Discussion on tower foundation load under extreme working conditions

[Yushun.Fu]

Dear,
I used an IEA15MW semi submersible wind turbine for simulation, and the configuration was completely set according to Task 37. I have doubts about the value of the tower base load in the x-direction. Below is my operating condition table.

Below is my extreme value statistics on tower foundation loads

My wind, waves, and current directions are all 0 degrees. In theory, the bending moment of the side to side tower foundation under DLC10 working conditions makes me wonder why there is such a large value. In theory, the before aft should be greater than the side to side direction. I thought that the only force that affects the side to side direction is the wind. My wind file was generated using Turbsim for EWM50year, and below are the wind speed time domain diagrams for the three directions

As you can see, there is no problem with the wind speed in the y direction. So, what is the reason for the large bending moment of the tower base in the side to side direction in dlc10?
I hope to receive everyone’s help!
Best regards,

[Jason.Jonkman]

Dear @Yushun.Fu,

Can you clarify how you are setting up dlc10? Presumably you are modeling an idling rotor with the generator disabled, the blade-pitch angles feathered, and simplified aerodynamics without BEM and unsteady airfoil aerodynamics disabled? Do you have a nacelle-yaw error? What is happening with platform yaw?

Not sure this matters to your question, but what is meant in your figure regarding QS, LM1 and LM2?

Best regards,

[Yushun.Fu]

Thank you for your reply.
1.My dlc10 settings are as follows:

---------------------- FEATURE SWITCHES AND FLAGS ------------------------------ 
      1   CompElast       - Compute structural dynamics (switch) {1=ElastoDyn; 2=ElastoDyn + BeamDyn for blades}  
      1   CompInflow      - Compute inflow wind velocities (switch) {0=still air; 1=InflowWind; 2=external from OpenFOAM} 
      2   CompAero        - Compute aerodynamic loads (switch) {0=None; 1=AeroDyn v14; 2=AeroDyn v15} 
      1   CompServo       - Compute control and electrical-drive dynamics (switch) {0=None; 1=ServoDyn} 
      1   CompHydro       - Compute hydrodynamic loads (switch) {0=None; 1=HydroDyn} 
      0   CompSub         - Compute sub-structural dynamics (switch) {0=None; 1=SubDyn; 2=External Platform MCKF}
      3   CompMooring     - Compute mooring system (switch) {0=None; 1=MAP++; 2=FEAMooring; 3=MoorDyn; 4=OrcaFlex} 
      0   CompIce         - Compute ice loads (switch) {0=None; 1=IceFloe; 2=IceDyn} 
      0   MHK             - MHK turbine type (switch) {0=Not an MHK turbine; 1=Fixed MHK turbine; 2=Floating MHK turbine}

---------------------- DEGREES OF FREEDOM --------------------------------------
True          FlapDOF1    - First flapwise blade mode DOF (flag)
True          FlapDOF2    - Second flapwise blade mode DOF (flag)
True          EdgeDOF     - First edgewise blade mode DOF (flag)
False         TeetDOF     - Rotor-teeter DOF (flag) [unused for 3 blades]
False         DrTrDOF     - Drivetrain rotational-flexibility DOF (flag)
False          GenDOF      - Generator DOF (flag)
False         YawDOF      - Yaw DOF (flag)
True          TwFADOF1    - First fore-aft tower bending-mode DOF (flag)
True          TwFADOF2    - Second fore-aft tower bending-mode DOF (flag)
True          TwSSDOF1    - First side-to-side tower bending-mode DOF (flag)
True          TwSSDOF2    - Second side-to-side tower bending-mode DOF (flag)
True          PtfmSgDOF   - Platform horizontal surge translation DOF (flag)
True          PtfmSwDOF   - Platform horizontal sway translation DOF (flag)
True          PtfmHvDOF   - Platform vertical heave translation DOF (flag)
True          PtfmRDOF    - Platform roll tilt rotation DOF (flag)
True          PtfmPDOF    - Platform pitch tilt rotation DOF (flag)
True          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)
          90  BlPitch(1)  - Blade 1 initial pitch (degrees)
          90  BlPitch(2)  - Blade 2 initial pitch (degrees)
          90  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)
          0   RotSpeed    - Initial or fixed rotor speed (rpm)
          0   NacYaw      - Initial or fixed nacelle-yaw angle (degrees)

---------------------- PITCH CONTROL -------------------------------------------
0                      PCMode      - Pitch control mode {0: none, 3: user-defined from routine PitchCntrl, 4: user-defined from Simulink/Labview, 5: user-defined from Bladed-style DLL} (switch)
0.0                    TPCOn       - Time to enable active pitch control (s) [unused when PCMode=0]

---------------------- GENERATOR AND TORQUE CONTROL ----------------------------
0                      VSContrl    - Variable-speed control mode {0: none, 1: simple VS, 3: user-defined from routine UserVSCont, 4: user-defined from Simulink/Labview, 5: user-defined from Bladed-style DLL} (switch)
1                      GenModel    - Generator model {1: simple, 2: Thevenin, 3: user-defined from routine UserGen} (switch) [used only when VSContrl=0]

I will set PCmode to 5 and VScontrol to 5 except for shutdown conditions

======  General Options  ============================================================================
False                  Echo        - Echo the input to "<rootname>.AD.ech"?  (flag)
default                DTAero      - Time interval for aerodynamic calculations {or "default"} (s)
0                      WakeMod     - Type of wake/induction model (switch) {0=none, 1=BEMT, 2=DBEMT, 3=OLAF} [WakeMod cannot be 2 or 3 when linearizing]
1                      AFAeroMod   - Type of blade airfoil aerodynamics model (switch) {1=steady model, 2=Beddoes-Leishman unsteady model} [AFAeroMod must be 1 when linearizing]

Except for shutdown conditions, I will set WakeMod to 1 and AFAeroMod to 2

2.The following figure shows the time-domain diagram of Yaw direction motion on my platform

3.As you can see, I did not encounter any errors while running the DLC10 operating condition. In order to make it clear, I have added NacYaw

4.My QS refers to the use of MAP++quasi-static, LM1 uses MoorDyn to lumped mass, and LM2 adds Water Kinematics file on the basis of LM1

We look forward to your reply！
Best regards，

[Jason.Jonkman]

Dear @Yushun.Fu,

Thanks for clarifying. Just a few comments:

• During parked or idling condition under storm conditions, I would expect the aerodynamic loads on the rotor to be quite small, unless the yaw error becomes sizeable. There are likely sizeable yaw errors under the turbulent inflow and platform-yaw motion you are seeing, but you can quantify how large by outputting the yaw error via AeroDyn output RtSkew.
• Do you have tower aerodynamic loads enabled in AeroDyn (TwrAero = TRUE), which I would expect to dominate over rotor aerodynamic loads when the yaw error is not sizeable?
• I see that you’ve parked the rotor rather than kept the rotor idling. Normally a brake would only be applied for such a large wind turbine during maintenance operations. I would suggest idling the rotor by setting GenDOF = TRUE in ElastoDyn with GenTiStr = TRUE and TimGenOn > TMax in ServoDyn.

Best regards,