ISSN (Print) Research Article. *Corresponding author Zhou Lijian

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1 Scholars Journal of Engneerng and Technology (SJET) Sch. J. Eng. Tech., 2016; 4(4): Scholars Academc and Scentfc Publsher (An Internatonal Publsher for Academc and Scentfc Resources) ISSN X (Onlne) ISSN (Prnt) Research Artcle The Mechancal Response Analyss of Large Horzontal LNG Storage tank Zhou Ljan, Wang Xue, Sun Mngyang School of Cvl Engneerng, Northeast Petroleum Unversty, Chna *Correspondng author Zhou Ljan Emal: Abstract: As a clean and effcent energy, LNG (Lquefed Natural Gas) s usng on a bg scale. For storage LNG, the desgn of the horzontal LNG storage tank s developng to large sze and complcated functons. Under the earthquake, the destructon of storage tank wll lead to explode, leak and make serous losses. Based on ths present stuaton, for 463m 3 large horzontal LNG storage tank, ths paper establshed fnte element models that one has no water and one has 50% water, and analyzed model analyss and sesmc response. Researchng on the nfluence of water sloshng on structure, the results shows, the structural free vbraton perod s larger wthout water, and when storng water, the water sloshng reduces the structural free vbraton perod effcently. The sesmc response analyss shows, horzontal tank structure s response under the earthquake s not only related to the dynamc performance of the structure tself, but also related to the characterstcs of sesmc wave. Keywords: large horzontal tank, fnte element model, model analyss, sesmc response. INTRODUCTION Due to usng LNG s safe and convenent, and easy to storage and transport, the requrement of LNG s ncreasng rapdly, so for storng LNG, the requrement for tank s ncreasng to and developng to large sze. Accordng to the structural type, tank can be dvded nto vertcal and horzontal. For large volume of low temperature storage tank, the desgners usually choose the horzontal. In 1914, the U.S.A publshed the frst patent of LNG and bult a small LNG ndustry. Japan s the largest LNG mporter country and ts mport volume accounts for 40% of the world. And Japan s one country wth buldng lots of large storage tanks n the world [1]. South Korea has begun to mport of LNG n 1986, and ts LNG ndustry has been rapd development. At present, South Korea has bult 36 LNG tanks and 12 tanks s under constructon [2, 3]. Compared wth other countres, Chna s LNG ndustry has just started. The LNG recevng statons that are plannng and buldng by Chna are more than 40 [4]. The study of Chnese desgners for LNG horzontal tank s only calculaton and analyss under statc force wthout performance analyss under earthquake. It s estmated that the energy when a m 3 LNG storage tank was full of lqud s 76 bgger than that n Hroshma atomc bomb [5]. Because of ths present stuaton, tme-procedure analyss was carred out on the large horzontal LNG storage tank on the bass of the statc analyss n ths paper, as to provde theoretcal bass for engneerng applcaton. The establshment of fnte element model Large horzontal LNG storage tank s made up of nsde contaner, shell, shell nsulaton, road lne, saddle, etc. Because the actual large horzontal LNG storage tank has lots of part, and ts structure s large and complex, the modelng and analyss calculaton of ADINA s complex, what we should frst do before dong smulaton s that make reasonable smplfcaton n the horzontal LNG storage tank [6, 7]. 1)For facltatng calculaton, the nsde contaner and the shell wll be calculate as a whole n ths paper, the allowable stress s 180N/mm². 2) These small parts wll be gnorng that has low stress and make less nfluence on structural strength. 3) The whole model wll be smplfed to symmetrc structure. 4) The actual weld parts and the surroundng condton are complex, so t wll be not consder some problems that welds brng to horzontal model. Tank capacty s 463m 3, cylnder dameter 5 m, cylnder length s 22 m, and ths s no change along the length. The shell cover s thckness s 26mm and depth s 1.2 m. Specfc related dmensons as shown n table

2 The materal of cylnder and shell over was Q345. Consderng the materal nonlnear; ths paper used the Table 1: Geometrc parameters of tank Parts Parameters of tank Sze( mm) Shell cover Depth H 1200 Thckness T f 26 Length L Cylnder Dameter D 5000 Thckness t 24 blnear strengthenng model. The lqud n the tank was LNG. Saddle materal was 16Mn. Fnte element model was establshed accordng to the sze of the structure and materals. The tank stored LNG, assumng lqud was potental flud. The boundary between lqud and sold was set for the lqud-sold couplng; ts basc theory was conssted of flud mechancs and sold mechancs [8]. Defned boundary condtons accordng to the actual constrants of storage tank, one saddle was set to full constrants, and the other saddle on the other sde was set to the vertcal, horzontal and three rotatonal constrants. After meshng, the fnte element model of horzontal tank was shown n fgure 1. ( a ) horzontal tank grd model ( b) the sde vew of horzontal tank model Fg-1: 50% lqud of LNG horzontal storage tank fnte element model The dynamc characterstcs analyss In actual test shows that n the model analyss, the dampng has lttle mpact on the natural frequency and vbraton mode of structure, so we don t consder the effect of dampng. Modal analyss solvng process wthout dampng s as follows. Whch, 2 M 0 K (1) K Stffness matrx, matrx, M Mass The frst order modal modal vector, The frst order natural frequency of the modal. In ADINA, modal extracton methods have determnant-search method; the Subspace Iteraton method and the Lanczos Iteraton method. In ths paper, the fnte element model s belong to large symmetrc egen value problem. Consderng the lqud level of lqud sloshng n the lqud storage tanks, we need to extract the low frequency vbraton of lqud and the vbraton of outer tank s hgh frequency, thus usng the Lanczos Iteraton method [9]. Modal analyss of storage tank wthout lqud Analyzng large horzontal LNG storage tank model, table 2 shows the mode of vbratons and the sde vew of the large horzontal LNG storage tank wall n empty tank (we only shows pctures that have a sgnfcant vbraton shape.) It can be seen form table 2 that dfferent natural frequences had the dfferent modes n horzontal LNG storage tank model. In ths example, the frst nherent vbraton mode that was nspred was 2 waves vbraton of storage tank n drecton of rng, the thrd order natural vbraton mode was the 3 waves vbraton of storage tank n the rng drecton, after that the wave vbraton n rng drecton was not obvous. The wave s number of eghth order natural vbraton mode was smlar to that n the thrd order natural vbraton mode, and the nne and ten order natural vbraton mode were the 4 waves vbraton of storage tank n the rng drecton. 194

3 Table 2: mode of vbratons of large horzontal LNG storage tank wthout lqud Order(n) mode of vbraton sde vew The frst order vbraton mode Frequency: Hz The thrd order vbraton mode Frequency: Hz The ten order vbraton mode Frequency: Hz Modal analyss of storage tank wth 50% lqud Table 3 shows the mode of vbratons and the sde vew of the Large horzontal LNG storage tank wall wth 50% lqud. Table-3: mode of vbratons of large horzontal LNG storage tank wth 50% lqud Order(n) mode of vbraton sde vew The frst order vbraton mode Frequency: 2.09Hz The thrd order vbraton mode Frequency: 2.332Hz The ten order vbraton mode Frequency: 2.387Hz 195

4 The mode of vbraton n tank wth 50% lqud was manly shown n mode of vbraton n lqud. Compared wth the frequency of LNG tank wthout lqud, the sloshng frequency of tank wth 50% lqud was lower than the tank wthout lqud, and the perod of tank wth lqud was long. The table 2 and 3 results showed that when tank wthout lqud, the basc frequency of outsde tank was 7.674Hz, when tank wth 50% lqud, the basc frequency of outsde tank was 2.09Hz.Through the frequency calculaton to tanks wth no lqud and 50% lqud, we can conclude that the frequency of empty tank was taller than that n tank wth 50% lqud, and the perod of empty tank was smaller. Its llustrate that lqud had much nfluence on Natural vbraton characterstcs of the storage tank system. When the tank had a half of lqud n tself, the overall flexblty was mproved a lot, t was equal to ncrease the lqud constrants n the nsde of that tank. THE EARTHQUAKE RESPONSE ANALYSIS Adopted the acceleraton sesmc ground moton method, we selected classc waves (ncludng two natural waves and one artfcal wave), made the earthquake exctaton on the large LNG horzontal tank, and researched on sesmc response of large LNG horzontal tank. 2. The adjustment of the peak acceleraton formula s A max A ( t) A( t) (2) A max Whch, A (t) sesmc wave curve, A max the structure s maxmum value of the sesmc ground moton n frequent earthquake and rare earthquake under the fortfcaton ntensty. A max A( t) the orgnal sesmc wave curve and the maxmum value. In earthquake acceleraton tme hstory curve, the maxmum acceleraton peak values for calculatng were unfed adjustment to 70 cm/s 2 under 8 degree frequent earthquakes. In front of nputtng the sesmc ground moton, we should process 10s preloadng under statc load, then calculate earthquake n restart procedure. The analyss of the dsplacement Table-4: The maxmum value of the horzontal tank s peak dsplacement under dfferent earthquake waves Tme that peak drect sesmc Space of Lastng peak dsplacement on wave tme( s) tme( s) dsplacement( m) occurred( s) EL wave E Taft wave E X axs artfcal E wave The horzontal tank s maxmal dsplacement appeared n the mddle of tank. Because under the acton of earthquake, the upper tank shakng and the mddle part don t have constrans, these generate larger relatve dsplacement. Through fnte element software, we found that node whch self-weght was the bggest had 9E-4 m ntal negatve X drecton dsplacement. EL wave dsplacement peak appeared on the largest sesmc ground moton. And Taft wave dsplacement peak appeared n s, compared wth the sesmc waveform, we found that n 14.24s, the vbraton was very strong, close to the maxmum value. The maxmum relatve dsplacement was 4.3E-4 m n X drecton, ts was far less than the dsplacement value that desgn requrements. From table 4, we easly found that even n dfferent earthquake types, under the same ntensty, the peak dsplacement was very close; the error was wthn 1%. The analyss of the acceleraton In El-centro wave, Taft wave and artfcal wave, the maxmum acceleraton value of the horzontal tank peak acceleraton tme hstory curve as shown n table 5. Table-5: the maxmum value of the horzontal tank s peak acceleraton under dfferent earthquake waves peak Tme that peak Space of Lastng sesmc wave acceleraton(m/s2 acceleraton tme( s) tme( s) ) occurred( s) drect on X axs EL wave Taft wave Artfcal wave In the ntal stages of earthquake wave, under the dfferent knds of sesmc waves, the acceleraton of a horzontal tank had the same change trend, after that shown a dfferent state of vbraton? In Horzontal tank, 196

5 the absolute maxmum acceleraton value of X drecton was n -0.69m/s m/s 2. Peak acceleraton appeared on the node 22626, where saddle jont tank. On the X drecton, due to the effect of the saddle, the maxmum acceleraton appeared n the stress concentraton. Generally, maxmum acceleraton peak value appeared near the maxmum acceleraton of earthquake. But the maxmum acceleraton of Taft wave appeared n 14.24s, the tme was same wth the maxmal dsplacement appeared and the appearng locaton was very close. The analyss of equvalent stress In El-centro wave, Taft wave and artfcal wave, the maxmum equvalent stress nephogram of the horzontal storage tanks was respectvely shown n fgure-2, 3 and 4 Fg. 2: The horzontal tank equvalent stress nephogram n El-centro wave Fg.3: The horzontal tank equvalent stress nephogram n taft wave Fg. 4 the horzontal tank equvalent stress nephogram n artfcal wave From the equvalent stress nephogram fg. 2 to 4 under the sesmc ground moton n the X drecton, the dstrbuton of equvalent stress were bascally dentcal, and maxmum value appeared n the juncton and local parts of saddle. It s not beng gnored that the X drecton earthquake wave had an effect on the juncton and local parts of saddle. The maxmum value of equvalent stress was MPa n EL wave, the maxmum was MPa n Taft wave and the maxmum was MPa n artfcal wave. ALL of the maxmum values were large, we shouldn t gnore. Above all, t proved that the saddle form, especally the connecton ways had sgnfcant effect on the horzontal tank under earthquake. The saddle of horzontal tank can be selected based on artcle [10], we can accordng to the nomnal dameter of equpment to select standards of saddle. The analyss of axal stress The horzontal tank axal stress nephogram, respectvely, as shown n fgure 5,fgure 6 and fgure 7. Fg. 5 the horzontal tank axal stress nephogram n EL-centro wave 197

6 Fg. 6 the horzontal tank axal stress nephogram n Taft wave Fg.7 the horzontal tank axal stress nephogram n artfcal wave From the axal stress nephogram fgure 5 to 7, under the sesmc ground moton n the X drecton, the dstrbuton of axal stress were bascally dentcal, and the maxmum negatve stress appeared n the jont and local parts of saddle, the maxmum normal stress appeared n the central part of tank above the perpheral saddle jont. Ths llustrated that under the horzontal X drecton earthquake wave, the tank central axal where on the saddle had a great normal stress and negatve stress appeared n the jont below the tank. The maxmum value of axal stress was MPa n EL wave, the maxmum was MPa n Taft wave and the maxmum was MPa n artfcal wave. The analyss of shear stress The horzontal tank shear stress nephogram, respectvely, as shown n fgure 8, fgure 9 and fgure 10. Fg. 8 the horzontal tank shear stress nephogram n EL-centro wave Fg. 9 the horzontal tank shear stress nephogram n Taft wave Fg. 10 the horzontal tank shear stress nephogram n artfcal wave From the axal stress nephogram fgure 8 to 10, dfferent sesmc waves had the same effect on the maxmum shear stress of structure. It llustrated that the sesmc form had no obvous nfluence on the shear stress, only the earthquake sze affected the maxmum shear stress. Shear stress concentraton poston was basc same wth equvalent stress dstrbuton, there was an obvous stress concentraton on the saddle and the bottom of jont. The maxmum value of shear stress was 12.06MPa n EL wave, the maxmum was MPa n Taft wave and the maxmum was MPa n artfcal wave. CONCLUSIONS At present, the research of LNG horzontal tank sesmc s stll n prmary stage n Chna. Ths paper 198

7 took 463 m 3 LNG horzontal tanks as the research object, and appled large fnte element software to process the modelng analyss of tank. Adopt the potental flow theory and consdered lqud-sold couplng effect, we researched the analyss of characterstc and sesmc response by smulated calculaton. Man conclusons are as follows: 1. Analyzed the dynamc characterstcs of the horzontal tank wth no lqud and lqud, researched the nfluence that the water sloshng made on the structure, calculatons shown that water sloshng greatly reduced the perod of the tank structure. 2. Based on the modal analyss research, horzontal tank model were set up n 8 degree frequency earthquake, respectvely got dynamc tme hstory analyss under three dfferent waves. The analyss shows that maxmum dsplacements under the dfferent earthquake were very small, all n a safe state. Acceleraton was related wth the earthquake type. The nfluence of equvalent stress shear stress nfluence was very close to that n shear stress, and both of them were not affected by the earthquake type. All of maxmum values appeared at the bottom of the tank and saddle jont. The axal stress appeared maxmum value n the central axal of tanks where saddle located, and negatve maxmum value appeared n the bottom half of the saddle, these were sutable for the structural characterstcs. REFERENCES 1. Bng W, Xuedong C, Guopng W; New progress of large low temperature LNG storage tank desgn and constructon technology. Natural gas ndustry, 2010; 30(5): Par ES; Techncal challenges and desgn features of the largest LNG tank n Korea. Journal of the Korea Gas Unon, 2006; Yang YM; Development of the world s largest above-ground full contanment LNG storage tank. 23rd World Gas Conference, Amsterdam, Q Z, Lanfu L; The development of world lquefed natural gas (LNG) and the enlghtenment to our country. Internatonal petroleum economy, 2004; (7): Guohu Z; LNG dffuson range analyss after the Large LNG storage tank catastrophc rupture. Safety, Health and Envronment, 2005; 5(7): Yunlu S; Medum-szed vertcal molecular seve adsorber of ANSYS fnte element analyss and desgn. Wuhan: Huazhong Unversty of scence and technology, Ranran L; Safety evaluaton and structure optmzaton based on ANSYS hydrogen adsorpton. Shandong: Chna Petroleum Unversty, Malhotra PK; New method for sesmc base solaton of lqud storage tanks. Earthquake Engneerng and Structural Dynamcs, 1997; 26: Langfeng L; Storage tanks fuzzy sesmc relablty and earthquake damage predcton. Huaqao Unversty, Zhdong H, Yutng W; Relablty analyss of pressure vessel based on the ANSYS. Forest Engneerng, 2011; 27(4):