Sloshing Model Tests Research Articles

Long-term approach for assessment of sloshing loads in LNG carrier, part II: Grouping method

Assessing the sloshing loads has been a significant issue in designing the cargo containment system (CCS) of a liquefied natural gas carrier (LNGC). The sloshing problem contains numerous physical and technical uncertainties. The long-term approach has been suggested to reduce the possible uncertainties, but it has not been a feasible option owing to the incalculable number of experimental cases. Daewoo Shipbuilding and Marine Engineering Co., Ltd., Hyundai Heavy Industry Co., Ltd., Samsung Heavy Industry Co., Ltd., Korean Register, and Seoul National University conducted extensive sloshing model tests for the 174 K S-LNGC. To consider the long-term approach, Part I describes the differences between short- and long-term approaches based on the guidelines of different classification societies. To address the significant number of experimental conditions required for the long-term approach, a grouping method is investigated (Part II). Although this grouping method is typically used in practical design, it has not been validated, and its applicability is yet to be determined. In Part II, the sea states are classified using 16 different grouping methods. The long-term exceedance probabilities and pressures are obtained individually and then compared with the results obtained from fully considering all sea states. After performing the general sloshing model test of the shipbuilding industry, the sloshing impact pressure of a liquefied natural gas cargo model is measured via 6DoF irregular simulations, where the upper section of the cargo hold structure is considered. Long-term pressures are locally and globally investigated and compared among 16 different sea states and all the sea states. Based on the results, the best grouping method is suggested for the long-term approach.

Long-term approach for assessment of sloshing loads in LNG carrier, Part I: Comparison of Short- and Long-term approaches

To assess the long-term sloshing loads of a liquefied natural gas carrier, Daewoo Shipbuilding and Marine Engineering Co., Ltd., Hyundai Heavy Industry Co., Ltd, Samsung Heavy Industry Co., Ltd. (SHI), Korean Register, and Seoul National University conducted a series of joint industrial experiments using a six-degree-of-freedom irregular sloshing model test. SHI provides a 174 K S-LNGC and its cargo hold information. To consider the numerous test conditions of the long-term approach, the present study focuses on the upper regions of the cargo hold structure, and feasible operating scenarios are assumed. A total of 3870 h at a real scale are simulated based on 387 sailing conditions. Extensive experimental results of sloshing loads are analyzed based on different guidelines suggested by different maritime classification societies. Part I of this study provides an assessment of sloshing loads based on a comparison of short- and long-term approaches distinguished by the guidelines, and the possible reference values are presented. A practical solution for the long-term approach will be discussed in Part II.

Outlier analysis of sloshing impact loads on liquid ship cargo

This paper introduces an outlier analysis which can improve the convergence of the statistical analysis results of sloshing model test data. The paper classify possible outliers in the sloshing model test into three categories and present a treatment method for each outlier. The developed outlier analysis is adapted to the model test results for the cargo of the liquefied-natural-gas (LNG) carrier in operation. The results of the present new method are compared with those of the conventional procedure, particularly focusing on long-term sloshing prediction. Through this study, the effectiveness of the present method is observed, and it is found that the present method provides is robust and reliable results in the application of experimental data for load prediction.

Data mining in sloshing experiment database and application of neural network for extreme load prediction

Database of the sloshing model test has been mined. More than 540 terabytes experimental data have been accumulated for various cargo holds, vessels, environmental conditions, operational conditions, and experimental conditions. The database was organized, cleaned, and analyzed for the floating units larger than standard size LNG carriers or LNG fueled vessels. The selected target data was used for the machine learning to predict the model test results from the test conditions. An artificial neural network has been developed. Many different types of parameters were scaled and transformed as the input attributes followed by the optimization of the hyperparameters and the architecture. The network predicted the test results that were not used in the training process. The prediction results were validated according to the changes in the environmental conditions, operational conditions, and model dimensions. The accuracy of the network was acceptable to be applicable to the designing perspective.

Structural safety assessment procedure for membrane-type LNG CCS considering hydroelasticity effect

As environmental regulations have become more onerous, the demand for LNG and LNG carriers has increased. The LNG cargo containment system (LNG CCS) is one of the most important facilities in LNG carriers, and many membrane types of LNG CCS have been developed so far. Traditionally, sloshing model tests are performed and a series of statistical approaches are used to obtain design sloshing pressures. Then, these design loads are utilized to analyze the structural safety of LNG CCS. In the sloshing model test, the flat and almost rigid plate is used as the LNG CCS instead of a real model, and thus, the hydroelasticity effect cannot be considered. In the present research, the effect of hydroelasticity on sloshing pressure was investigated in a fluid-structure interaction simulation. A wet drop simulation was conducted, and its results subsequently were compared with wet drop experimental test results to ensure their validity. Then, two types of structure model, namely a flat-rigid plate model and a flat-flexible CCS model, were generated to investigate the effect of hydroelasticity. Also, a fluid hitting analysis model was devised to realize the sloshing phenomenon, and the two generated structure models were applied as the structure domain in the analysis. In the fluid hitting simulations, it was found that the hydroelasticity effect on sloshing pressure is significant. Thus, correction factors, which are quantitative values of the effect of hydroelasticity, were derived (and are proposed herein). Finally, a structural safety assessment procedure for consideration of the hydroelasticity effect was derived (and is suggested herein).

Observation on sloshing flow and hydrodynamic pressures on cylindrical liquefied natural gas tank with swash bulkhead

In this study, a series of sloshing model tests were conducted for type-C tanks, particularly to observe the effects of the inner bulkhead and rings. In regular pitch motion, the internal flow by swash bulkhead and rings located inside the tank was observed. The frequency range near the resonance frequency was checked at filling heights of 70%, and sloshing-induced impact pressures were investigated. Through this study, the global flows inside the tank and local flows during impact occurrence at the hemispherical end of the tank were systematically observed, and the impact pressure pattern for each frequency ratio was compared. Due to the swash bulkhead located in the center of the tank, the flow does not move at once and the velocity of the flow is reduced by the inner rings. The flows passing through the swash bulkhead proceed with a time difference, overlapping with the first wave, generating various types of sloshing impact. The results of computational fluid dynamics calculation and the experiment were also compared for limited conditions.

Experimental Study and Method of Sloshing Model Test Considering Gas-Liquid Density Ratio

Experimental Study and Method of Sloshing Model Test Considering Gas-Liquid Density Ratio

Database of model-scale sloshing experiment for LNG tank and application of artificial neural network for sloshing load prediction

Seoul National University has conducted a considerable number of six degree-of-freedom irregular small-scale sloshing model tests 1/70–1/25 scales, particularly focusing on the tanks of liquefied natural gas (LNG) carriers. An experimental database has been created to provide information of sloshing load severity, which are obtained from a lot of the post-processed experimental results. In this paper, the summary of the database is described. The artificial neural network is trained based on the database to predict sloshing load severity. Various attributes that affect experimental results are considered. Management of these attributes and the machine learning architecture are illustrated. The prediction results are validated for several experiments that are not included in the training process. Further possibilities of using the database for model test planning and cargo hold design are discussed.

Investigation of the Motion Accuracy Influence on Sloshing Model Test Results

Investigation of the Motion Accuracy Influence on Sloshing Model Test Results

LNG-solid impacts with gas cushioning and phase change

LNG-solid impacts with gas cushioning and phase change are investigated theoretically in a coupled inviscid liquid and viscous gas regime. Condensation from the gas to the LNG is driven by local increases in the gas pressure above the saturation vapour pressure. This is modelled as a sink term on the kinematic boundary condition at the gas interface. To leading order, the bulk liquid motion is unaffected by condensation, with its evolution governed by the same boundary integral equation used in models of non-volatile gas-cushioned liquid–solid impacts. The proposed model extends the approach used to describe two-dimensional non-volatile gas-cushioned impacts by incorporating phase change and is applied to a range of physically relevant LNG-solid impacts associated with sloshing. As an LNG free-surface approaches touchdown with a solid wall, a gas pressure build-up occurs in the gap separating liquid from solid, which decelerates and deforms the liquid free-surface. This deformation of the free surface may result in gas entrapment. In non-volatile impacts, pockets of trapped gas are associated with oscillatory pressure signals, while previous experiments have shown that these oscillations may be damped by phase change in impacts involving volatile liquids. Compared to impacts with non-volatile liquids, gas condensation is shown to reduce both the impact pressures and the volume of gas trapped. Depending on the impact parameters, the proposed model differentiates between cases where a pocket of trapped gas may or may not be formed. A criterion on the critical normal impact velocity above which gas entrapment is not expected is obtained. This indicates that across a range of length scales that are physically relevant to LNG sloshing, gas entrapment is not expected for impact velocities greater than 0.05 m s−1. Impacts where gas compressibility is important are investigated, as well as impacts into corners of containment tanks with varying angles. The model developed is suitable for the analysis of small and medium-sized LNG-solid impacts, as well as larger-scale sloshing model tests involving impacts of water cushioned by water vapour. The importance of inertia in the gas is identified in larger scale impacts.

Sloshing design load prediction of a membrane type LNG cargo containment system with two-row tank arrangement in offshore applications

This paper addresses the safety of two-row tank design by performing the extensive sloshing model tests. Owing to the uncertainties entangled with the scale law transforming the measured impact pressure up to the full scale one, so called comparative approach was taken to derive the design sloshing load. The target design vessel was chosen as 230 K LNG-FPSO with tow-row tank arrangement and the reference vessel as 138 K conventional LNG carrier, which has past track record without any significant failure due to sloshing loads. Starting with the site-specific metocean data, ship motion analysis was carried out with 3D diffraction-radiation program, then the obtained ship motion data was used as 6DOF tank excitation for subsequent sloshing model test and analysis. The statistical analysis was carried out with obtained peak data and the long-term sloshing load was determined out of it. It was concluded that the normalized sloshing impact pressure on 230 K LNG-FPSO with two-row tank arrangement is higher than that of convectional LNG carrier, hence requires the use of reinforced cargo containment system for the sake of failure-free operation without filling limitation.

The effect of liquid viscosity on sloshing characteristics

A series of sloshing model tests for various viscous liquids were carried out and the effects of liquid viscosity on sloshing are investigated quantitatively in this paper. The experimental results show that the liquid viscosity has an important effect on sloshing pressure, which is the mean value of those peak pressures from several consecutive periods. Energy dissipation due to viscous friction leads to reduction of sloshing pressure and the dissipation effect is more remarkable, especially when liquid viscosity becomes higher. As a result of viscous damping effect, the rising time of impact pressure is longer in higher viscosity liquid.

Effect of ullage gas on sloshing, Part I: Global effects of gas–liquid density ratio

Previous analytical, numerical, and experimental studies have proved that properties of ullage gas influence sloshing wave impacts and induced pressures. One of those properties is ullage gas density (ρg), also considered in dimensionless form as gas–liquid density ratio (DR). Previous studies intended to study the effects of DR sometimes mixed DR effects with gas compressibility and ullage gas pressure effects and attributed them only to DR. This study is based on experiments and is meant to focus only on the effects of DR far from impact zones which will be addressed as global effects. Effects of DR near impact zones and before detection of any compressibility effects are addressed as local effects and will be treated in part II. Quantitative and statistical comparisons will be presented in part III.Test setup consisted of two 2D tanks as transverse slices of tank 2 of a membrane LNG carrier with total capacity of 152000 m3 at scales 1:20 and 1:40 at 20% fill level. Using two liquids of water and sodium polytungstate (SPT) and different ullage gases of helium (He), air, two mixtures of sulfur hexafluoride (SF6) and nitrogen (N2), and pure SF6, all at atmospheric pressure, provided a range of DRs from 0.0002 to 0.0060.For the tested range of DR, the global effects of DR on sloshing geometry (free surface) are small when comparing at the same scale or at two different scales. Small discrepancies exist even between two exact repetitions with the same DR at the same scale. Global flow keeps the same phase regardless of tested DRs which implies that all breaking wave impacts take place at the exact same time instants, considering a small time window (usually smaller than 100 ms). This was confirmed by verifying the recorded impact times.Based on this, scaling in sloshing model tests as well as the effect of changing the ullage gas can be investigated impact by impact as opposed to the solely statistical approaches adopted so far. It also helps to track down impacts measured at full-scale (on board the ship) and to further verify whether sloshing model tests are representative for them or not. The stochastic nature of sloshing can be studied more in depth also with the help of high-speed video recordings for corresponding wave impacts. Reevaluations of the current statistical sloshing assessment methodologies can be envisaged.

Comparison of Sloshing Pressures in 2D and 3D Tanks

This paper introduces the experimental results of sloshing model tests carried out at Seoul National University. Two 1/50-scale tanks, i.e., 2D and 3D models with the same shape, were manufactured for the comparative study. Particular interest was taken in the differences in impact pressures between the 2D and 3D models. Regular motion tests were conducted for different filling depths. For each filling depth, 500 cycles of regular excitation were imposed at each frequency. To observe the characteristics and severity of sloshing from the acquired pressure data, statistical analyses were performed, not only for the peak pressure, but also for the area-concept indices, which represented the amount of impulse.

Statistical analysis of sloshing-induced random impact pressures

This article presents the statistical analysis of sloshing-induced random impact pressure. To obtain the pressure signal, three-dimensional sloshing model tests were conducted at two different filling depths. Several different methods were applied to identify sloshing peaks and to define the rise and decay times of the peak pressure signals. Statistical properties acquired from these methods were compared, and their consistency and/or discrepancy were observed. In addition, 200-h duration test data were acquired by running forty 5-h tests under each test condition. Based on the long-duration test data, the uncertainty of the test results with respect to the test duration was studied using the bootstrap method.

LNG 화물창 단열구조의 슬로싱 충격응답 간이해석법에 관한 연구

To ensure structural integrity of membrane type LNG tank, the rational assessment of the sloshing impact responses of tank structures should be preceded. The sloshing impact pressures acting on the insulation system of LNG tank are typical irregular loads and the resulting structural responses show very complex behaviors accompanied with fluid structure interaction. So it is not easy to estimate them accurately and immense time consuming calculation process would be necessary. In this research, a simplified method to analyse the dynamic structural responses of LNG tank insulation system under pressure time histories obtained by sloshing model test or numerical analysis was studied. The proposed technique based on the concept of linear combination of the triangular response functions which are the transient responses of structures under the unit triangular impact pressure acting on structures. The validity of suggested method was verified through the example calculations and applied to the dynamic structural response analysis of a real Mark III membrane type insulation system using the sloshing impact pressure time histories obtained by model test.