Second Level Reliability Analysis of a Double Hull Oil Tanker

A second-level reliability analysis is carried out on a realistic case study, represented by a 150,000 DWT double-hull tanker on service on a North Europe – Persian Gulf route. The object of the investigation is the longitudinal strength of the hull girder subject to bending loads. The scope of the analysis is to evaluate different stochastic models for the load variables and various combination criteria for the static and dynamic load components. A reference period of one year is adopted in the analysis and, as regards the loading conditions, use is made of detailed records collected from the owner during the first year of activity of the ship. The collapse equation describes the failure mode due to pure vertical bending moment, considering the ultimate capacity of the hull girder in its “as-built” conditions. A FORM procedure is adopted to calculate the reliability index and failure point co-ordinates. The present work is intended as a first step for the set up of a second-level reliability procedure to be adopted in the calibration of partial safety factors formats for the verification of the longitudinal strength of ships. Values of the reliability index are obtained for an existing, typical and well designed oil tanker on a common route: such values can be used for future comparisons between different design solutions or practices.