GASTECH 2OO2

PROGRAMME

Kazuaki Yuasa, Group manager, Ship & Ocean Engineering Department, Mitsubishi Heavy Industries Ltd

KAZUAKI YUASA is the Group Manager of the Ship and Ocean Engineering Department of Mitsubishi Heavy Industries, Ltd. He graduated from the University of Tokyo and finished his master course of the same university. He joined the company in 1977, and since then he has been working mainly in the principal designing of merchant vessels especially gas tankers as project manager. He was involved in almost all LNG and LPG projects since 1980.

Kazuhiko Ohtake, Manager, Ship & Ocean Engineering Department, Mitsubishi Heavy Industries Ltd

KAZUHIKO OHTAKE graduated from Nagoya institute of technology and joined Mitsubishi Heavy Industries Ltd in 1982. He has been engaged in designing ship machinery equipment for merchant vessels, particularly for BOGÅ@re-liquefaction system for LNGC recently, and now he is the Engineering manager of the Ship and Ocean Engineering Department of Mitsubishi Heavy Industries, Ltd.

Masura Oka, Senior Engineer, Ship Designing Department, Mitsubishi Heavy Industries Ltd

MASARU OKA graduated with a master degree in Physics from the university of Kyushu and joined Mitsubishi Heavy Industries, Ltd in 1992. He has been engaged in designing ship machinery equipment, particularly for LNG carriers, and now he is the senior engineer of the Initial Designing Section of the Ship Designing Department.

Hiroyuk Ohira, Engineer, Ship Designing Department, Mitsubishi Heavy Industries Ltd

HIROYUKI OHIRA graduated with a Masters degree in Naval Architecture from the University of Kyushu and joined Mitsubishi Heavy Industries Ltd in 1984. He has been engaged in the initial design of ships, particularly for LNG carriers and now he is the engineering manager of initial designing section of the Ship Designing Department.

Subject : Proposals for LNGC Propulsion System with Re-Liquefaction PlantReview from the World's First Application

Hiroyuki OhiraManager, Ship & Ocean Engineering Department

Mitsubishi Heavy Industries Ltd., JapanMasaru Oka

Senior engineer, Ship Designing DepartmentMitsubishi Heavy Industries Ltd., Japan

Kazuhiko OhtakeManager, Ship & Ocean Engineering Department

Mitsubishi Heavy Industries Ltd., JapanKazuaki Yuasa

Group manager, Ship & Ocean Engineering DepartmentMitsubishi Heavy Industries Ltd., Japan

AbstractDiscussions on alternative propulsion systems for LNG carriers have taken place over many years, while,during this time, only steam turbine propulsion systems using gas/oil burning boilers have beenemployed.

“LNG JAMAL”, an LNG carrier, is a conventional vessel except for her boil off gas (BOG) re-

liquefaction plant.　 Since her delivery at the end of 2000, she has been used in LNG transportationservice from Oman to Osaka, with saving boil off loss of cargo.

The world’s first onboard BOG re-liquefaction plant was developed for the profit by mainly

using fuel oil for propulsion and preserving cargo LNG　 She is equipped with one BOG re-liquefactionplant using nitrogen coolant in the BRAYTON cycle, with enough capacity to process normal BOG atladen voyage. The re-liquefaction capacity is adjusted from about 33% to 100% continuously, and its

control is linked to the cargo tank pressure control.　　Full automation is realized for saving ship’s work,not only for normal liquefaction operation during both laden and ballast voyages, but also for system cooldown operation at start-up. No special staff is required for the operation of the BOG re-liquefaction plant.

This paper introduces the "Onboard BOG re-liquefaction plant" with its successful voyage, and suggests anew alternative propulsion system for an LNG carrier, especially focusing on a multi-diesel enginepropulsion system with a BOG re-liquefaction plant.

1. IntroductionMitsubishi Heavy Industries, Ltd., (MHI) is a pioneer in liquefied gas carriers, having built the world’sfirst large sized refrigerated LPG carrier, the “Bridgestone-maru”, in 1962. Currently, MHI holds the topmarket share in the construction of both large sized LPG carriers and LNG carriers.

Since 1983, MHI has constructed nineteen (19) Moss spherical tank LNG carriers and is designing nine(9) LNG carriers including three (3) membrane type LNG carriers. MHI introduced the so-called “secondgeneration LNG carrier concept,” characterized by a lower boil-off rate (BOR) with a forcing vaporizersystem, applying it in the Australian North West Shelf project in 1989. This concept has since becomethe world standard because of its economical merits and operational flexibility.

MHI has also been studying Gaz Transport membrane tank LNG Carriers in detail since a licenseagreement in 1973, in order to realize more advanced membrane ships. In 1999, MHI was awarded thecontract for the membrane LNG carriers of a Malaysian project as the lead yard. These membrane ships

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are now under construction at the shipyards.

In this paper, MHI’s technical development of the world’s first application of a boil-off gas (BOG) re-liquefaction plant and a proposal for a new alternative propulsion plant for the future will be explained.

2. Technical development of Mitsubishi LNG carriers

MHI introduced LNG technologies for the Technigaz membrane system in 1969, the Moss spherical tanksystem in 1971, and the Gaz Transport membrane system in 1973.Since then, MHI has undertaken many activities such as:¸ first generation LNG carriers;¸ second generation vessels with the Moss spherical tank system;¸ 135,000 m3 LNG carrier with a very low BOR of 0.10%/day;¸ Gaz Transport type membrane LNG carriers for a Malaysian project;¸ first BOG re-liquefaction plant on an LNG carrier;¸ 145,000 m3 LNG carrier for a SNOHVIT project; and¸ environmental friendly LNG carrier.An outline of these matters is explained below.

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2.1 First generation LNG carriersIn 1983 and 1984, as the first generation LNG carriers built in Japan, MHI delivered the “Banshu maru,”the “Echigo maru” and the “Dewa maru.”The principal dimensions and hull form were selected to optimize their resistance, propulsiveperformance, maneuverability, and fuel economy. Their hull form and rudder area were determined onthe basis of hull behavior analysis.Through these experiences, MHI obtained its quality control method in the design and building of LNGcarriers.

2.2 Second generation LNG carriers with the Moss 4 tank systemIn 1986, MHI was awarded the contract for a second generation LNG carrier with the Moss 4 tank vesselas the lead yard, and delivered a 125,000 m3 vessel to the Australian North West Shelf Project in 1989.Conventional LNG carriers were of a 5- or 6-tank design to maintain basic performance easily, but a 4-tank ship is much easier to operate, maintain and repair. It also leads to a reduction in BOR and in theinitial investment.The BOR in a conventional LNG carrier was 0.25%/day. Studies were conducted to optimize the BORunder overall operating conditions for this project, and it was determined to adopt a BOR of 0.15%/day.A booster fuel during a ship’s high speed can be chosen from fuel oil or fuel gas, whichever is moreeconomical at the time.To allow for a difference in the vertical transformation of the cargo tank and pipe tower, the bottom partof the pipe tower incorporates flexible construction.In order that the heat ingress from the skirt part is reduced, a stainless steel thermal brake is insertedbetween the aluminum alloy skirt and steel skirt.Horizontal stiffeners are also adopted in the skirt parts for design refinement and for cooling downquicker.

2.3 135,000 m3 LNG carrier with a very low BORSucceeding and developing the design know-how from the previous LNG carriers, MHI has realized the135,000 m3 Moss spherical tank type LNG carrier “EKAPUTRA,” which was the largest LNG carrier inthe world at the time and on which a very low boil-off rate (BOR) of 0.10%/day was realized for the firsttime in the world.A five-tank system was selected in this case and considering the results of the systematic study, whichwas carried out in various combinations of tank sizes and principal dimensions, a smaller diameter (35.74m) was selected for Nos. 1 & 5 tanks and a larger diameter (38.62 m) for Nos. 2, 3 & 4 tanks. The shipwas confirmed during the sea trial to have the expected propulsive performance and praised highly forhaving a superior hull form and high efficiency propeller.

2.4 Gaz Transport membrane LNG carriers for a Malaysian projectIn 1999, MHI was awarded the contract for the membrane LNG carriers of a Malaysian project as the leadyard.Computational fluid dynamic (CFD) has developed dramatically during the past two decades in allengineering fields where fluid flow phenomena takes place. CFD is used not only for research purposesbut also for the design of hull forms and propellers in the field of ship hydrodynamics.The CFD code can predict not only the flow field but also wave patterns, resistance, pressure distributionon a hull surface, and self-propulsion factors. These calculations result in important information for theimprovement of a ship’s hull form.Sufficient strength of the inner hull and insulation structure is particularly important for membrane LNGcarriers, and must be maintained throughout the vessel’s life. The basic strength of the inner hull hasbeen verified in the global stress analysis by the Finite Element method for the entire hull model thatc o m p l i e s w i t h L R S D A ( s t ruc tu ra l des ign a s se s smen t ) p rocedure .

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The fatigue strength of the inner hull and contiguous structure, which is one of the most importantfeatures of the membrane LNG carrier, has been verified by hot spot stress assessment using a fine-meshF.E. analysis that conforms to LR FDA (fatigue design assessment) notation. The fatigue strength ofsquare corners, as stress concentration parts, has been verified, as well as that of the hopper. In addition,the Discrete Analysis Method (DISAM), which has been developed by MHI, was carried out using thesame F.E. model. DISAM performs a simulation of direct wave pressure, and long-term prediction ofstresses on the specific trading route of a subject vessel, taking the complex effects of several dynamicload components into account.

2.5 First BOG re-liquefaction plant on an LNG carrier for the 21st centuryIn 2000, MHI delivered a 135,000 m3 LNG carrier for the Oman Project. This ship, which incorporateshigh safety, reliability and economical performance with innovative technologies, deserves to be the LNGcarrier of the 21st century.Special features are as follows.¸ Radar- t y p e l e v e l g a u g e

A radar-type level gauge is experimentally installed in the No. 5 cargo tank and is expected to reducemaintenance work. As approved level gauges, both a capacitance-type and a float-type level gaugeare also installed. Nowadays, a radar type level gauge is installed on an LNG carrier as standardequipment.

¸ I n t e g r a t e d b r i d g e s y s t e mFor a safe voyage, navigational information is centralized by integrating the navigational equipment,and providing a split level wheelhouse with a 360 degree view.

¸ S t r e s s m o n i t o r i n g s y s t e mThe acceleration of the ship and stress on critical parts are monitored. With this monitored data,external forces assumed in the design can be verified and the service life of the hull accuratelyevaluated.

¸ B O G r e - l i q u e f a c t i o n s y s t e mAll natural BOG can be re-liquefied by this system, which has been firstly adopted in the world forLNG carriers. BOG can be also used as fuel, the same as for conventional LNG carriers. Thissystem allows the ship to be operated in the most economical way with regard to fuel cost. This willbe explained in detail below.

In this paper, the alternative propulsion system is suggested especially focusing on that combined with theMHI developed BOG re-liquefaction plant, which has the possibility to improve the cost of transportationdrastically, and to simplify the ship’s propulsion system including BOG handling.

3. Onboard BOG re-liquefaction plant on LNG JAMAL

Osaka Gas, Nippon Yusen Kaisha, Chiyoda Corporations, and Mitsubishi Heavy Industries havedeveloped a new concept LNG carrier, the S/S LNG JAMAL, built at MHI and delivered at the end of2000. Her propulsion system is of conventional steam turbine with dual burning boilers, but she is alsoequipped with the world’s 1st onboard LNG boil off gas (B.O.G.) re-liquefaction plant. After her delivery, she has been mainly engaged in the LNG transportation from Oman to Japan onmonthly round trip voyages. Since a single carrier is used for the Osaka-gas/Oman project, highestreliability and redundancy is required. The basic design concept was to provide the same function as aconventional vessel, in the event during the carrier runs into problems on the BOG re-liquefaction systemunexpectedly.

Normally, BOG re-liquefaction is done all through the laden voyage, and also for ballast voyage tomaximize the benefit of LNG transportation. All through the voyage, the cargo tank pressure ismaintained by BOG re-liquefaction plant. The BOG re-liquefaction plant is completely stopped during

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loading/discharging. In her work pattern, the system is required to start/stop at about every two weeks.MHI developed a full automatic process control system for onboard use, and it is realized unmannedoperation during normal re-liquefaction. Thewhole system could be operated by a ship’sengineer at cargo control console, and a staff atthe machine side even at the start up of thesystem. No additional staff member or speciallytrained engineer is required.

3.1　PROCESS BOG is introduced from the cargo tank via apipeline, and then supplied into a cold box with apressure of about 3 bar(g) by means of doublestage compression. Double stage compression isdone by 2 sets of single stage compressorsconnected in series. (Figre3.1) This is to achieve ahigher efficiency by higher condensation pressure.Supplied gas is cooled by refrigeration nitrogen inthe box. Condensed gas at a temperature of about –150 deg.C is received in the pressure vessel, andseparated liquid led to the sub-cooler for sub-cooling until about –165 deg. C and then returned to thecargo tank by the pressure itself.

A nitrogen refrigeration circuit is applied for the generation cold for cooling and condensation of BOG.BRAYTON cycle is adopted for this circuit from the point of view of reliability, simplicity and safety,comparing with the process of direct compression and expansion. The idea of mixed refrigerant, which isnormally hydrocarbon mixture, was also omitted based on the same point of view.

Fig.3.2 Nitrogen circuit

Fig. 3.1 BOG supply system to boiler

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3.2 MAIN COMPONENT

Heat exchangersAluminum plate fin heat exchangers were used for cryogenic part. They are nitrogen pre-cooling, BOGcondenser, and LNG sub-cooler. In general, a larger heat exchanger area can achieve liquefaction higherefficiency, that is, reduce fuel consumption. The cores are so designed to be compact for the arrangementin an existing cargo machinery room.A cold box filled with insulating material is applied for the cryogenic heat exchanger, and nitrogenexpander, because the heat invasion on the cryogenic component badly effects on the performance of there-liquefaction system.

Steam turbine driving nitrogen compressorA recycle nitrogen compressor of centrifugal, two stage type and booster compressor of centrifugal, singlestage type is applied for nitrogen cycle.The nitrogen compressor consumes more than 90% of the system’s power consumption. The compressoris driven by a steam turbine with increasing gear-box, instead of electric motor driving, for a reduction ofenergy conversion loss.

Expander driving booster nitrogen compressorThe single stage booster compressor driven by an expander turbine is provided on the downstream ofnitrogen compressor, to utilize generated power during the nitrogen expansion process.

BOG compressor:S/S LNG JAMAL is not equipped forcing vaporizer because the her project has no intention to use cargoLNG, However, she also has the spare spaces to install the vaporizer with minimum modification in thefuture, considering the flexibility in unforeseeable future market change.The design condition, which complies the following required condition, is summarized on Table 3.1.- 100% fuel gas burning at MCR- Re-liquefaction of normal BOG.- Treat more than normal BOG by using

both the re-liquefaction and the gasburning simultaneously.

Table 3.1. Design condition of BOG compressor

Service condition BOG to betreated.

Condition

Less than3,000 kg/h

2 sets running in tandem(Disch.press.: 3 barg)

RL

service More than3,000 kg/h

One for boiler ,other for RL(Disch.press: 1/1.3 barg)

Less than3,400 kg/h

1 set running(Disch.press: 1 barg)

Boiler

service 3,400 to 6,800kg/h

2 sets running in parallel(Disch.press: 1 barg)

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Nitrogen generation systemRefrigerant nitrogen is generated from air by onboard system. No nitrogen bottle is required.

3.3 OPERATIONFig.3.3 shows the typical operation pattern.

n Start up of the plantBasically, the re-liquefaction plant is not running at the port. The re-liquefaction plant is started afterdeparture from the loading/unloading port, and takes about 3-4 hours, for the system to cool down.The cryogenic component, piping and heat exchanger for example, are first cooled down along with timeschedule, to avoid extreme heat stress. Cooling down shall be done by the system itself, that is, from coldgenerated by the nitrogen circuit. A full automatic control system is developed to enhance the operationalreliability using less manpower.Temperature profile at each point is automatically controlled within allowable range.System cool down can be done automatically by monitoring from the cargo control console.

n Laden voyageCargo tank pressure is automatically controlled in line with the pre-set time schedule at “normal” re-liquefaction operation after the completion of cool down.Monitoring of all necessary information can be done at the cargo control console.

n Ballast voyageTo maintain the cargo tanks temperature within the allowable range, small amount of LNG(hereafter, HeelLNG) is carried during ballast voyage by liquid spray operation in the tank.BOG evaporated from Heel LNG can be liquefied and then returned to the cargo tank through spraynozzle. The cargo tank pressure is controlled automatically in line with the pre-set time schedule providedby the ship operator, in the same manner as during a laden voyage, of course.

n Line cool downCargo loading line is to be cooled down for the preparation of in advance of cargo loading/dischargingoperation. The ship executes the line cool down operation before the next port, and the much more BOGthan normal is generated during this operation. When BOG exceeds the capacity of the re-liquefactionplant, any excessive BOG is introduced and burnt in the boiler simultaneously.

n Cargo loading/DischargingThe re-liquefaction plant is stopped before the ship arrives at port, according to the project requirement,and not running during loading/discharging operation.

Figure 3.3 Typical operation pattern

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4. Proposal of an alternative propulsion system with an onboard BOG re-liquefaction plant

4.1 Overview

Conventional systemA steam turbine and gas/oil-burning (hereafter, dual burning) boiler have been used for almost all LNGcarriers ever built because of the established reliability and flexibility inherent in dual burning capability.

Normally, BOG is generated by heat penetrationthrough the insulation, dynamic energy caused bythe ship’s motion, and cargo operations such ascargo tank spraying and line cool down.

For a 135,000m3 class LNG carrier with BOR of0.15%/day and ship speed of around 19.5 knots,the normal quantity of BOG for loaded voyagescorresponds to about a half of the requiredpropulsion energy at MCR by a steam turbineplant, so additional fuel oil or fuel gas is used forcovering any higher load. The quantity ofadditional fuel oil or fuel gas must be adjusted tocompensate for any difference between therequired steam load and BOG generation. As anexample, Figure 4.1 shows the balance of BOGand fuel oil consumption.

When the available heat for propulsion from BOGexceeds the required energy for propulsion, whenmaneuvering for example, the excessive heat mustbe dumped into steam condenser. Such a situationsometimes occurs during line cool down before theship approaches the loading/discharging port.

Some ships are also equipped with forcingvaporizer for utilizing the cargo LNG as fuel forcovering the excess boiler load. The use of bunkercan be minimized in this case, although cargoLNG has to be consumed as fuel.The forced vaporization of LNG is of courseuseless especially in a case where the LNG price ishigher than the bunker price. However, there is merit in the flexibility of fuel selection if bunker oilbecomes higher than the LNG price in the future.

BOG re-liquefactionHow to treat BOG has been a main issue not only for the modification of an existing system, but also forthe study of new propulsion systems. For more than ten (10) years, alternative versions of propulsionsystems for future use have been studied and discussed.

Existing LNG carrier and feasibility studies for alternatives have put emphasis on how to use LNG as fuelfor propulsion, because the value of BOG is sometimes estimated at less than that of LNG, even forced

Fig.4.1 Heat source and energy distribution

Fig. 4.2 BOG supply system (Conventional plant)

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BOG, and that was the only way to go. The S/S LNG JAMAL is the world’s first vessel using BOG re-liquefaction, and based on this experience, MHI developed a BOG re-liquefaction system suitable for ahigh efficiency engine propulsion system.The main features are summarized as follows:

Simple designProcess and process control are simple enough to be operated by normal LNGC onboard engineers whoare not specially trained for processing plant operation. No additional staff is required; the staff sizecount is the same as that for a conventional vessel.

Full automatic sequence & controlThe fully automatic control system was developed to save ship staff duty time needed for maintaining thereliability of operation.At the start-up/stopping: The entire operation can be carried out remotely and is supported by

automatic sequence control.At normal sea going: The liquefaction operation is automatically adjusted continuously from about

30% to 100%.

Cargo tank pressure and equator controlThe tank pressure is maintained automatically within the allowed range for both laden and ballastvoyages. Cargo tank cooling down during a ballast voyage is possible by using the heel LNG as a coolant.BOG is re-liquefied and sprayed back into the cargo tank using a spray nozzle. The evaporating gas coolsdown the cargo tank. The quantity of heel LNG can be reduced and the loading quantities maximized.

Simultaneous operation with gas burningGas burning is possible by using the auxiliary boiler for the generation of ship service steam of dual fueltype, and off gas from the liquefaction plant can be burned and utilized as a heat source of steam.The steam can be used for the power for the liquefaction itself.

An LNG carrier with a re-liquefaction plant can be has the merit of many different options for type ofpropulsion. The S/S LNG JAMAL, a conventional turbine ship, found economical advantages related tore-liquefaction by using cost advantages related to sea route. Bunkering prices are reasonable inSingapore, on the way from Japan to Middle East, and the economical advantages of a re-liquefactionplant could be realized, even though she has a steam turbine plant. Table 4.1 shows the possiblecombination of a propulsion system with a re-liquefaction plant. In order to simplify the discussion in thispaper, options requiring fuel other than HFO or BOG are omitted.

Table 4.1 Propulsion systems combined with re-liquefaction plantfCategory Steam turbine Diesel engine + RL plant Electric propulsion

Propeller & shaft.Single screw, FPP,

single shaft.

Twin screw, FPP

Twin shaft.

Single screw, CPP

single shaft.Single screw(CPP) or twin screw(FPP)

Steam turbine with

reduction gear2-stroke engine direct

4-stroke engine

with reduction gearElectric propulsion motor

Engine plantSteam turbine

generator4-stroke diesel engine (HFO) generator Dual fuel or gas engine

driven generator (*1)

*1 Dual fuel diesel engine, lean burn gas engine (Gas with pilot fuel), gas turbine etc…

The steam turbine plant, and the system equipped with a gas-use (LNG) internal combustion engineprovide flexibility in fuel selection for responding to the market price of HFO and LNG. BOG can be re-liquefied when its price is evaluated as being higher than HFO, and they can be used as fuel when it is

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evaluated as being lower.But the steam turbine has disadvantages concerning fuel consumption, and the internal combustion enginehas disadvantages related to simplicity and safety. In most cases, a gas-use engine requires much highergas pressure than the 20 bar for a gas turbine, and than the 120 bar for a dual fuel diesel engine forexample, while less than 1 bar is required for an existing steam ship.A pipeline of highly pressurized flammable gas on deck or in an engine room is involved in these options.

Motivation for new propulsion systemWhile many aspects, such as economy, reliability, safety, maintainability, and environmental issues, haveto be evaluated in the case of a new propulsion system, economy is the most important factor to berealized. A ship’s economy is measured by transportation cost, which depends on running cost, and theinitial investment. Apart from the initial investment, fuel economy, i.e., fuel consumption, is a dominantfactor not only for economy, but also in environmental evaluation.The high efficiency characteristics of a large bore 2-stroke diesel engine promoted its replacement of thesteam turbine for the main engine. For a steam turbine plant, about 30% of generated heat in the boiler isavailable for the propulsion. On the other hand, a diesel engine improves the efficiency of the propulsionsystem drastically, especially for 2-stroke engines of more than 50%. Also, they still have heat availablefor energy recovery in exhaust gases, accounting for more than 26 % of total input heat for 2-strokeengines, and 30% for 4-stroke engines.

The reasons for the use of steam turbines in the LNG sector are summarized as follows.

1) Onboard LNG re-liquefaction plants did not exist.

2) 2-stroke diesel engines were not regarded as reliable enough for LNG transportation.

Dual fuel boilers and the idea of a gas-use internal combustion engine have been derived from the firstprovision, that is, boil off loss of cargo is inevitable. It was a special aspect of LNG transportationbecause of its cryogenic properties. However, the situation is changing since the successful result for theS/S LNG JAMAL.

4.2 Proposal: Diesel engine propulsion system with re-liquefaction plant

Based on the experience of the world’s 1st re-liquefaction plant, MHI developed a BOG re-liquefaction plant suitable for a high efficiencyengine propulsion system. Table 4.1 shows themain features of the equipment, an example of anew LNG carrier following the specifications of thelatest typical vessel.

Propulsion and auxiliary system

Main engine…The concept of a multi-engine design is applied to maintain the equivalent reliability with existingpropulsion systems. There are two alternatives, one is the twin direct driving engine, and the other iselectric propulsion. (See Figure 4.1)

Table 4.1 Main featuresType: 135-145KM3 class LNGC

Ship speed: 19.5 knot

Specification

of the ship

Boil off rate: 0.1 - 0.15%/day at laden voyage

Main engine 2 or 4-stroke diesel engine(HFO type)

Electric powergeneration

4 stroke diesel engine generator(HFO type)

Auxiliary boiler Gas/Oil firing 1 setSteam condition : 2.0MpaG sat.

Engine plant

Economizersteam gene.

HP, LP, and SH section 1 set2.0 / 0.6MpaG

Re-liq. plant 1-cold box x 2-nitrogen circuit,2-BOG compressor.

Cargo

machineryCargo heater& vaporizer

1- LNG vaporizer1- LNG gas heater

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Figure 4.1 Multi-engine concept

Steam generating system and nitrogen circuit …LNG liquefaction requires higher power inputs than that of other gas carriers. The drive power of thenitrogen compressor, which is the main consumer for liquefaction, requires more than 2,000kW fornormal BOG. The idea of a dual fuel auxiliary boiler involves a heat recovery system (hereafter, DFHR)combined with a re-liquefaction plant and is proposed for the two purposes below.

1. Reduction of additional power (fuel) for re-liquefaction2. Enhancement of flexibility of fuel utilization, especially for liquefaction.

Originally, an auxiliary boiler and an exhaust gas economizer were provided for the heating duty of cargoheater, vaporizer and fuel oil tank. The exhaust gas economizer supplies steam to the F.O. heating duringnormal sea going. The exhaust heat from main diesel engine is recovered by low/high pressure steam typeeconomizer for efficient heat recovery to drive the steam turbine. The system could recover about 1,000kW of the power assistance for driving the compressor.On the other hand, because of the circumstances that the fuel market does not make re-liquefaction byusing HFO attractive, that is, price of HFO is evaluated higher than BOG, the dual fuel boiler suppliesdriving steam for re-liquefaction by burning a part of the BOG.

Fig.4.2 shows the several alternatives for the nitrogen component. Type T is developed particularly forsteam turbine ships, and was used for the S/S LNG JAMAL. MT is an advanced type of combined withheat recovery steam circuit. A steam turbine and electric motor drive the nitrogen compressorsimultaneously, which is the main power consumer. MX type, which is combined with an expander andbooster compressor, has the advantage in minimizing the mechanical and pressure loss of the pipingbetween them. The MT type is adopted for the proposal for the DFHR system.

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Re-liquefaction plant…The re-liquefaction plant, shown in Figure 4.3, is proposed from the point of view of redundancy andoperability. Two sets of nitrogen circuits are provided with one set with cold box involving one set ofplate fin cores. According to the requirements of IGC, a total of more than 125% of rated capacity andtwo or more sets must be installed. The total capacity of refrigeration must be more than 125% of thedesign quantity of B.O.G. Also, one set must be able to treat normal BOG during laden voyages.Two (2) sets of B.O.G. compressor equipment are provided, and the capacity of each is designed to treatnormal BOG under the condition of tandem operation (Double stage compression).The auxiliary boiler of the dual fuel type is to be free from the full dumping of BOG, because theliquefaction plant provides full redundancy to treat BOG.

Electric power generating system…H.F.O. fueled 4-stroke diesel engines are used for prime movers for electric generators.Electric power is consumed mainly for ship propulsion, ship service, and the drive power for the re-liquefaction equipment.For the case of electric propulsion, the natural gas fueled internal combustion engine, dual fuel (NG/HFO)diesel engines, NG/MDO fuel lean burn engines, and NG fueled gas turbines are examples.

Cooling water system …A central fresh water-cooling system is used for cooling duty for the heat exchangers for propulsion andauxiliary equipment, except for the following.- Auxiliary turbine condenser- Steam dump condenser

Comparison of economyFuel consumption is evaluated for these options, including existing systems.

▲　Steam turbine plant + RL

■ Twin, 2 stroke main engine and 2 shaft +RL

Fig. 4.2 Nitrogen component Figure 4.3 Liquefaction equipment

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◆ Twin, 4 stroke, diesel engine, single shaft +RL

●　Electric propulsion-use 4 stroke diesel generator engine +RL

The fuel cost for a round voyage is shown on Fig4.5a. (Fuel cost is based on HFO consumption for each,and is converted by using HFO price rates.) Both BOG re-liquefaction and BOG utilization forpropulsion are shown by dot line, for steam turbine and electric propulsion. BOG consumption isevaluated as the equivalent fuel oil rate, and added on the fuel price based on the price ratio.A vertical scale price ratio of “0” indicates the fuel oil consumption for options where BOG has nocommercial value, and a vertical line on “1” indicates total fuel consumption based on the input heat.The 2-stroke diesel option has the highest efficiency, and the difference between the re-liquefaction andgas utilization is negligible for steam turbine and electric propulsion.

Fig.4.5b is also the same, but takes into account the profit for preserved cargo as a reduction of fuel cost.Saved LNG is evaluated using the corresponding fuel oil rate based on the ratio between prices of fuels.Fuel cost is lessened by this profit, and its reduction is remarkable and can be seen from the positivedirection on the horizontal axis, i.e., higher BOG price. BOG utilization for propulsion is not attractive forwide ranges in the price ratio, from the standpoint of fuel cost.

ConclusionA BOG re-liquefaction plant can be combined with any propulsion systems, with an expectedimprovement in efficiency through heat recovery, and flexibility of fuel selection by the provision of dualburning auxiliary boilers. This idea is also effective for conventional steam turbine ships, such as the S/SLNG JAMAL.

A combination of an oil fueled 2-stroke diesel engine for propulsion and 4-stroke diesel generator engine,the same as an LPG carrier, may be the best solution as a future option, even though higher powerconsumption is necessary for LNG re-liquefaction. This system can realize a drastic improvement of

Fig.4.5a Fig.4.5b

Remarks:Dual fuel diesel engine is applied to electric propulsion for BOGutilization, as example.

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efficiency, and the separation of the BOG treatment system from the ship’s propulsion system.

Electric propulsion with HFO fueled 4-stroke diesel generator engine is the third. However, its high fuelflexibility and reliability may attract some projects. The longer investment recovery period as a result ofhigher cost of electric propulsion compared to the steam turbine or diesel direct ones, is another aspect ofthis option.

5 Summary

The steam turbine plant has maintained an unchanging position on LNG carrier, because of its highreliability supported by long experience, and this may continue.On the other hand, MHI have been paid much attention to alternative options to meet future needs, andthe world’s first trial is one example of our efforts. We recognize that alternative propulsion system havenow achieved a higher degree of reality than ever before because of the S/S LNG JAMAL.

MHI is building its latest LNG carriers with both major containment systems, namely, the MOSSspherical tank type and the Gaz Transport membrane tank type. Finally we would like to say that we lookforward to continuing to supply LNG carriers that meet the needs of owners worldwide.

Lastly, we wish to express our sincere appreciation to Osaka Gas, Nippon Yusen Kaisha, ChiyodaCorporations and the Classification Society for their cooperation in developing BOG re-liquefactionplant.