The vessel skirt has no internal pressure, it is under combination of loads such as weight, overturning due to wind/seismic etc. Hence, there is no direct formula to compute its required thickness.
You have to assume a starting thickness, compute the vessel natural frequency, compute the wind/seismic loads etc., then compute the stress in the skirt due to combination of these loads.
If you want to find the req. skirt thickness, you can set a very less thickness in the input, tell the program to automatically select wall thickness for axial stress (this is on Design Constraints tab).
Then analyze the file, you will get a message that the wall thicknesses have been updated. You can also see the new selected thicknesses in the "Element Original Thicknesses" report.
Next close the output, you will be asked to accept new thickness changes (be careful not to update other element thicknesses) or manually enter the new skirt thickness.
Then, re-run the analysis with the new skirt thickness. After few iterations you will find the required skirt thickness.
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Best Regards,
Mandeep Singh
CADWorx & Analysis Solutions
Hexagon PPM