Dear Wichuda,
The steps outlined by Minxi where to obtain the rigid-body mass matrix of the system, not including hydrodynamic added mass. In OpenFAST, it is possible to zero out the added mass completely, but this was not possible in FAST v7. So, the workaround was to specify WAMIT output files with all coefficients set to zero. These are not obtained by running WAMIT; instead, the files were created by hand to look like they were WAMIT-generated. I’ve attached the files to my post dated Oct 30, 2012 in the following forum topic: Questions about the HydroDyn Module - #4 by Jason.Jonkman.
Best regards,