An object-oriented expert system, aiding in the structural design of semi-submersibles, particularly those constructed with ring-stiffened shells subjected to external pressure, is studied in this paper. A ring-stiffened shell must have enough structural strength to avoid collapse. On the other hand, there are some requirements such as minimal weight and minimal construction cost. Since the two abovementioned requirements are mutually opposing, an optimal design technique must be considered. In this study, a sensitivity analysis is implemented inside of the expert system to achieve an optimal design. The developed expert system is basically written in Uti - LISP and implemented on an EWS - A ring-stiffened shell is designed by this system using the same requirements that were used in the design of an actual submersible. Total constructon cost and weight of the submersible designed by this expert system and that of the real submersible are compared here.