gel,
A couple of books describe the design and analysis of vertical vessel skirts:
1) Megyesy, E. F. 1986. "Pressure Vessel Handbook" 7th ed. Pressure Vessel Handbook Publishing Inc.: Tulsa.
2) Henry H. Bednar, P.E. "Pressure Vessel Design Handbook", Second Edition, Van Nostrand Reinhold Company Inc.115 Fifth Avenue, New York, NY
Good luck...... let us know how you make out
MJC