Dear Satish,
Your results make sense. You are getting a lower amplitude of mudline fore-aft bending moment because version of HyrdoDyn you are using in OpenFAST does not inherently consider wave stretching, which are included in the results you are comparing to.
If you need to match the OC3 results exactly, until wave stretching is added to OpenFAST*, you could apply wave stretching yourself and use the corresponding wave kinematics within OpenFAST using the WaveMod = 6 option of HydroDyn. This would require that you have wave kinematics at all hydrodynamic analysis nodes along the monopile.
Best regards,
Footnote:
*NREL did have a project to add wave stretching into OpenFAST, but the project ran out of funding before a successful completion, and the solution was not numerically stable for structural flexible substructures; and we have not yet been funded to restart and finish the project.