DAF random wave for jack up

Dear friends

I am following the SNAME method to estimate the DAF for jack up, based on 3 hours time domain random wave analysis.

As per SNAME, the DAF from overturning moment (OTM) should be higher than the DAF from base shear (BS). However, the 2 DAF from base shear and overturning I get from SACS are almost identical, even when the natural period of jack up (5.0s) are close to wave period (6.3s). I have tried many cases with different wave parameters and different natural periods, but DAF from BS and OTM are always the same.

Can anyone explain why is OTM DAF supposed to be higher than BS DAF in most cases?

And what might be the cause that SACS is producing results that give same DAF from BS and OTM? 

Thanks!!!

  • I am not familiar with the SNAME method for determining the DAF. In SACS we recommend the Max Dynamic BS or OTM be divided by the Max Static BS or OTM for each direction. I imagine that the difference in DAF's for the BS or OTM would be dependent upon the loading and the structure.

    The suggestion that the OTM DAF should be higher than the BS DAF I believe would be based upon the assumption that the 1st mode of the structure would be a global cantilever bending mode (flagpole) and that this dynamic response would be more pronounced when maximizing OTM. So the difference would be dependent upon the mode shape and the wave load.

    When SACS calculates the maximum static wave load we cycle through a number of phase angles for the defined waves and then select the load case that maximizes either BS or OTM. When SACS calculates the maximum dynamic wave we calculate the maximum BS or OTM at each time step and output the selected option. There should be no reason why the SACS calculated wave loads are inaccurate.

    Regards,


    Geoff