Dear @TianYuan.Wang,
I’m not really sure what you are referring to by “tutorial”.
The AddBQuad values documented for the OC4-DeepCwind semisubmersible in Table 4-5 of NREL/TP-5000-60601 were calculated by choosing coefficients that result in free-decay behavior that best fit the free-decay behavior calculated by a model with a more complicated distributed viscous drag formulation. I believe the same process was used to derive AddBQuad for the IEA Wind 15-MW RWT.
But again, if you already know the viscous drag coefficients that can be used in a strip-theory (Morison’s equation) solution distributed across the substructure, I would use that rather than use AddBQuad.
Best regards,