Liquefied gas is one of the most important sources of energy nowadays. Worldwide it is being transported by pipelines or in liquefied state in tanks on Liquefied Natural Gas or Liquefied Petroleum Gas carriers. The independent tanks of type C, which contain liquefied gas both under pressure and refrigerated, are most commonly made in cylindrical or bilobe shape and are designed according to the Rules of classification societies. Absence of any serious incidents in seagoing transport is good evidence of the reliability of these rules and the tank construction itself. However, simple rules may not provide insight into actual behavior of ship and tank construction nor do they provide answer to the specific practical questions arising from the tank design and assembly practice. At the same time the tanks are becoming larger and therefore their constructions is becoming more technologically demanding, the fixed sailing routes are being replaced by the worldwide charter sailing, efforts are made to assure safe transport in the case of partially filled tanks or tanks under higher pressure. These reasons give rise to the need for more accurate analysis and better understanding of ship structure behavior subjected to realistic hydrodynamic loading. At the same time, shipbuilding practice requires answers to open problems of tank structure reliability, particularly related to stress concentration in thin shell structures, imperfection of geometry due to technical and other limitations during the tank assembly process, fatigue strength assessment of critical tank structural details, etc. This thesis presents both analytical and numerical methods for direct calculation of LPG tank structure safety. An overview of the theory of thin shells of revolution is presented and approximate analytical expressions for forces and deformations determination are given for cylindrical, conical, spherical and toroidal shells. A full spectral analysis is described in detail for each of its four steps: the hydrodynamic load calculation, quasi-static analysis of ship structure response, evaluation of notch stress transfer functions at the weld toe, statistical analysis and longterm distribution of notch stress as function of ship sailing condition and the calculation of highcycle fatigue damage for the structural detail under observation. The application of the method is presented through illustrative example: full parametric spectral analysis of imperfect Y-joint of cylindrical shells and longitudinal bulkhead of a bilobe tank in 6500 m3 LPG carrier. Different values of Y-joint eccentricity are found and measured on the existing bilobe tank, leading to high stress concentration and the presence of bending moment in membrane theory designed structure. Therefore, a comprehensive analytical and numerical analysis of Y-joint is performed and remedies for this imperfection are proposed in a systematic manner. Furthermore, low-cycle fatigue analysis methods are presented since low-cycle fatigue governs a significant part of the fatigue life of structural details subjected to tank pressure variation during cargo loading and unloading cycles. The application of the method is illustrated by low-cycle fatigue damage estimation for different values of Y-joint eccentricity. Finally, cumulative high-cycle and lowcycle fatigue damage concepts are presented. The stress concentration has been analyzed both by analytical and numerical methods in the case of cylindrical and hemispherical shells and in the case of imperfect joint of tank dish shells where a small angle conical joint had been formed. In the end, the scientific contribution of the performed research has been pointed out as well as the verification of research results through their practical application in shipbuilding practice.