Reinforcement of imperfect bilobe cargo tanks in liquefied gas carriers

Short review of Liquefied Gas Carriers, i.e. Liquefied Natural Gas (LNG) and Liquefied Petroleum Gas (LPG) Carriers, is given. In general for gas transport integral tanks, membrane tanks, semimembrane tanks and independent tanks are used depending on design features. The independent tanks of different shapes, which are further subdivided into A, B and C type related to design pressure, are described. Special attention is paid to remedy of misalignment in a 3000 m(3) bilobe cargo tank of a 8350m(3) LPG Carrier as a result of manufacturing difficulties. Namely, some eccentricity in the Y-joint of tank shells and longitudinal bulkhead usually remain during fabrication that causes additional stress concentration. First, this problem is considered theoretically and then numerically by FEM as a 2D task. Complete and reliable solution is achieved by a 3D FEM analysis of a tank segment between two vacuum rings. Necessary reinforcement of the Y-joint by set of knees and bars, depending on value of shells eccentricity, is recommended in order to reduce stress level below the allowable value. The obtained results are presented in a practical diagram for general use. Following given instructions cargo tanks of the considered LPG are reinforced and submitted to the hydraulic test with water pressure 50% higher of the design pressure. Passing this test successfully the approval of the relevant Classification Society is obtained.