PRELIMINARY EARTHQUAKE RESPONSE ANALYSIS OF 200,000kl LARGE CAPACITY ABOVE-GROUND LNG STORAGE TANK FOR THE BASIC DESIGN SECTION

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1 PRELIMINARY EARTHQUAKE RESPONSE ANALYSIS OF 200,000kl LARGE CAPACITY ABOVE-GROUND LNG STORAGE TANK FOR THE BASIC DESIGN SECTION Byeong-Moo Jin, Institute of Construction Technology, DAEWOO E&C Co., Ltd. Chul Hun Chung, Dept. of Civil & Environmental Engineering, Dankook University Se Jin Jeon, Institute of Construction Technology, DAEWOO E&C Co., Ltd. Jun Hwi Kim, DASAN ENC Co., Ltd. Ki Hong Kim, DASAN ENC Co., Ltd. Jae Weon Yoo, DASAN ENC Co., Ltd. Hong-Sung Kim, LNG Project Team, Plant Division, DAEWOO E&C Co., Ltd. 1. INTRODUCTION The main loads to be considered in the design of LNG storage tank can be largely classified into the loads induced by the liquid pressure, temperature and earthquake etc. The basic dimensions such as the height and the diameter of the tank are determined from the LNG storage capacity volume. The safety evaluation for the general structure against earthquake loading is performed considering the local earthquake properties and the characteristics of the site where structure would be constructed. Moreover it is validated that the earthquake responses of liquid storage tank such as LNG (storage) tank are significantly affected by the interaction between the inner liquid and flexible structure, i.e., fluid-structure interaction and the interaction effects are being considered in the design. In this study, as a trend of enlarging the LNG tank, the dynamic characteristics of LNG tank are investigated and henceforth the earthquake response analysis of the 200,000kl large capacity aboveground LNG tank is performed. 2. STATE OF ARTS: EARTHQUAKE RESPONSE ANALYSIS OF LIQUID- STORAGE STRUCTURE In general, it is assumed that the liquid storage tanks have nonlinear behaviors against severe earthquake, but the elastic analysis methods are mainly used for the earthquake response analysis in the preliminary design. One method considering the nonlinear behaviors against severe earthquake is to reflect the nonlinear behavior characteristics by dividing the elastic earthquake loading by the response modification factor and taking as the design earthquake loading. Recently it is more emphasized on the importance of nonlinear analysis which can strictly track the nonlinear behaviors of liquid storage structure. The method that the liquid is assumed to be incompressible and inviscid and the solution of

2 governing Laplace equations are obtained by numerical method and the structure is modeled by using Rayleigh-Ritz method or finite element procedure is widely used in the earthquake response analysis of liquid storage tank [1-4]. Especially the boundary element method [2,4] which is very convenient to model the ideal fluid is widely used. Studies on the earthquake response analysis considering the site effect, i.e., so-called soil-structure interaction have been being realized on the cylindrical storage tanks since 1980[5]. Recently the trends of soil-structure interaction analyses prefer the hybrid method in which the soil-structure interaction is analyzed by coupling the near field adjacent to the foundations of structures as the finite element and the far field modeled by the boundary integration method [6-10], hyperelement method [11-14] and infinite element method [15-17]. Haroun et al. [18] have studied the full-scale liquid storage tank earthquake response test and forced vibration test and Chalhoub et al. [19] have accomplished shaking table test of base-isolated cylindrical water storage tank. Since the 1990s as the numbers of the nuclear fuel storage and the LNG storage facilities have been increased and as a result the interests in the safety of these facilities have been increased, researches related to these fields have been progressed in Korea[4,20]. The experimental and theoretical studies, for example, shaking table test of cylindrical liquid storage tank and rectangular water storage tank [21,22] which is base-isolated and non-isolated structure[4] have been performed. After the Kobe earthquake, Japan Ministry of International Trade and Industry (MITI) has determined the establishment of the finite element method on the nonlinear behaviors against severe earthquake, henceforth studies on buckling and uplifting analysis of liquid storage tank for the relatively high level earthquake (SSE)[23]. 3. BASIC DESIGN SECTION OF 200,000kl LARGE LNG STORAGE TANK Figure 1 shows the schematic view of basic design section of the 200,000kl large LNG storage tank under development by DAEWOO E&C LTD. The maximum operation level (H.L) and maximum design level (H.H.L) of LNG is m and m respectively. The diameters of inner tank and outer tank are 90.0m and 92.0m respectively. The diameter of the 200,000kl large LNG storage tank is 6.0m larger than 140,000kl tanks being constructed at Tongyoung, Korea. Basic design section was determined by main loadings which should be considered in design. Works on the final design section due to these loading are being processed and investigations of dynamic characteristics compared to Tongyoung 140,000kl tank are also being accomplished as a part of these works.

3 Figure 1: Schematic View of Basic Design Section of the 200,000kl Large LNG Storage Tank 4. CHARACTERISTICS OF LNG STORAGE TANK To evaluate the dynamic characteristics of outer tank structure, the outer tank is modeled as axisymmetric elements by using general-purpose finite element analysis program (ANSYS). The 1st fundamental frequency of the 200,000kl tank is 4.952Hz, while that of Tongyoung 140,000kl tank is 5.705Hz. The natural frequencies are summarized in Table1. The 1st fundamental frequency of the 200,000kl tank is about 1Hz less than that of Tongyoung 140,000kl LNG storage tank. So it is clear that the 200,000kl tank is more flexible than Tongyoung 140,000kl tank. Mode No ,000kl ,000kl Table 1: Fundamental Frequencies of LNG Storage Tanks To verify the feasibility of fluid elements provided by ANSYS, the fundamental periods resulting from ANSYS model were compared to those of the analytical method and it is concluded that the usage of fluid elements of ANSYS is very acceptable. The 1st fundamental sloshing period of the 200,000kl tank is about seconds while that of Tongyoung 140,000kl tank is about seconds. The difference is negligible for the 1st mode. When a storage tank is fully filled with liquid, the behaviors of structure are affected by the inner filled liquid. This phenomenon is so-called fluid-structure interaction. Generally, the fundamental

4 periods of sloshing motion are calculated assuming the structure to be rigid in the design of a cylindrical liquid storage tank, and this procedure is based on the fact that the fundamental period of sloshing is far longer compared to that of the structure itself, i.e. the period of sloshing motion is not affected by the flexibility of structure itself. However the structure s deformation is largely affected by the inner fluid properties and the flexibility of the structure itself. The interaction frequencies of Tongyoung 140,000kl tank are computed for some cases and presented in Table 2. Categories Inner structure + LNG liquid Inner structure + Water Outer structure + LNG liquid Outer structure + Inner structure + LNG liquid 1st Fundamental Frequency Hz Hz Hz Hz Table 2: Fundamental Frequencies for 140,000kl Tank including Fluid-Structure Interaction These results indicate that the properties of inner liquid have a significant effect on the total response of LNG tank. Also the 1st fundamental mode of deformation including the effect of liquid is different from that of structure itself. The interaction analyses of the 200,000kl large LNG tank against an earthquake are to be presented next. 5. MODAL ANALYSIS OF 200,000kl LNG TANK Figure 2 shows the finite element modeling of 200,000kl LNG storage tank. The outer concrete tank and inner steel tank are modeled as axisymmetric shell element provided by ANSYS. The inner filled liquid (LNG) is modeled as fluid element also provided by ANSYS. Table 3 shows the finite elements provided by ANSYS. Most general-purpose finite element analysis programs would provide these elements. Categories Simple Model Axisymmetric Model 3 Dimensional Model LNG Liquid Lumped Mass, Spring FLUID FLUID Inner Tank Beam-Stick ASHELL 3DSHELL Outer Tank Beam-Stick ASHELL / ASOLID 3DSOLID / 3DSHELL Soil Region* Spring, Dashpot ASOLID / Spring + Dashpot 3DSOLID / Spring + Dashpot Table 3: Modeling Method of LNG Storage Tank with ANSYS *Not considered in this study Because the outer structure has been modeled as axisymmetric shell element, some

5 kinematic constraints between the roof / wall and ring-beam or wall and foundation are considered in modeling the outer tank. The constraints were verified to be right by comparing the fundamental frequencies and modes with those of axisymmetric solid model of outer tank. Table 4 compares the fundamental frequencies of axisymmetric shell and solid elements model of outer tank. The 1st mode shape of axisymmetric solid model of the outer tank is shown in Figure 3. Mode No. ASOLID ASHELL Mode No. ASOLID ASHELL Table 4: Comparison of Fundamental Frequencies (Axisymmetric Solid and Shell Model, units: Hz) Figure 2: Finite Element Modeling of 200,000kl LNG Tank (Axisymmetric Modeling)

6 Figure 3: The 1st Mode Shapes of Axisymmetric Solid Model (1st Mode, f 1 = Hz) The constraint equations are also adequately established for the interaction between inner tank and outer tank. In present study, the insulating materials (between inner and outer tank) are not considered because their material characteristic survey is beyond the scope of this study. The mustbe-provided constraints for proper usage of fluid elements are also established. The total numbers of nodes are 139, 63 and 792 for outer shell, inner shell and inner fluid elements respectively. The total numbers of elements used for modeling are 135, 62 and 731 respectively. Table 5 shows the key node numbers and descriptions of finite element model of 200,000kl LNG tank. The material properties used for this study are the same as a conventional LNG tank and are shown in Table 6. Node No. Description Node No. Description 109 Roof Top 1 Foundation 74 Ring Beam High 262 Inner Tank Wall High 69 Ring Beam Low 216 Inner Tank Conner 23 Outer Tank Wall Middle 201 Inner Tank Center 21 Outer Tank Wall Low 1709 Liquid Surface at R=45.0m Table 5: Node Numbers and Descriptions of Finite Element Model of 200,000kl LNG Tank

7 Categories Density (kg/m 3 ) Young s Modulus (GPa) LNG Inner Steel Tank Outer Concrete Tank Roof Wall Bottom Slab Steel Pile* Table 6: Material Properties of LNG Storage Tank *Not considered in this study. Figure 4 shows the fundamental modes of outer PSC(PreStressed Concrete) tank. The 1st fundamental frequency of outer tank only is approximately Hz and the 1st mode shape is beam deformation shape. The 2nd ~ 4th modes are roof deformation modes. The total mass of outer PSC tank is approximately 74,396 ton. As shown previously, the inner-outer tank interaction effects are negligible. The total mass of inner and outer tank is approximately 76,525 ton, i.e., the mass proportion of inner tank to outer tank is only 2.8%. Figure 5 shows the fundamental modes of inner liquid sloshing. The 1st period mentioned earlier is about seconds and well identical to the theoretical value. The comparison of sloshing frequencies is presented in Table 7. From Table 7, it can be recognized that the inner liquid modeling used in present study is capable of performing dynamic analysis of LNG tank properly considering the fluid-structure interaction effects. Mode No. Theoretical ANSYS Table 7: Comparison of Sloshing Frequencies for 200,000kl LNG Tank Figure 6 shows the interaction modes of fluid-structure system. The 1st fundamental frequency of outer tank (including inner tank) is 4.12 Hz. This is about 0.8 Hz less than that of sole outer tank. It could be conceived that the interaction effects are rather minor. However the dynamic effect cannot be estimated hastily, because the fundamental frequency of fluid-structure interaction system is approximately 83% of empty structure (outer tank only), which is 17% less and may not be negligible. And the fundamental mode of fluid-structure interaction system is different from that of sole

8 outer structure system. (a) 1st Mode, f 1 = Hz (b) 2nd Mode, f 2 = Hz (c) 3rd Mode, f 3 = Hz (d) 4th Mode, f 4 = Hz Figure 4: Outer Tank Deformation Modes for the 200,000kl LNG Tank

9 (a) 1st Mode, f 1 = Hz (b) 2nd Mode, f 2 = Hz (c) 3rd Mode, f 3 = Hz (d) 4th Mode, f 4 = Hz Figure 5: Sloshing Modes for the 200,000kl LNG Tank

10 (a) 1st Mode, f 1 = Hz (b) 2nd Mode, f 2 = Hz (c) 3rd mode, f 3 = Hz (d) 4th mode, f 4 = Hz Figure 6: Major Fluid-Structure Interaction Modes for the 200,000kl LNG Tank

11 Table 8 lists up the modal analysis results for the 200,000kl LNG Tank. Categories LNG liquid Outer structure Outer structure + Inner structure + LNG liquid 1st Fundamental Frequency Hz Hz Hz Table 8: Fundamental Frequencies for 200,000kl Tank Including Fluid-Structure Interaction 6. SEISMIC RESPONSE OF 200,000kl LNG TANK Figure 7 and Figure 8 show the input ground motion: The El Centro Earthquake occurred at May 19, 1940 [24], whose maximum peak ground acceleration is about 0.31g and maximum ground displacement is about 13cm. We used this historical earthquake as input motion to the 200,000kl LNG tank. It is very important to consider the soil-structure interaction effect in the analysis of seismic responses of structure, which would be constructed on soft soil region. However in most cases the response of fixed-base structure would be the more critical than that of soil-structure system. Therefore the soil-structure interaction effects were not considered in this study. Figure 7: Acceleration Time History of El Centro Earthquake Figure 8: Displacement Time History of El Centro Earthquake

12 Figure 9 shows the sloshing height time history computed at the node 1709 (R=45m). The peak value of sloshing height is about 0.53m for intensity of 0.31g. The peak sloshing motion occurred at time seconds. The free surface shape at the time seconds is shown in Figure 10. From Figure 9, it can be seen that the fundamental period of sloshing motion is longer than 10 sec. Figure 9: Sloshing Height Time History of LNG Liquid (Node 1709) Figure 10: Free Surface Shape at Maximum Sloshing Height of 0.53m (Time=27.26 seconds) Figure 11 shows the relative displacement time histories at node 109, 74, 69 and 23 respectively. The maximum deformation of outer tank about base, i.e., the maximum relative displacement at roof (node 109) is 5.9mm at time 2.32 seconds. At this time, the maximum relative deformation occurred at wall. This coincided with the 1st fluid-structure interaction mode in Figure 6. The deformation shape at time 2.18 seconds and 2.32 seconds are shown in Figure 12. In Figure 13,

13 the stress distributions are plotted for meridian stress and hoop stress. The maximum meridian and hoop stresses occurred at the middle wall of inner steel tank, where the deformation of section is larger. (a) Roof Top (Node 109) (b) Ring Beam High (Node 74) (c ) Ring Beam Low (Node 69) (d) Wall Middle (Node 23) Figure 11: Relative Displacement Time Histories of Outer Tank

14 (a) Time=2.18 sec (b) Time=2.32 sec Figure 12: Deformation Shapes at Time 2.18 and 2.32 seconds

15 (a) Meridian Stresses (b) Hoop Stresses Figure 13: Stress Contours of at Time 2.32 seconds (Unit: MPa)

16 7. CONCLUSIONS In this study, a preliminary seismic analysis of LNG storage tank has been performed, where the LNG liquid, inner tank and outer tank are modeled as axisymmetric finite elements. From the results of modal analysis of liquid storage tank, the fundamental frequency of structure shifts to the lower as the effects of inner liquid. The analysis results in this study shows that the 1st structural fundamental frequency of 140,000kl tank shifts from 5.71Hz to 4.75Hz, while that of 200,000kl tank shifts from 4.95Hz to 4.12Hz. The fundamental frequency of the objective structure has major influences on the response of structure in dynamic analysis such as seismic response analysis, so that it is recommended to consider this effect in design of structures in a reasonable way. In design of outer structure tank, the stiffness or mass contribution of inner tank affecting the outer tank is very negligible so that it is reasonable to consider only the inner liquid. However, from a point of view in design of the inner tank, the method used in this study, which consider the liquid-inner tank-outer tank interaction, is appropriate and acceptable. Although the response spectrum analysis is generally used in seismic design or seismic response analysis, the time history analysis has been performed for a special earthquake ground motion. From the results of time history analysis, it was found that the dynamic behavior of structure corresponded to the 1st fluid-structure interaction mode that was computed from modal analysis. The responses of tank or sloshing motions are computed in time domain and the deformation, stress distribution in inner tank/outer tank are shown at the time of maximum sloshing height or maximum structural deformation. Feasibility studies of the 200,000kl capacity above-ground LNG storage tank developed by DEAWOO E&C are now in progress including the earthquake response analysis presented in this study.

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