Dear Casey,
I’m assuming you are running some variation of Test25 from the FAST CertTest, which is a model of the NREL 5-MW turbine atop the OC4-DeepCwind semisubmersible. In this model, the second-order hydrodynamic effects, including mean drift, are solved using DiffQTF = 12 i.e. using the full difference-frequency quadratic transfer function (QTF). This QTF contains the second-order wave-excitation loads per unit wave amplitude squared at pairs of first-order wave frequencies and directions, across a range of frequency and direction pairs, as derived from WAMIT. However, the *.12d file provided with Test25 only contains data for the zero-degree wave direction. It was the intent for HydroDyn to abort if the specified wave direction (WaveDir) is outside the range of the available WAMIT data, but there is a known bug in the upper bound on WaveDir in the WAMIT2 submodule of HydroDyn (unfortunately, we have not yet fixed the bug). See the following forum topic for details: Second order file WaveDir boundary check issue. (Please note that the second problem reported in that forum topic is not a bug; the direction pairs must be fully populated (not sparse).)
Regardless, unless you’ve changed the *.12d file to expand the QTF to other wave directions, this HydroDyn model should only be run with WaveDir = 0.
I hope that helps.
Best regards,