The world s first commercial LNG fuel tank made of high manganese austenitic steel Jiwon Yu 1), Kihwan Kim 1), Do-Won Seo 1), Sung Tae Yun 1), Geon Shin 1), Hyun Kug Lee 2) 1) POSCO 2) Ilshin Marine Transport Co. Ltd. Abstract In this study, we introduce the world s first commercial LNG fuel tank made of high manganese austenitic steel. This IMO type-c fuel tank is 500 m 3 in capacity and will be installed on a LNG fuelled 50,000 DWT bulk carrier, which is the world s largest LNG fuelled bulk carrier up to date. This vessel is designed for Tier III ready condition and will be delivered at the end of 2017. Both the vessel and LNG fuel tank is designed following IGF code, KR (Korean Register) and LR (Lloyd Register) rules as well as other international regulations. Inner shell of the tank is made of high manganese austenitic steel while outer shell is made of carbon steel. It is known that high manganese austenitic steel is the steel with high manganese concentrations at approximately 22 ~ 26 wt. % and has excellent physical properties at very low temperatures similar to nickel based alloys and aluminum alloy while the cost of the material is relatively low. Thus, it is expected that the world s first commercial LNG fuel tank made of high manganese austenitic steel will play a significant role to demonstrate the competitiveness of high manganese austenitic steel as a cryogenic material. 1. Introduction International Maritime Organization (IMO) has set 2020 as implementation date for 0.5% global sulfur cap on October 2016 and it is expected that this decision accelerates the use of Liquefied Natural Gas (LNG) as a marine fuel. Constructing safe and cost competitive cryogenic fuel tank is very important to adopt LNG as a marine fuel. Since material cost significantly influences overall price of LNG fuel tank, appropriate material should be selected considering both safety and economic feasibility. Materials that are applicable at cryogenic temperatures below -165 ºC are listed in Table 6.3 and Table 6.4 in the IGC Code [1] as well as in Table 7.3 and 7.4 in the IGF Code [2]. Those materials are 9% Ni steels, nickel-based austenitic stainless steels, and aluminum alloys. High manganese austenitic steel for cryogenic service (hereinafter referred to as high Mn steel ) is a steel with manganese concentration at approximately 22 ~ 26 % and has similar or superior physical properties at cryogenic temperatures compared to the materials listed in IGC and IGF codes while the cost of the material is relatively low. Mechanical characteristics of high Mn steel and its welding consumables such as tensile properties, Charpy impact energy, CTOD, fatigue, and corrosion performance at room temperature as well as cryogenic temperature have been evaluated [3] and certified by various classes including ABS, BV, DNV-GL, KR and LR. In addition, mock-up tests for various types of LNG tanks including cylindrical IMO type-c tank and Lattice tank have also been successfully completed. Structural integrity of cryogenic tank made of high Mn steel after repeated thermal loads has also been evaluated [4]. Welded joints are inspected by nondestructive radiographic testing
(RT) before and after the operation of tested tank and no defect had found, which means that the cryogenic storage tank made of high Mn steel keeps its integrity after experiencing repeated thermal cycles. Although extensive studies have been conducted regarding the characteristics of high Mn steel, there has been no actual track record to adopt high Mn steel for LNG fuel tank and operate it in real application. Once LNG fuel tank made of high Mn steel is successfully operated without any trouble, it would be the best evidence to prove the safety and mechanical characteristics of high Mn steel. In this sense, the project to use high Mn steel as a material of LNG fuel tank for actual application has been initiated. This project is target to adopt first commercial LNG fuel tank made of high Mn steel on LNG fuelled bulk carrier which will carry limestone from Donghae to Gwangyang in Korean Peninsula. The IMO type-c LNG fuel tank made of high Mn steel is capable to store LNG up to 500 m 3 and is going to be installed on the aft mooring deck of the ship as shown in Figure 1. Figure 1. General arrangement of 50,000 DWT LNG fuelled bulk carrier The LNG fuelled ship is 50,000 DWT bulk carrier and is under construction targeting to deliver on November 2017. Both KR (Korean Register) and LR (Lloyd s Register) approve the design and inspect the manufacturing process of the ship as well as the tank which are designed in accordance with IGF code. The ship is designed to satisfy 1) ocean-going, 2) ecofriendly, and 3) high efficiency concepts. As earlier illustrated, this ship is normally going to carry limestone from Donghae to Gwangyang in Korean Peninsula as demonstrated in Figure 2 (a). However, the capacity of the fuel tank is set so that the ship is able to navigate from Busan, South Korea to Singapore with only LNG fuel as shown in Figure 2 (b). Since one of the most important purposes of building this ship is to internationally demonstrate high Mn steel, the ship and the fuel tank is designed to meet all requirements of IGF code and other international regulations and navigation route is set as far as possible to extend the usage of the ship. In addition, this ship is prepared to satisfy environmental regulations which are
effective now as well as will be effective in near future. For example, EGR (Exhaust Gas Recirculation) is read to meet Tier III for NOx regulation and BWTS (Ballast Water Treatment System) is also ready. Furthermore, ME-GI engine will be installed to pursue high efficiency. The amount of fuel consumption of this ship is 22 % less than that of the ship which currently navigates the same route. Figure 2. Navigation route of 50,000 DWT LNG fuelled bulk carrier for (a) normal condition and (b) maximum design condition The objective of this paper is to introduce the world s first commercial LNG fuel tank made of high Mn steel which will be installed on the ship briefly introduced above. High Mn steel is used as a cryogenic material for inner tank of this IMO type-c fuel tank where LNG directly contacts with while outer shell is made of carbon steel. Details of the tank regarding from design to manufacturing will be introduced. 2. Design As illustrated earlier, the LNG fuel tank is designed in accordance with the class rules for both KR and LR as well as IGF code. The tank is horizontal multi-layered cylindrical tank and high Mn steel is adopted as the material for inner tank. LNG would be stored inside of inner tank and inner tank is thermally isolated with outer tank using vacuum and pearlite to block the heat ingress from the environment as much as possible. The performance of LNG fuel tank is evaluated by holding time which stands for the period that the vapor pressure of the tank is kept under design pressure (7.0 bar) from operating pressure (2.0 bar). The fuel tank introduced in this paper can hold LNG under design pressure for more than 60 days which are much longer than the requirement of IGF code (15 days). Other specifications of the tank are listed in Table 1 and general arrangement of the tank is demonstrated in Figure 3.
Table 1. Specifications of LNG fuel tank made of high Mn steel Description Inner Tank Outer Tank Design Pressure 7.0 bar Full Vacuum Material (Shell / Head) High Mn steel / High Mn steel SA 516-70 / SA 516-70 Diameter 6,175 mm 6,935 mm Length 17,777 mm 18,700 mm Figure 3. General arrangement of LNG fuel tank made of high Mn steel Numerical analysis had also been performed to assess the structural integrity of the tank under various loading conditions. Finite element analysis (FEA) was performed using a 3-D model to evaluate the stress of the tank under each loading condition. Finite element model of the LNG fuel tank used in this study is shown in Figure 4 and the boundary conditions of the tank are described in Table 2. Table 2. Boundary conditions of the tank Location Fixed Side Sliding Side Bottom Plate Fixed Y Fixed Y Bolt Area Fixed X and Z Fixed Z
Figure 4. Finite element model of LNG fuel tank made of high Mn steel Ultimate design condition criteria are illustrated in 6.4.15.3.3.1 of IGF code. One of the criteria is that equivalent stress, the summation of equivalent primary local membrane stress, equivalent primary bending stress, and equivalent secondary stress, should not exceed three times of allowable stress. The equivalent stress distribution of inner tank, outer tank, and support are shown in Figure 5 ~ Figure 7 and the results are summarized in Table 3. Figure 5. Equivalent stress distribution of inner tank and support for inner tank
Figure 6. Equivalent stress distribution of outer tank and support for outer tank Figure 7. Equivalent stress distribution of support for the tank inner tank and support for inner tank Table 3. Summary of the analysis result Member Material Maximum Equivalent Stress [MPa] Allowable Stress [MPa] Safety Factor Inner Tank High Mn Steel 356 540 1.5 Outer Tank STS 304 354 414 1.2 Support SA516-70 213 414 1.9
As described in Figure 5 ~ Figure 7, the locations of maximum equivalent stress are the region near supports. Nevertheless, all components of the tank satisfy the criteria of IGF code as shown in Table 3. Therefore, it could be concluded that the LNG fuel tank made of high Mn steel is safely designed. 3. Manufacturing Table 4 describes the results of mechanical tests for mill test certificate of high Mn steel used for manufacturing of LNG fuel tank which is introduced in this paper. As shown in this table, the mechanical strength of the material exceeds the minimum requirement of the specification (400 MPa, 800 MPa, 27 %, and 41 J for yield strength, tensile strength, elongation, and impact test, respectively) and both KR (Korean Register) and LR (Lloyd s Register) certified the result. Table 4. List of sizes and mechanical properties of high Mn steel used to manufacture LNG fuel tank Thickness (mm) Width (mm) Length (mm) Yield Strength (MPa) Tensile Strength (MPa) Elongation (%) 15 2,530 9,710 484 869 62 97 15 3,020 6,200 443 860 64 85 15 3,020 9,800 439 835 68 82 18 2,920 9,740 483 872 60 91 20 1,650 8,500 508 888 67 84 20 2,000 4,000 508 888 67 84 26 2,000 6,000 482 853 50 88 26 2,470 7,370 623 876 48 99 26 2,470 12,450 499 863 61 90 26 3,000 6,300 489 864 57 99 Impact Test (J) Procedure qualification record and welding procedure specifications are reviewed prior to begin manufacturing of the LNG fuel tank to verify the integrity of the welding of high Mn steel. The tests were performed for Submerged Arc Welding (SAW), flat (1G) Flux-Cored Arc Welding (FCAW), horizontal (2G) FCAW, and vertical (3G) FCAW and the pictures for each position and specimen are demonstrated in Figure 8 (a) ~ (d). In addition, Procedure qualification record and welding procedure specifications for dissimilar metal welding of high Mn steel and STS 304 is also reviewed since the pipes connected to the high Mn steel plates are made of STS 304. Figure 9 describes the picture and the specimen of dissimilar metal welding for fillet FCAW.
Figure 8. Procedure qualification record test of high Mn steel for various butt-weld positions (a) Submerged Arc Welding (SAW), (b) Flat (1G) Flux-Cored Arc Welding (FCAW), (c) Horizontal (2G) Flux-Cored Arc Welding (FCAW), and (d) Vertical (3G) Flux-Cored Arc Welding (FCAW) Figure 9. Dissimilar metal welding of high Mn steel plate and STS 304 pipe
Table 5. Results for mechanical test of butt-weld specimens for procedure qualification record test values are average of each specimen Process Tensile Test (MPa) Charpy Impact Test at -196 ºC (J) Bend Test SAW 853 51 No open defect FCAW 1G 801 53 No open defect FCAW 2G 787 58 No open defect FCAW 3G 801 42 No open defect Figure 10. Test report for SAW certified by KR and LR Results for mechanical test of welded specimens for procedure qualification record test are illustrated in Table 5 and one of the test reports are demonstrated in Figure 10. As shown in Table 5, values for tensile test and Charpy impact test exceed the minimum requirement 660 MPa and 27 J, respectively. LNG fuel tank made of high Mn steel is manufactured following certified welding procedures which are described above. 4. Summary High Mn steel for cryogenic service has been developed and the first commercial LNG fuel tank made of high Mn steel is introduced. This LNG fuel tank is designed and manufactured in accordance with the class rules for KR (Korean Register) and LR (Lloyd Register) as well as IGF code. This tank will be operated for more than 18 years which will be a good reference for the use of high Mn steel for cryogenic service. Expansion of the use of high Mn steel as a safe and economic material for LNG tank is expected.
5. Acknowledgement The authors wish to express deep appreciation to Daewoong Cryogenic Technology Co., Ltd., Hyundai Mipo Dockyard Co. Ltd., and Ilshin Logistics Co., Ltd. for their great cooperation to apply the high Mn steel to LNG fuel tank. 6. References [1] IGC Code resolution MSC.370(93) [2] IGF Code resolution MSC.391(95) [3] Choi, J. K., Lee, S. G., Park, Y. H., Han, I. W., & Morris Jr, J. W. (2012, January). High manganese austenitic steel for cryogenic applications. The Twenty-second International Offshore and Polar Engineering Conference. International Society of Offshore and Polar Engineers. [4] Yu, J. et al., (2015), "Experimental Study on the Cryogenic Storage Tank made of High Manganese Steel." The Twenty-fifth International Offshore and Polar Engineering Conference. International Society of Offshore and Polar Engineers.