Platform Forces and Moments, PtfmFxi, PtfmMxi etc

Wind & Water Structural Analysis
1

Dear Dr. Jonkman

I modeled the free vibration response of OC3Hywind turbine. All the environmental loading’s are disabled. I wanted to take check the platform output forces and moments PtfmFxi, PtfmFyi, PtfmFzi, PtfmMxi, PtfmMyi, PtfmMzi. However it shows Invalid and all the values are zeros. I do understand that all the environmental forces are zeros, but according to this post "Clarifications PtfmFxyz/mxyz in FAST - #7 by Jason.Jonkman these forces also include mooring line forces. So i am expecting that i should get mooring lines forces on platform. i want to understand the reason why i get invalid, zeros forces.

Regards,
Zahid

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2

Dear Zahid,

The forum post you linked to referred to an older version of FAST. Outputs PtfmFxi etc. are no longer available in FAST v8.16; they have been replaced with other outputs from ElastoDyn, HydroDyn, and the mooring modules. See the documentation of each module for a complete list of available outputs.

Best regards,

3

Thank you Dr. Jonkman. I will look into the changes that has been made.

4

Dear Jason,

I have run simulations for 5MW offshore wind turbine for the rated speed (11.4m/s ) and cutout speed (24 ) both operational conditions. The setting of control system is
5 PCMode = user-defined from Bladed-style DLL

and as it is blade-pitch-to-feather-controlled turbine, the thrust peaks at rated wind speed and then drops with increasing wind speed. This is correct in my simulation for thrust forces( the Fore-aft shear at top of tower (YawBrFxp) in rated speed is higher than in cutout speed). I expected the same trend for the fore-aft and side-side moment at top of tower (YawBrMxp and YawBrMyp) but the moments at top of tower for 24 m/s wind speed are much higher with more fluctutions. I have plotted them

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. Could you please kindly have a look at the plot and let me know if they make sense?

I got confused and I am wondering if I should change some parameters for the above rated wind speeds?

Thank you very much.

Best Regards,
Arash,

5

Dear Arash,

I’m not surprised by these results. While the mean thrust (represented in your plot as YawBrFxp) will drop with increasing wind speed above rated speed during pitch-to-feather controlled operation, other mean loads like the rotor torque (YawBrMxp) and rotor asymmetric loads (YawBrMyp) do not drop. Further, the standard deviation of most loads tend to increase with increasing wind speed (and associated increasing wind speed standard deviation).

Best regards,