Floating platform motions

Hello,

I am attempting to develop a coupled model of FAST+OrcaFlex using the FASTlink package. I am interested in looking at the behavior of the mooring system for a small (1kW) floating wind turbine. I am having a few problems with FAST that I hope you could help me resolve. First, I have been getting Small Angle Assumption violations due to large platform displacement. This seems to be caused by excessive yaw motions at high wind velocities. From other posts, I have seen that this is typically a time-step size issue, but I have done a sensitivity test in which I decreased the time step and got nearly identical results. Would you suggest that the time step continue to be decreased, or is there anything else I might try to mitigate these large yaw motions?

Second, I have compared the platform’s roll decay between the FASTlink model and a comparable OrcaFlex model which shows that the FAST model is much more highly damped than the OrcaFlex-only model. I would expect to get the same result between the two models, so I’m wondering if there is anything in the FASTlink module that could cause higher platform damping? As I understand it, previously there was artificial damping written into the OC3 Hywind spar model (http://forums.nrel.gov/t/additional-linear-hydrodynamic-damping-in-oc3-hywind/383/1). Do you know if this damping is hardcoded into the FAST+OrcaFlex module?

Thank you for your assistance.

Michael MacNicoll

Dear Michael,

We have experienced platform-yaw instability problems for models with too little yaw stiffness. For example, the OC3-Hywind spar system is commonly modeled with a simplified mooring model without the delta connection to the platform. The delta connection in real life greatly increases the yaw stiffness of the mooring system; thus, a simple yaw spring is commonly modeled to mimic the effect of the delta. But without this yaw spring, the model is likely to go unstable. Alternatively, the details of the delta connection could be modeled.

I’m not aware of any “extra” damping brought about by the FAST-OrcaFlex interface. Is aerodynamics disabled in the coupled FAST-OrcaFlex model? Aerodynamic damping could be significant. For a coupled FAST-OrcaFlex model without aerodynamics, I would expect consistent damping to an equivalent OrcaFlex-only model.

Best regards,

Dear Jason,

Thanks very much for your response. I see that this yaw stiffness is introduced manually in the WAMIT file for the OC3 Hywind FAST case. However it’s not clear to me where this stiffness is incorporated in the corresponding OC3 Hywind FAST+OrcaFlex model. Can you tell me where this is done so that I may work on implementing something similar in our model?

Thanks again for your response.

Michael

Dear Michael,

Yes, historically for FAST-only simulations we modeled the yaw stiffness of the OC3-Hywind mooring system by augmenting the (6,6) element of the hydrostatic stiffness matrix. However, OrcaFlex does not allow this element to be nonzero because there is no physically meaningful hydrostatics mechanism for inducing yaw stiffness. Instead, you’ll have to add the yaw stiffness to the OrcaFlex model in another way.

Best regards,