The design of vertical vessel & its skirt is not according to a given weight. Lot of considerations required superimposed loads, internals, liquid, attachments, loads,piping,seismic,wind, lifting, temp. material of your skirt, anchoring etc. these affects your skirt thickness, its time consuming & wish that somebody can help you to run thru software. Overall height is considered on thickness calc. Stiffeners for skirt is not necessary, you need to satisfy the loads without stiffener.