Dear Sungmoon,
I’m not sure what exactly you mean by “large deformation”, but the theory of FAST is based on a number of nonlinearities that allow for what I would call “medium deformation”. Examples of nonlinearities already included in FAST include:
*Corrections for coordinate system orthogonality
*Motions include radial shortening terms (geometric nonlinearity)
*Inertial loads include nonlinear centrifugal, Coriolis, & gyroscopic terms
With these nonlinearities, the tower (and blade) model in FAST permits deflections up to about 15 degrees slope; the model looses considerable accuracy when the slopes greatly exceed 20 degrees. Most wind turbine towers don’t reach nearly these limits (the limits are usually reached first by the blades). So, the conventional approach should be appropriate to most towers.
More inform on the FAST theory basis is available in the following forum topic: Coupled blade modes in FAST.
I’m sure with a bit of work it is possible to add a direct finite-element-based option for the tower. In fact, we at NREL are already currently working on adding such an option (more for the blades than the tower), although such a release is still a ways off.
Best regards,