Dear @Xing.Tan,
The buoyancy terms in the strip-theory solution of the HydroDyn module of OpenFAST are recomputed based on displacement as of OpenFAST v2.6 and newer. In addition to buoyancy, axial forces exist at member ends and along tapered members for drag, added mass, and fluid inertia forces, although I see that you’ve set some (but not all) of the associated axial coefficients to zero.
Best regards,