LNG carrier two-stroke propulsion systems: A comparative study of state of the art reliquefaction technologies

Energy 195 (2020) 116997

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LNG carrier two-stroke propulsion systems: A comparative study of state of the art reliquefaction technologies Dimopoulos G. George*, Koukoulopoulos D. Eleftherios, Georgopoulou A. Chariklia DNV GL, Piraeus, Greece

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 July 2019 Received in revised form 13 January 2020 Accepted 17 January 2020 Available online 19 January 2020

Transportation of Liquefied Natural Gas (LNG) by sea has been intensified in the current shipping environment, opening new markets and trade routes. LNG carriers are inherently complex vessels, featuring a high degree of integration of their energy conversion systems, all operating under time and load varying mission profiles, making design decisions non-trivial. To this end, DNVGL COSSMOS (Complex Ship Systems Modelling and Simulation) in-house process modelling framework is used to build digital twins of the propulsion and cargo module of LNG carriers, modelling the entire energy conversion process from cargo tanks to useful energy required for propulsion, electricity and heat. Emphasis is given on reliquefaction systems and the improvement they provide in performance, comparing the currently available technologies and giving deeper insight for the Joule-Thomson systems. The overall ship system performance is improved by 5e15% along the low vessel speed range and 25 e40% for the anchorage loaded port condition, when partial reliquefaction systems are considered, depending on the configuration they are compared to. Engine technology also plays an important role, with high-pressure engines exhibiting 3e10% better performance, depending on the reliquefaction technology coupled with, along the high vessel speed range, mainly because of their inherent better performance. © 2020 Published by Elsevier Ltd.

Keywords: LNG carrier Reliquefaction Partial reliquefaction Joule-thomson Integrated marine energy systems Performance Efficiency

1. Introduction Natural gas has always played an important role in the global energy market, constantly being strengthened nowadays. Strict environmental emission regulations for power generation, heating and transportation, has made LNG one of the most attractive and clean solutions ensuring compliance [1]. In order to cover the increased global demand, more and more shipping companies are investing in LNG carriers, whose fleet is expected to increase significantly within the next 10 years. LNG carriers have always been equipped with pioneering and innovative technologies offering high safety and quality standards, with new technologies emerging, focusing on energy efficiency and cargo containment [2]. Natural gas is transferred in liquid state in cryogenic cargo tanks at about
163  C with a portion of it being vaporized during voyage, due to the constant heat ingress from the surrounding environment. This boil-off gas (BOG) can potentially be used as fuel

* Corresponding author. E-mail address: [email protected] (D.G. George). https://doi.org/10.1016/j.energy.2020.116997 0360-5442/© 2020 Published by Elsevier Ltd.

for the main and auxiliary engines and/or liquefied and returned to the cargo tanks or dumped at a dedicated burner (GCU e Gas Combustion Unit). The dynamic landscape and increased competitiveness of the LNG market has caused a shift from the GCU usage, which was traditionally preferred by operators, to reliquefaction plants installed on-board on almost all newbuilt vessels. A large variety of reliquefaction technologies exist, based on different cryogenic refrigeration cycles, all sharing the common target of liquefying evaporated natural gas cargo [3]. Reliquefaction units guarantee that no cargo is lost, except the amount used to power the vessel, upgrading the vessel at the charters ranking. Therefore, apart from the overall fuel/cargo consumption (tons per day), that the shipowner can offer, and the charter will bear, charters recently emphasise on reliquefaction, aiming at almost zero cargo loss during transportation. With reliquefaction becoming the new standard, shipyards and vendors are investing to develop innovative technologies, aiming at increased flexibility and efficiency. Additionally, tailormade solutions are offered for different types of main engines (high/low gas feed pressure), in an effort to optimize the gas management onboard and increase the overall efficiency of the integrated ship

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Nomenclature AE BOG COSSMOS FBOG FG GCU HFO JT JT-Hyb LHV LNG ME MGO MR NBOG OPEX RLQ

Auxiliary Engine Boil-off Gas Complex Ship Systems Modelling and Simulation Forced BOG via forced cargo evaporation Fuel Gas Gas Combustion Unit Heavy Fuel Oil Joule-Thomson Hybrid JT reliquefaction system with methane refrigerant loop Lower Heating Value Liquid Natural Gas Main Engine Marine Gas Oil Mixed Refrigerant Natural Boil-off Gas OPerational EXpenses Reliquefaction

system [4]. Within this complex and wide technology landscape, identifying the optimal machinery propulsion configuration is a nontrivial procedure [5]. In addition, the vessels operate in highly varying operating profiles and trading patterns, carrying LNG cargoes of varying characteristics and compositions. To this end, advanced simulation tools are required to combine both the increased complexity of such systems and the varying operational profile, in order to make design decisions between alternative technology configurations [6]. In the present study we employ process modelling and simulation techniques, combined with an integrated systems approach to compare different technologies and system design configurations for LNG carriers, so as to support the decision making during the new building phase. Our focus is on the current state of reliquefaction plant technologies, including the latest innovations available in the market. The in-house process modelling framework DNVGL COSSMOS [7] is used to simulate and compare the considered configurations, aiming to capture the tight interrelations between components, sub-systems and processes in and LNG carrier marine energy system and derive improved design solutions at an integrated system level. In this paper, first the LNG carrier energy system is briefly described, along with the process modelling approach and the gas distribution philosophy among the sub-systems that comprise it. Then, reliquefaction systems are further elaborated, listing the currently available solutions and the possible combinations with other ship systems. The application case and the configurations that consists of are then presented, followed by the simulation results and the conclusions of the study. 2. LNG carriers integrated energy system

needs. The primary fuel found on-board is the evaporating cargo, in the form of boil-off gas (BOG), due to the heat ingress from the surrounding environment to the cargo tanks. Additionally, traditional marine fuels are also stored on-board and used; HFO (Heavy Fuel Oil) (mainly for auxiliary boiler) and MGO (Marine Gas Oil) (as pilot fuel or alternative fuel). According to the current industry practice, the burning of BOG in dual-fuel prime movers and auxiliary gen-sets is the most efficient and cost-effective way of operation [8]. In case where natural cargo evaporation is not sufficient to meet the consumption needs, forced evaporation of liquid cargo is applied. A sketch of the integrated ship energy system is shown in Fig. 1, including the main elements and subsystems required to convert primary fuels energy content to the on-board energy demand. The BOG from the LNG tanks is pre-processed passing through a mist separator and in-line mixer and fed to the electric-driven gas compression trains. Then the BOG is distributed to the combustion prime mover engines, either for power generation or propulsion. A BOG management module is used to distribute the gas fuel to the prime movers and redirect the surplus BOG either to a GCU or to a reliquefaction plant, if present. This flow diagram is not globally applicable, as different main engine types (low vs high pressure) and the employed reliquefaction systems define the final form of the gas flow diagram. The integrated system has to cover a time-dependent power demand in propulsion, electricity and heat based on its operational profile. Moreover, the LNG and boil-off gas have variable properties (heating value and composition) depending on LNG cargo type and in-voyage boil-off rate conditions, also strongly affected by the selection of the cargo, containment system and type. Further to that, cargo composition is an important parameter being captured by the developed model, as it varies between liquid cargo and BOG evaporation, due to the lighter hydrocarbons evaporating first [9]. In addition, the ships usually operate in several trading routes, under varying operating profiles in terms of speed, propulsion, electricity, and heat demand. The integrated energy system of Fig. 1 is tightly coupled, featuring multiple interrelations and feedback loops between its major subsystems, capturing the non-linear phenomena of natural gas different states at different points of the system. Both the gas compression trains and reliquefaction plant (if any) are major parasitic electric power consumers. Power demand variations affect both the parasitic power and the BOG usage, which in turn may affect its composition. Therefore, an integrated systems approach coupled with process modelling and simulation is used in this work to assess and compare various configurations of the LNG carrier machinery system, addressing the following issues:  Main engine two-stroke options: low or high gas feed pressure technology.  Comparison of electric driven, partial reliquefaction systems and latest hybrid technology.  Ship performance and reliquefaction efficiency at different vessel speeds and non-sailing modes.  Fuel/LNG cargo price sensitivity analysis.

2.1. Introduction 2.2. Process model In the present study the machinery and propulsion integrated system of an LNG carrier is considered, in terms of fuel availability on-board and energy demand from the different consumers. The power required on-board an LNG carrier consists of propulsion power, electricity to cover the auxiliary systems and accommodation needs and heat as low-pressure steam for steam driven auxiliary equipment and other heating accommodation

DNVGL COSSMOS (Complex Ship Systems Modelling and Simulation) modelling framework has been used for the process modelling of the LNG carrier integrated energy system and its alternative machinery configurations. COSSMOS framework is implemented in the gPROMS [10] process modelling environment and a detailed description, its features and potentials can be found

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Fig. 1. LNG carrier generic marine energy system [19].

in Ref. [11]. Two main COSSMOS models have been developed for the low and high pressure two-stroke mechanical propulsion systems, and each one coupled with two different reliquefaction systems, as seen from Fig. 7 to Fig. 10. The major sub-system components featured in these system process models are: the natural and BOG preprocessing module, the BOG compression trains, the BOG management module, the reliquefaction plant, the dual-fuel 4-stroke diesel generator sets (both configurations), the main dual-fuel 2stroke diesel propulsion engines (mechanical propulsion), the engine waste heat economisers, the auxiliary fired boiler, the propulsion, electricity and steam demand management modules, and, an operational profile characteristics module. The following paragraphs describe the modelling of each of these major sub-systems. 2.2.1. BOG processing module The BOG handling line consists of: the natural and forced BOG feed streams, a simplified model of the piping and pressure drop in these sections and a mixing/flash separation component model, where natural and forced BOG streams are mixed and any liquid phase natural gas is removed. Forced BOG line also consists of a high-pressure pump and an LNG vaporizer using steam to heat-up and vaporize cold liquid natural gas from the cargo tanks. Different cargo compositions have been considered for the above two lines. Forced BOG composition is equal to that of cargo LNG, which changes according to the trading route and point of loading. Natural BOG composition varies dynamically during voyage as lighter hydrocarbons and nitrogen in the LNG mixture evaporate first. This is captured by the employed dynamic LNG evaporation model. Using this model, BOG composition has been calculated for a characteristic trip and subsequently the mean value of it has been used in this study. 2.2.2. BOG compression BOG compression trains are used to compress the fuel to the required engine operational pressure. Electric-driven piston, screw or centrifugal compressors with intercooling are the most common solutions, with their number and capacity affected by cargo tank containment technology, safety and redundancy issues. For the low-pressure two-stroke main engine, two-stage centrifugal compressors with intermediate and after cooling are employed. For the

high-pressure main engine, five-stage piston compressors with intermediate and after cooling are employed. The Gas engine feed pressure mandates the choice between centrifugal or piston compression technology. Low-pressure twostroke dual-fuel engines require low gas fuel pressure (~16.5 bar), while high-pressure two-stroke dual-fuel engines require high gas fuel pressure (~300 bar). For the latter case, piston compressors are the only feasible technology for shipboard applications. For both cases, the possibility for a low-pressure extraction from the compression train is also considered, in order to feed four-stroke auxiliary engines (~6.5 bar). In both generic system configurations, two compression trains are modelled, mainly due to safety and redundancy considerations. The sizing of these trains is based on the natural boil-off gas quantity and in the case of partial reliquefaction systems is also based on the amount of recirculating gas (BOG recirculation is described in Section 3.3). The compressor stages are modelled using manufacturer performance maps correlating flow, speed, pressure ratio and efficiency. The electric motors drives are modelled as simple electric machines using manufacturer data to derive their efficiency curve. Finally, the intercooler heat exchangers are modelled as cross flow plate-fin heat exchangers. It is noted that the electric drive power consumption, the shaft losses and the cooling water pump consumption are accounted for in the compression train model. More information regarding the individual component models can be found in Ref. [12]. 2.2.3. BOG management Two different philosophies can be distinguished for the BOG management, one being used during real vessel operation and the other being an assumption for the design phase of the vessel. During the actual ship operation, heat ingress from the surrounding environment evaporates a portion of the liquid cargo, which accumulates as vapor at the top of the cargo tanks. The pressure within the cargo tanks increases, up to a certain limit where the crew has to open the relief valves and free the boil-off gas. This boil-off is primarily used as fuel gas for the dual-fuel engines and the surplus amount (if any) is either reliquefied or burned at the GCU. As the reliquefaction units can handle BOG up to a maximum capacity, in case this is reached, the leftover BOG is obligatorily burned at GCU. So, the cargo tank pressure is a

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parameter which defines the available amount of BOG, and is usually fine-tuned by the crew, based on the operational profile of the vessel, the demanded speed, the geographical region and the charterer time limits they must meet. For example, using a real scenario, if the crew knows that the ship is entering a piracy region, where they should sail at the maximum speed to minimize their stay within this area, the crew may have let the pressure increase in the cargo tanks for the period before entering this area. This will increase the BOG quantity in the tanks, which they can burn during sailing through the piracy region. The gas management philosophy of the design phase is quite different from real operation, because a consistent gas management strategy needs to be used so that all configurations are directly comparable with each other, but also because the real operation is based on the case specific crew decisions, which are unpredictable. To this end, the philosophy of constant cargo tank pressure has been used. This means that the amount of BOG extracted from the cargo tanks is such that, at any given time, the cargo tank pressure is kept constant and close to the ambient pressure. So, the available BOG quantity depends on the boil-off rate of the cargo containment system considered. Using the philosophy of the design phase, the BOG management module distributes the gas fuel to the prime movers and redirects the surplus BOG either to the gas combustion unit (GCU) or to the reliquefaction plant. It is noted that a GCU is always present due to safety/class requirements. The aim of our developed model is to determine the boil-off gas flow to the reliquefaction unit (if present), the GCU and the forced BOG requirement, according to the available natural BOG and the gas fuel demand from the engines. The reliquefaction plant can handle BOG flows between its design minimum and maximum operational limits, meaning that BOG values outside this range are obligatorily burned at the GCU. Finally, in the case that no reliquefaction plant is present in the configuration, all of the excess BOG is burned in the GCU. 2.2.4. Main & auxiliary engines For power generation 4-stroke dual fuel medium speed diesel engines are used. For mechanical propulsion, 2-stroke dual fuel slow speed diesel engines are used, implementing either the low or high gas feed pressure technology. Both engine types have two operating modes: gas mode, in which natural gas with a small quantity of pilot fuel oil is used, and, fuel mode, where marine fuel oil is solely used. Both engine types are modelled in COSSMOS using a lookup model based on linear interpolation of the most updated catalogued performance data of existing engines, as these are given by manufacturers (main engines [13,14], auxiliary engines [15]). The model is presented in detail in Refs. [11,12]. 2.2.5. Steam production Exhaust heat economizers are used for both the main and auxiliary engines, capable of covering the steam demand in the majority of vessel speed conditions. An oil-fired auxiliary boiler is also installed, which is used to cover the steam demands when the main and auxiliary engines exhaust gas does not have enough heat content to produce the necessary steam in the economizers, mainly at very low sailing speeds and port conditions. 3. Reliquefaction systems 3.1. Introduction Reliquefaction systems are the dedicated plants on-board an LNG carrier responsible for the liquefication of the excess boil-off natural gas, caused by cargo evaporation. They can be divided in three categories; the electric driven units, the Joule-Thomson

systems and the hybrid that combine the two previous categories. Each category lists many different variations, with the most common solutions of each category being the following:  Electric driven: Inverse Brayton cycle (Nitrogen loop) systems and Mixed Refrigerant (MR) loop systems.  Joule Thompson: Systems operating with one fuel-gas compressor (JT1) or with two compressors (JT1þ1).  Hybrid: Methane refrigerant loop system (JT-Hyb).

3.2. Electric driven units 3.2.1. Reverse Brayton cycle (nitrogen loop) The operation of this system is based on highly compressed and intercooled to ambient temperature nitrogen, which is then expanded in a turbine dropping its temperature to cryogenic levels [16]. This nitrogen is then used to cool-down and liquefy the BOG in a cryogenic heat exchanger. Regenerative cooling of nitrogen is also included in many set-ups, proposed by vendors, using the already cooled nitrogen, coming from the compressors, in the heat exchanger [17]. A simplified sketch of the system is shown in Fig. 2. 3.2.2. Mixed refrigerant systems (MR) This system, instead of using nitrogen as the cooling medium, it uses a mixture of hydrocarbon refrigerants, and a secondary cooling cycle. In more detail, a mixture of refrigerants, consisting usually of iso-butane, propane, ethylene and nitrogen, is used to cool-down and liquefy the excess BOG in a cryogenic heat exchanger. The mixture or refrigerants is either cooled by propane or via regeneration in an appropriate heat exchanger/condenser. A tertiary cooling loop is also used to cool-down propane using sea or fresh water at ambient temperatures. The proportion of each component in the mixture can be fine-tuned and match the BOG properties, adapting the reliquefaction efficiency at each different cargo [18]. A simplified sketch of the system is shown in Fig. 3. The common feature of all electric driven units is that they demand electric power from the ship’s electric network, in order to reliquefy BOG. They usually operate along a capacity-power curve, meaning that the power consumption depends on the BOG quantity they are called to reliquefy, up to their nominal/design point (usually not in a linear way). As for their location along the gas line, they extract low pressure BOG from the compression line, liquefy almost the whole amount of it (~99%) and return it to the cargo tanks. In the present study, a simplified model has been developed based on performance curves (electricity demand as a function of BOG reliquefaction capacity) provided by manufacturers from past projects. This model calculates the electric power demand, as a function of the BOG quantity it liquefies (varying per vessel speed and condition), which is then translated in an additional load to the auxiliary engines through the main switchboard. If no data are available from manufacturers, then a detailed model can be developed as reported [19], similar to a nitrogen loop system. 3.3. -Thomson systems 3.3.1. Joule Thomson e 1 compressor (JT1) These systems are suitable for two-stroke dual fuel highpressure main engines, as they take advantage of the existing high pressure (~300 bar) engine feed fuel gas. High-pressure BOG at near ambient temperature is cooled down to cryogenic levels in a suitable heat exchanger. Then, BOG is expanded in a gas valve (Joule-Thomson effect) and is then driven to a separator [20]. In the separator, approximately 70%e80% of the excess BOG is in liquid phase and is driven back to the cargo tanks. The remaining gas-

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Fig. 2. Inverse Brayton nitrogen loop reliquefaction system sketch.

Fig. 3. Mixed Refrigerant reliquefaction system sketch.

phase BOG is extremely nitrogen rich (20%e25% N2 content) is mixed with the natural BOG flow then driven to the cryogenic heat exchanger to cool the excess BOG and then to the BOG compression trains. This system philosophy, which allows for a flash gas recirculation to the compression system, has an impact on the overall methane number and lower heating value of the gas fuel that the engines are using, resulting in a negative impact in the gas fuel consumption of the engines. Moreover, despite not having a

dedicated electric driven motor, it induces an additional compression work requirement due to the recirculated BOG, translated in an indirect electric power demand and an increased consumption from the side of auxiliary engines. A simplified sketch of the system is shown in Fig. 4. As these systems intervene in the fuel gas line of the integrated system, a lot of complex, non-linear effects have to be captured.

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Fig. 4. Joule-Thomson reliquefaction system sketch.

3.3.2. Joule Thomson e 2 compressors (JT1þ1) Recently, shipyards have proposed an improved way of operation Joule Thomson systems, by deploying the second redundancy compressor [21]. Whilst JT1 makes use of one of the two fuel gas compressors being installed on-board, JT1þ1 alternatively uses the second redundant compressor. As JT1 capacity depends on the gas handling capacity of the BOG compressor, which is equal to the design volumetric flowrate limit of the compressor, there is an upper limit of the BOG handling capacity. Consequently, at low loads and port conditions, where the engine gas consumption is low and thus the overall mixture of excess BOG and recirculation is high, JT1 cannot handle the whole BOG amount, sending a BOG portion to the GCU. For this reason, according to JT1þ1, the second compressor is switched on and parallelized with the first one, doubling the system gas handling capacity resulting in zero GCU usage. This mode of operation is usually referred to as full reliquefaction. The term full reliquefaction may be misleading, as the reliquefaction still follows the Joule-Thomson expansion effect, which by definition results in partial reliquefaction. What the word “full” implies is that adding the second compressor in parallel with the first, increases the BOG handling capacity of the system, passing through the compression train, and then through the JouleThomson expansion valve, the whole amount of excess BOG, plus the recirculating stream. Operating the second fuel gas compressor adds a high electric load to the system, which can be considered as an indirect way of dumping/oxidising part of the excess BOG through the increased gas consumption of the auxiliary engines. However, as long as liquid cargo returns to the tanks, despite how small this quantity is, the overall system efficiency is still improved.

3.4. Hybrid system (JT-Hyb) According to the latest shipyard developments [22], a new technology has emerged, being a combination of the traditional electric driven units (methane refrigerant cycle) with the JT philosophy, resulting in a hybrid system. This system is mostly suitable for low-pressure two stroke dual fuel main engines. A similar philosophy can be used for high-pressure engine after appropriate changes of the basic equipment, which is out of the scope of the present study. JT-Hyb system comprises of a JT partial reliquefaction system and an independent methane refrigerant cycle, using the BOG as refrigerant. In the JT part, the pressure of the excess BOG not burned at the engines is raised from 17 bar (low-pressure gas feed pressure) to 150 bar, using a dedicated boosting compressor. Next, this BOG is expanded at a Joule-Thomson valve, achieving partial reliquefaction, similarly to what described at the previous section. However, due to compression, the BOG temperature has increased up to around
70  C, significantly reducing the efficiency of partial reliquefaction, because the heated BOG has moved further away from the liquid state and the enthalpy from expansion is not enough to liquefy it. For this reason, a methane cooling cycle is introduced, between the boosting compressor and before the JT valve expansion. The added methane cooling loop is based on the reverse Brayton cycle, using the naturally generated BOG as the refrigerant medium. The main benefit of this technology is that it does not require an independent refrigerant system, which would come along with separate circuits and storage for the new refrigerant, like nitrogen or mixed refrigerant, and an additional refrigerant compressor. Contrariwise, it exploits the cold cargo vapor and the second redundancy fuel gas compressor as the refrigerant compressor,

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both being already installed on-board. When the methane cycle is activated pressurized BOG after the boosting compressor is further cooled to about
150  C finally achieving full reliquefaction. At this point it should be clarified what partial and full reliquefaction implies, when the methane cycle is activated or not. After the JT expansion, a liquid and vapor stream are still separated in a dedicated tank, with the liquid stream returning to cargo tanks and the remaining vapor, recirculating to the fuel gas compressor. When the methane cycle is not activated, the BOG is far from the liquid state, resulting in a higher proportion of vapor after the JT expansion. When this recirculation is mixed with the natural BOG stream and directed to the fuel gas compressor, the mixture quantity surpasses the compressor maximum capacity, sending the surplus to be burned at the GCU, and thus finally resulting in what we call partial reliquefaction. On the other hand, when the methane cycle is activated, the recirculation quantity is significantly reduced and when mixed with the natural BOG flow, the mixture quantity is lower than the nominal of the fuel gas compressor, finally achieving what we call full reliquefaction. A simplified sketch of the system is shown in Fig. 5. Having described the philosophy of this hybrid system and the complex phenomena it entails, three modes of operation can be distinguished, which will also be used in the application case: HP mode: According to the HP mode, all the reliquefaction systems are engaged, with boosting compressor raising the BOG pressure (150 bar e High Pressure), and the redundant FG compressor creating a close cold loop to further cool-down pressurized BOG, which is then expanded at JT valve. During this mode, reliquefaction capacity is maximum, making it suitable for low vessel speed and anchorage modes, where excess BOG production is maximum. LP mode: The LP mode can be selected in case of boost compressor failure or for energy saving by switching off and bypassing the boost compressor, depending on the vessel speed

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and the available excess BOG. Excess BOG remains at a pressure equal to the engine feed pressure (16 bar e Low Pressure), further cooled by the closed BOG cold cycle and expanded at the JT valve. At this mode, the system achieves lower reliquefaction than in HP mode, but better than the PRS mode, so it is usually suitable for the mid speed range of the vessel. JT mode: According to JT mode, methane cooling loop is deactivated, reducing the energy consumption of the system, but also significantly reducing the reliquefaction capacity of the system. This mode is only suitable for the high-speed range of the vessel, where the excess natural BOG is minimum and zero GCU usage can be achieved. Consequently, an important benefit of this hybrid system is its flexibility, as it can adapt to each operational mode of the vessel, able to achieve zero GCU usage with the most energy efficient manner at each mode.

4. Application case 4.1. Case-study The present study focuses in the comparative assessment of the newly emerged reliquefaction technologies, serving the needs of two-stroke duel fuel main engines, considering both low and highpressure technologies. Gas distribution, reliquefaction and separation efficiency are examined in detail and the complex phenomena are captured and explained contributing to the better understanding of the alternative systems. An LNG carrier vessel with a cargo capacity of 173,400 m3 is considered. The main design characteristics of the vessel and the cargo system are given in Table 1. Two different two-stroke main engine technologies are compared, namely the low- and highpressure gas feed technologies, each one combined with different reliquefaction systems.

Fig. 5. Joule Thomson and methane cycle hybrid reliquefaction system sketch.

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ups are considered.

Table 1 LNG carrier main characteristics. Characteristic

Value

Cargo capacity Length Breadth Depth Design draft Service speed Design boil-off rate Number of propellers Installed main engines Installed auxiliary gen-sets

173,400 m3 288.5 m 46.4 m 26.5 m 11.5 m 19.5 kn 92.1 tn/day 2 LP:2  12600kW/HP: 2  12590kW 2  2880kWþ 2  3840kW 5000 kg/h

Auxiliary boiler capacity

4.1.1. Propulsion demand The propulsion demand curve of a typical LNG carrier hull may vary for different machinery configurations, mainly due to the engine room arrangement, affecting the aft body hull shape. However, as in this study only two-stroke dual fuel main engines are considered, the same hull shape has been assumed for both lowpressure and high-pressure two-stroke engine set-ups. The propulsion curve for both laden and ballast conditions is presented in Fig. 6, including 20% sea margin. 4.1.2. Electric & steam demand Typical values have been used for the saturated steam demand of auxiliary systems and accommodation needs, based on the vessel mode, as follows:  Sea-going: 1107 kg/h  Anchorage: 2961 kg/h The electricity demand is summarized in Table 2. The power demand of the components engaged in the fuel gas handling and reliquefaction have already been excluded from the below values, as they are directly computed by the COSSMOS simulation model. The nominal values of all three compressors are shown in Table 3. 4.2. Components configurations The application case simulations are divided in two large categories, based on the main engine technology, namely these of lowpressure and high-pressure engines. Each main engine is combined with different reliquefaction technologies and the following set-

4.2.1. High-pressure engine 1. JT system consisting of two identical fuel gas compressors, in a parallel arrangement raising the fuel gas pressure at 300 bar. Each one has a capacity of 6700 m3/h and a nominal power of 1650 kW. In JT1 operation, only one compressor is activated, while in JT1þ1 operation, both compressors are activated sharing the BOG handling load (see Fig. 7). 2. MR2.5 electric driven unit (as shown in Fig. 8): a. Reliquefaction capacity: 2500 kg/h b. Nominal power: 1500 kW

4.2.2. Low-pressure main engine 1. JT-Hyb system involving the following equipment (see Fig. 9): 2. MR2.5 electric driven unit (see Fig. 10): a. Reliquefaction capacity: 2500 kg/h b. Nominal power: 1500 kW The above nominal values are the design points of each component. COSSMOS models have been used to simulate the operation at partial loads, predicting the corresponding power consumption based on typical turbine efficiency curves and data from previous experience.

4.3. Cargo and fuel characteristics A typical LNG cargo has been considered having the composition provided in Table 4, with different values between the liquid state (cargo) and the vapor state (boil-off). The Lower Heating Values (LHV) of each fuel employed in the model is as follows:  MGO: 42,700 kJ/kg.  HFO: 40,100 kJ/kg. This value is also used for the calculation of the equivalent HFO consumption.  LNG: 50,000 kJ/kg. This is the reference value. The LHV of the cargo LNG is calculated using the composition of Table 4. The actual LHV of the fuel gas burned by the engines is different in the following two cases and accurately calculated by the model: o The BOG recirculation in partial reliquefaction configurations (the phenomenon is analyzed in Section 3.3). This recirculation being nitrogen rich, when mixed with the natural BOG flow penalizes the LHV of the resulting mixture. This phenomenon is also captured by the model. o When the BOG is not adequate to cover the engine energy demand, forced cargo evaporation occurs. Cargo is extracted and evaporated from the bottom of the tank and then mixed with the natural BOG flow. The molar composition and the LHV of the resulting mixture are different from its components, and accurately calculated by the model. The prices of the considered fuels and the cargo are assumed as follows:

Fig. 6. Propulsion demand for both ballast and laden conditions, including 20% sea margin.

 HFO: 325 $/tn  MGO: 700 $/tn  LNG: 10 $/mmBTU (~475 $/tn); sensitivity analysis of the LNG cargo price is also performed.

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Fig. 7. High pressure main engine configuration, coupled with the JT reliquefaction system, capable of operating either in JT1 of JT1þ1 mode (see Paragraph 3.3).

Table 2 Electricity demand of application case, excluding the power demand of BOG handling and reliquefaction equipment. Electricity demand (kW) High-pressure ME Sea-Going 2040

Anchorage 1405

Low-pressure ME Sea-Going 2000

Anchorage 1347

Table 3 Joule Thomson hybrid system main components. Component

Capacity

Pressure outlet

Nominal power

FG compressor I FG compressor II Boosting compressor

6400 m3/h 6600 m3/h 3600 kg/h

16 bar 25 bar 150 bar

1250 kW 1800 kW 1500 kW

5. Results The results of this study can be divided in three groups, as follows: 1. 2. 3. 4.

Gas distribution break-down Energy performance results Reliquefaction efficiency results Price sensitivity analysis

Simulations have been performed for the range of 12 kn up to 20.5 kn and for the two anchorage conditions, laden and ballast. 5.1. Gas distribution break-down In this section, Figs. 11e14 correspond to a separate mode of operation of each configuration, wherever separated operation

modes can be distinguished (JT cases). These figures depict the distribution of the BOG among the components on-board (engines, reliquefaction unit and GCU), as well as the need of the forced cargo evaporation wherever this is necessary. The following colour codes were used:  Blue represents the BOG consumed by the main and auxiliary engines.  Orange represents the excess BOG reliquefied  Red represents the excess BOG burned at GCU  Grey represents the cargo forced evaporation Figs. 11e14 give a better understanding of the pros and cons of each system, in terms of GCU usage, which is usually preferred to be close to zero. From the results presented in Figs. 11e14, the following key points can be derived. Anchorage loaded condition is of particular interest as it is the mode with the lowest gas consumption (only auxiliary engines operate), resulting in maximum excess BOG: 1. The electric driven MR reliquefaction system is capable to handle the whole BOG amount along the speed range, leaving a portion (~17%) for GCU at the anchorage loaded condition, for both main engine technologies. 2. The JT partial reliquefaction system combined with the highpressure main engines cannot reliquefy the whole excess BOG quantity at low speed range (<15kn) and at anchorage loaded, when operating with 1 FG compressor (JT1 mode). On the other hand, when 2 FG compressors operate (JT1þ1 mode), zero GCU is guaranteed for all vessel conditions. Note that despite the fact that simulations have been performed down to 12kn, as zero GCU is observed in anchorage loaded (the condition where BOG excess is maximum due to minimum power demand and thus minimum consumption), zero GCU is also guaranteed at speeds

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Fig. 8. High pressure main engine configuration coupled with the electric driven MR reliquefaction system.

Fig. 9. Low pressure main engine configuration coupled with the JT-Hyb reliquefaction system, capable of operating either in HP, LP or JT mode (see Paragraph 3.4).

lower than 12kn, where the propulsion demand will always increase the BOG consumption by main engines.

3. The JT-Hyb performance, combined with low-pressure main engines, depends on the selected mode of operation. In JT mode (methane cooling loop deactivated), a portion of excess BOG

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Fig. 10. Low pressure main engine configuration coupled with the electric driven MR reliquefaction system.

Table 4 LNG cargo and BOG composition. Species

Cargo Molar %

BOG Molar %

Methane Ethane Propane Butane Isobutane Nitrogen

98.01 1.44 0.23 0.00 0.01 0.31

96.6 e e e e 3.4

When operating in the HP mode (all systems are activated), the whole quantity of the excess BOG is reliquefied by the system along the whole speed range and for both anchorage points, guaranteeing zero GCU usage under all circumstances. Consequently, it can be deducted that JT-Hyb is a very flexible system, able to adapt its energy efficiency to the reliquefaction needs of each vessel operational mode and speed. 5.2. Energy performance results

cannot be reliquefied and is burned at GCU, for vessel speeds <15kn and for both anchorage conditions (loaded and ballast). When operating in the LP mode (boosting compressor bypassed), a very small portion of the excess BOG cannot be reliquefied and is burned at GCU, for vessel speeds lower than 13kn, which is expected to slightly increase, proportionally, for speeds lower than 12 kn, which have not been simulated. A certain portion of excess BOG is also sent at GCU for anchorage loaded condition and a much lower proportion at anchorage ballast condition.

In this Section all the systems are compared in terms of the cargo and pilot oil consumptions, the equivalent HFO consumption, OPEX and the reliquefaction efficiency. The compared configurations and their corresponding operational modes are listed below: 1. High pressure engine with JT a. JT1 mode: laden speeds >15kn and anchorage ballast b. JT1þ1 mode: laden speeds 15kn and anchorage loaded 2. High-pressure engine with MR2.5 (no specific modes per vessel speed) 3. Low-pressure engine with JT-Hyb:

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Fig. 11. BOG distribution for high-pressure main engine configuration coupled with the electric driven MR2.5 reliquefaction system: (a) laden speed-range, (b) anchorage.

Fig. 12. BOG distribution for high-pressure main engine configuration coupled with the JT partial reliquefaction system operating in: 1. JT1 and 2. JT1þ1 mode; (a) laden speedrange, (b) anchorage.

Fig. 13. BOG distribution for low-pressure main engine configuration coupled with the electric driven MR2.5 reliquefaction system: (a) laden speed-range, (b) anchorage.

a. JT mode: laden speeds >15kn b. LP mode: laden speeds 15kn, this may leave a very small amount for GCU but in terms of electric consumption this mode has been preferred. c. HP mode: anchorage loaded and ballast conditions. For much lower speeds, where excess BOG increases HP mode may be selected to guarantee zero GCU, but such low speeds have not been simulated.

4. Low-pressure engine with MR2.5 (no specific modes per vessel speed). Ballast condition results are not presented in the study, but in terms of reliquefaction, zero GCU is guaranteed for all speeds, as the natural BOG flow is half of that of the laden conditions. From the results of Fig. 15, it can be noted that different systems perform better at different vessel conditions. For each metric the following can be observed:

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Fig. 14. BOG distribution for low-pressure main engine configuration coupled with the JT-Hyb reliquefaction system operating in: 1. JT mode, 2. LP mode and 3. HP mode; (a) laden speed-range, (b) anchorage.

1. Cargo consumption: High-pressure engines have higher efficiency by design, explaining the lower LNG consumption. For both main engines, when coupled with electric driven MR2.5 the overall system exhibits slightly lower LNG consumption, compared to the JT systems, mainly along the high-speed range, still achieving zero GCU usage. At the anchorage loaded mode, both the JT systems consume less LNG, as they can reliquefy the total excess BOG, whilst the MR systems have reached their nominal capacity and leave a certain amount for GCU. The lowpressure MEs with JT-Hyb result in slightly lower cargo consumption. 2. Pilot oil consumption: High-pressure engines have much higher pilot oil consumption by design, compared to low-pressure engines. 3. HFO equivalent: All fuels are expressed under the same base fuel (HFO with LHV ¼ 40,100 kJ/kg), making it a representative metric for the total energy consumption. All four configurations perform very close. High-pressure engines coupled with JT reliquefaction performs marginally better along the high-speed range, but when coupled with MR2.5 it performs even better at low speed range. 4. OPEX: Each fuel stream is multiplied with the assumed prices, as found in Section 4.3. Along with the high speed range, the fuel OPEX of all configurations is marginally the same. However, it is interesting that it may be better to use an MR2.5 reliquefaction unit instead of the JT systems, as in low speed range, it is proved to be more efficient to dump the excess BOG at GCU, than

operate the heavy consumers of JT systems (extra compressor for JT and methane cooling cycle for JT-Hyb) These heavy consumers increase the electric power demand and consequently the auxiliary generators load, finally increasing the pilot fuel consumption. On the other hand, if the charters demand is to maintain as much LNG as possible in the cargo tanks, then JT systems should be used despite the additional OPEX cost. In order to more directly compare the 4 configurations, exact numbers are presented in Table 5 for 3 speeds (low, medium, high) and the anchorage loaded port condition. For each of the modes of Table 5 the following results can be drawn.

5.2.1. High-speed (19kn) Top performer: High-pressure ME with JT. The rest of the designs have lower performance by:  High-pressure ME, MR2.5: 1.5% more HFO eq. and almost 0% OPEX.  Low-pressure ME, JT-Hyb: 3.5% more HFO eq. and 1.7% more OPEX.  Low-pressure ME, MR2.5: 10.6% more HFO eq. and 2% more OPEX

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Fig. 15. System energy performance comparison: 1. Cargo consumption, 2. Pilot oil consumption, 3. HFO equivalent consumption, 4. Fuel OPEX; (a) speed range, (b) anchorage.

5.2.2. Medium-speed (16kn) Top performer: High-pressure ME with JT. The rest of the designs have lower performance by:  High-pressure ME, MR2.5: 4.4% more HFO eq. and almost 1.6% OPEX.  Low-pressure ME, JT-Hyb: 6.3% more HFO eq. and 3.8% more OPEX.  Low-pressure ME, MR2.5: 14.1% more HFO eq. and 3% more OPEX

5.2.3. Low-speed (12kn) For low speeds the results change depending on which parameter we want to optimize, HFOeq or OPEX, because the quantity of BOG excess is higher at low speeds and the way it is handled plays a major role. However, both designs with electric driven MR2.5 reliquefaction units perform better at this speed as they manage to handle all excess BOG with lower electric power consumption compared to their partial counterparts. However, for speeds lower than 12kn, where the excess BOG quantity is further increased,

partial systems are expected to be more promising as they are capable of reliquefying the high BOG quantities, while the electric driven ones cannot. This will also be seen in the anchorage loaded mode. 5.2.4. Anchorage loaded Low-pressure with JT-Hyb reliquefaction system has the best performance at this mode, as, along with the JT system of highpressure engines, they are the only systems that can reliquefy the total amount of excess BOG, which is maximum at this port condition. The rest of the designs have lower performance by:  High-pressure ME, JT: 17.8% in HFO eq. and 23.4% in OPEX  Low-pressure ME, MR2.5: 59.3% in HFO eq. and 40.2% in OPEX  High-pressure ME, MR2.5: 55.2% in HFO eq. and 46.8% in OPEX. 5.3. Reliquefaction efficiency results In this Section, graphs depicting the reliquefaction efficiency, as well as the separation efficiency of the partial JT reliquefaction systems are presented. The two expressions used are as follows:

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Fig. 16. Reliquefaction performance comparison: 1. Reliquefaction efficiency and 2. Separation efficiency; (a) speed range and (b) anchorage. From the results of Fig. 16, the following can be inferred for the system reliquefaction efficiency.

Table 5 Performance of4 LNG configurations in terms of equivalent HFO and fuel OPEX, for 3 vessel speeds and the anchorage loaded port condition. 19 kn

Low-pressure ME, MR2.5 Low-pressure ME, JT-Hyb High pressure ME, MR2.5 High-pressure ME, JT

16 kn

12 kn

OPEX ($/day)

HFO eq. (tpd)

OPEX ($/day)

HFO eq. (tpd)

OPEX ($/day)

HFO eq. (tpd)

OPEX ($/day)

104.9 98.2 96.2 94.8

40,092 39,971 39,558 39,290

74.4 69.3 68.1 65.2

28,500 28,727 28,125 27,668

47.2 51 44.7 46.9

18,100 21,414 18,519 20,464

43.8 27.5 42.7 32.4

17,460 12,454 18,282 15,365

5.3.1. Reliquefaction efficiency It is defined as the actual cargo energy that returns to the cargo tanks, after having subtracted the energy that is required to reliquefy the excess BOG quantity. In other words, it can be expressed by the following expression:

hRLQ ¼

RLQ $LHVLNG


Wel;RLQ effAE

RLQ $LHVLNG

Anch. Loaded

HFO eq. (tpd)

5.3.2. Separation efficiency It is defined as the ratio between the resulting liquid after the reiqfuefaction unit to the total excess BOG quantity entering the unit. In other words, it can be expressed by the following expression:

hS ¼ (1)

RLQliq;out RLQliq;out ¼ NBOGin RLQliq;out þ RLQvap;out

(2)

where. where.  RLQ: Reliquefied BOG quantity  LHVLNG : Lower heating value of LNG cargo  Wel;RLQ : Electric power required for the reliquefaction of the RLQ quantity  effAE : Efficiency of auxiliary engines W  effel;RLQ : It is the power consumed by the AEs corresponding to the AE reliquefaction work.  Wel;RLQ : The electric power consumption of the reliquefaction system. For electric driven units, like MR2.5, it is directly equal to the power demand of the system. For the JT system, it is calculated indirectly as the additional load the fuel gas compressors have to carry due to recirculation. For the JT-Hyb system, it also includes the extra compressors activated at each operational mode (boosting compressor and methane cooling loop compressor).

 RLQliq;out : liquid cargo after the RLQ unit  RLQvap;out : leftover vapor gas after the RLQ unit  NBOGin : excess NBOG entering the RLQ unit Separation efficiency is mainly used for the JT partial reliquefaction efficiency systems. This means that a certain amount of excess BOG remains as vapor after the Joule-Thompson expansion valve and is recirculated to the fuel gas compressors. The other BOG part, which has been liquefied, returns to the cargo tanks.

5.3.3. Partial JT systems Low-pressure main engine with the JT-Hyb reliquefaction system can achieve higher reliquefaction efficiency per speed range, compared to MR2.5, because JT-Hyb can adjust its operation mode at the vessel speed. In more detail, the system operates very

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efficiently along the low speed range (<15kn), where the boosting compressor is by-passed and only the methane cooling cycle is activated. Along the higher speed range, the two systems are swapped (boosting compressor is activated and methane cooling loop is deactivated) adapting its operation to the reliquefaction needs of the system, resulting in higher efficiency. For the high-pressure main engine with the JT system, the efficiency exhibits a constant behaviour where one FG compressor operates (JT1 mode), with a slight drop in efficiency at low speed range, where the second FG compressor is activated (JT1þ1 mode), because of the added of one heavy consumer.

5.3.4. Electric driven MR systems The electric driven reliquefaction systems operate along an efficiency curve, which is a characteristic of the stand-alone unit. The power consumption of the system is a reverse function of the reliquefied quantity, meaning that the specific power consumption is increased for low values of reliquefaction. This can also be observed in the dashed lines of Fig. 16 1(a). The difference between the low- and high-pressure main engines at high vessel speeds can be explained by the difference between the two technologies; by design, high-pressure main engines have higher efficiency compared to low-pressure engines. Regarding the liquid/vapor separation efficiency, this parameter is only relevant for the partial reliquefaction systems, as a metric to evaluate the recirculating vapor after the JT expansion. The JT-Hyb

system exhibits better separation (more liquid in the mixture after JT valve) because of the additional methane cooling loop, which brings the excess BOG closer to the liquid state. The electric driven MR units liquefy the whole (~99%) inlet BOG excess, up to their nominal capacity, sending the rest to GCU.

5.4. Price sensitivity analysis In this Section, the LNG cargo price ranges between 6 and 12 $/mmBTU, while the MGO price is kept constant and equal to 700 $/tn. The overall fuel OPEX of each configuration is plotted Figs. 17e19, in order to capture the impact of the LNG price. Each diagram refers to one vessel speed at the laden condition, namely one low speed (12 kn), one medium (16 kn) and the design speed of the vessel (19.5 kn). From Figs. 17e19, it can be deducted that the compared configurations exhibit very similar performance at higher speeds, where the major part of the natural BOG is burned by the engines. At lower speeds more excess BOG remains unburned, which is either reliquefied completely by consuming more energy (JT and JTHyb systems) or part of it is reliquefied by the electric driven units (MR2.5) and the rest burned at GCU. In more detail, at low speeds (<12 kn) the high and low pressure MEs coupled with the JT systems exhibit similar fuel OPEX when the LNG price is very low (<6 $/mmBTU), whilst their gap increases for higher LNG prices.

Fig. 17. LNG cargo price sensitivity analysis on total fuel OPEX for vessel speed 12kn.

Fig. 18. LNG cargo price sensitivity analysis on total fuel OPEX for vessel speed 16kn.

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Fig. 19. LNG cargo price sensitivity analysis on total fuel OPEX for vessel speed 19.5kn.

At mid-speed range (16 kn) and high LNG prices, the highpressure ME with the JT has the lowest OPEX and low-pressure ME with JT-Hyb has the highest OPEX, whilst when these MEs are combined with the MR2.5 the resulting cost is between the above values. For very low LNG prices, the JT systems perform marginally better than the MR ones. Finally, for high speeds (around design speed of 19.5 kn), high LNG prices slightly favour the JT reliquefaction systems, whilst for low LNG prices the behaviour is marginally inversed.

6. Conclusions This paper presented the existing reliquefaction technologies for LNG carrier vessels, focusing on the comparative assessment of the two main philosophies, electric driven (selecting the mixed refrigerant system) and the partial reliquefaction systems, including the latest developed hybrid design. Mechanical propulsion with two-stroke dual-fuel main engines has been considered, including both high- and low-pressure technologies. The model-based assessment of the integrated system presented in this paper proved to be particularly useful to capture the complex effects of natural gas on-board an LNG carrier. DNVGL COSSMOS modelling framework has been used encompassing different machinery systems and running simulations for timevarying operational profiles (different vessel speeds and anchorage port conditions). The developed models were used in a number of model-based design and technology comparative studies, according to the most current reliquefaction systems. High pressure main engines have traditionally been coupled with partial reliquefaction systems; when coupled instead with an electric driven MR unit, in low speed range it is proven more economically efficient to dump excess BOG at GCU (if zero GCU usage is not the target), than operate the heavy consumers of JT systems (second FG compressor). On the other hand, low-pressure main engines are traditionally coupled with electric driven MR units; partial reliquefaction JT-Hyb system has recently been developed to suit this type of engines, offering improved flexibility and efficiency per vessel mode. Latest partial reliquefaction systems have been found promising for low speeds operation and port conditions as they are capable of zeroing out GCU usage. However, they are more energy consuming compared to traditional electric driven reliquefaction units, acting indirectly as dumpers of excess BOG through electricity production. Indicatively, when using partial reliquefaction technologies, the

integrated ship system for low pressure engines exhibits 40% higher performance at anchorage loaded port condition, while for high pressure engines 25%, compared to the rest of the options that have been studied. The same result can be projected for low speeds (<12kn) but with a lower percentage difference. At medium speeds, high-pressure ME engine coupled with JT reliquefaction technology performs 5e15% better compared to the rest of the reliq. Options (depending on the option that is compared to). At high speeds (>19kn), low-pressure main engines become less attractive compared to high-pressure ones, as their lower efficiency makes BOG forcing compulsory, resulting in 3.5% lower performance when coupled with JT-Hyb system and 10% when coupled with MR2.5. The above numbers refer to pure fuel energy consumption, using the HFO equivalent KPI. When energy needs to be translated in money, the balance between the LNG and MGO price is crucial. This balance is also dynamic along the years, which may alter, even completely the above results. However, for the price assumptions of this study (page 12), the above trends are maintained but with lower percentage values. The complementary sensitivity analysis of Section 5.4 clarifies how the trends can change along the LNG price fluctuations. Concluding, it is clarified that when selecting between the wide variety of machinery options there is not a global optimal configuration, but the result may change case by case. What this study has clarified is that each system has different benefits at different modes (i.e. high-speed range, anchorage laden, etc.) and there is not one system that outshines the rest for all cases. To this end, each system must be studied under the scope of the considered trading route and mission profile, where the different performance at different vessel modes will be averaged, based on their frequency in the operational profile, making the available options easily comparable. References [1] Majo D, Alappat Ben Austin B, Sreekanth Sarma PV, Aravind M, Kiran M. Process modelling and validation of LNG reliquefaction. Global Res Develop J Eng 2018;3(2). January.
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