Yaw stiffness for Spar

Wind & Water Computer-Aided Engineering Software Tools
1

Dear Prof. Jonkman,

Could you please explain me how did you get the yaw stiffness value in the additional linear stiffness matrix in HydroDyn file of the OC3Hywind model. (I understand this term was added to take into account the mooring delta connection.) I try to build my own spar but I also get important yaw amplitude that makes the simulation crash. I tried many possibilities such as adding more mooring lines or adding additional damping in yaw, without much success.

But it turns out that when I put an additional stiffness in yaw it works, but I don’t know how to calculate this term or which coherent value to put considering my spar and mooring lines caracteristics.

Thank you and best regards,

Pierre-Adrien

2

Dear Pierre-Adrien,

The OC3-Hywind model in the FAST CertTest matches the OC3-Hywind specifications document identically: nrel.gov/docs/fy10osti/47535.pdf. We originally derived the additional yaw spring stiffness by the following steps:
(1) Approximating the location of the apex of the delta
(2) Building two mooring models: (a) with mooring lines extending from the anchor only to the apex of the delta (assuming the delta is rigid) and (b) with mooring lines extending from the anchor to the spar (bisecting the delta)
(3) Taking the difference in the resulting yaw stiffness between the two mooring models (a) and (b)
(4) We then confirmed that the yaw natural frequency in yaw free decay was correct.
You could probably skip steps (1)-(3) and simply tune the yaw spring stiffness to match a desired yaw natural frequency.

Best regards,

3

Dear Dr. Jonkman,

Thank you for your answer.

Best regards,

PA

4

Dear Dr.Jonkman,

Just to confirm, Is it because of this additional yaw stiffness, the OC3 Spar has lesser yaw natural period compared to that of the OC4 semisubmersible platform?

Thanks
Krishnaveni

5

Dear Krishnaveni,

Yes, although the yaw natural frequency is dictated both by this yaw stiffness of the mooring system and the full-system yaw inertia.

Best regards,

6

Thank you for clarifying it Dr.Jonkman. I was wondering how the semisubmersible with a larger lever arm compared to the OC3 spar could have a lesser natural frequency than spar. Now it makes sense when understanding the additional yaw stiffness provided by the delta connection/additional damping(In FAST) and also the roles played by the yaw moment of inertias of the respective systems.
Thank you for your insight. I appreciate it.

Regards
Krishnaveni