Liquefied Natural Gas Storage Research Articles

Research on temperature distribution in container ship with Type-B LNG fuel tank based on CFD and analytical method

The liquefied natural gas (LNG) fuel tank in a large container ship is loaded with liquid LNG at an ultra-low temperature (-163°C), there is a significant temperature difference in the cargo hold area where the entire fuel tank is located, which will have an important impact on the steel grade design and structural safety of the cargo tank in container ship. This paper develops two heat transfer models using Computational Fluid Dynamics method (CFD method) and an analytical method to analyze the temperature distribution in a large container ship equipped with Type-B LNG fuel tank. These models incorporate the arrangement and heat transfer characteristics of LNG fuel tanks. The temperature field analysis is conducted under typical the environmental conditions specified in the Code for the Construction and Equipment of Ships Transporting Liquefied Gas in Bulk (IGC Code) and the United States Coast Guard Code (USCG Code), based on the CFD method and the analytical method, and the results of temperature field distribution are compared. Additionally, a parametric analysis of the hull temperature field is further carried out, the results show that the thermal conductivity of the insulation layer in LNG storage tanks and the types of the loaded liquid cargo have a limited impact on the final temperature field distribution in hull structure. However, the selection of steel grade in the local structure of the cargo hold, especially in the inner hull part, may lead to significant changes.

A novel stability analysis method of single-layer ribbed reticulated shells with roof plates

The steel dome of liquefied natural gas (LNG) storage tanks comprises a curved roof plate welded onto a single-layer ribbed reticulated shell (SLRR shell). However, the roof plate may cause diaphragm effects, which may resulting in instability of the steel dome at a relatively low load level. In this paper, an innovative equivalent single-layer ribbed reticulated shell (ESLRR shell) was proposed to investigate the impact of the diaphragm effect. Specifically, the roof plate and H-shaped radial beam were equated as radial irregular beams based on the principle of plate-shell composite action. Next, we performed a systematic analysis of the instability mechanisms in the ESLRR shell, revealing the interaction between the beam buckling and the overall structural instability. Finally, through parametric study, we explored the effects of roof plate thickness and rise-to-span ratio on the ESLRR shell, and obtained the optimal structural parameters. The results show that the proposed method effectively solves the challenges associated with the presence of roof plates in LNG storage tanks steel dome stability analysis; Under the combination of weak boundary stiffness and high bending moment, radial irregular beams are more prone to buckling; Increasing roof plate thickness or rise-to-span ratio can reinforce the stability of the ESLRR shell. Overall, this research work provides theoretical guidance for the future design and optimization of LNG storage tank domes.

Experimental study of fracture behaviors of granite at ultra-low temperatures

A good understanding of fracture characteristics of rocks at ultra-low temperatures is essential in selecting and designing cavernous liquefied natural gas (LNG) storage facilities. In this study, three-point bending tests on Notched Semi-Circular Bend (NSCB) granite specimens with various grain sizes and saturation levels were conducted at temperatures of 25°C, −90°C, −120°C, and −165°C, respectively. The test results show that the fracture characteristics of granite are significantly weakened at −165°C under saturated conditions. As the temperature goes low, granite’s peak strength, fracture toughness, and fracture energy increase under dry conditions. A robust linear relationship between the mode-I fracture toughness (KIC) and the fracture energy of granite at ultra-low temperatures is established in this study. This relationship demonstrates that specimens with higher fracture toughness require more energy to reach their fractures. In addition, the relationship, along with its test constraints, forms a theoretical basis that can be used to explain the fracture damage modes and the characteristics of granite at different temperatures. Furthermore, granite at ultra-low temperatures was also investigated from the view of pores with frozen water and the change of particle sizes. The data obtained from our experiments can serve as valuable parameters for simulating underground LNG storage. The research findings from this study provide a valuable foundation that can be used for designing and installing cavernous LNG storage tank facilities.

Leakage monitoring and diagnosis of LNG storage tanks with temperature sensing network integration and artificial intelligence algorithm

Temperature is the most important safety monitoring indicator for leakage diagnosis during the operation phase of liquefied natural gas (LNG) storage tanks. Timely monitoring and accurate identification of LNG leakage events are crucial for accident prevention, loss reduction, and facility safety maintenance. This study integrates artificial intelligence (AI) algorithms and temperature sensing data to achieve intelligent monitoring and diagnosis of leakage in LNG storage tanks. Firstly, a comprehensive temperature sensing network is constructed by combining numerical simulation of the temperature field and temperature sensing experiments using fiber Bragg grating sensors. Secondly, Python is used to perform linear grid interpolation and flattening on the sensing network, generating 2D temperature nephogram samples that are conducive to AI algorithm recognition. Finally, sample features are extracted using machine vision, and leakage location calculation, leakage diagnosis and leakage volume calculation are implemented with the help of machine learning algorithms, achieving satisfactory accuracy on the test set. In addition, the ConvLSTM framework is introduced for deep learning and recurrent neural network training, enabling spatiotemporal prediction of the leakage area.

Cross-Scale Reliability Analysis Framework for LNG Storage Tanks Considering Concrete Material Uncertainty

The reliability of liquefied natural gas (LNG) storage tanks is an important factor that must be considered in their structural design. Concrete is a core component of LNG storage tanks, and the geometric uncertainty of concrete aggregate material has a significant impact on their reliability. However, owing to the significant size difference between the concrete aggregate compared to the LNG storage tank, structural analysis using an accurate finite element model that includes all the geometric characteristics of the aggregate incurs significant analytical costs. In particular, for reliability analysis requiring a large number of samples, the computational costs incurred by finite element models are infeasible. Therefore, a dual acceleration strategy based on the asymptotic homogenization method and surrogate model technology is proposed to improve the efficiency of LNG storage tank reliability analysis. In the cross-scale analysis of a LNG storage tank based on asymptotic homogenization, order reduction of the LNG storage tank analysis model was realized. Based on this, a surrogate model construction method with the aggregate fraction and mass moment as inputs was proposed to further accelerate the reliability analysis of LNG storage tanks. Subsequently, a Monte Carlo method was used to perform a reliability analysis of the LNG storage tank considering the uncertainty of the concrete aggregate geometry and distribution under the action of liquid weight and wind load. The analysis showed that the wind load has a significant influence on the safety of the design of the roof of a LNG storage tank. The directionality of the wind load has a significant impact on the distribution of the sample point response for reliability analysis and the failure mode of the LNG storage tank. Owing to the directionality of the wind load, the response distributions of the maximum displacement and maximum stress of LNG were more concentrated, and the reliability of the LNG storage tank decreased after considering the wind load. In particular, the stress reliability of the tank decreased by 5.86%. When only the liquid load was considered, the maximum displacement and stress exhibited asynchronous failure, and the two almost never occurred simultaneously. When the wind load was considered, the failure mode of the LNG storage tank was dominated by the maximum stress. Moreover, the numerical example also demonstrated that the degree of freedom involved in structural analysis, as well as the time of structural analysis can be significantly reduced. So, the proposed cross-scale analysis framework can significantly improve the efficiency of reliability analysis. The conclusions established in this study provide theoretical and methodological guidance for the reliable design of LNG storage tanks.

Study on the Comparison of Electron Beam Welding and Hybrid Laser-Arc Welding of Thick High-Manganese Steel Plate for Cryogenic Applications

Environmental protection policies have led to a focus on clean energy from Liquefied Natural Gas (LNG) and thus, its demand is increasing. Several studies for the development of cryogenic materials and fabrication technologies for storage and delivery of LNG are presented worldwide. Recently, International Maritime Organization (IMO) acknowledged that high-manganese steel was one such cryogenic material for application in LNG storage. Thus, high-manganese steel is expected to be increasingly used in the cryogenic storage industry because of its excellent material properties such as low-temperature toughness and tensile strength. A variety of researches on the welding technologies are available, including arc welding, laser welding and hybrid laser-arc welding. However, electron beam welding (EBW) has not been widely studied. In this work, EBW was investigated for the thick high-manganese steel plate and its mechanical properties were evaluated and compared with the characteristics of hybrid laser-arc welding. Our study indicates that EBW exhibits better mechanical properties than the hybrid laser-arc welding and it can be an effective potential candidate technology for welding of thick high-manganese steel plate with application in cryogenic storage tank.

Experimental investigation of liquid-tank interaction effects on full containment LNG storage tanks through shaking table tests

Liquefied natural gas (LNG) storage tanks play a crucial role as primary storage facilities in modern gas supply systems. This study investigates the seismic response of full containment LNG storage tanks through shaking table tests. A 1/14 scale model structure is meticulously constructed and subjected to testing. The test comprises two distinct phases: an empty tank phase and a water-filled tank phase. The primary emphasis of this paper is the exploration of the effects of liquid–tank interaction. A novel no-contact binocular video synchronization measurement system is developed by utilizing high-speed cameras to accurately measure the liquid sloshing height. The seismic responses of both the liquid and the tank, including acceleration and hydrodynamic pressure, are meticulously measured and analyzed. Findings reveal that the filling fluid has a significant effect on the tank, particularly the steel inner tank. The liquid sloshing phenomenon influences the hydrodynamic pressures, with the magnitude of influence decreasing as the liquid depth increases. In addition, a comparison is conducted between the experimental liquid sloshing height and hydrodynamic pressure results and those obtained from liquid linear forced sloshing theory. Liquid linear forced sloshing theory fails to accurately predict liquid sloshing height for large-amplitude sloshing but provides a rough estimate of hydrodynamic pressure distribution on the tank wall. Two simplified mechanical models for liquid sloshing are proposed and compared with the experimental results. The outcomes demonstrate that the Housner method exhibits relatively large errors, while the simplified mechanical models based on liquid linear forced sloshing theory accurately describe cases with small-amplitude liquid sloshing.

Liquefied Natural Gas Value Chain: A Comprehensive Review and Analysis

AbstractOver the past few decades, the liquefied natural gas (LNG) business has experienced astounding expansion and gained global relevance. LNG has emerged as a key element in meeting the world's rising energy demand while working to cut carbon emissions, since it is a cleaner and more adaptable energy source. The entire process from natural gas extraction to consumption is covered in the presented analysis of the LNG value chain, starting out by looking at the discovery and production of natural gas reserves, highlighting important innovations and methods used during the extraction stage. The procedures used to treat and purify natural gas in order to meet the strict standards for liquefaction are detailed. Analysis of the liquefaction process reveals numerous liquefaction technologies, e.g. the well‐known conventional liquefaction and the cutting‐edge floating LNG and small‐scale LNG facilities. LNG storage and transportation, as critical components in the value chain, are examined in great detail. When the LNG reaches its destination, regasification plays a crucial role in repurposing it as a gas for distribution and consumption. Regasification technologies, such as conventional onshore terminals, floating storage and regasification units, and land‐based regasification facilities, are covered. Concerns regarding methane emissions and the carbon footprint, the potential for reducing greenhouse gas emissions through new technology, the environmental impact of LNG, and the economics of LNG are also considered across its value chain.

Experimentally validated numerical analyses on the seismic responses of extra-large LNG storage structures

Extra-large liquefied natural gas (LNG) storage tanks are comprised by a steel inner tank and an outer protective tank of prestressed reinforced concrete (PRC), both being very thin. These important lifeline infrastructures are prone to damage and even failure under earthquake during their service life. Convincing numerical simulation, verified by shake table tests, is a feasible tool to quantify the seismic responses and to mitigate the potential damage/failure of LNG storage structures. This work addresses systematically the three-dimensional finite element (FE) modeling, experimental validation and numerical simulations of such structures, with the soil–pile interaction of the foundation, the fluid–structure interaction of the inner tank and damage of the outer tank all properly accounted for. For the sake of computational efficiency with no loss of too much precision, the soil–pile interaction is described by the mass–damper–spring model, and the fluid–structure interaction by the mass–spring model with both the convective and impulsive components accounted for. Moreover, the nonlinear mechanical behavior of concrete is considered by the damaged-plasticity model regularized with the fracture energy. The concrete–steel interaction in PRC is practically dealt with by modifying the constitutive relations of concrete and steel rebars/tendons properly with the equilibrium and compatibility conditions accounted for. The FE modeling strategy is validated against the recently conducted benchmark shaking table test of a scaled extra-large LNG storage structure. The capability in capturing the overall dynamic responses, e.g., the hydrodynamic pressure and structural vibrations under seismic actions, etc., is sufficiently verified. Finally, the dynamic responses and structural reliability of an extra-large LNG storage tank under deterministic and stochastic seismic actions are numerically studied in details.

Application of Hybrid Laser Arc Welding for Construction of LNG Tanks Made of Thick Cryogenic 9% Ni Steel Plates

AbstractHybrid laser-arc welding (HLAW) was applied for butt welding of 14.5 mm thick plates of ferritic cryogenic steel X8Ni9 containing 9% Ni, which is used for manufacturing storage and transport facilities of liquefied natural gas (LNG). The weld seam formation and the achievable metallurgical and mechanical properties of the hybrid welds were investigated experimentally for two types of filler wire, an austenitic wire dissimilar to the base metal (BM) and an experimentally produced matching ferritic wire. Safe penetration and uniform distribution of the austenitic filler metal in the narrow hybrid weld could only be achieved in the upper, arc-dominated part of the weld. The pronounced heterogeneous distribution of the austenitic filler metal in the middle part and in the root area of the weld could not ensure sufficient notched impact toughness of the weld metal (WM). As a result, a decrease in the impact energy down to 17 ± 3 J was observed, which is below the acceptance level of ≥ 34 J for cryogenic applications. In contrast, the use of a matching ferritic filler wire resulted in satisfactory impact energy of the hybrid welds of up to 134 ± 52 J at the concerned cryogenic temperature of -196 °C. The obtained results contribute to an important and remarkable conversion in automated manufacturing of LNG facilities. In other words, the results will help to develop a new laser-based welding technology, where both quality and productivity are considered.The efficiency of the developed welding process has been demonstrated by manufacturing a prototype where a segment of the inner wall of large size LNG storage tank was constructed. In this concern, hybrid laser arc welding was conducted in both horizontal (2G) and vertical (3G) positions as a simulation to the actual onsite manufacturing. The prototype was fabricated twice where its quality was confirmed based on non-destructive and destructive examinations.

A Comprehensive Review on Material Compatibility and Safety Standards for Liquid Hydrogen Cargo and Fuel Containment Systems in Marine Applications

As the maritime industry’s emphasis on sustainable fuels has increased, liquid hydrogen (LH2) has emerged as a promising alternative due to its high energy density and zero-emission characteristics. While the experience of using natural gas in ships can serve as a basis for the introduction of hydrogen, the different risks associated with the two fuels must also be considered. This review article provides a methodology for selecting suitable metal materials for shipboard LH2 storage and piping systems based on operational requirements. The effects of both liquid and gaseous hydrogen environments on metal materials are first comprehensively reviewed. The minimum requirements for metal materials in liquefied natural gas (LNG) storage systems, as stipulated in the IGC and IGF codes, were used as a baseline to establish minimum requirements for liquid hydrogen. The applicability of austenitic stainless steel, a representative metal material for cryogenic use, to a liquid hydrogen environment according to nickel content was examined. In order to apply liquid hydrogen to the marine environment, the minimum requirements for liquid hydrogen were organized based on the minimum requirements for metal materials in the LNG storage system covered by the IGC and IGF codes. Finally, to expand the material selection criteria for low-temperature cargo and fuel storage facilities at sea, slow strain tensile testing, fatigue life, and fracture toughness considering the hydrogen environment and cryogenic temperature were derived as evaluation items.

Conceptual design of floating storage and regasification unit (FSRU) for Eastern Part Indonesia

Liquefied Natural Gas (LNG) can be used as an alternative fuel with environmentally friendly solutions. In general, the domestic distribution of natural gas in LNG is more cost-effective than transporting gas through underwater pipelines considering Indonesia as archipelagic country. According to Indonesia’s Minister of Energy and Mineral Resources Regulation Number 13 of 2020, the government supports converting 52 power plants to LNG. However, the current challenge in the LNG industry is the lack of supporting facilities. Storage and regasification facilities are among the infrastructure that requires optimization. Floating Storage and Regasification Unit (FSRU) are viable options for energy distribution that offer opportunities and competitiveness compared to onshore terminals. Small and medium-scale FSRU can serve as an alternative solution due to their relatively fast, flexible, and affordable development process. Therefore, this research will develop a small-medium FSRU design with a capacity of 60 MW − 70 MW in Eastern Indonesia. FSRU design should consist of four segments including location considerations and environmental conditions, powerplants gas requirements, LNG storage tank sizing and LNG regasification process. Calculations for various supporting FSRU components, seakeeping analysis, loading-offloading processes, 3D modelling and the integration of solar panels were discussed. The results obtained include the principal dimension, technical aspects of FSRU design, and the technology for LNG regasification processes used.

Phased Array Ultrasonic Testing for Inspection of LNG Storage Tank

Liquefied natural gas (LNG) is liquefied at -162°C at atmospheric pressure and stored in the inner shell. Due to the transport and storage characteristics of such low temperature conditions, 9% high nickel steel, which shows excellent strength and toughness even at low temperatures, is used for the inner wall. On the other hand, the base material of the inner wall has a tempered martensite structure, whereas the weld metal has an Inconel- based austenite structure with high damping properties in acoustics. Also, the weld metal is composed of an anisotropic structure completely different from the base material. These are considered obstacles in non-destructive testing. We applied automated phased array ultrasonic testing(PAUT) as a replacement of radiographic testing in inspection of LNG storage tank. To obtain the reference value data, a reference specimen for each thickness was prepared, and an automatic scanning device was designed and manufactured. Also, a customized phased array(PA) probe was developed and applied. All parts are integrated and operated as one system, and the data is analyzed and stored through an operating program developed in-house. The current system has proven its performance through field application to a much-improved level in the existing scanning device, probe and application method. In conclusion, it was proved that our technology can be commercialized by showing excellent detection ability.

Study on the seismic mitigation effects of inerter isolated storage tanks

Liquefied Natural Gas (LNG) storage tanks can ensure national energy sufficiency, but they are vulnerable to earthquakes. Although ordinary seismic isolation bearings can effectively reduce the base shear force and overturning moment, they tend to amplify the height of the sloshing wave. In this paper, a hybrid vibration control method for LNG storage tanks is proposed, paralleling the series viscous mass damper (SVMD) devices with Lead-core rubber bearings (LRB). The seismic mitigation effects of this parallel seismic isolation scheme were investigated. First, three-dimensional finite element models of fixed base, LRB isolated and SVMD isolated tanks were established, and nonlinear time history analyses were performed for the three types of tanks to compare the seismic responses. It was concluded that SVMD can effectively control the sloshing wave height. In addition, the effects of site types, storage liquid heights, and SVMD's performance parameters on the seismic mitigation effects were also studied. It was found that the softer the site soil, the higher the storage liquid height would make seismic isolation less efficient. The increase of SVMD's additional equivalent mass reduces the sloshing wave height, increases the shear-to-weight ratio, and has little influence on the base displacement. The increase of SVMD's additional equivalent damping reduces the sloshing wave height, base shear force, and base displacement of the tank. The final conclusion is that SVMD can effectively control the sloshing wave height and has a good seismic mitigation effect.

A multi-zone thermodynamic model for predicting LNG ageing in large cryogenic tanks

During the storage and transportation of liquefied natural gas (LNG), generation of boiloff gas (BOG) and change in the LNG composition, also known as LNG aging, are inevitable. Thus, a thermodynamic model that can accurately predict LNG aging during long-term operation is critical. This study presents a novel aging model, in which the LNG tank is divided into five zones: the vapor phase region, thermal stratification, bulk liquid region, boundary layer, and surrounding thermal insulation region. The heat and mass transport into different regions were calculated separately, and the thermal conductivity of the solid walls was considered as a function of temperature. Following model validation, simulations were performed for various thermal insulation conditions, ambient temperatures, and initial liquid compositions. From the results, the errors of the pressure, BOG rate and total heat flux could be approximated to 17, 7, and 14% respectively, for the previous models omitting the variation of thermal conductivity with temperature. As the initial methane fraction increased (i.e., N fraction decreases), the BOG rate decreased at the initial stage, while the BOG rate became less sensitive with the initial LNG composition as evaporation proceeded. Such an effect is determined by the variation in the latent heat of vaporization and total heat loss. This study can provide guidance for the design and operation of LNG storage tanks.

Numerical study on the effects of bund on liquid pool spreading and vapor dispersion after a catastrophic LNG tank failure

Liquefied natural gas (LNG) storage tanks are usually equipped with bunds to provide an additional layer of protection. However, the catastrophic failure of an LNG storage tank usually results in a bund overtopping. The overtopped LNG will evaporate rapidly at ground surface to form the flammable gas cloud. Previous studies have developed computational fluid dynamics (CFD) models to investigate the ability of bunds to retain liquids; However, the models were developed based on ambient liquid, ignoring the evaporation phenomena and vapor dispersion of LNG. In this work, a multiphase CFD model which integrated with the evaporation model was proposed to simulate the complete overtopping process (including LNG overtopping, LNG spread, LNG evaporation and vapor dispersion). The data from two field experiments were used to validate the model. In addition, this paper studied a case of catastrophic failure that derived from an industrial LNG Tank. The effect of the bund configurations on the wind flow field, the overtopping fraction, the spread of LNG and the dispersion of LNG vapor cloud was analyzed. The present work can help relevant personnel to better assess the engineering risks associated with catastrophic failure of storage tanks for decision making in the process safety framework.

Research on digital twin based temperature field monitoring system for LNG storage tanks

The temperature distribution is a critical indicator of the health condition of liquefied natural gas (LNG) storage tanks. Point measurements are the main method of traditional LNG tank temperature field monitoring. To obtain more abundant temperature data and realize the digital transformation of temperature field monitoring, an LNG tank temperature field monitoring system was constructed, which consists of a model layer, a data layer, and an application layer, by combining sensing data with digital twin (DT) technology. First, the temperature distribution of the LNG storage tank under working and leaking conditions was simulated, and the sensing monitoring network was constructed by combining Fiber Bragg Grating (FBG) sensor temperature-sensing experiments. Second, a finite element calculation model of the steady-state temperature field of an LNG storage tank was developed, and the twin model was generated by fusing the sensing data to realize the inversion from local sensing data to the whole temperature field. Finally, an LNG temperature field monitoring system based on the DT was constructed. Based on physical field leakage simulations performed with the Fluent software, the functions of visual interaction, construction splitting, temperature chart analysis, leakage diagnosis, and historical data management of the twin models were realized, which provided system building samples for digital monitoring of new LNG storage tanks.

Energy, Exergy, and Economic (3E) Analysis of Boil-Off Gas Re-Liquefaction Systems Using LNG Cold Energy for LNG-Fueled Ships

Liquefied natural gas (LNG)-fueled ships have the effect of reducing most pollutants, which is advantageous for responding to strict regulations. Because boil-off gas (BOG) is generated in the LNG storage tank of an LNG-fueled ship, a BOG re-liquefaction system is required. The representative systems for LNG-fueled ships were proposed by Kwak and Shen, but their exergy efficiencies were only 19.6% and 24.9%, respectively. To improve the system, this paper proposes novel BOG re-liquefaction systems combined with the fuel gas supply system. The systems utilize LNG cold energy in the BOG stream and N2 reverse Brayton cycle, respectively. The proposed systems were simulated using a commercial program and were optimized using a genetic algorithm. The results of energy, exergy, and economic (3E) analyses performed for comprehensive evaluation of the proposed system show that the system in which LNG cold energy is applied to the BOG stream has the best performance. Specific energy consumption, exergy efficiency, and total annual costs of this system were improved by up to 78.6%, 69.2%, and 68.2%, respectively, compared to those of the existing systems. The overwhelmingly superior system is expected to greatly contribute to the improvement of the BOG re-liquefaction system for LNG-fueled ships.

Sizing-Based Flaw Acceptability in Weldments Using Phased Array Ultrasonic Testing and Neural Networks

Liquefied Natural Gas (LNG) is one of the major renewable energy sources and is stored and carried in a storage tank that is designed following international standards. Since LNG becomes highly unstable when it encounters oxygen in the air, a leakage from an LNG storage tank can cause a catastrophic industrial accident. Thus, the inspection of LNG storage tanks is one of the priorities to be completed before LNG is stored in a storage tank. Recently, the usage of Phased Array Ultrasonic Testing (PAUT) has been gradually increasing as the risks of RT emerge. PAUT has some obstacles to overcome in order to substitute RT, such as efficiency and accuracy. Specifically, the cost issue must be addressed. Therefore, many attempts to combine PAUT with Artificial Neural Networks (ANN) have been made. PAUT provides many types of 2D images of the inspected weldment. The S-scan is one of the 2D images provided by PAUT, and it displays the cross-sectional view of the specimen with a single transducer. The inspectors examine the S-scan image and other provided images of PAUT to detect, classify and size the flaw that exists in the weldment so that the decision of whether the inspected weldment with the flaw is acceptable can be made. Nowadays, most of the previous research on PAUT and ANN focuses on detecting and classifying the flaws in B-scan or S-scan images. However, the last step to determine the flaws’ acceptability is not yet covered. In this study, the flaw acceptance criteria of PAUT in various international standards are listed. EXTENDE CIVA is used to create the PAUT S-scan images. The S-scan images are labeled with the listed acceptance criteria. Then, they are used in Mask R-CNN training. After the training, some new S-scan images with flaws are used to test the performance, and this showed 96% precision and 87% recall. With the algorithm, the acceptability of a flaw in a weldment can be determined efficiently and it will reduce the burden of PAUT usage and reduce the time required for a full-length inspection.

Effect of Ultralow Temperature on Expansion, Strength, and Modulus of Cement-Stabilized Soil: Case Study for LNG Underground Storage in Singapore

Singapore has a relatively high water table. When liquefied natural gas (LNG) storage systems are built in soils below the water table in Singapore, the engineering properties of the surrounding soils could be improved through cement stabilization. Due to the extremely low temperature (T) of LNG (approximately −162°C), the effect of ultralow T on the physical and mechanical properties of cement-stabilized soil in case of accidental LNG leakage should be investigated. However, existing studies on cement-stabilized soils are limited to T above −20°C. Therefore, in this study laboratory experiments to quantify volumetric expansion, unconfined compressive strength (UCS), and Young’s modulus (E) of cement-stabilized soils at −40°C, −80°C, and −120°C were conducted. The experimental results indicated that the stabilized soils’ volumetric expansion reached ≤2.6% at −120°C. The UCS and E of the stabilized soils increased from 0.9 to 27.6 MPa and from 99.8 to 2,669 MPa, respectively, and the T decreased from 24°C to −120°C. In summary, the ultralow T induced limited volumetric expansion (<3%) for cement-stabilized soil and significantly increased its strength and E, which indicated that cement soil stabilization could be beneficial for underground LNG storage systems in Singapore when exposed to accidental LNG leakage.