# Experimental and Numerical Study of Orifice Coefficient of Cargo Tank Design of LNG Vessels
## 6 Conclusions  
The purpose of this study is to examine the validity of the design criteria applied in the design of liquid cargo storage systems of LNG vessels. This study examined the experimental and numerical results of the design of liquid cargo tanks, on the orifice coefficient of drip trays considered as a major design factor. In order to investigate the relationship between the leakage flow and the orifice coefficient in case of cracks in the tank, the distribution of leakage quantity was determined through an experiment in which the amount of leakage was directly measured for various crack areas by using a pressure tank filled with water. To ensure safety when designing the independent LNG tanks, a method of estimating the flow rate during drip tray design was studied. The flow rate of liquid cargo was measured by cracks using the pressure tank filled with water. Rectangular crack specimens were used, and changes in flow rate and orifice coefficients were investigated depending on the size of the crack.  
Figure 19: Orifice coefficients according to the pressure condition.  
Figure 20: Orifice coefficients according to crack area.  
Furthermore, to overcome an experimental limitation, the CFD method was applied to calculate the amount of leakage flow generated from the LNG filled tank by crack areas.  
The orifice coefficient was found and calculated to vary with the cross-sectional area of the crack and the loading conditions of the liquid cargo. Therefore, the leakage from the cracks depended on the shape and size of the cracks. The orifice coefficients varied from 0.22 to 0.59 depending on the size of the cracks. Additionally, looking at the friction and viscosity effects of fluids at the point where the walls of the fluid and cracks were tangent, it was confirmed that the friction effects of the fluid were significantly issued below a certain crack size. In this study, it was found that the area of cracks was 14 mm\({}^{2}\) or less, which had a frictional and viscous effect. Furthermore, the effects were negligible in areas over 14 mm\({}^{2}\). This proves that if the area of a crack is less than a certain size, the amount of discharging fluid can vary depending on its area, size, and shape. It can also be explained that there is an effective size that viscosity and friction of the fluid affect, depending on the crack condition.  
According to the guideline presented by the international guidance, it is described that a value of 0.1 for the orifice coefficient has been found to give good results compared with the test data [1]. However, the study confirmed that if the area of the crack is smaller than a certain size, the orifice coefficient changes nonlinearly due to the friction of the fluid. It was also found that the value of the coefficient was significantly greater than the value given in the guidelines. Based on the results of this study, it is, therefore, suggested that the orifice coefficient for the design of the LNG fluid tank needs to be reviewed.  
[PERSON] and [PERSON] suggested the concept of orifice coefficient of fluid cargo tank of LNG vessels; [PERSON] and [PERSON] designed and performed experimental measurements; [PERSON] analyzed the data and suggested a CFD model for estimating the orifice coefficient; [PERSON] and [PERSON] wrote the paper. All authors have read and agreed to the published version of the manuscript.  
This research was funded by KIAT(Korea Institute for Advancement of Technology), No. 10063532.  
This work was supported by the KIAT (Korea Institute for Advancement of Technology) grant funded by the Korea Government (Ministry of Trade Industry and Energy, No. P0001968; HRD program for R&D Human Resource Education Program for Green & Smart Ship and Industrial material core technology program, No. 10063532; development of steel application technologies against ice-induced crashworthiness and arctic temperature high toughness).  
The authors declare no conflict of interest.