mechbadem, the saddle ltself is usually just treated as a beam subject to external loading. Longitudinal due to wind, seismic, bundle pull, etc. Vertical due to seismic, dead weigtht, etc. Find the moment of intertial of the built up section, s = MC / I.
Influence of saddle loads on the shell may be done by a Zick-type analysis.
Recommend you get a standard handbook such as Moss, Bednar, etc.
The different handbooks expand on and explain this some. Typically handled in software analysis, but you have to understand what it's doing to make it work right.
You can refer Zick's method for calculating the effect of saddle support on vessel
Main stresses are Longitudinal bending, Circumferential and tangential shear. Longitudinal stress should be added with stress in longitudinal direction (girth seam) to get total stress in compression.
An alternative would be calculating the saddles according to AD 2000 Merkblatt S3/2.
This AD 2000 standard is from year 2004. This, just to see how calculating.
