I’ll try to answers your questions:
Are you asking how long you need to run FAST to get statistical significance in the results? If so, the answer likely depends on the nature of the sea state and substructure. At a minimum, the IEC 61400-3 offshore wind turbine design standard requires 6 seeds of 10-minute simulations per bin for the simplest cases, but this is only a minimum.
I’m not sure I understand this question. HydroFxi is the total integrated hydrodynamic force along the xi direction. While the reference point is important for moments, it is irrelevant for forces.
NREL has not yet been budgeted to add a wave-stretching model to FAST v8. However, we are working to add a user-defined wave-kinematics option for the next release of HydroDyn that will allow wave kinematics to be prescribed up to the instantaneous water elevation, assuming the user has access to data that can be applied. Alternatively, you could modify the source code yourself (although implementing a general wave-stretching solution applicable to all substructures would not be trivial).
The platform additional stiffness and damping in HydroDyn can be used to tune the substructure natural frequencies and damping ratios of any offshore FAST model. When SubDyn is enabled, the SubDyn manual recommends at least setting a nonzero value for AddBLin(3,3).
NREL has validated the offshore capability of FAST within several projects. FAST results have been compared with measurements taken from offshore basin tests at MARIN under the DeepCwind consortium, involving combined wind-wave tests of a wind turbine atop a spar buoy, tension-leg platform (TLP), and semisubmersible, and with measurements taken from open-ocean tests for the SWAY, WindFloat, and Hywind prototypes. Further validation of FAST’s new offshore features is ongoing under the IEA Wind Task 30 Offshore Code Comparison Collaboration, Continued, with Correlation (OC5) project. That said, I would highly encourage you to validate your own results, especially if you have access to the appropriate data or if you’ve created a model that is different from one of the default models NREL has provided.
PropSetID = 4 within the SubDyn model of FAST Test19 is not actually used by any member. We used that property set while debugging a model, but didn’t remove it from the file before releasing FAST.
The platform mass (PtfmMass) in ElastoDyn for a fixed-bottom offshore wind model can be used to define the point mass of the transition piece. However, the details of the transition piece have not been specified for the OC3-monopile model of Test19. The distributed mass of the tower and substructure are specified in the ElastoDyn and SubDyn input files of Test19, respectively.
I hope that helps.