Liquefied natural gas storage tank simplified mechanical model and seismic response analysis

https://doi.org/10.1016/j.soildyn.2020.106491Get rights and content

Abstract

In this study, a scale model of a liquefied natural gas (LNG) storage tank was designed to carry out a shaking table test, the main position of insulation layer on tank and the site effect were investigated under different ground motions. The test results show that there is an obvious interaction between the insulation layer and the liquid-solid coupling position of the storage tank, and the seismic response of the tank was related to the frequency of the ground motions, the site effect should be considered when analysing the seismic response of liquid-solid coupling structures. Based on the test results, a simplified mechanical model for the seismic design of LNG storage tanks that considered the insulation layer was constructed, and the simplified mechanical model was verified via a numerical simulation method. Moreover, based on the results of the 16 × 104 m3 LNG storage tank simplified mechanical model, the base shear and bending moment were reduced by 20%–25% on average, the acceleration and displacement of the inner tank wall were reduced by 40% on average after filling the insulation. In the seismic design of large scale LNG storage tanks, considering the damping effect of insulation is helpful to save design capital, and ignoring the damping effect of insulation will make the design conservative.

Introduction

16 × 104 m3 full containment liquefied natural gas (LNG) storage tanks require a special structure to store ultra-low-temperature media. They consist of a steel inner tank, which is the main container, a pre-stressed reinforced concrete outer tank can be considered as a secondary vessel if the failure of the inner tank, and resilient 0.3 m thick felt and expanded perlite powder with a thickness of 0.7 m is arranged in between the inner and outer tanks as the insulation layer [[1], [2], [3]]. A high seismic capacity of such storage tanks is required because of the leakage of LNG may lead to explosion or fire; therefore, carrying out seismic analyses of these storage tanks is crucial [4].

Many assumptions and simplifications were made to deduce a mechanical model of LNG storage tanks. For vertical storage tanks, Housner [5] used the two-lumped-mass model with the assumptions that the tank wall was absolutely rigid and that the liquid behaved as an ideal fluid. Veletsos et al. [6,7] and Haroun et al. [8,9] later modified the model by using the three-lumped-mass model after taking into consideration the elastic deformation of the tank wall. Subsequently, Zhang et al. [10,11] extended the vertical storage tank model to LNG storage tanks and deduced a simplified mechanical model of full-capacity LNG storage tanks. Numerical simulations have been used by many researchers to analyse the seismic responses of storage tanks; topics covered have included the pile-soil-tank interaction [12], the seismic responses of the inner and outer tanks [13], a seismic vulnerability of the tank [14], the effect of near-fault ground motion on a tank [15], the sloshing effect of the liquid [16], etc. Using the shaking table test, researchers have analysed the seismic performance of steel storage tank or rectangular storage tank [[17], [18], [19]]. However, as it is difficult to perform the shaking table test on LNG tanks owing their special structure, only the inner tank shell was modeled in the mock-up [20]. Hence, there are limited shaking table test results available for LNG storage tanks.

The previous research mentioned above can provide strong support for the design and construction of LNG storage tanks, but the role of the insulation was neglected in the aforementioned analyses. Over time, the insulation becomes dense [21]; therefore, the coupling effect that the insulation provides between the inner and outer tanks cannot be neglected in the event of an earthquake. By analysing the explosion of an LNG tank, it was determined that the annular insulation transfers load between the tank walls, enhancing their individual strengths [22]. Zhang et al. [23,24] considered the insulation when conducting their seismic analysis of LNG storage tanks using numerical simulations. Besides, relative researches indicated that the seismic response of storage tank is the largest at the full tank, and the effect of insulation on storage tank is not related to ground motion intensity [25,26]. In this study, the seismic response of full level tank under different ground motions was analysed, the main position of the tank affected by the insulation under the earthquake action was explored, which provided the basis for the simplified mechanical model of the LNG storage tanks. Based on the test results, a simplified mechanical model for the seismic design of LNG storage tanks was constructed, which considered the insulation, and its rationality was verified via a numerical simulation method. Moreover, the influence of site effect on seismic response of storage tank was also analysed.

Section snippets

Test design description

The unidirectional horizontal-displacement earthquake shaking table at Dalian Minzu University was used. It was a servo hydraulic system and the specific parameters were as follows: the table size was 3.00 m × 3.00 m; the displacement limit ±80 mm; the maximum load 50 t; and the frequency ranged from 0.1 Hz to 50 Hz. The scaled test model was based on a 16 × 104 m3 prototype LNG storage tank, a structural diagram of the tank is shown in Fig. 1.

The scale design idea and method of the test model

Simplified mechanical model of LNG storage tanks

The outer tank of the 16 × 104 m3 LNG storage tank is composed of a sidewall and dome, the sidewall is regarded as a shear cantilever beam in the simplification. Fig. 17 shows the simplified model of the sidewall, where mb and hb are the equivalent mass and height, respectively, and cb are the stiffness and damping coefficients, respectively. The dome was connected to the sidewall and had no contact with the insulation; therefore, it could be regarded as a concentrated mass acting on the top of

Model validation

Reference [26] performed a numerical simulation analysis on 16 × 104 m3 LNG storage tanks via ADINA, referring to this method, the numerical simulation of the tank is carried out to verify the simplified mechanical model. Fig. 20 and Table 2 display the finite element model and the element selection, respectively. Twenty ground motions are selected in this study, and the results are compared with those of the simplified model to verify its validity. Adjusting the peak ground acceleration (PGA)

Conclusions

Based on the shaking table test results, a simplified mechanical model of an LNG storage tank is proposed and verified by a numerical simulation method. The conclusions are as follows:

  • (1)

    The test results reviled that the passive pressure of the insulation on the inner tank wall is the largest in the middle position of the tank wall, which shows that there is a strong interaction between the insulation layer and the liquid-solid coupling position of the storage tank. The hydrodynamic pressure

CRediT authorship contribution statement

Dongyu Luo: Conceptualization, Formal analysis, Data curation, Methodology, Writing - original draft, Writing - review & editing. Chunguang Liu: Software, Formal analysis, Writing - review & editing. Jiangang Sun: Funding acquisition, Writing - original draft. Lifu Cui: Writing - review & editing. Zhen Wang: Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research work was supported by National Natural Science Foundation of China (51878124). The authors also would like to thank Dr. Yuan Lyu from Dalian Maritime University for his help in the research process of this project.

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