Reliability analysis of the primary strength of an oil tanker

A procedure is presented to perform a second order reliability-based structural analysis of the mid ship section of a vessel. The procedure accounts for the combination of the vertical and horizontal components of the wave induced bending moment as well as of the still water bending moment, in order to determine the actual angle gamma formed with the vertical plane by the total resultant bending moment. The ultimate strength of the midship section is considered as a function of the same angle. The proposed analysis is applied to a double hull tanker on a particular route (Persic Gulf - North Europe and ballast return trip) and directional scatter diagrams are used for the description of the sea states encountered during the voyage. The results are compared to those obtained on the same test case with a simpler model, which takes into account only the vertical components of loads and strength. The comparison shows that the introduction in the load model of the horizontal component of the bending moment does not reduce the structural reliability of the considered oil tanker ship. Such result is to be related to the shape of the curve of hull girder ultimate strength as a function of gamma, which shows a local minimum for pure vertical bending moment: in the examined case, the most dangerous condition is obtained in such situation.