Dear Marie,
Hello, I can’t open your link, and I am not familiar with DTU 10 MW Turbine, but as far as I know, the smallest controller response natural frequency must be lower than the smallest support-structure natural frequency to avoid the negative damping.
For example, you used the PI gains of OC3 with spar platform, and you applied it to DTU 10 MW Turbine with semi-submersible. These two platforms may have a different natural frequency, you have to tune the controller gains to ensure the smallest controller response natural frequency lower than the smallest support-structure natural frequency.
Best regards,
Cheng