1E+7 TeetSSSp - Rotor-teeter soft-stop linear-spring constant (N-m/rad) [used only for 2 blades and when TeetMod=1]
1.0401E+10 TeetHSSp - Rotor-teeter hard-stop linear-spring constant (N-m/rad) [used only for 2 blades and when TeetMod=1]
This is a part of my teeter file. I want to figure out a minimum value of SSSp. Because, firstable, i don’t consider about fatigue now. I just only want to see the decrease of load at the blade root and tower base. I calculated the c and k values by referencing other papers and used a value of about 2E+8. For reference, the model I’m using is the 22 MW turbine model. When I used that SSSp value, the loads in the x-direction at the two previously mentioned locations did not decrease; instead, they actually increased. I expected that enabling the teeter function would reduce the loads, but they did not decrease at all. Therefore, while I was trying to reduce the k value slightly, I found a strange result. When I set k below 1E+7, the OpenFAST simulation showed the teeter angle dropping rapidly to −11deg and failing to recover. Why does this happen?
If I understand correctly, you’ve modified the IEA Wind 22-MW RWT to be a 2-bladed rotor with the teeter degree of freedom enabled (NumBl = 2, TeetDOF = TRUE in ElastoDyn); is that correct? Can you share your full ROTOR-TEETER section of the ElastoDyn input file?
---------------------- ROTOR-TEETER --------------------------------------------
1 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]
2.645E+8 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]
00.0 TeetSStP - Rotor-teeter soft-stop position (degrees) [used only for 2 blades and when TeetMod=1]
11.0 TeetHStP - Rotor-teeter hard-stop position (degrees) [used only for 2 blades and when TeetMod=1]
1E+7 TeetSSSp - Rotor-teeter soft-stop linear-spring constant (N-m/rad) [used only for 2 blades and when TeetMod=1]
1.0401E+10 TeetHSSp - Rotor-teeter hard-stop linear-spring constant (N-m/rad) [used only for 2 blades and when TeetMod=1]
I have an additional question. When I ran the simulation with SSSp set to 1E+4 and then to 1E+7, the loads on the blades and the tower were almost identical, and TeetDeg was exactly the same. I don’t think it’s reasonable for these results to be unchanged even though the teeter stiffness was set differently. Is there a specific reason why the analysis would produce the same values in this case?
The teeter springs and stops are symmetric about 0 degrees, so, by setting TeetHStP = 11deg, you’ve placed a hard stop at -11 and 11deg. From your simulation, it looks like the rotor keeps bouncing around the lower limit.
I would expect different teeter stiffness to result in different response.
I expected the behavior of the TLP model to be similar to that of the fixed model, because in both models there is almost no angular motion in the roll, pitch, and yaw directions. However, when I ran simulations and analyzed the results in OpenFAST, I found that angular variations of up to about 1 degree still occurred. In this case, I observed that TwrBsFxt followed the behavior of the PtfmPitch graph almost exactly, and that the load in the TLP model became larger after applying Teeter than before applying it.
To address this increase in load after applying Teeter, I tried changing the SSSP value and the Dmp value, but I was unable to reduce the load. This made me wonder whether the teeter functionality is not being properly implemented in OpenFAST. If possible, could I ask for any additional advice or guidance regarding this phenomenon?
I’m not aware of any issue with the teeter implementation in ElastoDyn.
Is it that the teeter hitting its stops is trigger high tower loads? E.g., if you eliminate the teeter stops by increasing TeetSStP and TeetHStP, do the tower loads drop? How large of a teeter angle is obtained when you eliminate the stops?
I am not sure whether I have understood the overall situation correctly. To respond first, it seems that the main cause of TwrBsFxt among the tower loads is the platform pitch motion. In the early stage of my research, I thought that the aerodynamic drag acting on the blades would have a significant effect on TwrBsFxt. However, in practice, it did not have a large influence. Instead, I found that the platform’s pitch motion has a dominant effect on TwrBsFxt. Therefore, it seems difficult to meaningfully reduce the tower load simply by adjusting the Dmp and K values of the teetering system.
If TeetSStP is set to more than 11 degrees, the blades collide with the tower, making the simulation impossible. Also, I had not considered setting TeetSStP to a nonzero value. For example, if TeetSStP were set to 3 degrees, I assumed that within that 3-degree range, Dmp and K would not act on the teeter motion, which would make the simulation meaningless. I would like to understand why you suggested increasing TeetSStP and TeetHStP. At present, they are set to 0 degrees and 11 degrees, respectively. Please let me know if there is anything I have misunderstood.
In a floating offshore wind turbine (FOWT), the platform-pitch motion can have a large impact on the tower loads. And platform pitch is driven more by rotor thrust than by the bending moments possibly reduced by rotor teeter.
My point about changing the teeter stops is that if the teeter is constantly hitting the stops, the tower loads may be worse with teeter than without. So, my suggestion to change the teeter stops was to ensure that you are seeing the benefit of teeter in your model by checking tower loads with and without the teeter stops. Of course, if changing the teeter stops results in a tower strike, that is a different issue. Perhaps you can get around that be increasing the overhang or blade precone.
