Leakage characteristics of the glass fabric composite barriers of LNG ships

The consumption of LNG (Liquefied Natural Gas) has recently increased due to a substantial rise in the price of petroleum. One of the major carriers of LNG is the LNG ships whose containment systems are composed of corrugated stainless steel plates (primary barriers) and glass fabric composite sandwich constructions (secondary barriers). The primary barriers are constructed by welding many corrugated thin stainless steel plates to reduce thermal stress, while the secondary barriers are constructed by adhesively bonding glass fabric composite sandwich constructions.One of the key technologies for the secondary barriers is to thoroughly seal the adhesive joining area, which can retard LNG leakage when the primary barriers are failed. The sealing quality of the adhesive joint is dependent on the wetting characteristics between the sandwich constructions and adhesive, which is in turn dependent on the curing cycle for the adhesive.In this work, a new method to measure the gas leakage of adhesively bonded joint was devised. The adhesive joint specimens were prepared under several different curing cycles to investigate the impregnation of adhesive into the glass fabric composite. Also, the thermal residual stress in the adhesive joint was estimated by the double strip deformation experiment. Finally, an improved curing method was developed with high tightness of the secondary barriers without increase of thermal residual stress.     

基于液化天然气(LNG)冷量的废旧橡胶低温粉碎工艺流程

探讨了废旧橡胶低温粉碎中LNG冷量的利用问题.用空气作为中间冷媒,将LNG冷量先传给空气,再由空气去喷吹冷却胶粉.设计了此冷却换热的工艺流程,进行了物料及冷量的衡算,以实现冷量的有效利用.     

Design of the composite sandwich panel of the hot pad for the bonding of large area adhesive films

A light-weight hot pad system for curing large area adhesive films for the secondary barrier of cryogenic cargo containments of Liquefied Natural Gas (LNG) has been developed with a composite sandwich panel.In order to apply uniform pressure to the adhesive on unlevel insulation panels and to obtain an adequate adhesive thickness, a flexible stainless steel foil heater supported by a butyl rubber air pressure bag has been utilized for the lower part of the hot pad system, and a temperature controller provides reliable curing of the adhesive. To decrease the weight of the hot pad system and mitigate heat loss through the pad, the upper part of the hot pad system has been built with a composite sandwich panel composed of glass fiber epoxy composite face, polystyrene (PS) and polyvinyl chloride (PVC) foam cores with low thermal conductivity.Through finite element analysis and experimentation on hot pad systems, a light-weight hot pad system that is able to cure a 0.3 m by 3.5 m area has been developed with an autoclave cure quality.     

Analysis of temperature and pressure changes in liquefied natural gas (LNG) cryogenic tanks

Liquefied natural gas (LNG) is being developed as a transportation fuel for heavy vehicles such as trucks and transit buses, to lessen the dependency on oil and to reduce greenhouse gas emissions. The LNG stations are properly designed to prevent the venting of natural gas (NG) from LNG tanks, which can cause evaporative greenhouse gas emissions and result in fluctuations of fuel flow and changes of fuel composition. Boil-off is caused by the heat added into the LNG fuel during the storage and fueling. Heat can leak into the LNG fuel through the shell of tank during the storage and through hoses and dispensers during the fueling. Gas from tanks onboard vehicles, when returned to LNG tanks, can add additional heat into the LNG fuel. A thermodynamic and heat transfer model has been developed to analyze different mechanisms of heat leak into the LNG fuel. The evolving of properties and compositions of LNG fuel inside LNG tanks is simulated. The effect of a number of buses fueled each day on the possible total fuel loss rate has been analyzed. It is found that by increasing the number of buses, fueled each day, the total fuel loss rate can be reduced significantly. It is proposed that an electric generator be used to consume the boil-off gas or a liquefier be used to re-liquefy the boil-off gas to reduce the tank pressure and eliminate fuel losses. These approaches can prevent boil-off of natural gas emissions, and reduce the costs of LNG as transportation fuel.     

Prediction of liquefied natural gas (LNG) densities from new experimental dielectric constant data

A concentric cylinder capacitor has been used to measure the orthobaric liquid dielectric constants of multicomponent mixtures of the major components of liquefied natural gas (LNG) to an accuracy of approximately ± 0.05% at temperatures from 110 to 130 K. These mixtures ranged from a ternary mixture containing nitrogen, methane, and normal butane to four to eight component methane rich (74 to 90 mol %) mixtures containing up to 5 mol % of nitrogen, 16 mol % of ethane, 7 mol % of propane, 5 mol % of the butanes, and 0.44 mol % of the pentanes. Some of these mixtures were prepared to simulate commercial LNG compositions. Experimental densities previously reported for these mixtures have been combined with the mixture dielectric constant data to calculate values of the Clausius-Mossotti (CM) function and the excess CM function. Pure component experimental CM functions for LNG components except for propane and isobutane have been combined with the mixture data in the development of a simple calculational technique for the prediction of LNG densities to an uncertainty of approximately ± 0.15% based on a knowledge of the composition and dielectric constant of the liquid mixtures. In fitting the data, pseudo values of the CM function are derived for the slightly polar components, propane and isobutane, while constraining the mixture excess CM function to be zero.     

Numerical model to predict deformation of corrugated austenitic stainless steel sheet under cryogenic temperatures for design of liquefied natural gas insulation system

Austenitic stainless steel exhibits nonlinear hardening behavior at low temperature and under various strain rate conditions caused by the phenomenon of transformation-induced plasticity (TRIP). In this study, a uniaxial tensile test for 304L austenitic stainless steel was performed below ambient temperature (−163, −140, −120, −50, and 20 °C) and at strain rates (10⁻⁴, 10⁻³, and 10⁻² s⁻¹) to identify nonlinear mechanical characteristics. In addition, a viscoplastic damage model was proposed and implemented in a user-defined material subroutine to provide a theoretical explanation of the nonlinear hardening features. The verification was conducted not only by a material-based comparative study involving experimental investigations, but also by a structural application to the corrugated steel membrane of a Mark-III-type cargo containment system for liquefied natural gas. In addition, an accumulated damage contour was represented to predict the failure location by using a continuum damage mechanics approach.     

Fatigue crack propagation behavior in AISI 304 steel welded joints for cold‐stretched liquefied natural gas (LNG) storage tank at cryogenic temperatures

AISI304 steel welded joints are used in cold‐stretched liquefied natural gas (LNG) storage tanks used for storing and transporting of liquefied gases. Compared with a conventional liquefied natural gas storage tank, a cold‐stretched liquefied natural gas storage tank has many advantages such as reduced thickness, light weight, low cost and low energy consumption. However, liquefied natural gas storage tanks can be subjected to alternative loads at cryogenic temperatures; thus, it is important to investigate the fatigue crack propagation behavior in AISI 304 steel welded joints at cryogenic temperatures. Specimens were machined from a cold‐stretched liquefied natural gas storage tank with a welding structure. The crack length was determined using compliance method and confirmed by examination with traveling microscope. Fatigue crack propagation rates were evaluated at various stress ratios and temperatures. The fatigue crack growth rate of all specimens a little appears the effect of stress ratio, but it has a great influence at a cryogenic temperature. The fatigue crack growth rate of longitudinal welded joint is the fastest at room and cryogenic temperature. Fracture mechanism in the specimen is examined using a scanning electron microscope.     

Interaction between plasticity and damage in the behaviour of [+φ, −φ]n fibre reinforced composite pipes in biaxial loading (internal pressure and tension)

In this paper, we propose a new model which describes the behaviour of [+φ, −φ]_n composite laminates. Tests were performed on glass-epoxy pipes subjected to biaxial tensile and internal pressure loading. Experiments showed that [+55, −55]_n pipes exhibit varying types of damaged elastoplastic behaviour depending on the stress ratio σ_zz/σ_θθ (axial stress/hoop stress). A plastic model is based on the definition of a yield criterion and an associated flow rule. Damaging occurs when transverse microcracks appear in the layer. A micromechanical model defines the anisotropy of the damage. Interaction between plasticity and damage was of major importance in the definition of damage kinetics. This effect was observed on proportional loadings as well as on sequential tests: a preliminary loading in pure internal pressure (σ_zz=0) induced large plastic phenomena which blocked crack propagation in additional internal pressure with closed ends effect (IPCEF) tests (R=σ_zz/σ_θθ=1/2), even though IPCEF caused considerable damage on an unloaded specimen.