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.
I understand that we have to give both Integral Length (Lu) and Terrain Roughness (z0), Lu can be calculated according to Kaimal recommendation which is referred to Roughness value. Btw, I am not sure how SACS calculate Lu when we give Rougness value. Is it according to Kaimal ? May be not ? We can also give Lu directly i.e. 180m as per DNV-RP-C205 recommendation. However, refer to Harris spectrum, you still need to give SACS Sigma U which is standard deviation of wind velocity. Some document or paper mentioned that this Sigma U is function of Roughness value as well. Then, even we give Lu directly. We might still need to give Roughness value for SACS to calculate Sigma U. Btw, how SACS calculate Sigma U is still mystery for me as well. Is SACS still need Roughness value to calculate Sigma U ? Geoff, can you help to explain a bit about the background calculation of Sigma U (Standard Deviation of Wind Velocity) and Lu (Integral Length) in SACS??If it is possible, I would recommend SACS to have Theory manual similar to other software. Then, user can understand where they are when using SACS to solve their problem. Regards, Kasiphon
HoanW.,
The Harris Spectra that you refer to is actually referred to as the Von Karman spectrum in SACS. When SACS is referring to the Harris spectrum it is the Davenport version of the Spectrum referenced in 3.4 of the Dynamic Response manual.
In this equation the reference length (LH) is not dependent upon the surface roughness (k) and must be entered separately. Neither the Von Karman or Harris spectra calculations in SACS support the Kaimal reference length calculation described in the reference you have provided so you will have to enter those values. Of course, in the Von Karman calculation the surface roughness is not required and will not be used in the calculations.
Regards,
Geoff
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.