STRENGTH ASSESSMENT OF MEMBRANE-TYPE LNG CONTAINMENT SYSTEM

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1 STRENGTH ASSESSMENT OF MEMBRANE-TYPE LNG CONTAINMENT SYSTEM B Wang, J W Kim, Y Shin, American Bureau o Shipping, USA Originally presented at the RINA Conerence ICSOT 6 on LNG held September 14-15, 6, in Busan, Korea, and reprinted with the kind permission o the Royal Institution o Naval Architects SUMMARY A numerical analysis procedure has been presented or the strength evaluation o a membrane-type LNG containment system (CS). The procedure includes the idealization o sloshing impact load as well as static and dynamic structural analyses. In this study, an example analysis using NO 96 CS is presented. Three levels o structural analyses have been employed or strength evaluation, which include: static stress and buckling FE analyses perormed on the simpliied model or NO 96 CS; linear transient FE analysis conducted on the simpliied model with a dashpot system; and nonlinear dynamic FE analysis considering luid-structure interaction carried out on the ully coupled model o NO 96 CS. Results o parametric study are presented to demonstrate the eect o loading particulars on structural response. FE results identiy critical locations where the structure ails. Based on acceptance criteria, numerical results have been used to evaluate the structural integrity o NO 96 CS. 1. INTRODUCTION For LNG carriers, the hull structure has been designed and constructed in terms o applicable classiication rules. However, the containment systems in membrane-type LNG carriers are made o very dierent materials (such as plywood, mastic, and oam) and are exposed to a cryogenic environment. In the past, a simple assessment method, which is the comparative method based on existing service experiences and previous damage cases, has been used and accepted as a design practice [1, ]. As the global LNG market and demands grow, LNG carriers with much larger capacity and new operational conditions are being designed. To meet the special needs o the industry, a direct calculation-based strength assessment procedure is essential to evaluate the structural strength o LNG containment systems. In advanced structural analysis, dynamic and hydro-elastic eects on the strength o LNG containment systems need to be investigated through FE modelling considering the luidstructure interaction. The strength o an LNG containment system (CS) can be characterized as the maximum stress that the system can sustain beore it ails. The material property testing has been carried out or evaluating the strength o each component in the insulation system. The procedure determining the design sloshing load and evaluating the strength o the LNG CS includes: evaluation o the resultant sloshing impact load on the insulation system considering luid-structure interaction and dynamic structural response, as well as strength, deormation, and buckling assessments based on acceptance criteria. In the strength assessment, the saety actor is deined as the ratio o the ultimate strength o material to the maximum stress in the structure, namely, SaetyFact or = Strength MaximumStress In the recently published ABS guidance notes [3], the idealization o sloshing impact load has been introduced, and the three levels o strength assessment have been developed in structural analysis o LNG CS based on previous work [4-6]. The lowchart o the three levels o strength assessment is shown in Fig. 1. Level 1 is the simplest one, in which static FE analysis is required. I Level 1 is not passed, Level needs to be done, in which linear transient FE analysis is required. Furthermore, i Level is not passed, Level 3 needs to be done, in which nonlinear dynamic FE analysis is required considering luid-structure interaction. Alternatively, any o the three levels o strength assessment might be independently applied or the strength evaluation o LNG containment systems. Figure 1: Flowchart o three levels o strength assessment or LNG CS In terms o the luid-structure interaction, static stress and buckling analyses will be perormed on the simpliied model without considering LNG, and linear transient Strength Assessment o Membrane-Type LNG Containment System 195

2 analysis will been conducted on a simpliied model with a dashpot system representing LNG, using NASTRAN; and a ully coupled FE model considering the luidstructure interaction will be developed to investigate the hydro-elastic eect in NO 96 CS using DYTRAN. The sloshing load will be introduced or LNG CS in the next section.. SLOSHING LOAD Pressure T rise / T duration / t Pmax Pmax / To reach the design objective o a nominal 5-year service lie in the North Atlantic Ocean, the extreme sloshing loads during the lietime o an LNG carrier need to be evaluated or the structural assessment o the LNG containment system. ABS recommends the sloshing model test be perormed to estimate the extreme sloshing load. Because o the sensitivity o the sloshing impact load in location and area, panel pressure averaged over a speciied area is preerred to the local pressure measured by individual pressure sensors. The measured panel pressures are statistically processed to estimate the design load. It should be noted that not only the magnitude but also the time ootprint o the sloshing load is important or the dynamic response o the LNG CS. For the simplicity o structural analysis, the time history o a panel load around its peak is idealized by a triangular pulse, which can be characterized by three parameters magnitude, rise time and duration. Figure 3: Idealization pressure by a triangular pulse The skewness parameter varies rom to, depending on the shape o the impulse in such loading case. Based on the sloshing load, the strength assessment o LNG CS can be implemented at dierent levels. As a conservative evaluation, static analyses will be perormed on LNG CS in Sections 3 and STATIC STRESS FE ANALYSIS 3.1 FE Model p () t = P ( T t)( / T T ), max duration P max duration t /Trise, t < Trise; rise Trise t < T, otherwise. duration Fig. shows the deinition sketch o a triangular pulse. Skewness parameter, S = Trise / Tduration, is introduced to deine the loading pattern. For a general impact signal, the duration and rise time are deined rom the time moments when the impact pressure reaches hal o the maximum value, as depicted in Fig 3. p ( t ) p max ; Figure 4: Schematic drawing o NO 96 CS L W H 1 Primary Box H T rise T duration t Figure : Triangular pulse or uniorm pressure l Secondary Box Figure 5: Simpliied model or NO 96 CS H Strength Assessment o Membrane-Type LNG Containment System

3 The NO 96 insulation system is made o plywood and mastic. Young s modulus and Poisson s ratio o plywood and mastic are 7, MPa and.17, as well as 5 MPa and.3, respectively. 3. FE Results and Discussion A linear static analysis has been perormed to ind out the critical yielding load in NO 96 CS. As an example, a static uniorm pressure with a magnitude o 1.45 MPa is applied on the top plate o the primary box. Figure 6: FE Mesh or NO 96 simpliied model NO96 containment system consists o three main components primary box, secondary box and mastics, shown in Fig. 4. A 3D simpliied FE model without considering LNG or the NO 96 containment system is used to evaluate structural strength and stability, shown in Fig. 5. L is the length o primary and secondary boxes, W is the width o primary and secondary boxes, H 1 is the height o primary box, H is the height o secondary box, and H 3 is the height o mastic. In FE mesh as shown in Fig. 6, 4-node plate elements in MSC/NASTRAN are used or all components in the NO 96 CS structure. Figure 7: Loading and boundary conditions Pressure (bar) GTT static test 4 linear static analysis mean vertical displacement (mm) Figure 8: Comparison o numerical and experimental results Fig. 8 compares the vertical displacements rom both the static compressive test [7] and the numerical simulation results. In the linear elastic region, both results are very consistent. Thus, the simpliied FE model has been validated or static analysis. The discrepancy in the remaining may be due to the modeling simpliication. For example, the ventilation holes in the side panels are all neglected and not included in the inite element model, and the stiened wedges at the corners are also neglected. Thereore, the idealized structure in the numerical analysis is a little stronger than the real one. FE results also indicate that the stress in the secondary box is higher than the stress in the primary box. Based on linear static stress analysis, the NO 96 insulation box does not yield under the compression load with 1.45 MPa. However, the static test on NO 96 CS shows that the ultimate strength o the insulation structure is about 1.3 MPa. This means that the ailure mode in the NO 96 insulation structure under static compression is not yielding ailure. Instead, buckling ailure is dominant in the NO 96 CS under compressive loading. Thereore, the critical buckling load has to be determined rom FE eigenvalue analysis on this box type structure. Fig. 7 shows loading and boundary conditions or the simpliied model. Applied static load on the top surace o the primary box is a uniorm pressure. The bottoms o all mastics in the containment system are simply supported in the normal direction and only our corner nodes are constrained in all three directions. Strength Assessment o Membrane-Type LNG Containment System 197

4 4. STATIC BUCKLING FE ANALYSIS In the strength evaluation using static analysis, LNG can be ignored. However, the luid-interaction should be considered in dynamic analysis at Level and Level 3. The ollowing two sections will address the implementation o luid-structure interaction in structural analyses at two dierent levels. 5. LINEAR TRANSIENT FE ANALYSIS 5.1 FE model Figure 9: Static buckling mode shape or NO 96 CS Based on the same simpliied model in Fig. 6, a static buckling FE analysis has been carried out to ind out the critical buckling load or the NO 96 containment system using MSC/NASTRAN. A static unit pressure is applied on the top plate o the insulation structure to obtain the critical buckling load rom FE eignvalue analysis. Fig. 9 presents the buckling mode shape o the NO 96 containment system under compressive loading rom FE results. The same buckling ailure mode has been ound in static test. In both numerical and experimental results, the side panel o the secondary box is buckled leading to the ailure o the insulation structure. Fig. 1 shows the detailed buckling in the second box including middle stiness inside the box. The critical buckling load obtained rom numerical calculation is.99 MPa. Static compressive test results show the insulation structure starts buckling at the critical load with.95 MPa. Both numerical and experimental results are very close at buckling. It can be concluded that the NO 96 insulation system is going to ail due to buckling. FE results indicate that the load causing the onset o buckling is approximately.99 MPa, which is much smaller than the critical yielding load. Buckling ailure takes place beore the structure yields. This conclusion is veriied by the static test, which yields similar results. Figure 11: Simpliied model with a dashpot system or NO 96 CS To investigate the hydro-elastic eect in the NO 96 containment system, LNG should be considered in the FE analysis. At Level, the dynamic response o LNG is assumed to be equivalent to that o a simpliied dashpot system. The damping behaviour o the dashpot system is expressed as p = K 1 u& 1 where p is the pressure, u& 1 is the velocity along the normal direction, and K 1 is the damping coeicient. K1 = ρ Cd, where ρ is the density o luid and C d is the dilatational wave speed. For LNG, C d = 1,7 m/s. ρ = 474 kg/m 3 In the FE model, the NO 96 containment system is modeled using a 4-node plate element in MSC/NASTRAN. The dashpot system attached to the top plate is made o spring elements representing LNG, shown in Fig. 11. In loading conditions, a uniorm pressure load is linearly ramped up and down to represent the sloshing impact load (See Fig. or a description o the loading pattern). Three parameters are used or the triangular impulse load: magnitude, duration and skewness. A uniorm pressure with the magnitude o 1Pa is applied to the top plate. 5. FE Results and Discussion Figure 1: Buckling shape in secondary box At Level, linear transient stress analysis has been carried out on the LNG NO96 containment system. The 198 Strength Assessment o Membrane-Type LNG Containment System

5 dynamic behavior o the structure is investigated through the parametric study o structural response under dierent loading patterns. The eect o duration and skewness will be presented rom numerical simulations. The case parameters are shown in Table 1. The area o interest or this study is at the center o the side panel o the secondary box, where buckling and cracking occur in the drop tests. The stress-time history at this critical location due to the unit impact is shown in Fig. 1. Stress [Pa] Stress [Pa] Table 1 Parameters o loading patterns Load Case S T duration T rise [-] [-] [s] [s] Center o Lower Side Panel, Duration. s Skewness=. Skewness=1. Skewness= Time [s] Center o Lower Side Panel, Duration.6 s Skewness=. Skewness=1. Skewness= Time [s] Stress [Pa] Stress [Pa] Center o Lower Side Panel, Duration=.1 s Skewness=. Skewness=1. Skewness= Time [s] Center o Lower Side Panel, Duration.14 s Skewness=. Skewness=1. Skewness= Time [s] Figure 1: The stress-time history at center o side panel The loading pattern o impact is given in terms o skewness parameter, S. The eect o impact duration and shape on the hydro-elastic response o the simpliied model with a dashpot system is investigated by varying duration and skewness. To show the load reduction due to hydro-elastic and dynamic eect more clearly, hydroelastic load actor is deined as: max p t Hydro elastic Load Factor max p t p lex t lex rigid () t () t where max ( t) is the maximum hydro-elastic stress response in the containment system and max ( t) is the maximum pressure in the rigid t wall. p rigid Hydro-elastic Load Factor S=. S=1. S= Duration [s] Figure 13: Hydro-elastic eect o dierent loading patterns Strength Assessment o Membrane-Type LNG Containment System 199

6 The maximum stress at the center o the lower side panel rom a linear transient case is 17.5 Pa. The hydro-elastic eect on varying the skewness and durations o the load is shown in Fig. 13. This igure indicates that with increasing duration, the hydro-elastic eects decrease. For long durations, the eect on hydro-elastic response at large skewness is more than that at small skewness; but or short durations, the eect on hydro-elastic response at large skewness is less than that at small skewness. It can be explained that the short durations are not long enough or hydro-elastic response at large skewness. The results also show that the duration has a stronger eect on the structural response than the skewness. It can be concluded that hydro-elastic eect cannot be neglected, especially in the case o a short duration. The lower the skewness is, the more the eect on the hydro-elastic response will be. Table shows the comparison o stresses at critical location under static stress analysis, transient analysis without dashpot system, and transient analysis with a dashpot system. The results indicate that the hydro-elastic eect is signiicant as well. Table Comparison o FE results rom dierent methods Method Applied Load (MPa) Max Stress (MPa) Static Transient 1.75 (peak) 3.56 without dashpot Transient with dashpot 1.75 (peak) In Sections 4 and 5, some simpliications have been made in structural analysis o LNG containment systems. However, ully coupled luid-structure modelling is still necessary or analyzing LNG containment systems in practice. It will be more realistic to simulate both LNG (luid) and containment system (structure) in the FE model. In the next section, nonlinear dynamic analysis will be conducted on the ully coupled model or the NO 96 containment system, considering the luid-structure interaction. 6. DYNAMIC FE ANALYSIS CONSIDERING FLUID-STRUCTURE INTERACTION 6.1 Governing Equations in Coupled Acoustic Structure Modelling The equilibrium equation or small motions o a compressible, adiabatic luid with velocity-dependent momentum losses is taken to be p + ρ u&& x = where p is the excess pressure in the luid (the pressure in excess o any static pressure), x is the spatial position o the luid particle, u& & is the luid particle acceleration, ρ is the density o the luid. The constitutive behaviour o the luid is assumed to be inviscid, linear, and compressible, so p = K u x where K is the bulk modulus o the luid. For solid structures, the explicit dynamics analysis procedure in the inite element analysis is based upon the implementation o an explicit integration rule together with the use o diagonal or lumped element mass matrices. The equations o motion or a solid body are written as F I = M u& where M is the diagonal lumped mass matrix, F is the applied load vector, and I is the internal orce vector. The above equations are integrated using the explicit central dierence integration rule ( i+ 1) ( i) ( i+ 1/ ) ( i 1/ ) t + t ( i) u & = u& + u& ( i+ 1) ( i) ( i+ 1) ( i+ 1/ ) u = u + t u& where u& is velocity and u& & is acceleration. The superscript (i) reers to the increment number and i 1/ and i +1/ reer to midincrement values. The central dierence integration operator is explicit in that the kinematic state can be advanced using known values ) o ( i u& 1/ and u& & (i) rom the previous increment. In coupled acoustic-structure simulations, the continuity conditions o displacement as well as stress and pressure along the normal direction (x - direction) at the interace needs to be satisied by u ) ( u ) ( solid = acoustic ( σ ) solid = p + p orcing where u and σ are normal displacement and stress components in x - direction, respectively. p is the pressure in acoustic medium and p orcing is an applied pressure at the interace. 6. FE Model A ully coupled acoustic-structure model is to be used in nonlinear dynamic FE analysis considering the luidstructure interaction. In this coupled model, LNG is modelled as an acoustic medium and the box structure is modelled as elasticity. All dimensions o the coupled model based on the midsurace in the box structure are shown in Fig. 14. L is the length o LNG, primary and secondary boxes. W is the width o LNG, primary and secondary boxes, H is the height o LNG, and all other geometric parameters are the same as those in Fig. 5. Strength Assessment o Membrane-Type LNG Containment System

7 FE mesh is shown in Fig node shell elements are used or all plates in primary and secondary boxes including stiness inside two boxes and 8-node Lagragian solid elements are used or mastics at the bottom o the structure. 8-node Eulerian solid elements are employed or LNG (luid). Loading and boundary conditions in the NO 96 containment system are also shown in Fig. 15. The bottoms o all mastics in the containment system are simply supported in the normal direction. Four side suraces o the acoustic medium are constrained in the normal. Based on sloshing load analysis, applied impact load at the interace between LNG and the top surace o the primary box is a uniorm triangular impulse pressure, shown in Fig.. In this example, p max =.8 MPa, T rise =.5s, and T duration =.1s. L W LNG Primary Box Secondary Box l Figure 14: Coupled model or NO 96 CS H H 1 H H 3 Figure 15: FE mesh and loading/boundary conditions or NO 96 CS For ully coupled acoustic-structure simulations in the strength assessment o the NO 96 containment system, the inviscid, linear and compressible constitutive model is to be employed or the acoustic medium as: p = K u x where p is the pressure, u is the displacement vector, K = ρ Cd is the bulk modulus o luid, the density o luid, ρ = 474 kg/m 3 and the dilatational wave speed, C d = 17 m/s. 6.3 FE Results and Discussion Nonlinear dynamic FE modeling was conducted on the coupled luid-structure model or the LNG NO 96 containment system using DYTRAN [8]. Stress, displacement and strain distributions in each layer are obtained rom numerical results. Thus, the strength and deormation o the containment system can be evaluated based on acceptance criteria. Stress - von Mises stress contour in the containment system at.6s is shown in Fig. 16. Fig. 16 indicates the critical location, which is located in the central area o the side panel in the secondary box in the containment system. At this critical location, the maximum stress takes place possibly causing the strength problem. It also shows that there is stress concentration around the connection between the bottom plate o the secondary box and mastics. The stress-time history at the central Strength Assessment o Membrane-Type LNG Containment System 1

8 point o the side panel o the secondary box is shown in Fig. 17. The oscillation (or so-called noise ) in the curve might be caused by the explicit method in dynamic analysis. It will be urther investigated in the uture work. The maximum stressing in the NO 96 insulation structure is 1 MPa, which takes place at the time o.65s. Ultimate strength o plywood is 4 MPa. The saety actor in stress analysis is deined as Strength SaetyFactor MaximumStress Under a uniorm impulse pressure with a peak value o.8 MPa, the saety actor is 1.9, based on the acceptance criterion. Thereore, nonlinear dynamic stress analysis indicates that the NO 96 containment system is sae in strength under the uniorm impulse pressure with the maximum value o.8 MPa. the primary box. The displacement-time history curve at the central point o the side panel in the secondary box is shown in Fig. 19. Figure 18: Deormation contours o primary and secondary boxes at t =.5s Figure 16: Snapshot o von Mises Stress Contour in NO 96 CS at.6s Fig. 19 shows that the irst peak value o displacement is 7.5 mm, which takes place at.65s. This is consistent with stress. However, the 4 th peak value o displacement in this curve is 1 mm at the time o.145s. It seems very close to the inal ailure. The large deormation at this critical location may lead to the buckling ailure at the inal stage. FE results also indicate that the displacement at the central point o the top plate in the primary box increases with time but it is much less than that at the central point o the side panel in the secondary box. It can be concluded that the central area o the side panel in the secondary box is the most critical location or inal ailure in the NO 96 containment system. This conclusion rom numerical simulation is consistent with the experimental one in which the NO 96 insulation structure ails at the central area o the side panel in the secondary box due to buckling. Thus, the ully coupled luid-structure model developed or nonlinear dynamic FE analysis has been validated or the LNG NO 96 containment system. Figure 17: Stress time history at central point o side panel in secondary box Deormation The deormation contours o the structural part in the containment system at the time o.5s are shown in Fig. 18. This igure shows that the maximum displacement takes place at the central area o the side panel in the secondary box. This large deormation may cause the buckling ailure at the inal stage under dynamic loading. Fig. 18 also shows that large deormation occurs in the center o the top plate in Figure 19: Displacement-time history curve at central point o side panel in secondary box Strength Assessment o Membrane-Type LNG Containment System

9 7. CONCLUSIONS An analysis procedure has been developed or the strength evaluation o LNG containment system. Three levels o structural analyses have been developed in the strength assessment. In this study, 1) Static stress and buckling FE analyses have been conducted on the simpliied model without considering LNG or the NO 96 containment system. Numerical simulation shows that the insulation structure ails due to the buckling o the side panel o the secondary box, which is compatible with experimental results. The critical static buckling load obtained rom eignvalue analysis is very close to the test result. The ailure mode shown in static analysis matches the experimental one, which validated the inite element model. For the strength evaluation o the NO 96 containment system, a linear static analysis is suicient to make a conservative evaluation. ) Linear transient stress FE analysis has been conducted on the simpliied model with an equivalent dashpot system representing LNG in the NO 96 containment system. A parametric study using dierent loading patterns has been perormed to investigate the eect o the duration and skewness o the applied load on structural response. The results show that the duration has a stronger eect on the structural response than skewness. Since the ailure o the NO 96 containment system is due to buckling instead o yielding, linear transient analysis is not enough to show the high nonlinearity o the applied impact load. 3) A ully coupled luid-structure model has been developed to investigate the hydro-elastic eect in the NO 96 containment system. Nonlinear dynamic FE analysis has been conducted on the coupled model under the uniorm impulse pressure using DYTRAN. The hydro-elastic eect has been investigated in coupled luid-structure modeling. FE results indicate that the maximum stress, which occurs at the central area o the side panel in the secondary box, is less than the ultimate strength o materials. The saety actor or the NO 96 containment system is 1.9, showing the NO 96 containment system is sae in strength under the uniorm impulse pressure with the magnitude o.8 MPa. FE results also indicate that the most critical location is the central area o the side panel in the secondary box and the deormation delays ater the maximum stress takes place at this critical location. It can be concluded that hydroelastic eect cannot be neglected in the strength evaluation o the NO 96 containment system. 8. ACKNOWLEDGEMENTS The authors would like to thank Dr. Han Chang Yu and Dr. Roger Basu at ABS or their valuable comments. The authors are also grateul to University o Illinois at Urban-Champion (UIUC), GAZ TRANSPORT & TECHNIGA (GTT), and Daewoo Shipbuilding & Marine Engineering Co. (DSME) or providing the containment system data and drop test data and or supporting the dynamic material property test. 9. REFERENCES 1. Shin, Y., Kim, J.W., Lee, H. and Hwang, C., Sloshing Impact o LNG Cargoes in Membrane Containment Systems in the Partially Filled Condition, ISOPE, Hawaii, USA, 3.. Kim, J.W., H.S. Lee, and Yung S. Shin, Sloshing Impact Load and Strength Assessment o Membrane- Type LNH Containment System in Large LNG Carriers, 14th Oshore Symposium- LNG, Houston, USA, American Bureau o Shipping, Guidance Notes on Strength Assessment o Membrane-Type LNG Containment Systems under Sloshing Loads, Shin, Y., Kim, J.W., Wang, B., First Principle-Based Analysis Procedure or Strength Assessment o Membrane-Type LNG Containment System due to Sloshing Impact, World Maritime Technology Conerence (WMTC), London, UK, Lee, H., Kim, J.W., Hwang, C., Dynamic Strength Analysis or Membrane Type LNG Containment System due to Sloshing Impact Load, The international Conerence on Design and Operation o Gas Carriers, London, UK, American Bureau o Shipping, Guide or Building and Classing Membrane Tank LNG Vessels,. 7. GTT, LLNG Tank Qualiication Study Static Tests Assessment, GTT Report, 4 8. MSC, MSC Dytran User s Manual, Version 4.7, AUTHORS BIOGRAPHIES Bo Wang is a Research Engineer in the ABS Technology Department. He received his Ph.D. in Engineering Mechanics in 199. Dr. Wang worked as a post-doctor at Delt University o Technology in The Netherlands and a JSPS research ellow in Japan. Dr. Wang was a proessor in Tsinghua University in China rom 199 to. He worked as a research aculty in the School o Mechanical and Aerospace Engineering at Oklahoma State University beore joining ABS in 5. His research interests include nonlinear FE modeling, structural analysis, racture & atigue, and material science. Jang Whan Kim is a Senior Research Engineer in the ABS Technology Department. He earned his Ph.D. degree in Naval Architecture rom the Seoul National University, Korea in He worked as Post Doc. and Researcher in the Seoul National University, University o Caliornia at Berkeley and University o Hawaii. He joined ABS in 1. His research interests are in the ields o linear and nonlinear wave modeling, hydrodynamic impact, luid-structure interaction and sloshing phenomena in LNG tanks. 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10 Yung Sup Shin is a Senior Sta Consultant, ABS Technology. He received a Ph.D. in Hydrodynamics rom the Department o Naval Architecture rom the University o Michigan. Since joining ABS 7 years ago, Dr. Shin has led a number o hydrodynamic projects or theoretical development o ships and oshore structures o semisubmersible, tension leg platorm, and FPSO. Also, he has led an R&D team or the development o tanker and LNG sloshing analysis, pump tower analysis system and procedure. He serves as chairman o the PRECAL working group o MARIN Cooperative Research or Ships and SNAME H-7 Seakeeping panel. 4 Strength Assessment o Membrane-Type LNG Containment System