Liquefied Natural Gas Containment System Research Articles

A monotonicity-preserving higher-order accurate finite-volume method for Kapila’s two-fluid flow model

In preparation of the study of liquefied natural gas (LNG) sloshing in ships and vehicles, we model and numerically analyze compressible two-fluid flow. We consider a five-equation two-fluid flow model, assuming velocity and pressure continuity across two-fluid interfaces, with a separate equation to track the interfaces. The system of partial differential equations is hyperbolic and quasi-conservative. It is discretized in space with a tailor-made third-order accurate finite-volume method, employing an HLLC approximate Riemann solver. The third-order accuracy is obtained through spatial reconstruction with a limiter function, for which a novel formulation is presented. The non-homogeneous term is handled in a way consistent with the HLLC treatment of the convection operator. We study the one-dimensional case of a liquid column impacting onto a gas pocket entrapped at a solid wall. It mimics the impact of a breaking wave in an LNG containment system, where a gas pocket is entrapped at the tank wall below the wave crest. Furthermore, the impact of a shock wave on a gas bubble containing the heavy gas R22, immersed in air, is simulated in two dimensions and compared with experimental results. The numerical scheme is shown to be higher-order accurate in space and capable of capturing the important characteristics of compressible two-fluid flow.

Pressure-resisting capability of the knot area of the primary barrier for a LNG containment system

Stainless steel corrugations of the primary barriers of a liquefied natural gas (LNG) containment system have been supported with pressure-resisting structures (PRS) composed of an aluminum and glass fiber composite to improve their buckling resistance against shock loading due to LNG cavitation. Because the knot area is composed of two corrugations and is not reinforced due to its complicated shape, its pressure-resisting capability may be lower than that of the corrugations. In this study, new hydraulic test equipment has been developed with butyl rubber to evaluate the pressure-resisting capability of the knot area of the primary barrier. In addition, the pressure-resisting capabilities of the knot area with and without PRS under the corrugation near the knot area will be investigated.

Strength and stiffness assessment of an LNG containment system – Crushing and buckling failure analysis of plywood components

A liquefied natural gas (LNG) containment system (LCS) No. 96 type is built up as a box system consisting of plywood plates and bulkheads. Plywood components are connected using staple connectors.The aim of this paper is to explain the crushing and buckling failure of plywood component of an LCS No. 96. A systematic study of crushing and buckling behavior of plywood components is conducted by means of experiments and finite element (FE) analysis. The FE study is based on numerical material modeling using Hashin’s theory and fracture energy approach as damage initiation and evolution, respectively.Observations on strength and progressive deformation during the crushing and buckling tests are discussed. It is observed that crushing failure of plywood bulkheads determines the ultimate crushing capacity. Crushing failure of plywood plate which is due to shear through the thickness failure of the veneer, causes reduced stiffness before the ultimate crushing capacity is reached.It is found from buckling test that staple connectors have no influence on structural strength nor stiffness, i.e. they keep the structure together during manufacturing. During buckling of bulkheads, withdrawal of staple legs from the plywood bulkhead is observed. Thus, plywood bulkheads are supported on their edges and creates hinge like support. The numerical simulations – including the use of the damage model of plywood components – agree well with the test results.

Adoption of new advanced computational techniques to hazards ranking in LNG carrier operations

The risks of hazards of liquefied natural gas (LNG) carrier operations and their fundamental causes are investigated in this study. A new framework is developed through the combination of a risk matrix approach and a fuzzy evidential reasoning (FER) method. The risk matrix approach is employed to identify the major hazards associated with the LNG carrier operations. In the risk matrix methodology, linguistic terms are used to estimate how likely hazards may occur in LNG carrier operations at sea, and the consequences of the hazards. The failures of an LNG spill from a transfer arm and an LNG containment system are identified as the hazards of “very high risk”. The fault tree analysis (FTA) diagrams of “very high risk” hazards (top events) are used to identify their causes (basic events) and failure logics. Due to the unavailability of historical failure data, the safety/risk level of each cause is investigated using the FER approach. In the FER approach, the safety levels of the basic events of the top event are assessed using three safety parameters (i.e. occurrence likelihood, consequence severity and failure consequence probability) and combined using evidential reasoning (ER) in fuzzy environment in order to reveal the top events’ safety/risk levels.

A fuzzy genetic algorithm approach for analysing maintenance cost of high risk liquefied natural gas carrier systems under uncertainty

A Fuzzy Genetic Algorithm (FGA) is used to treat uncertainties associated with unit costs of maintenance of Liquefied Natural Gas (LNG) carrier systems such as a containment system and a transfer arm. A Fuzzy Rule Base (FRB) is established to identify the unit costs of maintenance of the LNG containment system and transfer arm. It includes 125 LNG carrier maintenance cost rules, with technical consultancy cost, maintenance duration, and spare part cost as the antecedents and maintenance cost as the consequent. The outcome from the FRB is used to optimise a risk model using a Genetic Algorithm (GA) approach to find the optimal maintenance cost of each system with provided information on their respective time of interest, failure probability, failure frequency and maintenance cost of the whole LNG carrier system.

Performance improvement by glass fiber of adhesively bonded metal joints at the cryogenic temperature

The performance of adhesively bonded joints composed of aluminum sheet and stainless steel foils was investigated at the cryogenic temperature for the secondary barrier LNG (Liquefied Natural Gas) containment system. Since the huge adhesive bonding area such as LNG carriers or floaters cannot be surface-treated economically by chemical treatment, new flame surface treatment method for the metal foils was evaluated and compared with other conventional surface treatments such as mechanical or chemical surface treatments at the cryogenic temperature. The surface wettability by contact angle measurement of the metal foil was investigated with respect to surface treatments and environmental aging conditions. The effects of adhesive thickness on the adhesive bond strength and fracture toughness have been also investigated at the cryogenic temperature.The experimental results showed that the bonding performances depended strongly on the surface treatment method, adhesive thickness and fiber reinforcement for the adhesive, such. It was also found that the thick adhesive joint had a reduced bonding strength at the cryogenic temperature, but the glass fiber reinforcement improved much the bonding strength and fracture toughness for thicker adhesive at the cryogenic temperature.

Improvement of the adhesive peel strength of the secondary barrier with level difference for LNG containment system

Cryogenic Containment Systems for LNG (Liquefied Natural Gas) ships are required to satisfy durability and safety for 40years of operation. Membrane type LNG containment ships generally consist of three parts such as the primary barrier which is made of stainless steel, the secondary barrier which is made of composite or metal foil, and the insulation board which is made of polymer foam. The secondary barriers which are the face parts of sandwich insulation boards are adhesively bonded with metal strips to keep the gas tightness. When LNG is loaded in the containment system, large thermal stresses are generated in the metal strips due to the cryogenic temperature of LNG (−163°C). If there is a level difference between the insulation boards, the adhesively bonded metal strips are subjected to peel stress due to the level difference.In this work, FEM analysis was performed to calculate the critical peel load with the level difference of 3mm between the insulation boards due to the non-uniform flatness of the inner hull structure of LNG ships. To verify the adhesive strength, a new peel test method representing height differences of the secondary barrier was developed. Finally, the adhesive was reinforced with E-glass fiber to increase the peel strength of adhesive and the optimal areal mass of E-glass fiber reinforcement was determined by the new peel test method.

Cryogenic reliability of the sandwich insulation board for LNG ship

Liquefied natural gas (LNG) is mainly carried by the LNG ship, whose cargo containment system is composed of dual barriers against the leakage of LNG and insulation board. The insulation board of LNG containment system should have cryogenic reliability and high thermal insulation performance for safe and efficient transportation of LNG. The polyurethane foam (PUF) reinforced with glass fiber (called RPUF) has been used for the insulation board to increase the fracture toughness of PUF. However, the RPUF not only increased the manufacturing cost of LNG ship, but also decreased the thermal insulation characteristics of PUF, which is a remaining concern in LNG ship building industries.In this study, the surface of PUF was reinforced selectively at the area under high thermal stress with glass composite to decrease the production cost and to increase the thermal insulation characteristics. Finite element analysis was conducted to estimate the thermal behavior of insulation board at the cryogenic temperature and a new experimental method was developed to investigate the reliability of insulation board based on fracture mechanics.From the experimental results, it was found that the selective reinforcing of the insulation board with the glass composite increased considerably the reliability at the cryogenic temperature.

Cryogenic Performance of Adhesively Bonded Metal Joints for LNG Containment System

The cryogenic performance of adhesively bonded joints composed of aluminum sandwich structures and stainless steel foils for LNG (Liquefied Natural Gas) containment system was studied. Surface treatment processes such as mechanical treatment, flame treatment, and sulfuric acid etching were applied to the stainless steel foil, whose morphology and chemical composition were determined by SEM (Scanning electron microscopy) and XPS (X-ray photoelectron spectroscopy) with respect to surface treatment methods. Also, the surface free energies of the stainless steel foils were determined with respect to surface treatment conditions. The bond strengths of the single lap joints composed of aluminum sandwich structures and stainless steel foils were measured using modified test specimens at cryogenic temperature with respect to adhesive thickness and repeated thermal shocks, considering real application conditions. Finally, the applicability of metal foil for the barrier tightness of LNG containment systems was validated and an optimum flame treatment condition was suggested for improving the cryogenic performance of adhesively bonded metal joints with thin stainless steel foils.

Cryogenic reliability of composite insulation panels for liquefied natural gas (LNG) ships

The major carrier of liquefied natural gas (LNG) is LNG ships, whose containment system is composed of dual barriers and composite insulation panels. The LNG containment system should have cryogenic reliability and high thermal insulation performance for safe and efficient transportation of LNG. The secondary barrier composed of adhesive bonded aluminum strips should keep tightness for 15 days, when the welded stainless primary barrier fails. However, cracks are generated in the composite insulation panels due to the local stress concentration and the brittleness of insulation materials at the cryogenic temperature of −163 °C. If cracks generated in the insulation panel propagate into the secondary barrier, LNG leakage problem might occur, which is a remaining concern in ship building industries. In this study, crack retardation capability in the composite insulation panel was investigated with glass fabric reinforcement. Finite element analysis was conducted to estimate the thermal stress at the cryogenic temperature and a new experimental method was developed to investigate the failure of secondary barrier of composite insulation panel. From the experimental results, it was found that the glass fabric reinforcement was effective to retard the crack propagation into the aluminum secondary barrier from the polyurethane insulation foam at the cryogenic temperature.

Application of genetic algorithm to risk-based maintenance operations of liquefied natural gas carrier systems

The concept of genetic algorithm (GA) is used to model the cost of maintenance and repair of a liquefied natural gas (LNG) containment system and its transfer arm, after assessing the total risk of the systems using the probabilistic risk assessment technique. The failure frequency data of the basic events of the fault tree developed to model the LNG containment system and transfer arm, which is implemented and evaluated to estimate the failure frequencies of the systems, are derived from a careful literature search. A total risk formula is developed, which is dependent on hazard severity weight, failure frequencies, and the time and cost of maintenance and repair of the LNG carrier systems. The formula serves as the objective function, while new total cost allocated for the maintenance and repair of the LNG carrier systems as a whole is the constraint with the boundaries of presenting initial/unit cost of maintenance and repair of each containment system and transfer arm. Optimization is carried out on the objective function and its constraint for the identification of new cost of maintaining and repairing the containment system and transfer arm independently with the powerful tool of GA using Matlab version 7.7 software for improvement of the system's safety level.

A case study of the Flex LNG/Rift Oil floating LNG project in Papua New Guinea

Flex LNG Limited is a producer of units for the production, storage and off-take of liquefied natural gas (LNG). It currently has four of these units committed for construction by Samsung Heavy Industries in Korea, utilising the SPB LNG containment system. The world’s first floating liquefaction unit will be delivered to Flex in 2012. Floating LNG facilities have unique potential for monetising uncommitted gas reserves. In June 2008 Flex and Rift PLC entered into a co-operation agreement under which they agreed to work together to develop a floating liquefaction project offshore Papua New Guinea (PNG). The project will utilise Rift’s gas reserves and one of Flex LNG’s floating liquefaction units. Annual production capacity will be 1.5 million tonnes of LNG and start-up is targetted for 2012. The paper will be a case study of this project including: critical path, project structure and contractual matrix, upstream reserves and facilities, feed gas quantity and quality, pipeline issues and, key design parameters and liquefaction operations for the LNG producer. The paper will also cover: direct and indirect stakeholders in the project; economics and financing; PNG-specific issues such as geography, permitting, fiscal regime, local employment opportunities, marine conditions, infrastructure and sovereign risk; LNG demand in the Pacific and LNG marketing and off-take arrangements.