The present paper describes a reliability analysis carried out for two oil tanker-ships having comparable dimensions, but different design. The scope of the analysis was to derive indications on the value of the reliability index obtained for existing, typical and well-designed oil tankers, as well as to apply the tentative “rule checking” formulation developed within the CEC funded SHIPREL Project. The checking formula was adopted to redesign the midships section of one of the considered ships, upgrading her in order to meet the target failure probability considered in the rule development process. The resulting structure, in view of an upgrading of the steel grade in the central part of the deck, leads to a convenient reliability level. The results of the analysis clearly showed that a large scatter exists presently in the design safety levels of ships, even when the Classification Societies’ unified requirements are satisfied. A reliability-based approach for the calibration of the rules for the global strength of ships is therefore proposed, in order to assist designers and Classification Societies in the process of producing ships which are more optimized, with respect to ensured safety levels. Based on the work reported in the paper, the feasibility and usefulness of a reliability-based approach in the development of ship longitudinal strength requirements has been demonstrated.