OC4 Phase II: Yaw activity in load case 3.8 (Mooring line loss)

Dear all,

as this is my first post in this forum I would like to say a few words before getting into the topic:
This forum helped me al lot to get started with FOWT simulations. I appreciate that members and administrators take their time to help others. Thanks for that.

Currently I am validating our panel method for FOWT simulations using the OC4 Phase II data.
Unfortunately I could not figure out if a yaw controller was used within the simulations. The only information I found comes from the definition of the NREL 5MW turbine (Jonkman):
“We did not establish additional control actions for nonpower-production operations, such as control actions for normal start-up sequences, normal shutdown sequences, and safety and protection functions. Nor did we develop control actions to regulate the nacelle-yaw angle.”

Still using a yaw controller would be reasonable in load case 3.8 (Mooring line loss) as the yaw angle exceeds 10° by far (see picture from the OC4 Meeting presentation in Frankfurt).
Most participants generally predict an asymptotic behaviour of the platform yaw angle. Nevertheless small ‘dents’ occur in the graphs (eg. NTUA, SEW, 4Subsea_P). Yaw activity would be a possible explanation for these ‘dents’. Non-uniform usage of the yaw actuator could also be responsible for the differences in the steady state (nearly) reached at the end of the simulation.

Therefore my question is: Does anyone know if there is a definition of the yaw controller used (or not used) for load case 3.8?

And in case a participant reads this: Is it possible that different controllers were used by the participants?

Thanks for your time in advance and greetings from Hamburg,
Christian Schulz

Dear Christian,

As you quoted from the NREL 5-MW specifications report, there was no yaw-control logic developed for the NREL 5-MW baseline wind turbine. Neither was there yaw-control logic supplied for load case 3.8 from OC4 Phase II. As far as I know, none of the OC4 Phase II participants applied a yaw controller to this load case.

Best regards,

Dear Jason,

thanks for your reply. Now looking from the modelling point of view:
My understanding is that the platform yaw angle in ‘steady state’ (long time after mooring line loss) should be determined by a balance of mooring forces and forces and moments from the rotor. As the situation is nearly steady (apart from the waves) the different mooring models should not deliver largely varying forces. The same should apply for the yawed rotor: The different implementations of BEM methods normally do not vary that much in steady state.

Do you have any idea why participants’ predictions of the ‘steady state’ platform yaw vary that much?

Thanks in advance and greetings from Hamburg,

Dear Christian,

I’m not sure I know. I suggest reviewing our paper that describes the results for some insights: nrel.gov/docs/fy14osti/61154.pdf. However, platform-yaw for load case 3.8 is not mentioned there–I think more work would be needed e.g. to enable/disable various features of the different tools in order to understand more what is causing the differences. The platform surge/sway motions agree reasonably well between the tools.

Best regards,

Dear Jason,

of course I examined your paper before asking here:) I think my interest in this topic is not that strong that I start running own simulations using FAST or other methods used in OC4 as I am trying to validate our in-hose panel method. Thus this remains an interesting topic for me but I will not invest much more effort into finding an explanation for that.

Thanks very much for your help and your time. It is really a luxury that one can ask detailed questions like this here and promptly gets advice from you and the other.

Without wanting to start a discussion on that and just for completeness:
I think when having a closer look the surge and sway motions vary a lot especially when considering the to equilibrium position, but this is difficult to see in the picture.

Greetings from Hamburg,