External pressure design

One of proposed procedures may be as follows.

For your design conditions (pressure, temperature, shell diameter, thickness)

(1) Find L/D, the maximum length of unstiffened shells with D/t according the code.

(2) Assume a stiffener span L' and calculate required inertial moment of stiffener.

(3) Assume stiffener ring material, shape and location (outside or inside of shell) considering cost, fabrication and weld to shell and ring itself. 

(4) Recalculate the code requirements with stiffener area As, inertia moment Is and Is'.

(5) Finalize the ring shape and span/location considering the integrity of shell under consideration.