A new re-liquefaction system of MRS-F (Methane Refrigeration System Full re-liquefaction) for LNG Carriers DongKyu Choi *, Hyunjun Shin DAEWOO SHIPBUILDING AND MARINE ENGINEERING Co., Ltd. Dual fuel slow-speed two-stroke diesel engines are emerged as new propulsion systems because of their high efficiency and low operational cost. ME-GI engines were installed or will be installed on large numbers of LNGC s ordered recently, and X-DF engines are being applied on several LNGC s. Because of their high efficiencies and slow steaming operation, BOG re-liquefaction is necessary for handling of surplus BOG. DSME has developed PRS (Partial Re-liquefaction System), and applied it to LNG carriers with ME-GI engines, which received positive assessment from the market. In line with the development, DSME has developed MRS-F (Methane Refrigeration System Full Re-liquefaction) which can effectively and flexibly handle NBOG (Natural Boil Off Gas). By the development of the MRS-F, DSME has the optimized re-liquefaction systems for both ME-GI and X-DF applications. MRS is cryogenic refrigeration system using BOG as refrigerant and the system uses a redundancy FG compressor as a refrigerant compressor for X-DF application. By combination of PRS and MRS, full re-liquefaction of the surplus BOG is possible for all speed range including anchoring state. The system is called as MRS-F and it will be applied for DSME s first X-DF powered LNGC. By the system, the vessel operation becomes more flexible with OPEX savings by preventing any GCU operation. For the development, a pilot plant was built and tested to evaluate its performance and operational characteristics. In this paper, details of the MRS-F including process, performance, efficiency and operation concept are introduced. The pilot plant was designed to fully demonstrate the MRS-F system on LNGC. Its details and data from operations are discussed for understanding of the system. 1. Introduction LNG market forecasts that world-wide LNG demand and trade will be steadily and gradually increased in long-term point of view. Even though there could be short-term recession of LNG trading from low oil price, global economic recession and so on, long-term growth of LNG market is supported by several reasons, for example, trends of NG substitution for coal because of demand of clean energy, steady growth of NG consumption and increased world LNG production capacity by Australia and North America. The word-wide transition and movement related to LNG is making the growth of short-term and spot market portion, recently. * Corresponding Author: Tel. +82-2-2129-3751, e-mail: dkchoi@dsme.co.kr
Nowadays, high efficient dual fuel two-stroke engines, ME-GI from MAN D&T and X-DF from Win-GD, are mainly selected as a propulsion system to save operation cost. Because of the high efficient two-stroke engine application and slow steaming operation, surplus BOG becomes critical issue. The surplus BOG means remained BOG after engine consumption and its treatment is very important. For the purpose, efficient re-liquefaction system has been emerged as a key technology of LNGC design. Increase of spot trading further requires full re-liquefaction capability at anchoring state. DSME has developed a new re-liquefaction system, MRS-F, which can fully re-liquefy the surplus BOG as per operation mode including prolonged anchoring state. Besides full re-liquefaction capability, MRS-F has unique features of flexibility and economic operation combined with FGSS (Fuel Gas Supply System). In this paper, features of MRS-F will be introduced based on X-DF engine applied LNG carrier but the system is equally applicable to ME-GI engine application with small change of equipment configuration. 2. MRS-F Re-liquefaction System for LNGC When LNG carriers are transferring LNG cargo, BOG is continuously generated by heat ingress and causes cargo tank pressure rise. The pressure of cargo tanks of LNG carrier should be managed to meet the import terminal limitation for unloading operation. When tank pressure is higher than import terminal limitation, BOG should be released through GCU (Gas Combustion Unit) or re-liquefied via re-liquefaction system. From the economic point of view, re-liquefaction is recommended because BOG burning is wasting money and energy. <Figure 1. Schematic Diagram of MRS-F applied LNGC for X-DF Engine >
MRS-F is full re-liquefaction system combined with FGSS and is composed of PRS (Partial Reliquefaction System) and MRS independent cycle. MRS which is based on reverse Brayton cycle uses naturally generated BOG as refrigerant. With the reason, additional refrigerant inventory system is not necessary even though most of independent refrigeration cycles should have separate inventory system. Typically independent refrigeration system requires additional refrigerant compressor, whereas, MRS-F does not because the MRS uses a redundancy FG compressor as a refrigerant compressor for X-DF application. <Basic concept of system> The MRS-F fulfills full re-liquefaction performance with combination of PRS and MRS. PRS is partially re-liquefying generated BOG first, and then, additionally supplied cold energy from MRS is to further lower the BOG temperature. When LNG carrier is on normal seagoing condition, all generated BOG can be managed by PRS (Re-liquefaction) and fuel gas consumption at engine side (propulsion & power generation). On the other hands, at very low speed operation or anchoring state, it is not enough to manage all the surplus BOG using PRS. In this kind of situation, MRS will be operated to handle the remained surplus BOG. MRS is only applied slow speed operation below about 15knots not to waste power consumption from rotating equipment at MRS. Thanks to very efficient re-liquefaction by PRS, separate refrigeration system operation can be minimized to achieve economic operation. Previous reliquefaction system should be operated even for small amount of BOG which causes large operation cost. <Process Description> At anchoring state, cold BOG of -120 is supplied to HRU (Heat Recovery Unit, PCHE I) and counter flow (pressurized BOG, 150 barg) is cooled down to approximately -70 at supercritical status. It is then expanded at JT(Joule-Thomson) valve to near atmospheric pressure. At the same time, temperature goes down to LNG temperature by JT effect. Even though the temperature is as same as LNG, only partial re-liquefaction can be achieved by PRS operation because enthalpy is not enough to re-liquefy all pressurized BOG. When MRS is activated, pressurized BOG at upstream of JT valve is further cooled down to below -150 at PCHE II and full re-liquefaction can be achieved. In the MRS, pressurized refrigerant by refrigerant compressor is cooled down, and then, expanded by expander with generating power. After expansion, lowest temperature is achieved and this stream cools down the above pressurized BOG at upstream of JT valve. Using generated
power from expander is transferred to boost compressor which is located before the refrigerant compressor and makes inlet pressure of the refrigerant compressor higher than atmospheric pressure. The expander and boost compressor at MRS is connected with shaft directly which is called as compander. <Operational Flexibility> MRS-F is basically operated in conjunction with BOG Management System to keep cargo tank pressure stable. MRS-F has advantages in operational flexibility point of view. Typically, most of commercial re-liquefaction process applied to LNG carrier should be stopped in case of equipment failure, whereas, all generated BOG should be treated in GCU or released to atmosphere to prevent tank pressure rise. On the other hand, MRS-F is designed and configured to minimize cargo waste by recovering BOG through re-liquefaction process even in equipment failure cases. MRS-F applied LNG carrier offers three operation modes as per unexpected equipment failure as Figure 2 and Table 1. (1) (2) (3) <Figure 2. Operation Mode/ (1) MRS-F mode, (2) MRS-P mode, (3) PRS mode> Operation Mode Occasion Note MRS-F Mode No Equipment Failure Full Re-liquefaction MRS-P Mode BOG Boost Compressor Failure Partial Re-liquefaction PRS Mode Compander or Refrigerant Compressor Failure Partial Re-liquefaction <Table 1. Equipment Status for Each Operation Mode>
MRS-F Mode MRS-F mode is executed with all re-liquefaction related equipment in operation in order to maximize the capacity of re-liquefaction. This mode is mainly activated at anchoring state or low speed laden voyage. MRS-P Mode MRS-P mode can be selected in case of BOG boost compressor failure. HP(High Pressure) BOG stream at PRS side is working at supercritical pressure region in order to enhance performance of re-liquefaction at MRS-F mode operation. For MRS-P mode, BOG stream is compressed up to 16 barg for the fuel gas delivery to engine and remained flow is sent to PCHE I bypassing BOG boost compressor, and then, introduced to PCHE II to be cooled down further. The capacity of reliquefaction of MRS-P mode is expected to be higher than PRS one. PRS Mode MRS-F application includes PRS basically and supplies additional refrigeration energy to BOG stream from MRS in case of short of cold energy from PRS. PRS mode is activated with priority because of its easy and efficient operation at sea-going status. When one of FG compressors (used as a refrigerant compressor) or compander has trouble in operation, MRS can be stopped intentionally by operator and re-liquefaction is executed with PRS mode. Full re-liquefaction may be not possible but NBOG can be managed in accordance with sea-going status. 3. Performance of MRS-F in accordance of service speed of LNG carrier The performance of MRS-F applied LNG carrier is evaluated from BOG management point of view. Three different schemes are considered as per laden voyage service speed as follows: Full Capacity Control Scheme with MRS-F Mode Full Capacity Control Scheme with PRS Mode Tank Pressure Control Scheme with Combination of MRS-F & PRS Mode The analysis is conducted with Aspen HYSYS based on following information: Containment System GTT NO.96 GW Propulsion X-DF engine BOG temperature -120 <Table 2. Basic Information of simulation> In this analysis, the amount of remained BOG as per service speed of LNG carrier is main factor
for operational mode change and is directly connected to cargo tank pressure rise. If the remained BOG is positive value, cargo tank pressure starts to increase, and vice versa. Remained BOG is resultant amount of BOG after re-liquefaction and fuel gas consumptions of propulsion & power generation from NBOG. The value can be calculated as follow: Remained BOG = NBOG (Re-liquefied amount + ME & DG Consumption) <Full Capacity Control Scheme> Full capacity control scheme is defined to be operated with maximized capacity of re-liquefaction system regardless of BOG management concern of LNG carrier. As can be seen Figure 3, the amount of re-liquefied BOG at anchoring state and low speed service area has far more deviation between MRS-F mode and PRS mode. The amount of remained BOG for MRS-F mode is negative value at all range of service speed including anchoring state since there are always fuel gas consumptions at main propulsion and power generation engine. So, tank pressure can be controlled to be decreased for all operation cases. <Figure 3. Comparison for Re-liquefied BOG Amount Full Capacity Control Scenario> When PRS mode is applied to all range of service speed from anchoring to 19.5 knots, the amount of remained BOG is switched from positive value to negative value at about 15 knots. It is expected that there is no problem to let tank pressure down intentionally combined with fuel gas consumption above about 15 knots.
<Tank Pressure Control Scheme> Tank pressure control scheme is BOG management method to keep the tank pressure constant by operation of re-liquefaction related equipment. As described in Full Capacity Control Scenario, MRS-F mode is able to manage all generated BOG by controlling refrigeration load of MRS. From 15 knots or above, PRS mode can handle remained BOG to zero as fuel consumption increases. It is possible to make zero remained BOG by activating MRS less than 15knots. Speed Remained BOG Operation Mode (knots) (kg/h) 19.5 0 19 0 18 0 PRS 17 0 16 0 15 55 Note 1 14 0 MRS-F 0 0 Note1 The value is based on simulation result. It can be managed by operator s choice of operation mode. <Table 3. Performance for Combined Operation - Tank Pressure Control Scheme>
4. DSME MRS-F Pilot Plant DSME has developed and tested MRS-F pilot plant to verify the performance of the system and its operability in 2016. Demonstration of the pilot plant was held in front of ship owners, classification societies and other participants. Total eight (8) skids were integrated to demonstrate MRS-F system. Among systems, three (3) major skids are marked as below: (1) (2) (3) <Figure 4. (1) DSME MRS-F Pilot Plant, (2) MRS Skid, (3) HMI of MRS Skid> HIiVAR Skid: HP BOG stream, 300 barg/ 43, is generated. PRS Skid: 300 barg BOG stream is cooled down to -90 before entering MRS. MRS Skid: HP BOG stream, 300 barg / -90, is further cooled down for full re-liquefaction
Trends of test data are displayed for duration of 7,800 seconds as shown in Figure 5. The compander was started to run at 1,600 seconds and reduced RPM at 7,300 seconds. <Figure 5. Trends of RPM & Mass Flow for MRS> <Figure 6. Trends of Temperature for MRS and HP BOG> The test condition was manipulated as same as design basis of process in order to verify its performance. Design condition of HP BOG inlet stream is 1533 kg/hr, 300 barg and -90. And the HP BOG inlet condition was kept to approximately, 1500 kg/h and 300 barg. The temperature was ranged from -70 to -90. The test temperature was kept a little higher range than design. It means that heat duty required for MRS was more than design duty. Even heat duty was higher than design, the test results showed its performance effectively covered increased re-liquefaction load. The HP BOG outlet temperature was well following the trend of expander outlet temperature, moreover, temperature difference of the two streams was less than 1. The test has been
successfully completed since higher than 95% of total supplied BOG flow was re-liquefied by MRS. <Figure 7. DSME MRS F Pilot Plant> 5. Conclusion DSME has developed cost effective and operationally flexible full re-liquefaction system called MRS-F and its performance was fully verified by pilot plant operation. MRS-F is basically configured with PRS and separate cycle of MRS and has following features. - MRS uses BOG as refrigerant and refrigerant inventory and management system are not necessary - For X-DF engine application, redundancy FG compressor can be used as refrigerant compressor and additional refrigerant compressor is not necessary. - At normal operation of vessel speed higher than 15knots, BOG can be effectively handled by PRS without additional power requirement of MRS. - Efficient full re-liquefaction is possible with MRS operation at low speed and anchoring. - Cargo tank pressure can be reduced by MRS operation if necessary.