LNG Cargo Tanks Research Articles

Study of Applicability of Triangular Impulse Response Function for Ultimate Strength of LNG Cargo Containment Systems under Sloshing Impact Loads

The LNG cargo containment system used in membrane-type LNG cargo tanks must have sufficient dynamic strength to withstand the impact of sloshing loads. However, performing direct dynamic nonlinear transient finite element assessments against design sloshing impact loads with different design specifications can be complicated and time-consuming. To address this, it is effective to use linear superposition methods, such as the triangular impulse response function (TIRF) method, to conduct dynamic transient FE assessments of LNG cargo containment systems. However, as LNG cargo containment systems have a high level of nonlinearities in terms of geometry, material, and boundary effects, it is necessary to evaluate the applicability of the TIRF method in advance. This study investigates the dynamic responses of an LNG cargo containment system using the TIRF method and compares the ultimate value of the structural responses and impulses with that obtained using direct dynamic nonlinear transient assessments. Based on a comparison of a series of FE analyses, the study proposes a design for the partial safety factors for calculating the ultimate bending and shear capacities of an LNG cargo containment system, taking into consideration the dynamic impact of sloshing loads using the TIRF method. Finally, the ultimate shear and bending capacities are calculated using the proposed method and compared with those obtained through direct dynamic nonlinear transient assessments. The results show that the proposed method provides conservative estimates against direct nonlinear finite element simulations, with a difference of around 10% for the mean minus two standard deviations. This approach can be practically applied for early basic design purposes in the shipbuilding industry.

Evaluation of the ventilation system in an LNG cargo tank construction platform (CTCP) by the AHP-entropy weight method

Evaluation of the ventilation system in an LNG cargo tank construction platform (CTCP) by the AHP-entropy weight method

멤브레인형 LNG선박의 리퀴드 돔 체어 구조개발

A membrane type LNG cargo tank is equipped with a pump tower and a liquid dome for loading and unloading of LNG. However, the membrane running continuously on the tank wall to prevent leakage of LNG is interrupted by the liquid dome, hence care should be taken in the design of liquid dome and its substructures. In case of GTT NO96 membrane type cargo containment system, chair structure is arranged along the periphery of the liquid dome targeting to support the membrane which is exposed to the both hull girder and thermal load. This paper proposes a new and simple chair structure, which outperforms traditional design from productivity point of view maintaining same level of structural safety. Strength assessment on the new design was performed to guarantee the structural safety of the new design, which includes strength, fatigue and crack propagation analysis.

Computational study of LNG evaporation and heat diffusion through a LNG cargo tank membrane

The seepage of the cryogenic LNG through the cracks in membrane insulation may result in cold spots within the hull structures of LNG vessels. At the cold spot, the hull structure will be crystalized and brittle. Also, the loss of LNG containments may lead to a potentially significant hazard. The estimation of the temperature distribution along the insulation panel and the heat diffusion speed through the insulation materials are crucial to assess the safety of LNG vessels. The prediction of the flow and thermal behaviors of the leaked LNG requires complex multiphase flow numerical simulation. The CFD (computational fluid dynamics) model is proposed to simulate the diffusion behavior and the heat transfer characteristics of leaked LNG including the liquid-to-gas phase change through porous structure. The CFD model considers phase change, gas–liquid reactions in the porous media and the accompanied rates of heat transfer. It also considers the geometry of NO96 membrane storage facilities with glass wool and plywood. In the numerical simulation, the LNG pool spreading, heat diffusion, and the evaporation are investigated. The simulation indicates that the predicted speed of seepage is too high to evaporate the LNG after the leakage.

기체-액체 밀도차에 대한 슬로싱 충격압력의 실험적 고찰

This paper presents the results of sloshing experiments having different fluids in model tanks with various density ratios. The experimental model consisting water and air at ambient, which has been commonly used, is not consistent in density ratio with that of an actual LNG cargo tank. Therefore, an advanced experimental scheme is developed to consider the same density ratio of LNG and NG by using a mixed gas of sulfur hexafluoride (<TEX>$SF_6$</TEX>) and nitrogen (<TEX>$N_2$</TEX>). For experimental observation, a two-dimensional model tank of 1/40 scale and a three-dimensional model tank of 1/50 scale have been manufactured and tested at various conditions. Two different fillings with various excitation frequencies under regular motions have been considered for the two-dimensional model tank, and three different filling levels under irregular motions have been imposed for the three-dimensional model tank. The density ratio between gas and liquid varies from the ratio of the ambient air and water to that of the actual LNG cargo container, and the different composition of gas is used for this variation. Based on the present experimental results, it is found that the decrease of sloshing pressure is predicted when the density ratio increases.

선수파 중 슬로싱을 고려한 병렬배치된 두 부유체의 거동 특성에 관한 실험 연구

The motions and drift forces of side-by-side moored FSRU and LNGC including the sloshing effect, were studied using experiments. The FSRU and LNGC contained LNG cargo tanks and the LNG sloshing could affect the motions and drift forces of the structures due to its coupling with floating body motion. The effect of coupling can vary with the LNG filling level, and the effect of the filling level was investigated. The coupling effect was stronger at lower filling level. It was confirmed that longitudinal sloshing influenced the surge and surge mean drift force in head sea. In addition, gap flow affected the sway and mean drift forces. Sloshing attenuated the sway and yaw excited by gap flow in side-by-side configuration.

GTT MARK III 스테인리스강 STS304L의 파이버 레이저 용접특성

레이저는 고밀도 열원으로써, 입열을 최소화하면서 고속용접이 가능한 장비이다. 특히 최근 상업화된 고출력 파이버 레이저는 특유의 발진원리 때문에 높은 빔품질과 소형화된 시스템이 가능하다. 이러한 장점들로 인해 파이버 레이저는 LNG 카고탱크에 대해 선박 내부에서 용접이 가능한 가장 적합한 열원이라고 판단된다.<BR> 따라서 본 연구에서는 선박산업에 레이저 용접기술을 적용하기 위해 파이버 레이저를 이용하여 LNG 선용 스테인리스강의 용접을 진행하였다. 연구에서 사용한 재료는 카고탱크에 사용되고 있는 오스테나이트계 스테인리스강인 두께 1.2mm의 STS304L였다. 레이저 출력 및 용접속도를 변화시켜가면서 겹치기 및 맞대기 용접을 실시한 후 용입특성을 분석하여 최적의 접합조건을 도출하였다. 그 결과 용접부에서 모재보다 우수한 기계적 성질을 얻을 수 있었다.

Analysis of the sloshing flows of a LNG cargo tank

The sloshing flows in a LNG cargo tank model (1/50 scale) of a ship are measured by an embedded panoramicPIV system. The measurement system consists of a Nd-Yag laser(120mJ, 15Hz), two cameras(1k x 1k) and a host computer. Four experimental cases were tested for the tank model, in which swaying motions are made by a 6 DOF-motion platform. The amplitudes of swaying are 9.76mm and 29.29mm, and the swaying frequencies are 0.633Hz and 0.828Hz. The measurement regions are the vertical plane 50mm away from the front wall of the tank where a pump tower is installed. It was verified that the flow patterns of the sloshing are similar to each other when the swaying amplitudes are similar.

Predicting Resistance of Spherical-Type LNG Carrier Structures to Ship Collisions

The modern design of steel structures uses limit states classified into four types, namely, serviceability limit state, ultimate limit state, fatigue limit state and accidental limit state. The aim of this paper is to propose and illustrate a procedure to assess the capability of a spherical-type LNG carrier of 135 000 m3 to resist ship collisions. An assessment procedure based on the energy dissipation capability relating to structural crashworthiness can, in this case, be applied to protect spherical LNG cargo tanks from fracture in minor or moderate collision accidents as shown in the paper. In the procedures developed, while the loss of kinetic energy during the collision accident is approximately estimated by applying the momentum equilibrium of the two implicated ships, a series of relatively sophisticated numerical computations is under- taken to simulate the structural crashworthiness of the object ship in the full load condition at standstill, varying the loading conditions and types of the striking ship, when the LNG carrier side structure is considered to be struck by the bow of either a VLCC or another LNG carrier. Based on the computed results, the collision resistance indices (CRI), which in the present study are defined as a function of the ratio of the structural energy dissipation capability to the initial kinetic energy loss, are calculated for the object ship for various collision scenarios. Some important insights useful for preventing collision damage of the spherical LNG cargo tanks of the ship are also discussed.

Properties of thick 5083-0 aluminum alloy plates for the equatorial ring of a spherical LNG cargo tank

The characteristics of a newly developed 160-mm thick 5083-O aluminum alloy plate have been examined with particular emphasis placed on the through thickness (ST) properties. Elongation of the plate in the ST direction is higher than for any other available data. The fatigue properties are also satisfactory and fatigue strength in the ST direction is 11 kgf/mm2 at 107 cycles. The deep notch test result demonstrates that the plate behaves in a fully ductile manner even with a load in the ST direction at −196 °C. The tendency of the plate to microfissuring in HAZ is very low. All these excellent properties are attributed to the improved internal soundness of the material.