슬로싱 효과를 고려한 압력식 각형 LNG 탱크의 피로 수명에 관한 연구

Abstract

Sloshing is a phenomenon in which a partially loaded fluid inside a tank causes changes to the free surface because of external load. These impact pressure may be momentary and localized, but it can cause significant issues in LNG fuel tank strength due to repetitive loading. In the case of LNG fuel tank design, due to the difficulty in accurately calculating dynamic loads, there is a concern about internal stiffener damage caused by sloshing during ship operation. For these reasons, LNG fuel tank need to withstand high pressure and have an easily adaptable shape for ship installation. The LNG fuel tank that satisfies these requirements is the pressurized prismatic LNG fuel tank, which is the subject of this study. In order to verify the safety of the LNG fuel tank, sloshing analysis, thermal structural analysis, buckling analysis, and fatigue life analysis (cumulative damage) were conducted. Additionally, the materials of the LNG fuel tank are 9% nickel steel and high Mn steel, and the material suitability verification of the LNG fuel tank was conducted through each analysis. Each analysis was performed according to the analysis procedure, and through the results of each analysis, the following conclusions were obtained. In the sloshing analysis of the pressurized prismatic LNG fuel tank, it was observed that the average pressure results for the four models, despite different scale ratios, exhibited similar trends. However, predicting the maximum pressure proved challenging as it showed non-uniform behavior regardless of the scale ratio. As a result, it was determined that relying on sloshing analysis to estimate the maximum pressure of the fuel tank is difficult. To ensure safety verification, it was concluded that the maximum pressure values obtained from the sloshing analysis should be applied in structural analysis and fatigue life analysis. The buckling analysis of the pressurized prismatic LNG fuel tank revealed that the first eigenvalues of the two materials decreased due to the influence of internal pressure and temperature. However, no issues related to buckling were identified. During the thermal structural analysis, differences were observed between the two materials in terms of their coefficient of thermal expansion under varying temperature conditions. Nonetheless, these differences were not deemed significant. The weld joints between the tank and the support structure experienced the highest stress. However, evaluation based on ASME standards indicated that the maximum stress remained within the allowable limits of the materials, confirming the absence of structural issues. The fatigue life analysis demonstrated that both materials exhibited cumulative damage values below 1, indicating no concerns regarding fatigue life. In the analysis of material suitability for the pressurized prismatic LNG fuel tank, both materials were deemed suitable from a safety perspective. However, high Mn steel stood out as a favorable choice from an economic standpoint due to its 30% lower price compared to 9% nickel steel, thanks to its abundant resources. In summary, based on the conducted analyses, the pressurized prismatic LNG fuel tank proved to be structurally sound and suitable for its intended application. While challenges existed in predicting maximum pressure through sloshing analysis, the other evaluations—buckling, thermal structural, and fatigue life—yielded satisfactory results. Both 9% nickel steel and high Mn steel were deemed appropriate materials from a safety perspective, with high Mn steel offering an economic advantage due to its lower cost.