Thermal Study on LNG Cargo Tank

Thermal Study on LNG Cargo Tank Li Rong*1, Jin Wenbing2, Zhang Juyong3, Jin Jialei4, Xu Endao5 School of Mechanical Engineering, Hangzhou Dianzi University, Hangzhou, China, 310018 *1

lirongjx@hdu.edu.cn; 2644973807@qq.com; 3zhang_juyong01@163.com; 4jinjialei005@163.com; 5xuendao90@126.com

Abstract The LNG (Liquefied Natural Gas) cargo tank is a key component in the maritime transport of ‐163℃ LNG. The cryo‐insulation performance of cargo tank directly affects the security of LNG transport greatly. Based on a 147000m3 LNG cargo tank, a structural type of the LNG cargo tank is certain, which includes its materials and parameters. With the safety requirements of cryopreservation of ‐163℃ LNG, the temperature fields and the thermal stress distribution of LNG cargo tank with full load are simulated by a 3d mathematic model by finite element software. Lastly, a cooling scheme of LNG cargo tank is completed according to its thermal performance. The cooling method has some practical meaning to some engineering.

Keywords LNG; Cargo Tank; Heat‐insulating Layer; Thermal Design

Introduction Nowadays, as one of the three pillars of the world energy, natural gas is the most important part in the world. At present, natural gas become the fastest growing energy with an average annual growth rate 20%, though it cannot meet the demand of the market, which has to rely on imports. LNG (Liquefied Natural Gas) is a product of natural gas, which can solve the transportation from overseas origin. Since the boiling point of LNG is ‐163℃ under standard pressure, it needs maintain the ultra‐low temperature during its transportation. The LNG cargo tank is used to load the ‐163℃ LNG, which must be protected strictly from ocean shipping environment. Therefore, the cryo‐ insulation performance of cargo tank directly affects the security of LNG transport greatly. And the thermal design of cargo tank becomes its design point.

Based on a 147000m3 LNG cargo tank, a structural & thermal design of the cargo tank is put out. In order to ensure its security of cargo tank, under thermal design requirements, the thermal performance of cargo tank must be checked in order to make out its cooling scheme.

LNG Cargo Tank In order to ensure the safety and efficiency of the

LNG ship during its transportation, there are many strict requirements on its cargo tank in the design of LNG ship. At present the common types of LNG ship are as follows.

Firstly, LNG ship with film type cargo tank. There is a main shield wall with a very thin film to construct a cargo tank. The cargo tank is supported by an insulating layer through the hull. For the sake of safety, there are dual insulating layers outside of cargo tank.

The outside of cargo tank are two layers, which make of heat insulating material. And the inside of cargo tank is a thin film, which make of metal film.

“Technigaz” and “Gaz Transport” are two typical LNG film cargo tanks, which are shown in FIGURE 1 (a) and (b). There is a leakproof interlayer in the LNG film cargo tank; it needs not to calculate thermal stress distribution of cargo tank. On the other hand, the thin film of metal can reduce the weight of the cargo tank, and can shorten the time of precooling before loading LNG.

(a)“Technigaz”film (b)“Gaz Transport”film (c)global FIGURE 1. THREE KINDS OF LNG CARGO TANK

Secondly, LNG ship with global type cargo tank. This cargo tank belongs to “B type” independent liquid cargo tank, which is used almost exclusively for liquefied natural gas ship. And recently it is widely used in LNG ship. Usually, there are 4~6 global cargo tanks on one LNG ship. For each one, there is a global main shield wall of metal to construct a cargo tank.

And each one is protected with insulating system on its outside. The global cargo tank is supported by a cylindrical tubular ball at its equator. Therefore, all the weight of LNG cargo tank is supported by its plate membrane through the hull. Two kinds of LNG cargo tank above are different in characteristics, design difficulties and practicability. Their advantages and disadvantages are list in Table 1 in detail.

International Journal of Automation and Control Engineering, Vol. 4, No. 1—April 2015 5 2325‐7407/15/01 005‐4 © 2015 DEStech Publications, Inc. doi:10.12783/ijace.2015.0401.02

6 Li Rong, Jin Wenbing, Zhang Juyong, Jin Jialei, Xu Endao

TABLE 1. COMPARISON OF TWO TYPE CARGO TANKS Item Construction cost Space utilization rate Driving vision Air resistance Course stability Anti‐collision ability

Global cargo tank high low Big blind area large poor good

Polyurethane foam and expanded perlite are two kinds of material to apply into LNG cargo tank based on comprehensive consideration. Therefore, two design schemes of LNG cargo tank are set up clearly as follows.

Film cargo tank low high Small blind area small good good

TABLE 3. PROPERTIES OF INSULATION MATERIALS USED IN LNG CARGO TANK

Compared with construction cost, space utilization rate, driving vision, air resistance, and course stability, and anti‐collision ability, film cargo tank is better with comprehensive performance. Considering film cargo tank is widely applicable nowadays, this paper focuses on thermal and structural design of LNG film cargo tank.

Thermal Bulk Water conductivity density absorption (W/ m▪K) (kg/ m3) rate (%)

Item Rigid polyurethane foam Polystyrene foam Expanded perlite

Cryogenic Insulation Design of LNG Film Cargo Tank Cryogenic insulation design of LNG film cargo tank is to reduce the heat from environment into tank with conduction, convection and radiation. Based on the cryogenic insulation design of tank, it can ensure the LNG in its tank at ‐163℃ with normal pressure levels during transformation.

Combustion

0.19

30~40

1.5

Self‐ extinguishing

0.043

79

29‐30

Nonflammable

0.051

112‐115

0.5

Nonflammable

Foam glass

0.037

25‐35

2. 0

PVC foam

0.031

30‐70

1.0

Phenolic foam

0.031

30‐40

4.0

Self‐ extinguishing Self‐ extinguishing Fire‐retardant

First scheme: there is a 1.2mm stainless steel plate to make of the main film. And the membrane is made of glass fiber cloth, aluminum foil and glass fiber cloth.

Primary insulation layer and secondary insulation layer are made of reinforced polyurethane foam block.

The structure of insulation layers are shown in FIGURE 2 and Table 4.

With the reference of independent LNG cargo tank, it is necessary to cover an insulation layer on cargo tank.

How to design its appropriate thermal insulation material and its thickness becomes the important content of its cryo‐insulation design. The insulation materials used in LNG cargo tank must meet the needs listed in Table 2.

TABLE 2. REQUIREMENTS OF INSULATION MATERIALS USED IN LNG CARGO TANK Item

Requirement

Thermal conductivity

small

Anti‐absorbent hygroscopicity

excellent

Resistance to water vapour permeability

excellent

Low thermal expansion coefficient

small

FIGURE 2. DIAGRAM OF CARGO TANK INSULATION STRUCTURE

Flame retardant

excellent

TABLE 4. DETAILS OF INSULATION STRUCTURE

In addition to the above, insulation material should also meets good chemical stability, high mechanical strength, cheap price and convenient construction and other requirements. Polyurethane foam, expanded perlite, foam glass, polystyrene foam, PVC foam, phenolic foam are commonly used as insulating materials in LNG ship largely. Their physical properties are shown in Table 3.

a b c d e f g

material Stainless steel film Laminate Enhanced foam Aluminum reinforced plate Enhanced foam Laminate Hull shell plate

Thickness(mm) 1.2 12 100 1 170 9 15~22

Second scheme: there is a 0.7mm 6% nickel alloy non‐ expanded plate. And the membrane is made of plywood box filled with perlite powder. The structure

Compared with the thermal insulation material in Table 3, each insulation material has its advantage.

Thermal Study on LNG Cargo Tank 7

of insulation layers are shown in FIGURE 3 and Table 5.

Compared with advantages and disadvantages of the two schemes above, considering with their temperature field distributions come from finite element models, the second scheme of LNG cargo tank is better of a long‐term trend.

TABLE 6. COMPARISON OF TWO INSULATION SCHEMES Second insulation scheme (1) insulation is cheap (1) low stress but should and simple; be consistent with the (2) welding is relatively assembly tolerance; simple; Advantages (2) thin film with 1.2mm (3) two film layers are thickness increases the same; strength; (4) anything on it can be removed; (1) the INVAR is very (1) the insulation layer expensive; will be destroyed by (2) it takes a long time to any removal because of Disadvantages install the film; its glue assembly; (3) INVAR thin film is (2) the foam is very fragile, it cannot be expensive; touched; Item

FIGURE 3. DIAGRAM OF CARGO TANK INSULATION STRUCTURE TABLE 5. DETAILS OF INSULATION STRUCTURE

a b c d e

material 36% nickel steel Wooden insulating layer 36% nickel steel Wooden insulating layer The cabin floor

Thickness(mm) 0.7 230 0.7 300 ‐‐‐

Thermal Simulation of LNG Cargo Tank With the help of “Ansys” finite element software, the numerical simulation of LNG cargo tank “first scheme” is finished. And its effectiveness is also verified. In view of LNG tank is axial symmetry structure, the numerical model of tank is simplified symmetrical 1/4 part during its thermal analysis. The element type is solid 87, after setting material properties and thermal conductivity, the chamber wall temperature is ‐163℃, and the outer wall temperature is 29℃. Applying the boundary conditions, the temperature field distribution of LNG cargo tank is shown in FIGURE 4. Similarly, with the help of the same software to set up a numerical simulation of LNG cargo tank of “second scheme”, with the same boundary conditions, the temperature field distribution of LNG cargo tank is shown in FIGURE 4.

First insulation scheme

Cooling Schemes of LNG Cargo Tank Based on insulation design of LNG film cargo tank, the cooling scheme of LNG cargo tank before loading is also important to LNG transportation. There are two cooling schemes below.

First cooling scheme: cool the pipeline to ‐120℃~‐ 110℃ with liquid nitrogen before LNG ship arrives.

Then, cool and affuse small flow LNG after LNG ship’s arrival. Till cargo tank is filled, the LNG cargo tank is cooled with spray ring cooling system.

Second cooling scheme: preliminary cool and replacement the pipelines, storage tank/vault/annulus with evaporation gas firstly. When the replacement of natural gas content is more than 30% and the temperature of standpipe reaches ‐120℃, the replacement is finished. Finally, cool and affuse small flow LNG into pipeline and cargo tank. Two cooling schemes are compared in Table 7.

TABLE 7. COMPARISON OF TWO COOLING SCHEMES

(a) First scheme (b) second scheme FIGURE 4. TEMPERATURE DISTRIBUTION MAP OF TWO SCHEMES

Based on the temperature field distribution of two schemes, two thermal insulation schemes of LNG cargo tank can be compared more clearly, which are listed in Table 6.

Item First cooling scheme Second cooling scheme Cooling temperature of liquid temperature of temperature nitrogen is ‐196℃; evaporation gas is ‐140℃; Price of cooling liquid nitrogen is evaporation gas is medium cheap; expensive; it can shorten the the pre‐cooling must be cooling time with after ship arrival, it Cooling time pre‐cooling before cannot shorten the ship abord; cooling time; the limited space of the implementation is Danger rate operation causes a convenient, and the risk

8 Li Rong, Jin Wenbing, Zhang Juyong, Jin Jialei, Xu Endao

greater risk of implementation;

is small;

of LNG ship. Technology foundation of national defence.

2006, 24(6):25‐35.

It is shown in Table 7, two cooling schemes can achieve large LNG cargo tank to a low temperature. In the first cooling scheme, it can pre‐cooling pipelines with liquid nitrogen before LNG ship arriving. But this scheme is a greater risk than second scheme, because its limited operation space of nitrogen tank.

The second cooling scheme is convenient with small risk, but it needs more time of cooling. Therefore, the second cooling scheme becomes the final cooling solution.

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Conclusions

Wang Yongwei, Wang Chuanrong, Wang Jing. Analysis on three types of LNG storage system on ship. 2007,

This paper mainly analyzes two types of cargo tank on LNG ship. Then two typical cargo containment systems are studied from their structural materials and insulation structures. Based on this, we focus on film cargo tank of a 147000m3 LNG ship to design its insulation structure and its cooling scheme. It is of certain reference value to actual manufacture film cargo tank on LNG ship.

32(2):15‐20.

Rong Li is a lecturer at the School of Mechanical Engineering, Hangzhou Dianzi University. She studies thermal control of larger equipment currently. She received a PhD in astronomical techiques and methods from National Astronmical Observatories, Chinese Academy of Sciences in 2010.

ACKNOWLEDGMENT

Webing Jin is a scholar candidate in mechanical engineering at the School of Mechanical Engineering, Hangzhou Dianzi University. He calculates thermal models of LNG cargo tank.

The authors are grateful to the “Zhejiang Provincial Natural Science Foundation of China (No. LQ13E060004)”; and the “Zhejiang Open Foundation of the Most Important Subjects (No. 201205)” for their financial supports.

Juyong Zhang is an associate professor at the School of Mechanical Engineering, Hangzhou Dianzi University. He studies structural design of larger equipment currently. He received a PhD in astronomical techiques and methods from National Astronmical Observatories, Chinese Academy of Sciences in 2007.

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Jialei Jin is a Master candidate in mechanical engineering at the School of Mechanical Engineering, Hangzhou Dianzi University. He studies thermal models of LNG cargo tank.

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