Dear Lingling,
I’m not sure I can fully answer your question. It is difficult to support BModes now that the developer has left NREL.
Please note that if you wish for the platform stiffness matrix to be applied at the base of the tower in BModes, then I’m quite sure you should define your input file such that hub_conn = 2 (all 6 tower-base DOF enabled) or 3 (surge, sway, roll, and pitch tower-base DOF enabled). Also, I’m quite sure that you should set draft = ref_msl (= 30 m in your case). I’m actually not sure what BModes would do in the case you’ve specified in SNL_pltfrm6.bmi. Please see the CS_Monopile example available from here for a similar example: wind.nrel.gov/public/jjonkman/BModes/.
I have not used the version from wind.nrel.gov/public/jjonkman/BModes/ (BModesJJ).
My understanding is that BModesJJ and BModes use differen’t eignesolvers, which may explain some of the differences you’re seeing. Additional differences are discussed in the following forum topics: Tower Eigenfrequencies of NREL 5MW Turbine.
Best regards,