parameters.  See OutListParameters.xlsx for a listing of available output channels, (-)                 
"Azimuth"                 
"BldPitch1"               
"GenSpeed"                 
"IPDefl1"                 
"LSSGagMya"               
"LSSGagMza"               
"OoPDefl1"                 
"PtfmSurge"               
"PtfmSway"                 
"PtfmHeave"               
"PtfmRoll"                 
"PtfmPitch"               
"PtfmYaw"                 
"RootFxb1"                 
"RootFyb1"                 
"RootFzb1"                 
"RootMxb1"                 
"RootMyb1"                 
"RootMzb1"                 
"RootFxb2"                 
"RootFyb2"                 
"RootFzb2"                 
"RootMxb2"                 
"RootMyb2"                 
"RootMzb2"                 
"RootFxb3"                 
"RootFyb3"                 
"RootFzb3"                 
"RootMxb3"                 
"RootMyb3"                 
"RootMzb3"                 
"TipDxb1"                 
"TipDyb1"                 
"TipDxb2"                 
"TipDyb2"                 
"TipDxb3"                 
"TipDyb3"                 
"NcIMUTAxs"               
"NcIMUTAys"               
"NcIMUTAzs"               
"YawBrTDxt"               
"YawBrTDyt"               
"RotSpeed"                 
"RotTorq"                 
"RotThrust"
"RotPwr"                                 
"Spn2MLxb1"               
"Spn2MLyb1"               
"TwrBsFxt"                 
"TwrBsFyt"                 
"TwrBsFzt"                 
"TwrBsMxt"                 
"TwrBsMyt"                 
"TwrBsMzt"                 
"TTDspTwst"               
"TwHt1TPxi"               
"TwHt1TPyi"               
"TwstDefl1"               
"YawBrFxp"                 
"YawBrFyp"                 
"YawBrFzp"                 
"YawBrMxp"                 
"YawBrMyp"                 
"YawBrMzp"                 
"TwrBsFzt"                 
"NacYaw"
END of input file (the word "END" must appear in the first 3 columns of this last OutList line)
---------------------------------------------------------------------------------------


After extracting the output parameters of the Tower Base Loads, the tower base pitching moment (TwrBsMyt) shows to deviate (significantly) from the pitching moment calculated by the aerodynamic thrust (RtAeroFxh) times the hub height of 150.0 meter (see figure, including another aerodynamic software). It seems that there is a fixed offset between the TwrBsMyt and the pitching moment calculated by RtAeroFxh times HubHt, resulting from a difference in the length of the reference points.

What could explain this (constant) difference in the TwrBsMyt?

Best regards,

Peter Veldman
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Jason.Jonkman
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Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
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Re: 15MW towerbase pitching moment discrepancy

Postby Jason.Jonkman » Tue Jan 26, 2021 1:31 pm

Dear Peter,

The applied aerodynamic thrust force times moment arm will be reduced a bit by the 6deg shaft tilt, but this effect may be small.

In addition to the thrust force times arm, another large contributor to the tower-base moment is the weight of the nacelle and rotor times their CM offsets from the tower centerline. Given that CMs of both are upwind of the tower (negative OverHang and negative NacCMxn), these combined will shift TwrBsMyt downward, counteracting the moment from applied aerodynamic thrust.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov


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