For OC3-Hywind, suppose the generator torque is constant in upper rated wind speed. In this way how increasing the blade pitch angle will affect the platform roll motion?
I expected it will increase, but as u see it didnt.
Note the yaw DOF disabled.
You never explain why you think the roll motion would increase with increasing blade-pitch angle.
I think it makes sense that the roll motion decreases with increasing blade-pitch angle because the side-to-side damping likely increases with increasing blade pitch angle (the side-to-side damping is very small when the blade-pitch angle is small and larger when the blade-pitch angle is large).
For barge type platform the following sentences appeared in research article “Individual blade pitch control of floating offshore wind turbines”, by H. Namik and K. Stol
“Because of the increased blade pitching, the controller causes the turbine to roll even more, thus increasing tower side–side fatigue loads. Interestingly, the blades affect the platform roll the same way it affects platform pitch, and the effect is almost exactly in phase.This means that when the controller generates the platform restoring pitch moment by increasing rotor thrust, it also induces a rolling moment by an asymmetric torque load. Although the rotor is symmetric and an increase in symmetric thrust should result in no torque imbalance on the rotor, the asymmetry is created by having blade pre-cone and shaft tilt angles”.
Can the above sentence be generalized on OC3-Hywind?
Also, i thought that increasing blade pitch will increase the rotor rotational torque that may increase the roll motion of platform.
Both the ITI Energy barge and OC3-Hywind systems use the NREL 5-MW turbine, so the precone and shaft tilt are the same between the systems. However, the ITI Energy barge has far more motion than the OC3-Hywind spar, so, my guess is any problems are exacerbated in the ITI Energy barge.
That said, the quote from Namik’s paper doesn’t say that roll motion is worse with higher blade-pitch angles, just that blade pitch influences platform roll.
I recall from Namik et al’s work that individual blade-pitch control may induce platform-roll motion if the controller has no knowledge of the roll motion, but this can be mitigated if minimization of the roll motion is added as a controller objective.
The goal of the collective blade-pitch controller is to maintain constant power above rated, which is equivalent to maintaining constant speed and constant torque. That is, the mean torque does not increase with blade pitch above rated.