Liquefied Natural Gas Cargo Containment System Research Articles

Procedure for evaluation of LNG CCS with reference vessel using nonlinear dynamic analysis

This paper presents a procedure for evaluating the structural safety of Liquefied Natural Gas (LNG) Cargo Containment System (CCS) against sloshing load by comparing the results of linear and nonlinear dynamic structural strength analysis with those of verified reference vessels. This comparative procedure indirectly considers (1) limitation of implementing real physical phenomena in model experiments including the Froude scaling method, density ratio, etc., (2) assumptions in statistical analysis for calculating design loads, and (3) complicated effects not implemented in nonlinear dynamic analysis such as phase transition between liquid and gas, hull deflection, and the detailed geometry of the CCS, among others. The uncertainties in sloshing model experiments and statistical analysis can be addressed by introducing the pressure correction factors, which make the linear design loads of the reference vessel satisfy the CCS’s capacity. Finally, the structural nonlinearity, which was not accounted for in the nonlinear dynamic analysis, can be taken into consideration through a comparison of the usage factors from nonlinear dynamic analysis. To apply the proposed structural assessment procedure, the GTT NO96 series were selected and applied to two failure modes. In the application of this procedure, the Triangular Impulse Response Function (TIRF) method was adopted, to efficiently calculate numerous linear dynamic structural strength analyses required for Dynamic Amplification Factor (DAF). Through this application applying larger sloshing loads to the reinforced NO96 CCS of the target vessel compared to the reference vessel, the feasibility of this procedure was shown.

Numerical investigation of subcooled two-phase flow and heat transfer characteristics in a liquefied natural gas cargo containment system

The increase demand in liquefied natural gas (LNG) leads to an increase in the import and export of LNG cargo using LNG carriers with composite insulation and double-hull structures for membrane-type storage tanks. Therefore, the transient characteristics of cryogenic heat transfer in LNG cargo containment systems have become important for understanding the physics when LNG vaporizes into the gas phase to minimize the cargo loss of boil-off gas (BOG). However, public papers on fundamental research on long-term computational fluid dynamics (CFD) simulations are still lacking. In the present study, the transient flow behaviors of LNG fluids and the heat transfer characteristics were numerically studied by applying the simplified single-layer insulation in an LNG cargo containment system, based on an initial temperature of TLNG,0 = 110.5 K and saturation temperature of TLNG,sat = 111.5 K in the boundary condition of IGC code. The proposed analysis procedure can serve as a reference for relative comparison of predicted BOG difference in LNG cargo holds.Consequently, the heat transfer mode between the two-phase methane and the solid insulation could be typically classified into two different regimes: (1) a subcooled regime before reaching the saturation temperature from the initial temperature, and (2) a boiling regime after reaching the saturation temperature. In the subcooled regime, the subcooled liquid exhibited bulk flow motion induced by natural convection. The loss rate of LNG mass per time interval was extracted from the best-fit correlations as −0.0448 kg/s at 1 d (105 s) to cool down the insulation system and −0.0072 kg/s at 5 d (5 × 105 s) to reach the LNG saturation temperature. When the predicted value of the loss rate of LNG mass by the CFD simulation was regarded as the generation rate of boil-off gas, it was smaller than the designed value of the boil-off gas of 0.464 kg/s corresponded to boil-off rate (BOR) about 0.2 %/day. In the boiling regime and under sufficiently cooled conditions, the loss rate of LNG mass per time interval did not consistently exceed the BOR value. The liquid flow exhibited a turbulent-like motion despite the relatively low velocity at 10 d (106 s). Especially, the increase in the effective thermal conductivity was a good indicator for distinguishing between the boiling and subcooled regimes.

Impact failure characteristics of LNG carrier cargo containment system

The most critical issue related to the maritime transportation of liquefied natural gas (LNG) is the structural safety of the cargo containment system (CCS), because LNG is stored at temperatures below –163 °C. During the unloading of LNG, several types of structural components can fall into CCSs from significant heights. Thus, the structural safety of LNG CCSs against dropped objects should be verified; however, the actual failure characteristics of the primary barrier are not well understood. In this study, the failure mechanism of the primary barrier of an LNG CCS was investigated by conducting falling-object-induced structural failure tests. To clarify the effect of the failure response of the primary barrier, impact tests were performed using various membrane types and impactor types. In addition, to ensure the wide applicability of the results, a finite element analysis-based design-aid numerical method was developed by comparison with experimental observations. Excellent numerical verification results were obtained using the Johnson–Cook material model, which can be applied to the dynamic failure problem for the corrugated plates used in LNG CCSs. Finally, guidelines that could set safety standards for the primary barrier were identified, considering the shape, weight, and height of potential falling objects.

에폭시 수지가 적용된 유리섬유 복합재료의 극저온 환경 기계적 특성 분석을 통한 LNG CCS 적용성 평가

Consumption of Liquefied Natural Gas (LNG) has increased due to environmental pollution; therefore, the need for LNG carriers can efficiently transport large quantities of LNG, is increased. In various types of LNG Cargo Containment System (CCS), Membrane-type MARK-Ⅲ composed of composite materials is generally employed in the construction of an LNG carrier. Among composite materials in a Mark-Ⅲ system, glass-fiber composites act as a secondary barrier to prevent the inner hull structure from leakage of LNG when the primary barrier is damaged. Nevertheless, several cases of damage to the secondary barriers have been reported and if damage occurs, LNG can flow into the inner hull structure, causing a brittle fracture. To prevent those problems, this study conducted the applicability assessment of composite material manufactured by bonding glass-fiber and aluminum with epoxy resin and increasing layer from three-ply (triplex) to five-ply (pentaplex). Tensile tests were performed in five temperature points (25, -20, -70, -120, and -170°C) considering temperature gradient in CCS. Scanning Electron Microscopy (SEM) and Coefficient of Thermal Expansion (CTE) analyses were carried out to evaluate the microstructure and thermos-mechanical properties of the pentaplex. The results showed epoxy resin and increasing layer number contributed to improving the mechanical properties over the whole temperature range.

Transient pool boiling of liquid nitrogen (a safe analogue of LNG) on AISI 304 stainless steel flat surfaces

Reducing the boil-off rate of liquefied natural gas (LNG) stored in an LNG cargo containment system (CCS) is one of the key concerns in the design of CCSs in terms of both safety and economy. Correlations for transient boiling on steel surfaces, which show the relationship between the heat flux and the wall superheat, are beneficial for the estimation of the boil-off rate of LNG in a CCS during a loading process. However, these boiling correlations are not yet available. In this study, an attempt was made to fill this gap. Specifically, transient boiling experiments of liquid nitrogen on AISI 304 stainless steel were performed. Liquid nitrogen was used because it has been commonly considered a safe analogue of LNG in laboratory tests. It was observed that the boiling curves for transient boiling deviated significantly from that for steady boiling. The boiling correlations were obtained by adjusting the correlations for steady boiling using the experimental results for the temperature difference range of 5–200 K. In addition, it was experimentally confirmed that the assumption of perfect thermal contact between a liquid and a boiling surface is inapplicable to boiling on a metal surface. The findings of this study are expected to be beneficial for the estimation of the boil-off rate of LNG and the analyses of the thermal stress distribution in a CCS during a loading process.

Impact behavior of hollow glass bubble reinforced foam core LNG insulation panel in cryogenic temperature

Composite sandwich panels are applied to numerous structures owing to their excellent mechanical properties and thermal performance. In marine craft, sandwich panels are located on the inner walls as an insulation system for the liquefied natural gas (LNG) cargo containment systems (CCSs). Polyurethane foam (PUF), as an insulation material, is commonly used as the core material against a cryogenic temperature of –163°C for the LNG CCSs. Owing to its excellent thermal performance that prevents heat flow from the outside, and because LNG is transported at cryogenic temperatures, PUF is the preferred choice as a core material. However, the LNG CCS is subjected to fluid impact load due to sloshing during a laden voyage, and therefore, the core material needs to be reinforced. In this study, a reinforcement material made of hollow glass bubbles (HGBs) was added to PUF to help withstand the sloshing loads, and the performance of the specimens was investigated in terms of their impact strength and absorption energy. The mechanical impact strength of the final products was investigated through an impact test at cryogenic temperatures and the absorption energy was analyzed along with the dynamic compressive behavior of the sandwich panels.

Effect of temperature on the mechanical performance of plywood used in membrane-type LNG carrier insulation systems

In this study, the mechanical performance of melamine–urea–formaldehyde (MUF) resin plywood composed of an orthotropic material, which is used as a structural material in liquefied natural gas (LNG) cargo containment systems (CCSs), is evaluated. With a decrease in temperature, the plywood changes from ductile to brittle under compressive loads; thus, it may fail to distribute the compressive loads caused by sloshing impact as well as lose its stiffness, which helps maintain the shape of the structure. However, only a few studies investigated the mechanical characteristics of MUF resin plywood under compressive loads caused by sloshing impact as well as the crack propagation and change in material features with decreasing temperatures. Therefore, the present study investigated the mechanical performance of MUF plywoods of different thicknesses under different temperatures and grain orientation parameters. The results indicate the mechanical properties of MUF plywood for compression with decreasing temperatures. Furthermore, based on thermomechanical analysis, this study shows that the critical temperature at which the plywood material tends to transition from ductile to brittle behavior is − 110 °C. This finding will help in the design of MUF plywood-based LNG CCSs considering its low-temperature brittleness.

Cumulative damage of hollow glass microsphere weight fraction in polyurethane foam in response to cryogenic temperatures and repeated impact loading

The sloshing impact load of liquefied natural gas (LNG) carriers is an important issue that can cause failure of the insulation system of LNG cargo containment systems, leading to cryogenic leakage from the hull of the ship and generation of boil-off gas. Thus, insulation systems should be designed by considering the repeated impact loading based on the design life as well as the cryogenic temperature. Polyurethane foam (PUF), epoxy resin, and plywood are acceptable materials that can be used to fabricate a typical insulation system to protect the load from the environment. In this study, we investigated the effect of adding hollow glass microspheres (HGMs) to the epoxy resin on the reinforcement of the insulation material. Specimens were fabricated using PUF, plywood, and an HGM/epoxy composite matrix with varying weight fractions of embedded HGM (0, 10, and 20 wt.%). The sloshing impact loads were estimated by the statistical method of the Weibull model, and the top 2% level of the Weibull distribution was selected. The calculated sloshing impact load was 3 J as calculated from the experimental results, and then repeated impact loadings under ambient (290 K) and cryogenic temperature (110 K) were realized in a drop tower facility. To obtain details of the dynamic behavior, microstructural analyses were performed using scanning electron microscopy (to detect the agglomeration of HGM embedded in the epoxy resin). Experimental results show that 10 wt.% HGM showed the best performance in absorption and dynamic behavior. The 10 wt.% HGM specimen also exhibited superior thermal conductivity.

Effects of Environmental Conditions on the Mechanical and Degradation Behavior of Polyurethane Foam Subjected to Various Deformation Histories

Since its development, polyurethane foam (PUF) has been widely used as an insulator, including in the marine industry. Liquefied natural gas (LNG) carriers have the highest market share in the LNG-related shipbuilding industry. PUF is used as an insulator in membrane-type cargo containment systems (CCSs) as one of the most important components of LNG carriers. Because PUF is subjected to constant load and deformation when used in insulation for LNG CCSs and is exposed to the cryogenics of the seawater environment for extended times, it is essential to evaluate the mechanical behavior of PUF in such environments. In this study, to investigate the effects of applying an initial strain and of immersion in various media on the mechanical properties of PUF, compression tests were carried out at ambient and cryogenic temperature using a universal testing machine. The mechanical behavior of PUF after deformation recovery upon subjection to various initial strain levels was estimated experimentally. In addition, the microstructure of the PUF specimen prior to testing was analyzed to investigate the effects of the initial strain level and immersion in different media on the mechanical behavior of PUF.

Experimental Investigation of Structural Response of Corrugated Steel Sheet Subjected to Repeated Impact Loading: Performance of LNG Cargo Containment System

The primary barrier in contact with LNG (Liquefied natural gas) cargo is manufactured from stainless steel, and has a corrugated membrane structure due to thermal deformation caused by extreme temperature differences. Because the primary barrier is tailored to be LNG watertight, the structural integrity of the primary barrier must be maintained despite the increase in LNG sloshing load. Among the various types of fluid impacts in a cargo hold, jet flow runs up along the wall of the cargo hold and extreme impact load is exerted on the side of the corrugation of the primary barrier. However, the impact behavior of the primary barrier under the prevailing environment has not been investigated due to the limitation of test methods. Thus, in the present study, the inelastic structural behavior of thin-walled and curved-shaped structures for liquefied natural gas cargo containment systems under repeated impact loading was investigated experimentally. Custom-built structural impact test equipment with large-scale weight drop test facilities was fabricated, and a series of structural impact tests were carried out. The impact energy and the number of repetitions were taken into account in order to investigate the characteristics of the inelastic structural behavior, such as plastic deformation and reaction force time history according to impact energy.

Preventive Maintenance Topic Models for LNG Containment Systems of LNG Marine Carriers Using Dock Specifications

The high demand for liquefied natural gas (LNG) requires more LNG carriers (LNGCs) to be in operation. During transportation, there is a high risk due to the required extremely low temperatures and the explosive nature of LNG cargo. Moreover, when there is a lack of experience in operating old LNGCs, there is a serious concern regarding operational accidents. A systematic maintenance strategy, especially for LNG cargo containment systems, is crucial for maintaining safe LNG transportation at sea. The purpose of this study is to develop preventive LNG cargo containment system maintenance models by using LNGC dock specifications from LNGCs of various ages. The dock specifications from a conventional LNGC repairing dock were analyzed using natural language processing techniques in order to develop preventive maintenance models of the LNG cargo containment system. From these results, and by considering the ship’s age, it was found that for young LNGCs, the priority for repair should focus on checking routine consumable spare parts by tank inspections, whereas for older LNGCs, the focus should be on tank condition maintenance rather than on other facilities. These results are expected to be useful in the development of a maintenance strategy of preventive LNG cargo containment systems in maritime LNG transportation.

Investigation of fatigue performance for new membrane-type LNG CCS at cryogenic temperature

In recent years, various membrane-type liquefied natural gas (LNG) containment systems have been developed to improve the performance related to the insulation system and boil off rate (BOR). In this respect, investigations into the mechanical properties and fatigue strength of the welded joints in a new LNG cargo containment system (CCS) are required to ensure the structural integrity of LNG carriers. In this study, a series of fatigue tests were carried out for five different types of test specimens manufactured using Invar alloy and SUS 304L at ambient and cryogenic temperatures, respectively. It was clearly observed that the fatigue strength at cryogenic temperature is higher than that at ambient temperature. Finally, the FAT value and slope for the welded joints in the new LNG CCS were suggested based on the effective notch stress approach.

Buckling strength of GTT NO96 LNG Carrier cargo containment system

The present study investigates the design buckling capacities of a Gaztransport and Technigaz (GTT) NO96-type liquefied natural gas (LNG) cargo containment system (NO96 CCS), one of the most critical failure modes for a box-type LNG cargo containment system, against sloshing dynamic loads from an LNG carrier cargo tank. The design load for NO96 CCS (sloshing) is a typical dynamic event; hence, the dynamic effect on the buckling capacity is investigated as the defining dynamic strength factor. A series of finite-element (FE) analyses for the insulation system are performed using commercial FE software (i.e., ABAQUS and ANSYS) and considering the environmental temperature and material properties of the system. The design buckling strength under both static and dynamic conditions is provided based on the FE calculations and compared with the as-designed criteria of DNV GL for a membrane-type LNG cargo tank.

Application of Gurson Model for Evaluation of Density‐Dependent Mechanical Behavior of Polyurethane Foam: Comparative Study on Explicit and Implicit Method

Recently, liquefied natural gas (LNG) is a focus of interest around the world for several reasons, and LNG cargo containment systems (CCS) increase in quality to prevent loss of LNG during shipping. For insulation of CCS, polyurethane foam (PUF), an outstanding insulation material, is commonly used. However, until now, although its mechanical properties are relatively good, the material is not considered as a structural member under compressive loading, principal load direction in CCS. Moreover, as PUF is a porous material by mixing and foaming, its mechanical properties depend on voids, which is a dominant parameter for density. Therefore, in the present study, nonlinear behavior of PUF is described using Gurson model with a novel technique, i.e., an acceleration factor. The model expresses the behavior through the volume fraction of voids with consideration of the density effect to evaluate structural performance using user‐defined material subroutine with explicit and implicit methods. image

Comparative Study on the Thermal Insulation of Membrane LNG CCS by Heat Transfer Analysis

This study discusses the thermal insulation capacity of variant of NO96 LNG (liquefied natural gas) cargo containment insulation system. Changing the insulation materials and the insulation layers of conventional GTT NO96 containment system, The thermal resistance and BOR(boil off rate) caused by the heat transfer between cryogenic and environmental temperature is discussed. Therefore, thermal analysis of LNG CCS(cargo containment system) is carried out to determine the insulation capabilities. Also, BOR is evaluated in terms of the total amount of heat invaded into CCS(cargo containment system). Variant of NO96 CCS such as NO96, NO96GW and NO96L3 membrane type during laden voyage is selected for the comparative study. Finite element model for heat transfer analysis is conducted by employing the equivalent thermal resistance model to simplify the complex insulation layers. Finally the results for each variant model are relatively compared and discussed to minimize the BOR.

Effects of cryogenic thermal cycle and immersion on the mechanical characteristics of phenol-resin bonded plywood

The main objective of the present study is to investigate the performance degradation of the plywood used in a liquefied natural gas (LNG) cargo containment system (CCS). A plywood sheet features an odd number of thinly layered wooden plies bonded perpendicularly to the previous layer to give it a very strong and durable structure. Owing to this strong point, plywood is applied to a variety of interior and exterior applications. Above all, it is widely adopted as insulation panels in an LNG CCS owing to a high stiffness with low density and its superior mechanical capabilities. As an insulation material of an LNG CCS, plywood is constantly exposed to repeated wave-induced thermal variations caused by the loading (−163°C) and unloading (20°C) of LNG during general operating periods of 25years on average. Therefore, the effects of cryogenic-level thermal loads on the material characteristics of plywood must be analyzed with respect to the design and safety aspects of LNG CCSs. In the present study, the influences of the estimated thermal load, testing temperature, and grain orientation on plywood adopted in an LNG CCS are investigated. In terms of safety and design, the repeated thermal loads in a LNG CCS must be considered because the modulus of elasticity (MOE), tensile strength (TS), and modulus of rupture (MOR) are degraded by thermal treatments, such as cyclic thermal-shock and cryogenic immersion.

Study on tank shape for sloshing assessment of LNG vessels under unrestricted filling operation

Conventional liquefied natural gas (LNG) vessels with membrane cargo containment systems have tank filling restrictions from 0.10 to 0.70 H (with H the internal tank height). The main reason for such restrictions is high sloshing loads around these filling depths. The new designs of prismatic LNG cargo tanks are proposed by increasing the lower chamfer length. Numerical sloshing analysis was used to optimize tank shape out of the several candidates. To validate the effectiveness of the modified tank, 1/50 scaled model tests were conducted. In these tests, 24 different irregular seaways were tested for both the conventional and the optimized tank and their statistical pressures were compared. Modified tank design was quite effective in reducing sloshing loads for a 0.30 H filling depth, and it did not significantly increase sloshing loads at other filling conditions. This study demonstrated the possibility of all filling operations for an LNG cargo containment system.

직교이방성 복합재료의 극저온 재료 물성치를 고려한 LNG CCS의 강도 평가에 관한 연구

Liquefied natural gas(LNG) cargo containment system(CCS) has the primary function of ensuring both adequate structural safety with respect to sloshing load which is defined as a violent behaviour of the liquid contents in CCS due to external forced motions and thermal insulation keeping natural gas below its boiling point. Among different LNG CCS types such as independent B-type and membrane ones, Mark III CCS is considered in this paper to perform its strength assessment. Mark III CCS plate is designed and constructed by stacking various non-metallic engineering materials such as plywood, triplex, reinforced PU foam that are supported by series of mastic upon inner steel hull structure. From the viewpoint of structural analysis, this plated structure is treated as a laminated composite structure showing complex structural behaviour under external load. Advanced finite element models of Mark III CCS plate is generated and used in conjunction with ultimate strength based failure criteria from laminated composite mechanics for the strength assessment. The strength assessment is performed within the initial failure state of Mark III CCS plate. Results provide failure details such as failure locations and loads. Finally obtained results are reviewed using the loads from acceptance criteria suggested by classification.

Glass composite vibration isolating structure for the LNG cargo containment system

The primary barrier of the cargo containment system (CCS) for the liquefied natural gas (LNG) carriers is subjected to large impact loads due to the cavitation of LNG. Therefore, the impact loads should be reduced to protect the insulation foam from cracking because the foam has very low fracture toughness at the operating temperature of −163°C.In this study, a corrugated-plate type vibration isolating structure (VIS) has been developed using glass fiber reinforced composites to reduce the load transmissibility from the primary barrier to the insulation foam of the CCS at cryogenic temperature. The uniform spring constant characteristic was achieved by optimizing the stacking sequence of the composite laminate and the wavelength of the corrugation. The crack resistance of the VIS against the impact load was measured by the drop weight impact test with respect to the stacking sequence of the glass composite laminate. The specifications of the VIS were obtained by the finite element analysis and experiments. Also, the durability against repetitive impacts on the VIS was investigated.

Rapid response calculation of LNG cargo containment system under sloshing load using wavelet transformation

Reliable strength assessment of the Liquefied Natural Gas (LNG) cargo containment system under the sloshing impact load is very difficult task due to the complexity of the physics involved in, both in terms of the hydrodynamics and structural mechanics. Out of all those complexities, the proper selection of the design sloshing load which is applied to the structural model of the LNG cargo containment system, is one of the most challenging one due to its inherent randomness as well as the statistical analysis which is tightly linked to the design sloshing load selection. In this study, the response based strength assessment procedure of LNG cargo containment system has been developed and proposed as an alternative design methodology. Sloshing pressure time history, measured from the model test, is decomposed into wavelet basis function targeting the minimization of the number of the basis function together with the maximization of the numerical efficiency. Then the response of the structure is obtained using the finite element method under each wavelet basis function of different scale. Finally, the response of the structure under entire sloshing impact time history is rapidly calculated by synthesizing the structural response under wavelet basis function. Through this analysis, more realistic response of the system under sloshing impact pressure can be obtained without missing the details of pressure time history such as rising pattern, oscillation due to air entrapment and decay pattern and so on. The strength assessment of the cargo containment system is then performed based on the statistical analysis of the stress peaks selected out of the obtained stress time history.