Remedy for misalignment of hemispherical head of bilobe cargo tank in liquefied petroleum gas carrier

Abstract

This work describes the remedy for a misalignment at the joint of two segments of hemispherical heads in a bilobe cargo tank. The misalignment is the result of a relatively thin spherical shell and production difficulties. Measurements of out-of-roundness show a rather large deviation of meridional camber and variation of radius in the head cross-section plane. At the ends of the segments the spherical form changes into a conical one. As a result a peak occurs in the meridional generatrix, which causes high stress concentration. The cross-section of the hemisphere at the segment joint is reinforced with a ring in order to overtake the bending moment caused by the variation of circular radius. The ring dimensions are determined based on an approximate consideration. Stress concentration at the segment joint is reduced by two additional rings connected with a set of bars in the meridional direction. Angular distance between the bars is specified in such a way that the strength criteria, i.e. stress and fatigue requirements according to the Classification Rules, are satisfied. Stress distribution in the actual imperfect and reinforced structure is determined by the finite element method. Total static plus dynamic tank pressure is taken into account.