# Seismic Analysis of a Large LNG Tank Considering Different Site Conditions
## 4 Seismic Analysis of a Large Liquefied Natural Gas (LNG) Tank
### Results
The more vulnerable sections of the tank may need to be specifically strengthened. When the tank is filled with 100% LNG, the maximum stress of the inner tank is above 500 MPa in all four site classes, which jeopardizes the safety of the structure considering the fact that the yield strength of 9% steel is between 500 MPa and 600 MPa [38]. Because the consequences of large LNG tank failure are severe, it is imperative to reduce the stress on the inner tank to ensure their safety. At the same time, an early warning system should be implemented to control the maximum liquid level. Different from the outer tank, the stress of the inner tank changes significantly with the liquid level. When the liquid level is at 25% and 50%, the maximum stress of the inner tank is below 300 MPa. At a liquid level of 75% and 100%, maximum stress exceeds 300 MPa and 500 MPa, respectively. With an increase in the amount of LNG, the stress at the base of the inner tank gradually increases to notable levels. Therefore, the bottom of the tank warrants close attention during design and construction. The structural behaviour of the inner tank at different location types is also individually analyzed. Under the same liquid level, the inner tank behaves similarly to the outer tank with the stress in Class I being the largest, Class II being second largest, and Class III and IV being the smallest. Under 100% liquid level, the stress of the inner tank differs significantly under different site classes. Therefore, considering their behaviour at different location types, close attention should be paid to the design of LNG tanks under high liquid levels. Figure 8: The maximum von Mises stress distribution on the inner tank at different LNG liquid levels under San Fernando seismic wave; (**a**) At 25% full, (**b**) At 50% full, (**c**) At 75% full, (**d**) At 100% full. The rationale for selecting applicable conditions for displacement analysis of the outer tank is similar to the aforementioned rationale for stress analysis. To compare the effect of site conditions on the tip displacement of the outer tank, each case is modelled with 100% LNG as the most critical liquid level in Figure 11. In addition, to compare the tip displacement of the outer tank for different liquid volume, Class IV is selected for having the largest tip displacement, as shown in Table 10. The maximum tip displacements of the outer tank at different liquid volume and different site conditions are summarized in Table 10. A more concise summary is shown in Table 11 by averaging the maximum displacements produced by seismic waves within the same site condition. In most cases, under the same liquid height, Class IV experiences the largest displacement, followed by Class III, Class II, and Class I. This phenomenon indicates that less ideal site conditions correspond to greater tip displacements of an LNG storage tank. In addition, this phenomenon is consistent with the Chinese seismic code [37], which states that site differences directly affect the seismic design of the structure. An exception to this phenomenon is the El Centro seismic wave of site Class II with a displacement similar to the waves in site Class IV. Because the characteristic period of the El Centro wave is closer to the natural vibration period of the structure, the vibration of structures under the El Centro wave is more sensitive to increasing liquid levels. Other exceptions include Kobe and Landers waves under site Class III, where displacement decreases with the increase in liquid volume. More specifically, tip displacement of the outer wall is greater at 25% liquid level than at 50% liquid level. This phenomenon may be a result of the complicated relationship between frequency of solid-liquid coupling, sloshing frequency, and the characteristic period of seismic waves.