Min Sung Chun

A Comparative Evaluation of Fatigue and Fracture Characteristics of Structural Components of Liquefied Natural Gas Carrier Insulation System

density R-PUF (130kg/m 3 ) and high density R-PUF (210kg/m 3 ). The fracture toughness of R-PUF and STS 304L was investigated in terms of the density effect of R-PUF and the difference in the nickel composition of STS 304L, STS 304L (10.2%Ni) versus STS 304L (9.4%Ni) at both ambient and cryogenic temperatures. In this study, the high density R-PUF (210kg/m 3 ) and STS 304L (9.4%Ni) were proposed to improve the structural strength of the LNGC insulation system and reduce the cost. The fracture toughness was characterized in terms of the critical strain energy release rate (GIC) in the context of linear elastic fracture mechanics (LEFM). The geometries of the fracture toughness test used were the center-cracked tension (CCT) and double-edge-cracked tension (DECT) specimens according to ASTM STP381 standard. [DOI: 10.1115/1.4007473]

A Structural Capacity Evaluation for Corrugated Membrane Structure of LNG Cargo Containment System

Corrugated membrane structure is adopted as 1st barrier of membrane type LNG cargo containment system of LNG carrier to reduce thermal stress caused by cryogenic temperature of LNG. The corrugated structure, however, can lead to buckling and large deformation followed by plastic strain at a local area of the 1st barrier under sloshing load. Therefore in order to evaluate structural capacity of the corrugated membrane, structural response against various environmental loading such as hull deformation, sloshing load as well as thermal loading should be investigated. In this study, failure criteria for corrugated membrane is proposed based on rupture strain of the material considering the objective of the 1st barrier of LNG cargo containment system with respect to functionality. And the structural responses of corrugated membrane, for each environmental loading, are carefully evaluated. To evaluate structural response under sloshing load and thermal loading, a series of finite element analysis is carried out and plastic strains for each loadings condition are estimated. In addition, to investigate the effect of plastic strain developed during metal forming process for corrugated membrane on structural response, plastic strain on corrugated membrane developed by metal forming is measured through real manufacturing process. By comparing accumulated plastic strain due to thermal, sloshing load as well as material forming to material rupture strain obtained from material tensile test, structural capacity of the corrugated membrane structure of LNG cargo containment system is evaluated.Copyright

Cryogenic Fatigue Strength Assessment for MARK-III Insulation System of LNG Carriers

The objective of this study is to investigate the typical failure mode and to obtain the stress range versus number of cycles to failure (S-N) data of MARK-III type liquefied natural gas (LNG) insulation system under the fatigue loading at actual cryogenic environment. A systematic experimental research is carried out for the assessment of the fatigue strength of MARK-III insulation system at cryogenic temperature. Three different types of test specimens are tested for the evaluation of fatigue performance of MARK-III insulation system. Test specimens are determined considering the fatigue vulnerable locations such as mastic area, slit area, and top bridge pad area inside the actual LNG cargo tanks. All test specimens are fabricated as close as possible to the actual yard practice. A series of fatigue test results is represented as S-N curves. Cyclic fatigue loadings were carefully considered similar to the actual sloshing loads. The effect of sloshing impacts is considered by selecting the stress ratio (R=−10). The load levels have been determined based on the ultimate strength of reinforced polyurethane foam as 12.2 bars. Different cryogenic temperatures are employed according to the test locations in consideration of temperature gradient within the insulation system. All test results including fatigue life, as well as failure locations of MARK-III insulation system at cryogenic temperatures, are reported and compared with those at room temperature. Consistent S-N curves of MARK-III insulation system at both room and cryogenic temperatures are obtained and compared. The slopes of S-N curves from both fatigue test results are observed to be almost identical, and the fatigue strengths are found to exhibit similar trend. The results from this research can be used for the fatigue assessment of the LNGC insulation system, as well as a design guideline of LNG CCS at cryogenic temperature.

Fatigue Damage Model for Numerical Assessment of Fatigue Characteristics

The aim of this study is the development of a numerical technique applicable for the fatigue assessment based on the damage mechanics approach. The generalized elasto-visco-plastic constitutive equation, which can consider the internal damage evolution behavior, is developed in order to numerically evaluate the material fatigue responses. Explicit information of the relationships between the mechanical properties and material constants, which are required for the mechanical constitutive and damage evolution equations, are derived. The performance of the developed technique has been verified using the S-N relationship assessment for STS304 stainless steel.

A Comparative Study on the Impact Damage of Membrane Type LNGC Insulation System

To verify the structural safety under impact load caused by sloshing of LNG is one of the main issues in the design of membrane type LNG cargo containment system of LNG carrier. In order to estimate structural response under sloshing impact load, many kinds of studies including experimental test and numerical simulation have been done by various research groups such as shipyards, oil companies, universities as well as classification societies. In spite of these efforts, many uncertainties still exist to predict the structural behavior of LNG insulation system under sloshing impact load. Therefore, it can be regarded as a challenge to investigate dynamic response of LNG cargo insulation system against sloshing load. In this paper, Cooperative research results obtained by SHI-HHI-PNU-Lloyd-ABS-DNV JDP focused on the impact damage or failure mode of membrane type LNG cargo insulation system are summarized. A systematic experimental research is carried out to find out failure mode of the insulation system under impact load and criteria which can be applied for the design of LNG carrier. A series of dry drop tests as well as static compressive tests are carried out. The structural behavior of the specimen under impact load is recorded using ultrahigh-speed camera and reaction force is measured using load cells which are installed under bottom of the test facility. By analyzing recorded video, deformation history of the specimen at impact moment is obtained. The numerical simulations are also carried out for the dry drop test for verification purpose, It is expected that the insights observed from the systematic experiments and numerical simulations for the structural response of the LNG cargo insulation system subjected to the impact load can be effectively used as design guide for evaluation of the integrity of structural components of LNG cargo hold system.Copyright

Fatigue and Fracture Performance of Insulation System in LNG Carriers

With the increase of the size of LNG (Liquefied Natural Gas) carriers, LNG cargo containment systems are exposed to more frequent severe sloshing impact actions. In design and construction of LNG carriers, dynamic failure of LNG cargo containment system (CCS) under repeated sloshing impact actions is a very important issue [1]. While the importance of dynamic strength characteristics of LNG cargo containment system is well recognized, no consideration with regard to the fatigue and fracture performance of insulation system toward the cryogenic environment is readily available. In this respect, a systematic experimental research is carried out for the assessment of fatigue and fracture strength of LNG insulation system at both ambient and cryogenic environments. This study begins with the fatigue test of LNG insulation system. In particular, the fatigue characteristic at the secondary barrier is explicitly considered. The effect of the density of R-PUF (Reinforced Poly-Urethane Foam) with respect to fatigue performance is investigated. In the later section, the fracture toughness of LNG Insulation system is characterized in terms of the critical strain energy release rate (GIC ) in the context of linear elastic fracture mechanics (LEFM). The specimen geometries used in this study are the center-cracked and double-edge-cracked tension specimen according to ASTM standard [2]. Fracture toughness tests have been carried out for structural components of MARK III type LNG insulation system such as Reinforced Poly-Urethane Foam (Insulation material) and SUS 304L (Primary barrier) under ambient and cryogenic temperature.Copyright

Damping Effect of Reinforced Polyurethane Foam under Various Temperatures

Reinforced polyurethane foam (RPUF) is one of the important materials of Mark III type insulation systems used in liquefied natural gas (LNG) cargo containment systems. However, RPUF is the most difficult material to use with regard to its safety assessment, because there is little public and reliable data on its mechanical properties, and even some public data show relatively large differences. In this study, to investigate the structural response of the system under compressive loads such as sloshing action, time-dependent characteristics of RPUF were examined. A series of compressive load tests of the insulation system including RPUF under various temperature conditions was carried out using specimens with rectangular section. As a result, the relationship between deformation of RPUF and time is linear and dependent on the loading rate, so the concept of strain rate could be applied to the analysis of the insulation system. Also, we found that the spring constant tends to converge to a value as the loading rate increases and that the convergence level is dependent on temperature.

Assessment of Cryogenic Fatigue Strength of MARK-III LNG Carrier Insulation System

The importance of dynamic strength characteristics of LNG cargo containment system, various research efforts including dry-drop and wet-drop tests as well as sloshing test have been carried out by GTT, shipyards, oil companies, universities as well as classification societies. However, no explicit consideration with regard to the structural performance of insulation system toward the cryogenic environment is yet given. Besides only limited information regarding experimental procedure and analysis results is shared among stakeholders, and it acts as obstacles for the development of consistent CCS (Cargo Containment System) design criteria in an effective manner. Demand for LNG carriers with increased capacity requires further development of the design technology for LNG cargo tanks to meet the safety requirements due to the larger size LNG tanks. In this study, a systematic experimental research is required for the assessment of fatigue strength at cryogenic environment as well as impact strength of insulation system. The different sized mastic support has been adopted to compare the cryogenic fatigue performance. One of the main challenges in this work is to generalize the test results for use in case of other mastic support sizes. The aim of the study is to investigate the typical failure mode and obtain the S-N data under cryogenic fatigue loading. This study will be used as a fundamental study for the cryogenic fatigue assessment for the LNGC insulation system as well as a design guideline. The fatigue tests of Mark-III insulation system are carried out at room temperature and cryogenic temperature, and considered sloshing effect (R = −10). The load levels have been determined based on ultimate strength of reinforced polyurethane foam 12.2bar.Copyright

An Experimental Study on Damping Effect of RPUF Under Compressive Load

Reinforced polyurethane foam (RPUF) is one of important materials of Mark III type insulation system used in LNG cargo containment system. But RPUF is the most difficult material to deal with in the numerical approach for the safety assessment, because there is little public and reliable data on its mechanical properties, and even some public data have relatively large difference between them. In this study, in order to investigate the structural response of the system under compressive loads such as sloshing action, time dependent characteristics of RPUF were examined by experiments. A series of compressive load tests of the insulation system including RPUF under various temperature conditions was carried out by using specimens with rectangular section. As the results, the relationship between deformation of RPUF and time is linear and dependent on the loading rate, so the concept of strain-rate could be applied to the analysis of the insulation system. Also, it was found that the spring constant tends to converge to a value as the loading rate increased, and the convergence level is dependent on temperature.Copyright

Effect of Residual Stress on the Mechanical Capacity of Layered Composite

For enhanced mechanical properties of ceramics for structural application, a great deal of attention has concentrated on preparation of layered composites. In this study, numerical simulation technique, which is applicable for the evaluation of mechanical performance for layered composite material, was developed. A generalized material constitutive equation coupled with material damage model based on the continuum damage mechanics approach was proposed and has been implemented to an in-house type finite element analysis code. The material behaviors for each component of layered composites can be simulated by the pre-defined material model on the simulation. A series of finite element analyses was carried out in order to elucidate the effect of fabrication related residual stress on the structural capacity of the layered composites.