Hi,
What would be the way to define a Storm Factor (STF) in MOSES, following the DNV-RP-H102 rules, which state that: Hmax = STF * Hs, where STF = 2.0.
Thank you,
Berislav
Hi Berislav,
Thank you for your question. It is nice to hear from you again.
My interpretation of the RP is to change the spectrum that will be used in the motion analysis. Let me know if my interpretation is different from yours.
MOSES does not have an option to change the Hmax/Hs ratio on a spectrum.
Is the RP asking that you first find the area under the curve then shape the peakness factor so that you achieve the Hmax/Hs specified?
Let me know your thoughts.
Georgina Maldonado
Hi Georgina,
Thank you for your reply.
Does MOSES then use 1.86 as a standard value?
My understanding of the RP is similar to yours, I think the area under the curve should be the same, but with a different peakedness factor that will give the Hmax calculated from the original Hmax/ Hs ratio.
BR,
Actually I think it is more related to the maximum wave height in any given spectrum that will be exceeded only once in a 3-hour storm. Following the Rayleigh probability distribution, and assuming that there will be 1000 waves in 3-hour storm, the ratio Hmax/ Hs is 1.86.
So in my opinion we need to create a wave elevation time signal where the Hmax will be 2.0*Hs, but coming out of the spectrum defined by Hs, Tp and gamma.
Do you have any suggestions?
Thanks and regards,
Well, I would have to take out the probability and statistics book I used in college. After reviewing that and getting refreshed on the areas and moments under the curve I would start with the JONSWAP spectrum.
I would start with a gamma of 1 and see what is the Hmax / Hs ratio. ( First confirming my understanding of what I read in the textbook.)
Then I would change gamma and see what the ratio is and separately calculate the area and moments.
I would include the wave elevation as part of the output. After the analysis I would FFT the wave elevation data to confirm my calculations.
What are your thoughts?
georgina
Just my two cents.
A classical "extreme" value for offshore operation is defined as the event with probability less than 1/1000 (the typical 3-hours storm, even if this is correct only if the wave mean period is about 11 s). From the Longuet-Higgins theoretical expression, this corresponds to a factor 1.86 that muliplies the significant value (that is exactly how the STF is defined). And the significant value is 2 times the standard deviation. So, MOSES uses, by default, a maximum value equal to 2*1.86 = 3.72.
If you perform frequency-based analyses based on the DNV codes (the one you cite and, for instance also the DNVGL-ST-N001), you instead have to use a different factor, depending on the duration of the operation. This is rather obvious, a shorter duration corresponds to a reduced STF coefficient, and a larger duration to a larger coefficient.
To make the story short: just use 2 * STF as maximum value factor for frequency-based analyses in moses (4.0 in your case).
Regards
Thank you The Ig
It is good to think of the problem the way you have presented it.
Thank you Georgina and the ig, I agree that this is the most appropriate way to deal with the DNV requirement, especially since I am doing a frequency domain check.
Best regards,