Dear Sir,
Refer to SACS fatigue manual, only 'Harris' wind spectrum is available in SACS.
However, both API RP2A and NORSOK recommended to use 'Froya' Wind Spectrum.
Is SACS able to model 'Froya' wind spectrum?
Many thank !
The Froya wind spectrum is an available option on the WINSPC line columns (40-41):
Please refer to the Dynamic Response Manual provided with your SACS installation for more information.
Answer Verified By: Kasiphon Kurojjanawong
Geoff McDonald,
If Harris spec is used, SACS required "Reference Length" and "Roughness Coefficient". My understanding is that "Reference Length" is Integral Length Lu that is calculated using Roughness Coefficient (same as Kaiman Spec Integral Length), therefore either Roughness Coefficient input or Referenc Length input is enough. Please advise!
Thanks & Regards,
HoanVV.
Hoan V.V.
Kasiphon,
I don't believe that any standard deviation of the wind velocity is used in our calculations. We use a weibull distribution for the wind spectra and the user inputs the k and a parameters to determine the shape and magnitude of the distribution. The k parameter essentially contains the standard deviation information since the shape of the distribution is dependent upon this parameter.
Most of the theory used in our calculations is available in the commentary section of each manual, however sometimes the information is incomplete. We will endeavor to improve our manuals to provide all of the information that is necessary to our users.
Regards,
Geoff
Hi Geoff,
It is a litle bit deviated from the topic, but I would like to have questions about the wind spectral fatigue analysis in SACS.
1. Wind load cases are required in wind spectral fatigue analysis, something look like as below:
I'm wonderring how many points of wind speed should be enough for each wind direction.
My understanding is: those wind speeds together with the Weibull distributions shall be used to determined the long-term distribution of Stress for each wind direction, Rayleigh distribution, I guess. Then the number of input wind speeds must be as many as a Rayleigh distribution is well fitted.
Please advise a number!
2. Can you add a function to do the Weibull Distribution fitting to Seastate or Dynamic Response? I think this is the most time consuming in this type of analysis if you don't have a tool do it for you. Moments fitting is the most simple way to do it, I guess.
Hoan,
There isn't an easy answer to that question since the answer depends upon your analysis. The wind speeds you define in your wind data are used to calculate the fatigue damage based upon the distribution you have input in the dynamic response input file. The more wind speeds that you use, the more stress ranges you will have to determine the fatigue damage and the more accurate your result will be. You can certainly perform a sensitivity study to determine if you have adequately captured the fatigue damage.
In regards to determining the weibull distribution fitting, would you determine the distribution from a wind time history? I can add something along those lines to the list of requested enhancements.