I am repeating the barge platform case in FAST. First, I would like to conduct a case of the barge platform without neither tower nor mooring lines. The tower is removed by setting a very small density of the tower and blades. Only 6 degrees of freedom of the platform is allowed in ElastoDyn. The environmental condition is set to be a regular wave (T=6.283s, H=2m) only. The plot below is the results of the time-series of the case:
I was hoping to see the oscillations of the platform in 6DOF would reach a stable state at some point, but it seems that the oscillations of sway, roll, and yaw have a significant increase from about 2,100s.
I also run some other cases with different wave frequencies. The strange increase of motions is significant when the wave frequencies close to 1 rad/s (6.283s). The figures below show the plots of two other cases. The wave conditions can be found in the title of each figure.
Are you using the FAST model of the ITI Energy barge or something else? Which version of FAST (or OpenFAST) are you using?
Without any mooring system, there is no restoring of the platform-yaw degree of freedom. So any excitation in yaw will cause the system to move and not come back. While generally there is no hydrodynamic excitation in yaw, there could be a small amount due to numerical round-off in the calculations. The yaw motion can then induce roll motion, which does have some restoring. Of course, the FAST / OpenFAST solution will become inaccurate once the platform-yaw angle greatly exceeds about 15 degrees. I believe this is what you are seeing; it is just taking a long time for the yaw motion to be sizeable.
I’m not sure that there is anything particularly concerning here. With a small amount of yaw restoring (from moorings), the yaw angle will not grow and induce other problems.