Dear Yuqi,
The OC3 project finished before FAST v8 was available and NREL applied an older version of FAST (v6) and ADAMS, with the ADAMS models derived using the FAST-to-ADAMS preprocessor. This functionality is well documented in the old FAST User’s Guide: nwtc.nrel.gov/system/files/FAST.pdf. These old models–while for the most part having less functionality than FAST v8–had some linearization functionality applicable to offshore floating systems.
Best regards,