Dear @Kun.Jiang,
Well, I’m not sure either. Your model appears to behave differently than other FOWT models with SubDyn enabled than I’ve seen before whereby setting NumCrctn = 1, DT_UJac = 1/10 of the floater-pitch period, and DT as small as necessary results in a stable solution with convergence. You should choose solver parameters (DT, NumCrctn, DT_UJac) such the solution remains stable and that lowering DT and/or DT_UJac and increasing NumCrctn doesn’t affect the solution. I’m not sure what else to suggest other than to try combinations of each of these.
Best regards,
Dear Jason,
Thank you for your suggestion. I sincerely appreciate your help!
Best regards,
Kun.Jiang
Dear @Kun.Jiang,
Were you able to solve this issue? If not I have a few questions.
I looked at your plots a bit further up in this post and noticed what looks like a strong transient response around 800 seconds. Is there a discontinuity at 800 seconds in the wave elevation or hydrodynamic forces?
Regards,
Andy
Dear Andy,
First of all, I would like to thank you for your advice.
I am currently only able to run the program by reducing DT_UJac in the main input file of openfast, but it only ends normally when I set a value smaller than the recommended value.
I followed your suggestion and looked at the curves of wave elevation and hydrodynamic forces when the program did not end properly. As you said, there is an unusually pronounced fluctuation in the hydrodynamic force along the y direction. It fluctuated within plus or minus 5000N before 800s, and suddenly rose to 8.18e+05N after 800s. I’m guessing it might be a problem with the model’s hydrodynamic shape design. I have attached a graph for your reference. Do you have any guidance?
Best regards,
Kun.Jiang
Dear @Kun.Jiang,
Can you clarify what you changed DT_UJac to for the model to run stably, versus what you expect it should have been?
How are you modelling the mooring system (MoorDyn or MAP++) and what solver settings have you used in the mooring module?
Best regards,
Dear Jason,
Thank you for your concern.
I changed DT_UJac to 1/10 of the minimum period in SubDyn that is prompted when the program is running. This value is 0.185110, and the program can run stably.
For the modeling of the mooring system, I used the MAP of ITIBarge4, and I did not change the solver settings.
Best regards,
Kun.Jiang
---------------------- LINE DICTIONARY ---------------------------------------
LineType Diam MassDenInAir EA CB CIntDamp Ca Cdn Cdt
(-) (m) (kg/m) (N) (-) (Pa-s) (-) (-) (-)
Material 0.0809 130.403 589.0E6 1.0 0 0 0 0
---------------------- NODE PROPERTIES ---------------------------------------
Node Type X Y Z M B FX FY FZ
(-) (-) (m) (m) (m) (kg) (mˆ3) (N) (N) (N)
1 fix 386.92 171.985 depth 0 0 # # #
2 Vessel 20.0 20.0 -4.0 0 0 # # #
3 fix 171.985 386.95 depth 0 0 # # #
4 Vessel 20.0 20.0 -4.0 0 0 # # #
---------------------- LINE PROPERTIES ---------------------------------------
Line LineType UnstrLen NodeAnch NodeFair Flags
(-) (-) (m) (-) (-) (-)
1 Material 473.312 1 2 tension_fair
2 Material 473.312 3 4 tension_fair
---------------------- SOLVER OPTIONS-----------------------------------------
Option
(-)
repeat 270 180 90
