Dear Wystan,
The platform added mass (PtfmAM) within UserPtfmLd() could be used to represent acceleration-dependent loads transfered to the pile from the soil. However, I’ve seen soil models that ignore these terms; so, you can probably set PtfmAM to zero.
Enabling the platform degrees-of-freedom (DOFs) will introduce additional natural modes in the FAST model. My guess is the model is not running because these new modes are at a high enough frequency that that the structural time step must be reduced to resolve them without numerical instability. (And introducing PtfmAM lowers the natural frequency of these modes, making the model numerical stable.) See my post dated Nov 4, 2010 in the forum topic found here for rules of thumb on how to choose proper time steps: Error in FAST, working in ADAMS.
Best regards,