Seismic Response Of Semi-underground LNG Storage Tanks With Embedment Depth

At present,all Liquefied Natural Gas(LNG)storage tanks having been built in China mainland are above-ground,and the application of inground/semi-underground tanks is still at the beginning stage.There are no terms related to design approach for inground/semi-underground LNG storage tanks listed in Chinese codes and specifications.As an essential aspect of structural design,safe seismic design is based on the clear understanding of seismic response characteristics of LNG storage tanks.This research works focus on semi-underground LNG storage tanks with partial embedment,and seismic analysis of the specific type LNG storage tank is carried out from three aspects: shaking table test,numerical analysis,and conceptual model.1)Shaking table test-Shaking table tests with laminar box of semi-underground and inground LNG storage tanks are carried out.Considering the influence of burial depth and focusing on dynamic soil-structure and fluid-structure interactions,dimensional similarity analysis of dynamic fluidstructure-soil interactions is developed.Based on a large capacity LNG storage tank project in China mainland,an experimental model is designed and made of three types of materials,i.e.liquid,structure and soil(including bedrock).Two series of shaking table tests with laminar box are carried out,i.e.semi-underground and inground embedment type.Seismic response of structure and liquid with different burial depth,input Peak Ground Acceleration(PGA),and liquid levels are obtained,and seismic response characteristics of semi-underground LNG storage tanks are revealed.Tests results can be validated by and used to calibrate the numerical analysis,so as to support the proposal of conceptual model for semi-underground LNG storage tank.Tests results indicate that seismic response characteristics of semi-underground LNG storage tanks are influenced by both kinematic and inertia interactions.On the one hand,the embedded structure is influenced by kinematic interactions,and seismic response characteristics of conventional underground structure are exhibited;On the other hand,the upper structure is influenced by inertial interactions,and seismic response characteristics of conventional above-ground structures are exhibited.Semiunderground LNG storage tank with embedment depth is wrapped by surrounding soil,and the structure of storage tank suppresses the ground motion of surrounding soil due to relatively larger stiffness.Compared with semi-underground,characteristics of seismic response of inground storage tank are more likely to be a fully underground structure due to the wrapping of surrounding soil.2)Numerical analysis-The dynamic time-history analysis method of semiunderground LNG storage tank is proposed and implemented using Abaqus,which can be used for the verification of seismic performance.The dynamic time-history analysis method of semi-underground LNG storage tank is proposed and implemented using Abaqus,in which fluid-structure-soil interactions are considered.Specific technical essentials are explained: modelling fluid-structure interaction by using Coupled Acoustic-Structural(CAS)approach,defining free surface with boundary impedance,calculation of sloshing wave height,shell-to-solid coupling in multi-scale modelling,et al.The results show that timehistory acceleration response,dynamic amplification effect,dynamic pressure,and sloshing wave height can be well simulated relatively by using the fully coupled fluidstructure-soil model,so as to develop the dynamic time-history analysis of a realsized semi-underground LNG storage tank model further.In addition,added mass method is further improved by introducing Housner and Veletsos-Shivakumar model subject to horizontal earthquake: added mass surface element is developed and added mass point with cylindrical coordinates is proposed,so that the simplified analysis approach of cylindrical liquid storage tank is implemented.3)Conceptual model-A mass-spring-dashpot conceptual model with four mass nodes for semi-underground LNG storage tank is proposed and established.Considering structural and burial features of semi-underground LNG storage tanks,a simplified conceptual model for semi-underground LNG storage tank with dynamic fluid-structure-soil interactions is abstracted.A mass-spring-dashpot model with four mass nodes and its governing motion equation are proposed and established.The approach of determination of parameters/coefficients is also provided.Comparative analysis is conducted for different embedment types.The results show that the proposed mass-spring-dashpot model can be used to model the seismic behavior of semi-underground LNG storage tank.The deeper the storage tank,the stronger the surrounding soil constraint,and the weaker the acceleration amplification effect of outer tank wall.The applicability of the proposed mass-spring-dashpot model for semi-underground LNG storage tanks can cover virous burial depth: above-ground,semi-underground,and inground.