Dear Carmen,
In FAST’s HydroDyn module, the wave direction input (WaveDir) is defined counter-clockwise when looking from above (that is, a positive rotation about the Z axis of the inertia frame, which points verticically upwards). Zero degrees corresponds to waves propogating along the X axis of the inertia frame, and 90 degrees corresponds to waves propogating along the Y axis of the inertia frame.
For definition of the wind direction and nacelle-yaw error, please see the following forum topic: Is positive Yaw error clockwise?.
In FAST, the platform translational displacements surge, sway, and heave are positive along the X, Y, and Z axes of the inertia frame, respectively. The platform rotational displacements roll, pitch, and yaw are positive about the X, Y, and Z axes of the inertia frame, respectively. See Figure 20 from the FAST User’s Guide for more information.
The mooring line angles in FAST v7 follow the same convention as WaveDir.
Best regards,