Development of
Next-Generation
LNGC Propulsion Plant
and HYBRID System
MASARU OKA* 1
KAZUYOSHI HIRAOKA* 1
KENJI TSUMURA* 1

In recent business of new building LNG carriers, demands have frequently been made for a alternative propulsion system to replace the steam turbine propulsion. In the background to this are sharp increases in worldwide LNG demands and the advent of new style of LNG trade, leading to a strong intention to improve transportation costs by introduction of the enlargement of cargo tank capacity and alternative propulsion systems inmany new LNG projects. Furthermore, an abrupt increase in number of ships in service has led to a serious
shortage of propulsion system operators, who need to be skilled for steam turbine propulsion plant. These
occasions in recent years stimulated the development of new techniques represented by the boil-off-gas (BOG)
re-liquefaction plant and the duel fuel gas engine has been put into practical use in quick succession. Developments of alternative propulsion system are now being promoted mainly focused on diesel engine application,
while Mitsubishi Heavy Industries, Ltd. (MHI) is going ahead with the development of various types of propulsion systems. This paper introduces the HYBRID LNG system originated and developed by MHI, making mention also of the latest trends in substitute propulsion systems.

However, use of this propulsion plant has becomeless common in general merchant ships lately. In addition, it has become much more difficult to train andsecure crew members capable of meeting the sharp
increase in newly built ships because of the high levelof skill required. Mainly for these reasons, development of a substitute propulsion system has beenpromoted.

1. Introduction
All LNG carriers(Note 1) now in service adopt the steam
turbine propulsion system with duel fuel boiler. Since
the oil crisis, conversion of the propulsion system has
been made in almost all merchant ships, by the application of low-speed diesel engine (Note 2) directly coupledwith propeller. In the case of LNG carriers, however,the current system is adopted in all such carriers because it is the best method of treating BOG safely andefficiently.
In LNG carriers that transport LNG in the refrigerated condition, BOG is inevitably produced as a resultof heat coming from outside the tanks. In order to control the tank pressure rise, BOG is utilized as boiler fuelto get propulsive force by the steam turbine.
In cases where the combustion heat of BOG is in excess of the required propulsive force, surplus heat is
disposed of in the sea water. In cases where the combustion heat of BOG is less than the required propulsive
force, the required heat is made up by additional burning of heavy fuel oil (bunker oil) or forcing vaporizationof LNG. The current system is capable of using both
LNG and heavy fuel oil as fuel, and thus of selecting themost economical fuel mixture.
Moreover, it has a better serviceability ratio, reliability and maintainability among the propulsion systems
and is in no way inferior to the latter except in the aspect of thermal efficiency.

Note 1: The "LNG carrier" referred to here means aship for large-scale transportation of LNG in
the saturated condition at atmospheric pressure and does not include small-sizedpressurized tank type.
Note 2: 2-cycle long-stroke diesel engine

2. Alternative propulsion plant
2.1 Requirements for propulsion plant
For the LNG carrier propulsion system, there is arequirement not only for improved thermal efficiency but
also for safe and efficient treatment of BOG.
For efficient treatment of BOG, there are two methods; one is to use BOG as "fuel for various types ofengines" and the other is to "save it by re-liquefaction."
Techniques related to the re-liquefaction of BOG and gas
fuel utilized engines have recently been put into use rapidly in the marine field.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 6 (Dec. 2004)*1 Nagasaki Shipyard & Machinery Works

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Fig. 1
(1) BOG reliquefaction plant (Fig.
1)
The S/S LNG Jamal, built at MHI's Nagasaki Shipyard and put into service in November 2000, is an
LNG carrier equipped with the world's first LNGC
onboard re-liquefaction system. Having successfully
passed its overhaul inspection in first dry dock in
August 2003, this system has the operation result fornearly four years. With the conventional type steam
turbine adopted as its propulsion system, this carrieris operated using cheap heavy fuel oil, and saving BOG
by re-liquefaction on voyage with cargo loaded.
The re-liquefaction process adopts the intermediate cooling system using the Brayton cycle in which
nitrogen is used as refrigerant. In this process, BOG
is liquefied and subcooled in the pressurized condition, and is then returned to the tank. There is no
consumable supply including refrigerant nitrogen, forthe nitrogen is produced from the air on board. For
compression of BOG, a system in which two units ofcentrifugal single-stage compressors (motor-driven)
are adopted for the purpose of sharing it for boiler
supply, and they can also be connected in series forboosting supply to re-liquefaction.
For compression of nitrogen, centrifugal, threestage compression is adopted in which the 1st and2nd stages are driven by a steam turbine and the
3rd stage one is driven by a nitrogen expansion turbine for the purpose of the power recovery of
expansion process. Also, by development of a dedicated control device for re-liquefaction system,
automation of starting procedure and unmanned continuous operation during normal navigation wererealized, leading to a reduction in work load and highskill of operators.
(2) Dual fuel engine
This four-cycle internal combustion engine (Note 3)
for generator drive is capable of achieving high thermal efficiency (48% on lower calorific value basis) andlow NOx emission rate (up to 3 kg/kWh: on gas fuelburning). It was adopted as the main engines of anelectrical propulsion LNG carrier being built (as of2004) at ALSTOM's Shipyard in France.
This engine, which is capable of being operated by
Gas Mode (Otto-cycle) using fuel gas as main fuel anddiesel oil as pilot fuel, and Oil Mode (diesel-cycle), iscalled the dual fuel engine (DFE).
Since this engine is very low in gas supply pressure (ca. 5 bar) and features excellent safety ascompared with the high-pressure gas injection type
diesel engine, it is well suited for use as a marine
engine. Moreover, it is different from so called "gasengine" which uses gas only, in that it is possible tosimplify the engine configuration because they canbe operated by diesel oil on voyage with no LNG
loaded.

Outline drawing of
S/S LNG Jamal's BOG
reliquefaction system
Cargo tank

To main boiler
Vapor return
compressor
N2 compressor

BOG
compressor

M

Steam turbine

M

N2 expander& compressor
Cold box
Fig. 1 Outline of LNG Jamal reliquefaction system

Note 3: Based on the performance data of 50DF of
Wartsila Corporation.
2.2 MHI's R&D for alternative propulsion systems
Promising alternative propulsion systems now underdevelopment around the world are based mainly on theapplication of diesel engine to propulsion plants. In addition to these, however, MHI has developed a wide
variety of alternative systems including a high-efficiency
steam turbine propulsion system and utilization of gas
Fig. 2
turbine (Fig.
2).
From the view of engine power conversion to propulsive force (propeller driving force), they are categorized
into the direct system and indirect system.
(1) Direct coupled system (directly coupled with engine)
The low-speed diesel engine, thanks to its excellent operating performance at high power and its wide
power range, can be driven directly coupled with afixed-pitch propeller. Moreover, since it is popular for
use in large-scale merchant ships, there is no problem in how to secure operators. However, since thisengine is inferior to the steam turbine engine in termsof the serviceability ratio and maintenance load, twounits of the same type engines (twin-screw vessel) are
required in many cases in order to secure redundancy
for maneuvering.
While the high-pressure gas injection type lowspeed diesel engine has also been put to practical use
in a power-generating facility installed on land, but
it is not preferred as a marine engine for reasons ofreliability, operating results and high-pressure gas
handling. Therefore, its combination with a BOG
treating system such as BOG re-liquefaction plant isadditionally required.
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 6 (Dec. 2004)

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Propulsion plant

Propulsion power
Emergency generator
ST-BY G

[Direct system]
Steam turbine propulsion system(High-efficiency plant)

LP
SST
HP

G

ST

DFMB DFMBGST

2ST-DE[Direct system]
Propulsion system directly
coupled with diesel engine+ BOG reliquefaction plant

DRL/
TWIN

E/G G

Stand-bydieselgenerator

Steam turbine
generator

Main dual
fuel boiler

ST-BY G

E/G G

G

ST
2ST-DE

RL

G

ST

E/G G

ST-BY G
Compound HYBRID/
HRTC
turbine

POD propeller

2ST-DE

Hybrid propulsion system
DFcompoundengineHYBRID/DF

2-stroke diesel

EP/
GTCC

DF engine
plant

EP/
DFE

[Indirect system]
Electrical propulsion system

G

ST
DFE

G

DFE

G

ST

GTCC(*1)
plant

G

ST

DFXB

RL

DFXB

BOG reliquefaction system
DF engine
generator

G

E/G G

RL

TOx

Main gas(*1) GTCC: gas turbine
E/G G
ST-BY Gturbine generator
combined cycle
High-speedG
propulsion motor
GT
DFXB DFXBHeat recoveryPM
G type steam
ST
Auxiliary dual
turbine generator fuel boiler
PM
DFE G
E/G G
ST-BY G
DFE G
Low-speed
DFXB TOx
propulsion motor
DFE G
Main DF
BOG
engineDFE G
incinerator
PM PM
generator

Fig. 2 Substitute propulsion candidate systems(2) Indirect system (electrical propulsion)
Since it is considered unsuitable to couple DFE anda gas turbine directly with the propeller in view ofthe operating performance, the indirect system byelectrical propulsion is selected.
For the propeller driving propulsion motors, ahighly redundant structure consisting of two motors
coupled together is required. Therefore, a system inwhich two high-speed type motors are decelerated,
and a low-speed type directly coupled system, will betaken as candidates (actually this system is adopted
in the case of the newly built ship of electrical propulsion type mentioned above).
The electrical propulsion system is in common use

as the engine for passenger ships because of its highreliability and low vibrations/noise, but is not usual
for large-scale merchant ships because it is expensive solution for many cases.
For this reason, it is usually taken importance on
the evaluation of the balance between the fuel cost
improvement and the increase in initial investment.

3. Hybrid propulsion system
The HYBRID LNG system proposed originally by MHI
is a system in which diesel engine propulsion and auxiliary electrical propulsion as well as re-liquefaction
system and gas combustion system are combined in the
propulsion plant and BOG treating plant, respectively.
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 6 (Dec. 2004)3

3.1 Hybrid propulsion plantFig. 3)
3 adopted for the first time
The CRP POD system (Fig.
in the world for a ferry boat built by MHI and put into
service in June 2004 is considered to be a suitable propulsion system for high-speed vessels and high-powered ships
such as next-generation high-speed container ships. Thissystem can achieve high power, fuel cost improvement andhigh maneuverability with comparative ease.
The propulsion plant in the HYBRID LNG system isalso of similar structure consisting of a low-speed diesel
engine and electrical propulsion plant.
The ratio of power share between the diesel engine and
POD is determined by the maneuvering around coastal andportal area (low-steaming navigation), with POD alone
taken as the basic requirement and in consideration of theadditional propulsive force during ocean going navigation.
3.2 Hybrid BOG treatment plant
The auxiliary power system in the HYBRID LNG system can treat BOG in a safe and efficient manner by
adjusting the quantity of BOG to be re-liquefied and thatto be fired, thus adapting to the circumstances in combiFig. 4
nation with the re-liquefaction system (Fig.
4). Thecogeneration system, in which waste heat from the main
engine and the heat generated upon partial combustion of
BOG are utilized, supplies electric power to POD and liquefying power, which are the main source of power
consumption. In case of LNG carriers, the liquefying power
is considerably higher than any other ship service power(3 to 5 MW when the entire quantity of BOG is liquefied in
a 135 000 to 200 000 m3 hull form).

Since existing marine (commercial) techniques are applied to this system, including the re-liquefaction system,
dependence on the skill of crew members is considered tobe less than in other alternative propulsion systems.
3.3 Optimization of liquefaction plant by peak shaving
The quantity of BOG shows significant fluctuations
Fig. 5).
5 In cases where a large quantity ofduring a voyage (Fig.
BOG is generated temporarily, the HYBRID LNG systemutilizes BOG in excess of the liquefying capacity as boiler
fuel (heat source). The re-liquefaction system is optimized
by shaving of liquefaction to natural BOG on voyage withcargo loaded, thus enabling high-efficiency operation at
most period of voyages.
3.4 Environmental protection measures
In recent years, there has been a marked tendency toward compulsory use of low-sulfur fuel and intensified
control of emissions of nitrogen oxides (NOx) from engines,
particularly in the coastal areas of Europe and West Coastof the USA.
LNG is a clean fuel having no sulfur content and is lower
in price than marine diesel oil and low-sulfur heavy oil.
LNG carriers with HYBRID LNG system are operated
mainly by diesel engines during navigation at sea, but
cruise with POD at low steaming during coastal and portal navigation, while BOG is used as boiler fuel andexcessive BOG is saved by re-liquefaction.

BOG at guaranteedvalue for tank insulation

Loading

POD

Cargo loaded voyage

Unloading

Typical BOG pattern

4-stroke diesel
Ratio of powershare = 50:50: Combustion in boiler: Reliquefaction

Excessive BOG resulting fromcargo handling (cool down, etc.)

Ballast voyage

Variable
pitch propeller

POD

DAYS
Fig. 5 BOG generating patterns and peak shaving of liquefaction
The capacity of the reliquefaction system is designed with natural boil-off(guaranteed value for tank insulation) used as reference, and temporarily
generated excessive BOG is used as boiler fuel.

The "HAMANASU" for Shin Nihonkai Ferry Co., Ltd.
Fig. 3 Hybrid propulsion system adopted for ferry boats
Schematic flow diagram of LNG hybrid system
Fuel oil filter
Exhaust gas

To other cargo tanks
Heavy fuel oil(from setting tank)

Cargo tank

Marinediesel oil

Vapor return
compressor

N2 compressor
Air

Cold box

Generator
EngineDual fuel boiler
Sea water

Steamcondenser

Electric power

BOG compressor

Other ship's consumer
Fig. 4 Outline of hybrid system

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 6 (Dec. 2004)

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: Without BOG (at anchor): With BOG (at anchor)

10

5

ST

DRL Hybrid Ep/DF

ST

+1
+2
+3
DRL
+4
+5
0

HybridEp/DF
5

10

Increase in
shipbuilding cost (%)

0

0.00

Super steam
turbineFreight rate improvement (%)

0.50

15

DRL Hybrid Ep/DF

Fig. 6 Comparison of CO2 and NOx emission rates: Increase intransportation
capacity0: Improvement infuel cost-1: Improvement inmanagement
cost

-2
-3
-4
-5

0

5 10

15 20 25 30

Improvement ratio (%)

Fig. 7 Freight rate improvement effect and ship priceLine of balance between improved fuel cost and increased ship pricecost on a freight rate basis.

This system requires neither excessive gas treatment
nor use of expensive marine diesel oil and low-sulfur heavy
oil, and achieves zero emission of sulfur oxides (SOx). Moreover, the gas combustion process in the boiler has a very
low NOx emission rate as compared with that of internal
Fig. 6
combustion engines (Fig.
6).

Evaluation of fuel cost

Basic conditions: Hybrid (or DRL)
: Steam turbine: Gas-fired: Dual fuel: Ep/DF

Equivalent to 145
MOSS-type LNG carrier
M/E power:
27 MW (on ST basis)

4. Evaluation of freight rate reduction effectNumber of days at sea
on average: 30

4.1 Ship price and fuel cost improvement
Significant fuel cost improvement is expected for all alternative propulsion systems, but initial investment may
increase in all cases. No alternative systems are considered attractive unless relative merits and demerits lead toa reduction in total freight cost. In the case of the HYBRID
LNG system, the line of balance lies at the fuel cost reduction rate of around 15 to 20% as against the increase in
Fig. 7
ship price (Fig.
7).
4.2 Evaluation of fuel cost
On the other hand, the fuel cost depends on the unit
Fig. 8).
8 Since the unit pricecost (ratio) of LNG/fuel oil (Fig.
ratio may fluctuate depending on the mode of LNG transaction (CIF, FOB, etc.) and the fuel oil market conditions,
it is desirable to achieve fuel cost improvement over as widea range as possible. In the case of the HYBRID LNG system, freight rate improvement can be expected if the BOG(LNG) unit price is around half or lower than the unit priceof heavy fuel oil.

Reduction ratioof fuel cost

Fuel cost

1.00

Effect of fuel cost improvement andshipbuilding cost on a freight rate basis

Freight rate increment (%)

Ratio of NOx emission rate during
coastal navigation or at anchor in
port to power (g/kWh)

Ratio of CO2 emission rate during
navigation at sea to power (g/kWh): With BOG: Without BOG

Approx.
20 %Unit price ratio of bunker oil
HFO : MDO : MGO=100 : 155 : 160

0

0.5

1

1.5

Unit price ratio(on calorific value basis)Fig. 8 Comparison of fuel cost
The abscissa expresses the price ratio of LNG/Heavy fuel oil. Its realistic
value in recent market conditions is 1 or less. BOG to be stored by reliquefaction is deemed to be equivalent to LNG and subtracted from thefuel cost upon evaluation.

For BOG reliquefaction plants, proper guidance wasprovided by many persons concerned including those from
Osaka Gas Co., Ltd., Nippon Yusen K.K. and Chiyoda Corporation, for which MHI expresses its sincere gratitude on
this occasion.
References(1) Hatanaka, N. et al., Osaka Gas Co., Ltd., A Challenge to Advanced LNG Transport for the 21st Century - LNG Jamal: NewLNG Carrier with Re-Liquefaction plant, LNG 13 (2001)(2) Oka, M. et al, Mitsubishi Heavy Industries, Ltd., Proposals for
LNGC Propulsion System with Re-Liquefaction Plant: Review
from the World's First Application, GASTECH 2002 (2004)(3) Oka, M. et al., Mitsubishi Heavy Industries, Ltd., Advanced LNG
Carrier with Energy Saving Propulsion Systems: Two Options,
LNG 14(2004)

5. Conclusion
For LNG carrier propulsion systems, the steam turbine
has occupied a monopoly position in the market. In future,
however, there is a likelihood that several types of propulsion systems will coexist. MHI has been developing various
types of alternative propulsion systems. In particular, the
HYBRID LNG system is a system capable of meeting theneeds of every customer because of its excellent featuresin terms of thermal efficiency, degree of freedom in choiceof fuel and environmental considerations. MHI intends topropose this system positively to the newly-built LNG carrier market from now on.

Masaru Oka

Kazuyoshi Hiraoka

Kenji Tsumura

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 6 (Dec. 2004)

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