P9105

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ME-GI Engines for LNG ApplicationSystem Control and Safety

Introduction ........................................................................................................ 3

Propulsion Power Requirements for LNG Carriers ................................... 3

Boiloff Gas from LNG Cargo ......................................................................... 4

Design of the Dual Fuel MEGI Engine ........................................................ 5

General Description ........................................................................................... 5

System Description .......................................................................................... 7

Engine Systems ................................................................................................. 7

– Exhaust receiver .................................................................................................. 7

– Fuel injection valves ............................................................................................ 7

– Hydraulic Cylinder Unit (HCU) ............................................................................. 8– Valve block .......................................................................................................... 8

– Gas pipes ........................................................................................................... 9

– Fuel oil booster system ...................................................................................... 9

– Miscellaneous ...................................................................................................... 9

Safety Aspects .................................................................................................... 9

– Safety devices – external systems ........................................................................ 10

– Safety devices – internal systems ......................................................................... 10

– Defective gas injection valves ............................................................................. 10

– Ignition failure of injected gas ............................................................................... 10

– External systems ................................................................................................. 11

– Sealing oil system .............................................................................................. 11

– Ventilation system ................................................................................................ 11

The Gas Compressor System ........................................................................ 12

– Gas supply system – capacity management ........................................................ 14

– Safety aspects ..................................................................................................... 14

– Maintenance ...................................................................................................... 14

– External systems .................................................................................................. 14

– Safety devices – internal systems ......................................................................... 14

– Inert gas system .................................................................................................. 14

Dual Fuel Control System ............................................................................... 14

– General ................................................................................................................ 14

– Plant control ........................................................................................................ 14

– Fuel control .......................................................................................................... 15– Safety control ....................................................................................................... 15

– Architecture of the dual fuel control system .......................................................... 15

– Control unit hardware ........................................................................................... 16

– Gas main operating panel (GMOP) ....................................................................... 16

– GECU, Plants control ........................................................................................... 16

– GACU, Auxiliary control ...................................................................................... 16

– GCCU, ELGI control ............................................................................................. 17

– The GSSU, fuel gas system monitoring and control.............................................. 17

– GCSU, PMI online............................................................................................... 17

– Safety remarks ..................................................................................................... 17

Summary ............................................................................................................. 17

References .......................................................................................................... 17

Abbreviations ..................................................................................................... 18

Content Page

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ME-GI Engines for LNG ApplicationSystem Control and Safety

Introduction

Until the end of 2004 there was still one

market for oceangoing cargo ships to

which the twostroke engine had not yet

been introduced: i.e. the LNG market.

This market has so far been dominated

by steam turbines, but the rst orders

for twostroke diesel engines were

given at the end of 2004. Today, 16 ME

engines to LNG carriers have been or-

dered for eight LNG carriers, which are

to be built in Korea, Ref. [1].

For these plants, the boiloff gas is retur

ned to the LNG tanks in liqueed form via

a reliquefaction plant installed on board.

Some operators are considering an

alter native twostroke solution, which

is the MEGI (Gas Injection) engine op-

erating at a 250300 bar gas pressure.

Which solution is optimal for a givenproject depends primarily on the price

of HFO and the value of natural gas.

Calculations carried out by MBD show

that additional USD 3 million can be

secured as prot per year when using

twostroke diesel engines, irrespective

of whether the HFO or the dual fuel

engine type is chosen. When it comes

to rst cost, the HFO diesel engine com-

bined with a reliquefaction plant has the

same cost level as the steam turbinesolution, whereas the dual fuel MEGI

engine with a compressor is a cheaper

solution.

This paper will describe the application

of MEGI engines inclusive the gas sup-

ply system on a LNG carriers, and the

layout and control system for both the

engine and gas supply system.

First, a short description is given of the

propulsion power requirement of LNG

carriers, and why the twostroke diesel

engine is winning in this market.

Fig. 1: Typical propulsion power requirements or LNG carriers

20.000

30.000

40.000

50.000

1 25.000 150.0 00 17 5.0 00 20 0.000 225.000 250.000

(m3)

Engine Power

(kW)

21.0 knots

20.0 knots

19.0 knots

Fig. 2: Typical thermal efciencies o prime movers

35

30

40

25

50

45

Medium speeddiesel engine

20

Capacity ( MW)501 10

55

Thermal efficiencies %

Gas t urbine

Combined cyclegas turbine

Steam turbine

Low speed diesel engine

5

LNG carrier

Propulsion power requirements for LNG carriers

Traditionally, LNG carriers have been

sized to carry 130,000 – 140,000 m3

liqueed natural gas, i.e. with a carrying

capacity of some 7080,000 tons, which

resembles that of a panamax bulk carrier.

The speed has been around 20 knots,

whereas that of the panamax bulk carri-ers is around 15. Now, even larger LNG

carriers are in project up to a capacityof some 250,000 m3 LNG. Such ships

will be comparable in size to a capesizebulk carrier and an aframax tanker but,

again, with a speed higher than these.

In an analysis of the resulting power

requirements, a calculation programme

normally used by MBD has been used,Ref. [2].

The result appears in Fig. 1, which showsthat a power requirement of 30 to 50 MW

is needed.

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Fig. 3: Propulsion alternative – energy need or propulsion Fig. 4: Fuel Type Modes – MAN B&W twostroke dual uel low speed diesel

Boiloff Gas from LNGCargo

The reason for having a continuousevaporated rate of boiloff gas is that itis generated by heat transferred fromthe ambient temperature through theLNG tanks and into to cold LNG. Theboiloff gas is the consequence if theLNG cargo should be staying liquid atatmospheric pressure and at a temperature of some minus 160 degrees Cel-sius. To keep the evaporated rate of boiloff at a minimised level, the cargo iskept in proper insulated tanks.

The LNG is a mixture of methane, ethaneand nitrogen. Other natural gases likebutane and propane are extracted dur-ing the liquefying and are only present invery small quantities.

In a traditional steam turbine vessel, theboiloff gas is conveniently sent to twinboilers to produce steam for the propul-sion turbine.

Due to the proper insulation, the boiloff is usually not enough to provide the energyneeded for propulsion, so the evaporatedgas is supplemented by either forcedboil off of gas or heavy fuel oil to pro-duce the required steam amount.

In a diesel engine driven LNG carrier,the energy requirement is less thanksto the higher thermal efciency, so thesupplementary energy by forced boil off

or heavy fuel oil can be reduced signi-

cantly, as shown in Fig. 3

As mentioned, diesels are now being

seen as an alternative to steam, rst of

all because of the signicant difference

in thermal efciency reected also in the

system efciency, as illustrated in Fig. 2.

With a power requirement of the mentio

ned magnitude, the illustrated efciency

difference of up to 20 percentage

points amounts to signicant savings

both in terms of energy costs and in

terms of emissions.

The desired power for propulsion can

be generated by a single, double, or

multiple fuel or gas driven diesel engine

installation with either direct geared or

dieselelectric drive of one or two pro-

pellers.

The choice depends on economical and

operational factors.

Over time, the evaluation of these factors

for the options of propulsion technol-

ogy, for ordinary larger cargo vessels

(viz. container vessels, bulk carriers

and tankers), has led to the selection

of a

single, heavyfuelburning, low speed

diesel engine in more than 90% of

contemporary vessels.

The aim of this paper is to demonstrate

that low speed propulsion is fully fea-

sible for LNG carriers.

100%

60%

50%

Steam

NBO

Gas

FBO

Gas

orFuel

NBO

Gas

or

Fuel

Diesel

Fuel

Fuel

Gas

Fuel

100% load 100% load

100% load

“Specified gas” mode

8%

Gas

Fuel 100% Fueloilonly mode “Minimum fuel” modeFuel 100%

Fuel 100%

Fuel

8%

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General Description

Fig. 5 shows the crosssection of a

S70MEGI, with the new modied parts

of the MEGI engine pointed out, com-

prising gas supply piping, largevolume

accumulator on the (slightly modied)

cylinder cover with gas injection valves,

and HCU with ELGI valve for control of the injected gas amount. Further to this,

there are small modications to the ex-

haust gas receiver, and the control and

manoeuvring system.

Apart from these systems on the en-

gine, the engine auxiliaries will comprise

some new units, the most important

ones being:

Fig. 5: New modifed parts on the MEGI engine

Fig.6: General arrangement o doublewall piping system or gas

Exhaust receiver Cylinder cover with gas valves

LargeVolume accumulator

Gas supply piping

HCU withELGI valve

. g pressure p pe rom gas compressor

2. Main gas valve

3. Main venting valve

4. Main gas pipe (double pipe)

5. Main venting pipe (double pipe)

6. Inert gas valve in main gas pipe

7. Suction fan

8. Flow control

9.HC sensors in double wall pipes

10.HC sensors in engine room(optional)

Air outlet

Outside engine room

Engine side

Inert gas

(N ) inlet2

Pilot oil outlet

Pilot oil inlet

Sealing oil inlet

Sealing oil outlet

Design of the Dual FuelMEGI Engine

In terms of engine performance (i.e.:

output, speed, thermal efciency, ex-

haust gas amount and temperature,

etc.) the MEGI engine series is gener-

ally identical to the wellestablished and

type approved ME engine series. This

means that the application potential

for the MEengine series applies to theMEGI engine series as well – provided

that gas is available as a main fuel. All

ME engines can be offered as MEGI

engines.

Consequently, the following description

of the MEGI engine design only deals

with new or modied engine com-

ponents with the different fuel mode

types, as illustrated in Fig. 4.

The control system will allow any ratio

between fuel and gas, with a preset

minimum fuel amount to be used.

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•Highpressure gas compressor sup-

ply system, including a cooler, to raise

the pressure to 250300 bar, which

is the pressure required at the engine

inlet.

•Pulsation/buffer tank including a con-

densate separator.

•Compressor control system.

•Safety systems, which ex. includesa hydrocarbon analyser for checking

the hydrocarbon content of the air

in the compressor room and in the

doublewall gas pipes.

• Ventilation system, which ventilates

the outer pipe of the doublewall pip-

ing completely.

•Sealing oil system, delivering sealing

oil to the gas valves separating the

control oil and the gas.

•Inert gas system, which enables

purging of the gas system on the en-

gine with inert gas.

Fig. 6, in schematic form, shows

the system layout of the engine.

The highpressure gas from the

compressorunit ows through the main

pipe via narrow and exible branch pipes

to each cylinder’s gas valve block and

largevolume accumulator. The narrow

and exible branch pipes perform two

important tasks:

• They separate each cylinder unit

from the rest in terms of gas dynam-

ics, utilising the wellproven design

philosophy of the ME engine’s fuel oil

system.

• They act as exible connections be-tween the stiff main pipe system and

the engine structure, safeguarding

against extrastresses in the main and

Fig. 7: MEGI uel injection system

branch pipes caused by the inevitable

differences in thermal expansion of

the gas pipe system and the engine

structure.

The largevolume accumulator, contain-

ing about 20 times the injection amount

per stroke at MCR, also performs two

important tasks:

• It supplies the gas amount for injection

at only a slight, but predetermined,

pressure drop.

• It forms an important part of the safe-

ty system (as described later).

Since the gas supply system is a com-

mon rail system, the gas injection valve

must be controlled by another system,

i.e. the control oil system. This, in

principle, consists of the ME hydraulic

control (servo) oil system and an ELGI

valve, supplying highpressure control

oil to the gas injection valve, thereby

controlling the timing and opening of

the gas valve.

As can also be seen in Fig. 7, the nor-

mal fuel oil pressure booster, which

supplies pilot oil in the dual fuel opera-

tion mode, is connected to the ELGI

valve by a pressure gauge and an on/off

valve incorporated in the ELGI valve.

By the control system, the engine canbe operated in the various relevant

modes: normal “dualfuel mode” with

minimum pilot oil amount, “specied

gas mode” with injection of a xed gas

amount, and the “fueloilonly mode”.

The MEGI control and safety system

is built as an addon system to the ME

control and safety system. It hardly re-

quires any changes to the ME system,

and it is consequently very simple to

implement.

The principle of the gas mode control

system is that it is controlled by the

error between the wanted discharge

pressure and the actual measured dis-

charge pressure from the compressor

system. Depending on the size of this

error the amount of fuelgas (or of pilot

oil) is either increased or decreased.

If there is any variation over time in the

caloric value of the fuelgas it can be

measured on the rpm of the crankshaft.

Depending on the value measured, the

amount of fuelgas is either increased

or decreased.

The system provides:

Pressure, timing, rate shaping,main, pre & post injection

200 bar hydraulic oil.

Common with

exhaust valve actuator

Low pressure fuel supply

Fuel return

Position sensor

Measuring and

limiting devicepressure booster

(800 900 bar)

.

Injection

FIVA valve

ELGI valve

800

600

400

200

00 5 10 15 20 30 3525 40 45

Bar abs

Pilot oil pressure

Control oil pressure

Deg. CA

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Fig. 8: Engine control system diagram

The change in the caloric value overtime is slow in relation to the rpm of theengine. Therefore the required changeof gas amount between injections isrelatively small.

To make the engine easy to integratewith different suppliers of external gasdelivering systems, the fuel gas controlsystem is made almost “stand alone”. The exchanged signals are limited to Stop,Go, ESD, and pressure setpoint signals.

System description

Compared with a standard engine forheavy fuel operation, the adaptation tohighpressure gas injection requires thatthe design of the engine and the pertain-ing external systems will comprise anumber of special external componentsand changes on the engine.

Fig. 9 shows the principal layout of thegas system on the engine and some

of the external systems needed fordualfuel operation.

In general, all systems and componentsdescribed in the following are to be made“fail safe”, meaning that componentsand systems will react to the safe side if anything goes wrong.

Engine systems

In the following, the changes of the

systems/ components on the engine,as pointed out in Fig. 5, will be de-scribed.

Exhaust receiver

The exhaust gas receiver is designed towithstand the pressure in the event of ignition failure of one cylinder followedby ignition of the unburned gas in thereceiver (around 15 bars).

The receiver is furthermore designedwith special transverse stays to with-

stand such gas explosions.

Fuel injection valves

Dual fuel operation requires valves for both

the injection of pilot fuel and gas fuel.

The valves are of separate types, and

two are tted for gas injection and two

for pilot fuel. The media required for

both fuel and gas operation is shown

below:

• Highpressure gas supply

• Fuel oil supply (pilot oil)

• Control oil supply for activation

of gas injection valves

• Sealing oil supply.

The gas injection valve design is shown

in Fig. 10.

This valve complies with our traditional

design principles of compact designand the use of mainly rotational sym-

metrical parts. The design is based on

the principle used for an early version of

a combined fuel oil/gas injection valve

as well as expe rience gained with our

normal fuel valves.

Gas is admitted to the gas injection valve

through bores in the cylinder cover. To

prevent gas leakage between cylinder

cover/gas injection valve and valve

housing/spindle guide, sealing rings

made of temperature and gas resistant

material are installed. Any gas leak-

age through the gas sealing rings will

be led through bores in the gas injec-

tion valve and the cylinder cover to the

doublewall gas piping system, where

any such leakages will be detected by

HC sensors.

The gas acts continuously on the valve

spindle at a pressure of about 250300

bar. In order to prevent the gas fromentering the control oil activating system

via the clearance around the spindle,

the spindle is sealed by means of seal-

ing oil led to the spindle clearance at a

Emergencystop engine

BOG evaporatedEngine on morethan 30% load

Not enoughBOG for full

Dual fueloperation

TBOG amount

evaporated

oo high

LNG tankers Oxidiser

Start up onHFO/DO

Momentaryshut off of gassupply system

HP compressor

Gas burnedin ME GI

Gas burning +supplementary

fuel oil between5-100%

95%gas +5% HFO/DO

Engine

N flushed

in gas pipes2

Engine momentarilychange to HFO when gas

pressure is reduced to lessthan 200 bar (Gas pipes and

valves are flushed with N )2

Gas led tooxidiser when

too much BOGis available

Excess BOGburned inoxidiser

Gas led tooxidiser

Gas burned inoxidiser

Compressor internal bypass

of remaining gas

Compressor up to 250 bar



Compressor

LP compressor

Compressor starts up

Recirculationof gas

to buffertank

Compressor

100%BOG

100%BOG

100%BOG

100%BOG

100%BOG

AvailableBOG

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pressure higher than the gas pressure

(2550 bar higher).

The pilot valve is a standard fuel valve

without any changes.

Both designs of gas injection valves will

allow operation solely on fuel oil up to

MCR. lf the customer’s demand is for

the gas engine to run at any time at 100

% load on fuel oil, without stopping the

engine for changing the injection equip-

ment, the fuel valve nozzle holes will be

as the standard type for normal fuel oil

operation. In this case, it may be neces-

sary to use a somewhat larger amount

of pilot fuel in order to assure a good in-

jection quality and safe ignition of the gas.

Cylinder cover

In order to protect the gas injection nozzle

and the pilot oil nozzle against tip burn-

ing, the cylinder cover is designed with

a weldedon protective guard in front of

the nozzles.

The side of the cylinder cover facing

the HCU (Hydraulic Cylinder Unit) block

has a face for the mounting of a special

valve block, see later description.

In addition, the cylinder cover is pro-vided with two sets of bores, one set

for supplying gas from the valve block

to each gas injection valve, and one set

for leading any leakage of gas to the

subatmo spheric pressure, ventilated

part of the doublewall piping system.

Hydraulic Cylinder Unit (HCU)

To reduce the number of additional hy-draulic pipes and connections, the ELGIvalve as well as the control oil pipe con-nections to the gas valves will be incor-porated in the design of the HCU.

Valve block

The valve block consists of a squaresteel block, bolted to the HCU side of the cylinder cover.

The valve block incorporates a largevolume accumulator, and is provided

Fig. 9: Internal and external systems or dual uel operation

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with a shutdown valve and two purgevalves on the top of the block. Allhighpressure gas sealings lead intospaces that are connected to thedoublewall pipe system, for leakagedetection.

The gas is supplied to the accumulatorvia a nonreturn valve placed in the ac-cumulator inlet cover.

To ensure that the rate of gas ow does

not drop too much during the injectionperiod, the relative pressure drop in theaccumulator is measured. The pressuredrop should not exceed about 2030 bar.

Any larger pressure drop would indicatea severe leakage in the gas injection valveseats or a fractured gas pipe. The safetysystem will detect this and shut downthe gas injection.

From the accumulator, the gas passesthrough a bore in the valve block to the

shut down valve, which in the gas mode,is kept open by compressed air. Fromthe shutdown valve (V4 in Fig. 9), thegas is led to the gas injection valve viabores in the valve block and in the cyl-inder cover. A blowoff valve (V3 in Fig.9), placed on top of the valve block, isdesigned to empty the gas bores whenneeded.

A purge valve (V5 shown in Fig. 9), which

is also placed on top of the valve block,

is designed to empty the accumulator

when the engine is no longer to operate

in the gas mode.

Gas pipes

A common rail (constant pressure) sys-

tem is to be tted for highpressure gas

distribution to each valve block.

Gas pipes are designed with double walls,

with the outer shielding pipe designed

so as to prevent gas outow to the ma-chinery spaces in the event of rupture

of the inner gas pipe. The intervening

space, including also the space around

valves, anges, etc., is equipped with

separate mechanical ventilation with a

capacity of approx. 10 – 30 air changes

per hour. The pressure in the interven-

ing space is to be below that of the

engine room and, as mentioned earlier,

(extractor) fan motors are to be placed

outside the ventilation ducts, and the

fan material must be manufactured fromsparkfree material. The ventilation inlet

air must be taken from a gas safe area.

Gas pipes are arranged in such a way,

see Fig. 6, that air is sucked into the

doublewall piping system from around

the pipe inlet, from there into the branch

pipes to the individual cylinder blocks,

via the branch supply pipes to the main

supply pipe, and via the suction blower

to the atmosphere. Ventilation air is to

be exhausted to a safe place.

The doublewall piping system is desig

ned so that every part is ventilated. how

ever, minute volumes around the gas

injection valves in the cylinder cover are

not ventilated by owing air for practical

reasons. Small gas amounts, which in

case of leakages may accumulate in

these small clearances, blind ends, etc.

cannot be avoided, but the amount of

gas will be negligible. Any other leak-

age gas will be led to the ventilated part

of the doublewall piping system andbe detected by the HC sensors.

The gas pipes on the engine are designedFig. 10: Gas injection valve

Cylinder

cover

Gas inlet

Gas spindle

Sealing oilControl oil

Connection to

the ventilatedpipe system

Sealing oil inlet

Control oil inlet

for 50 % higher pressure than the normal

working pressure, and are supported

so as to avoid mechanical vibrations.

The gas pipes should furthermore be

protected against drops of heavy items.

The pipes will be pressure tested at 1.5

times the working pressure. The design

is to be allwelded as far as practicable,

with ange connections only to the nec-

essary extent for servicing purposes.

The branch piping to the individual cylin-

ders must be exible enough to cope with

the thermal expansion of the engine from

cold to hot condition.

The gas pipe system is also to be desig

ned so as to avoid excessive gas pressure

uctuations during operation. Finally, the

gas pipes are to be connected to an inert

gas purging system.

Fuel oil booster system

Dual fuel operation requires a fuel oil

pressure booster, a position sensor, a

FIVA valve to control the injection of pilot

oil, and an ELGI valve to control the

injection of gas. Fig. 7 shows the design

control principle with the two fuel valves

and two gas valves.

No change is made to the ME fuel oil

pressure booster, except that a pressure

sensor is added for checking the pilot oil

injection pressure. The injected amountof pilot oil is monitored by the position

sensor.

The injected gas amount is controlled by

the duration of control oil delivery from

the ELGI valve. The operating medium is

the same servo oil as is used for the fuel

oil pressure booster.

Miscellaneous

Other engine modications will, basi-cally, be limited to a changed position of

pipes, platform cutouts, drains, etc.

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Safety aspects

The normal safety systems incorpo-

rated in the fuel oil systems are fully

retained also during dual fuel operation.

However, additional safety devices will

be incorporated in order to prevent situ-

ations which might otherwise lead to

failures.

Safety devices – External systems

Leaky valves and fractured pipes are

sources of faults that may be harmful.

Such faults can be easily and quickly

detected by a hydrocarbon (HC) analyser

with an alarm function. An alarm is given

at a gas concentration of max. 30% of

the Lower Explosion Limit (LEL) in the

vented duct, and a shut down signal is

given at 60% of the LEL.

The safety devices that will virtually

eliminate such risks are doublewall

pipes and encapsulated valves with

ventilation of the intervening space. The

ventilation between the outer and inner

walls is

always to be in operation when there

is gas in the supply line, and any gas

leakage will be led to the HCsensors

placed in the outer pipe.

Another source of fault could be a mal-

functioning sealing oil supply system. If

the sealing oil pressure becomes too low

in the gas injection valve, gas will owinto the control oil activation system

and, thereby, create gas pockets and

prevent the ELGI valve from operating

the gas injection valve. Therefore, the

sealing oil pressure is measured by a set

of pressure sensors, and in the event of

a too low pressure, the engine will shut

down the gas mode and start running in

the fuel oil mode.

Lack of ventilation in the doublewall

piping system prevents the safetyfunction of the HC sensors, so the

system is to be equipped with a set of

ow switches. If the switches indicate

no ow, or nearly no ow, an alarm is

given. If no correction is carried out, the

engine will be shut down on gas mode.

The switches should be of the normally

open (NO) type, in order to allow detec-

tion of a malfunctioning switch, even in

case of an electric power failure.

• In case of malfunctioning valves (not

leaky) resulting in insufcient gas sup-

ply to the engine, the gas pressure

will be too low for gas operation. This

is dealt with by monitoring the pres-sure in the accumulator in the valve

block on each cylinder. The pressure

could be monitored by either one

pressure pickup, or by a pressure

switch and a differential pressure

switch (see later for explanation).

As natural gas is lighter than air,

nonreturn valves are incorporated in

the gas system’s outlet pipes to ensure

that the gas system is not polluted,

i.e. mixed with air, thus eliminating the

potential risk of explosion in case of a

sudden pressure increase in the system

due to quick opening of the main gas

valve.

For LNG carriers in case of too low

a BOG pressure in the LNG tanks, a

stop/off signal is sent to the MEGI

control system and the gas mode is

stopped, while the engine continues

running on HFO.

Safety devices – Internal systems

During normal operation, a malfunction

in the pilot fuel injection system or gas

injection system may involve a risk of

uncontrolled combustion in the engine.

Sources of faults are:

• Defective gas injection valves

• Failing ignition of injected gas

These aspects will be discussed in detail

in the following together with the suitable

countermeasures.

Defective gas injection valves

In case of sluggish operation or even

seizure of the gas valve spindle in the

open position, larger gas quantities

may be injected into the cylinder, and

when the exhaust valve opens, a hot

mixture of combustion products and

gas ows out and into the exhaust pipe

and further on to the exhaust receiver.

The tempe rature of the mixture after

the valve will increase considerably, andit is likely that the gas will burn with a

diffusion type ame (without exploding)

immediately after the valve where it is

mixed with scavenge air/exhaust gas

(with approx. 15 per cent oxygen) in

the exhaust

system. This will set off the high exhaust

gas temperature alarm for the cylinder

in question. In the unlikely event of larger

gas amounts entering the exhaust re-

ceiver without starting to burn immedi-

ately, a later ignition may result in violent

burning and a corresponding pressure

rise. Therefore, the exhaust receiver is

designed for the maximum pressure

(around 15 bars).

However, any of the abovementioned

situations will be prevented by the de-

tection of defective gas valves, which

are arranged as follows:

The gas ow to each cylinder during one

cycle will be detected by measuring the

pressure drop in the accumulator. Thisis to ensure that the injected gas amount

does not exceed the amount correspon

ding to the MCR value.

It is necessary to ensure that the pres-

sure in the accumulator is sufcient for

gas operation, so the accumulator will

be equipped with a pressure switch

and a differential pressure switch. An

increase of the gas ow to the cylinder

which is greater than corresponding to

the actual load, but smaller than corre-sponding to the MCR value, will only give

rise to the abovementioned exhaust gas

temperature alarm, and is not harmful.

By this system, any abnormal gas ow,

whether due to seized gas injection valves

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Fig. 11: Gas system branching

or fractured gas pipes, will be detected

immediately, and the gas supply will be

discontinued and the gas lines purged

with inert gas.

In the case of slightly leaking gas valves,

the amount of gas injected into the cylin-

der concerned will increase. This will be

detected when the exhaust gas temper-

ature increases. Burning in the exhaust

receiver will not occur in this situation

due to the lean mixture.

Ignition failure of injected gas

Failing ignition of the injected natural gas

can have a number of different causes,

most of which, however, are the result of

failure to inject pilot oil in a cylinder:

• Leaky joints or fractured highpressure

pipes, making the fuel oil booster inop-

erative.

• Seized plunger in the fuel oil

booster.

• Other faults on the engine, forc-

ing the fuel oil booster to “Oindex”.

• Failing pilot oil supply to the engine.

Any such faults will be detected so quickly

that the gas injection is stopped imme-

diately from the rst failure to inject the

pilot oil.

In extremely rare cases, pilot fuel can be

injected without being ignited, namely in

the case of a sticking or severely bur ned

exhaust valve. This may involve such large

leakages that the compression pressure

will not be sufcient to ensure ignition

of the pilot oil. Consequently, gas and

pilot fuel from that cylinder will be sup-

plied to the exhaust gas receiver in a fully

unburned condition, which might result

in violent burning in the receiver. How-

ever, burning of an exhaust valve is arather slow process extending over a long

period, during which the exhaust gas

temperature rises and gives an alarm well

in advance of any situation leading to risk

of misring.

A seized spindle in the pilot oil valve is

another very rare fault, which might in-

uence the safety of the engine in dual

fuel operation. However, the still operat-

ing valve will inject pilot oil, which will

ignite the corresponding gas injection,

and also the gas injected by the other

gas valve, but knocking cannot be ruled

out in this case. The cylinder pressure

mo nitoring system will detect this con-

dition.

As will appear from the above discussion,which has included a number of very

unlikely faults, it is possible to safeguard

the engine installation and personnel

and, when taking the proper counter-

measures, a most satisfactory service

reliability and safety margin is obtained.

External systems

The detailed design of the external sys

tems will normally be carried out by the

individual shipyard/contractor, and is,

therefore, not subject to the type ap-

proval of the engine. The external sys-

tems described here include the sealing

oil system, the ventilation system, and the

gas supply and compressor system.

Sealing oil system

The sealing oil system supplies oil, via a

piping system with protecting hoses, to

the gas injection valves, thereby provid-

ing a sealing between the gas and the

control oil, and lubrication of the mov-

ing parts.

The sealing oil pump has a separate

drive and is started before commenc-

ing gas operation of the engine. It uses

the 200 bar servo oil, or one bar fuel oil,

and pres surises it additionally to the

operating pressure, which is 2550 bar

higher than the gas pressure. The con-

sumption is small, corresponding to a

sealing oil consumption of approx. 0.1

g/bhph.

After use, the sealing oil is burned in the

engine.

Protective hose Soldered

Bonded seal

High pressure gas

High pressure gas pipe

Outer pipe

Ventilation air

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Ventilation system

The purpose of the ventilation system

is to ensure that the outer pipe of the

doublewall gas pipe system is ventilated

with air, and it acts as a separation

between the engine room and the

highpressure gas system, see Fig 11.

Ventilation is achieved by means of an

electrically driven mechanical fan or

extractor fan. If an electrically drivenfan is chosen, the motor must be placed

outside the ventilation duct. The ca-

pacity must ensure approx. 10 – 30

air changes per hour. More ventilation

gives quicker detection of any gas leak-

age.

The gas CompressorSystem

The gas supply system is based on Flo-

tech™ packaged compressors:

• Lowpressure GE Oil & Gas RoFlo™

type gas compressors with lubricated

vanes and oil buffered mechanical seals,

which compress the cold boiloff gas

from the LNG tanks at the temperature

of 140oC to 160oC. The boiloff gas

pressure in the LNG tanks should

normally be kept between 1.061.20

bar(a). Under normal running conditions,

cooling is not necessary, but during

start up, the temperature of the boiloff

gas may have risen to atmospheric

temperature, hence preheating and

aftercooling is included, to ensure

stabilisation of the cold inlet and inter-

mediate gas. temperature

• The highpressure GE Oil & GasNuovo Pignone™ SHMB type gas

compressor; 4 throw, 4stage hori-

zontally opposed and fully balanced

crosshead type with pressure lubri-

cated and watercooled cylinders

& packings, compresses the gas to

approximately 250300 bar, which is

the pressure required at the engine

inlet at full load. Only reciprocating

piston compressors are suitable for

this highpressure duty; however the

unique GE fully balanced frame layout

addresses concerns about transmit-

ted vibrations and also eliminates the

need for heavy installation structure, as

is required with vertical or Vform unbal-

anced compres sor designs. The dis-

charge temperature is kept at approx.

45oC by the coolers.

•Buffer tank/accumulators are installedto provide smoothing of minor gas

pressure uctuations in the fuel sup-

ply; ± 2 bar is required.

• Gas inlet lter/separator with strainer

for protection against debris.

• Discharge separator after the nal stage

gas cooler for oil/condensate remov-

al.

• Compressor capacity control system

ensures that the required gas pres-

sure is in accordance with the engine

load, and that the boiloff gas amount

is regulated for cargo tank pressure

control (as described later).

• The compressor safety system handles

normal start/stop, shutdown and

emergency shutdown commands.

Fig. 12: Gas supply system – natural BOG only

65%

65%

65%

65%

65% 65%

65%

natural BOG

LNG

Tank

LP.+ comp.

LP.+ comp.

HP comp. MEGI

GCU

Redundant gas supply system comprising

2 xLow Pressure compressors.1 x gas combustion unit GCU

1 xHighPressure piston compressor.

Add up with 35 % HFO

+ pilot oil

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Fig. 13: Gas supply system– natural and orced BOG

The compressor unit includes a pro-

cess monitoring and fault indication

system. The compressor control

system exchanges signals with the

MEGI control system.

• The compressor system evaluates

the amount of available BOG and

reports to the MEGI control system.

Redundancy for the gas supply system

is a very important issue. Redundancy

in an extreme sense means two of all

com ponents, but the costs are heavy

and a lot of space is required on board

the ship. We have worked out a reco-mendation that reduces the costs and

the requirement for space while ensur-

ing a fully operational MEGI engine. The

dual fuel en gine concept, in its nature,

includes redunancy. If the gas supply

system falls out, the engine will run on

heavy fuel oil only.

The gas supply system illustrated in Fig.

12 and 13 are based on a 210,000 M3

LNG carrier, a boil off rate of 0.12 and

equipped with 2 dual fuel engines: 2 x7S65MEGI. For other sizes of LNG car-riers the setup will be the same but the

% will be changed. Figs. 12 and 13 show

our recommendations for a gas sup-ply system to be used on LNG carriers,

Fig. 14: Typical HP uel oil gas compressor

and gure 15 shows the compressor

system in more detail. Depending onwhether the ship owner wishes to run

on natural BOG only, Fig. 12, or run onboth natural BOG and forced BOG, Fig.13 is relevant.

Both systems comprise a double (2 x

100%) set of Low Pressure compres-

sors each with the capacity to handle

100% of the natural BOG if one falls out

(alternatively 3 x 50% may be chosen).

Each of these LP compressors can in-

dividually feed both the High Pressure

Compressor and the Gas Combustion

Unit. All compressors can run simulta-

neously, which can be utilised when the

engine is fed with both natural and

forced BOG.

The HP compressor section is chosen to

be a single unit. If this unit falls out then

the MEGI engine can run on Heavy Fuel

Oil, and one of the LP compressors can

feed the GCU.

Typical availability of these electrically

dri ven Flotech / GE Oil & Gas com-pressors on natural gas (LNG) service is

98%, consequently, an extra HP com-

pressor is a high cost to add for the 2%

extra availability.

Gas supply system –capacity

management

The minimum requirement for the regu-

lation of supply to the MEGI engine is

a turndown ratio of 3.33 which equals a

regulation down to 30% of the maximum

ow (For a twin engine system, the TRis 6.66). Alternatively in accordance with

the requirements of the ship owners

Both the LP and HP compressor pack-

ages have 0 => 100% capacity variation

systems, which allows enormous ex-

ibility and control.

Stable control of cargo tank pressure is

the primary function of the LP compres-

sor control system. Dynamic capacity

variation is achieved by a combination

100%

65%

65%

65%

100% 100%

65%naturalBOG

35%forcedBOG

LNG Tank

LP.+ comp.

LP.

+ comp.

HP Comp. MEGI

GCU

Redundant gas supply system comprising

2 x Low pressure compressors.

1 x gas combustion unit GCU

1 x High pressure piston compressor.

No Add up with HFOExpect from pilot oil

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14

of compressor speed variation and

gas discharge to recycle. The system

is responsible for maintaining the BOG

pressure set tank pressure point within

the range of 1,06 – 1,20 bar(a) through

0 => 100% compressor capacity.

At full load of the MEGI engine on gas,

the HP compressor delivers approxi-

mately 265 bar whereas at 50% load,

the pressure is reduced to 130180 bar.

The discharge pressure set points are

controlled within ±5%. Compressor

speed variation controls the capacity

range of approximately 100 => 50% of

volumetric ow. Speed control is the

primary variation; speed control logic

is integrated with recycle to reduce

speed/capacity when the system is re-

cycling under standby (0% capacity) or

part load conditions.

LP & HP compressor systems are coor-dinated such that BOG pressure is safely

controlled, whilst however delivering all

available gas at the correct pressure to

the MEGI engine. Load and availability

signals are exchanged between com-

pressor and engine control systems for

this purpose.

Safety aspects

The compressors are delivered generally

in accordance with the API11P standard

(skidpackaged compressors) and are

designed and certied in accordance

with relevant classication society rules.

Maintenance

The gas compressor system needs an

annual overhaul. The overhaul can be

performed by the same engineers who

do the maintenance on the main engines.

It requires no special skills apart from

what is common knowledge for an en-

gineer.

External systems

External safety systems should include

a gas analyser for checking the hydro-

carbon content of the air, inside thecompressor room and re warning andprotection systems.

Safety devices – Internal systems

The compressors are protected by aseries of Pressure High, Pressure Low, Temperature High, Vibration High, Liq-uid Level High/Low,

Compressor RPM High/Low and Oil LowFlow trips, which will automatically shutdown the compressor if fault condi-tions are detected by the local controlsystem.

Pressure safety valves vented to asafe area guard against uncontrolledoverpressure of the fuel gas supplysystem.

Inert gas system

After running in the gas mode, the gassystem on the engine should be emp-tied of gas by purging the gas system

with inert gas (N2, CO

2 ),

Fig. 16: Gas compressor system – indicating capacity control & cooling system

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15

Dual Fuel Control System

General

In addition to the above a special dual

fuel control system is being developed

to control the dualfuel operation when

the engine is operating on compressed

gaseous fuels. See g. 17. The control

system is the glue that ties all the dual

fuel parts in the internal and the external

system together and makes the engine

run in gas mode.

As mentioned earlier the system is

designed as an addon system to the

original ME control system. The conse-

quence is that the Bridge panel, the Main

Operating Panel (MOP) & the Local Op-

erating Panel (LOP) will stay unchanged.

All operations in gas mode are therefore

performed from the engine room alone.

When the dual fuel control system isrunning the existing ME control and

alarm system will stay in full operation.

Mainly for hardware reasons the control

of the dual fuel operation is divided into:

• Plant control

• Fuel control

• Safety Control

Plant control

The task of the plant control is to handle

the switch between the two stable

states:

• Gas Safe Condition State ( HFO only)

• DualFuel State

The plant control can operate all the fuel

gas equipment shown in g. 10. For theplant control to operate it is required

that the Safety Control allows it to work

otherwise the Safety Control will overrule

and return to a Gas Safe Condition.

Fuel control

The task of the fuel control is to deter-

mine the fuel gas index and the pilot oil

index when running in the three different

modes shown in g.4.

Safety control

The task of the safety system is to monitor:

•All fuel gas equipment and the relatedauxiliary equipment

• The existing shut down signal from

the ME safety system.

• The cylinder condition for being in a con

dition allowing fuel gas to be injected.

If one of the above mentioned failures is

detected then the Safety Control releases

the fuel gas Shut Down sequence be-

low:

The Shut down valve V4 and the master

valve V1 will be closed. The ELGI valves

will be disabled. The uel gas will be blow

out by opening valve V2 and fnally the

gas pipe system will be purged with in-

ert gas. See also fg. 9

Architecture of the Dual Fuel Control

System

Dual Fuel running is not essential for the

manoeuvrability of the ship as the enginewill continue to run on fuel oil if an un-

intended fuel gas stop occurs. The two

fundamental architectural and design

demands of the fuel gas Equipment are,

in order of priority:

• Safety to personnel must be at least

on the same level as for a conven-

tional diesel engine

• A fault in the Dual Fuel equipment

must cause stop of gas operationand change over to Gas Safe Condi-

tion.

Which to some extent complement each

other.

The Dual Fuel Control System is designed

to “fail to safe condition”. See Fig. 18.

All failures detected during fuel gas run-

ning and failures of the control system

itself will result in a fuel gas Stop / Shut

Down and change over to fuel opera-

tion. Followed by blow out and purging

of high pressure fuel gas pipes which

releases all gas from the entire gas sup-ply system.

If the failure relates to the purging sys-

tem it may be necessary to carry out

purging manually before an engine

repair is carried out. (This will be ex-

plained later).

The Dual Fuel Control system is a single

system without manual backup con-

trol.

However, the following equipment is

made redundant to secure that a single

fault will not cause fuel gas stop:

• The communication network is doub

led in order to minimize the risk of in-

terrupting the communication between

the control units.

• Vital sensors are doubled and one set

of these sensors is connected to the

Plant Control and the other to the Safety

System. Consequently a sensor failurewhich is not detectable is of no con-

sequence for safe fuel gas operation.

Control Unit Hardware For the Dual Fuel Control System twodifferent types of hardware are used:the Multi Purpose Controller Units and the GCSU , both developed byMAN B&W Diesel A/S.

The Multi Purpose Controller Units are

used for the following units: GECU, GACU,GCCU, and the GSSU see also g. 17.In the following a functionality descrip-

tion for each units shown in g. 17

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Gas Main Operating Panel (GMOP )

For the GI control system an extra panelcalled GMOP is introduced. From here

all manually operations can be initiated.

For example the change between the

different running modes can be doneand the operator has the possibility to

manually initiate the purging of the gaspipes system with inert gas.

Additionally it contains the facilities to

manually start up or to stop on fuel gas.

GECU, Plants control

The GECU handles the Plant Controland in combination with GCCU it also

handles Fuel Control.

Example: When “dual fuel” Start is initiated

manually by the operator, the Plant

Control will start the automatic start

sequence which will initiate startup of

the sealing oil pump. When the engine

condition for Dual Fuel running, which

is monitored by the GECU, is conrmed

to meet the prescribed demands, the

Plant Control releases a “Start Dual Fuel

Operation” signal for the GCCU (Fuel

Control).

In combination with the GCCU, the

GECU will effect the fuel gas injection if

all conditions for Dual Fuel running are

fullled.

The Plant Control monitors the condi-

tion of the following:

• HC “Sensors”

• Gas Supply System

• Sealing Oil System

• Pipe Ventilation

• Inert Gas System

• Network connection to other units of

the Dual Fuel System

and, if a failure occur, the Plant Control

will automatically interrupt fuel gas start

operation and return the plant to Gas

Safe Condition.

The GECU also contains the Fuel Con-

trol which includes all facilities required

for calculating the fuel gas index and

the Pilot Oil index based on the com-

mand from the conventional governor

and the actual active mode.Based on these data and including in-

formation about the fuel gas pressure,

the Fuel Control calculates the start

Fig. 17: MEGI Control System

On Bridge

In Engine Control Room

In Engine Room/

On Engine

ECU A

EICU A EICU B

ECU B

ADMINISTRATION PC

BACK-UP FOR MOP

BRIDGE PANEL

LOCAL OPERATION

PANEL - LOP

ECR PANEL

CRANKSHAFTPOSITION

SENSOR - MSA

CCU

Cylinder 1

CCU

Cylinder n

ALS

SAV

Cylinder n

HCUCylinder n

ALS

SAV

Cylinder 1

HCUCylinder 1

MAIN OPERATION

PANEL - MOP

Cylinder 1 Cylinder n

GCCU

Cylinder 1-6

GCCU

Cylinder 7-12

ELGI

Cylinder 1 = n= 6

ELGI

Cylinder 7 = n= 12

P U M P 3

P U M P 2

M M M

P U M P 1

F i l t e r

P U M P 1

M

P U M P 2

M

HPS

AUXILIARY

BLOWER 1

AUXILIARY

BLOWER 2

ACU 1 ACU 3ACU 2

GACU 1

Inert

gas

Sealing

oilFAN

GACU 2

10 Amp

Sipply

GMOP

GECU GSSU 1

1-6 cyl.

GSSU 2

7-12 cyl.

GCSU 1 GCSU 2

PMI

(on-line)

PMI

(on-line)

5-8 cyl.1-4 cyl.

ME - Control

GCSU 3

PMI

(on-line)

9-12 cyl.

Hardwire interface

with ME

ME

GI

Angle Encoders

Angle Encoders + MSA = Tacho system

TSA A/B

MEECU Engine Control UnitEICU Engine Interface Control Unit

ACU Auxiliary Control UnitCCU Cylinder Control UnitHPS Hydraulic Power SupplySAV Starting Air ValveCPS Crankshaft Position Sensors

ALS Alpha Lubricator SystemMOP Main Operation PanelLOP Local Operation Panel

GIGCSU Gas Cylinder Safety Unit per 4 cylinderGSSU Gas System Safety Unit per 6 cylinderGECU Gas Engine Control UnitGMOP Gas Main Operation Panel

GACU Gas Auxiliary Control UnitGCCU Gas Cylinder Control Unit per 6 cylinder

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17

and duration time of the injection, then

sends the signal to GCCU which ef-

fectuates the injection by controlling the

ELGI valve.

GACU, Auxiliary Control

The GACU contains facilities necessary

to control the following auxiliary systems:

The fan for ventilating of the double wall

pipes, the sealing oil pump, the purgingwith inert gas and the gas supply system.

The GACU controls:

• Start/stop of pumps, fans, and of the

gas supply system.

• The sealing oil pressure set points

• The pressure set points for the gas

supply system.

GCCU, ELGI control

The GCCU controls the ELGI valve on

the basics of data calculated by the

GECU.

In due time before each injection the

GCU receives information from the GECU

of start timing for fuel gas injection, and

the time for the injection valve to stay

open. If the GCCU receive a signal ready

from the safety system and GCCU ob-

serves no abnormalities then the injec-

tion of fuel gas will starts at the relevant

crankshaft position.

The GSSU, fuel gas System Monitor-

ing and Control

The GSSU performs safety monitoring

of the fuel gas System and controls thefuel gas Shut Down.

It monitors the following:

• Status of exhaust gas temperature

• Pipe ventilation of the double wall

piping

• Sealing Oil pressure

• Fuel gas Pressure

• GCSU ready signal

If one of the above parameters, referring

to the relevant fuel gas state differs from

normal service value, the GSSU over-

rules any other signals and fuel gas shut

down will be released.

After the cause of the shut down has

been corrected the fuel gas operation

can be manually restarted.

GCSU, PMI online

The purpose of the GCSUs is to moni-

tor the cylinders for being in condition

for injection of fuel gas. The following

events are monitored:

• Fuel gas accumulator pressure drop

during injection

• Pilot oil injection pressure

• Cylinder pressure:

Low compression pressure

Knocking

Low Expansion pressure

• Scavenge air pressure

If one of the events is abnormal theELGI valve is closed and a shut down of

fuel gas is activated by the GSSU.

Safety remarks

The primary design target of the dual

fuel concept is to ensure a Dual Fuel

Control System which will provide the

highest possible degree of safety to

personnel. Consequently, a failure in the

gas system will, in general, cause shut

down of fuel gas running and subsequent

purging of pipes and accumulators

Fuel gas operation is monitored by the

safety system, which will shut down fuel

gas operation in case of failure. Additio

nally, fuel gas operation is monitored by

the Plant Control and the Fuel Control,

and fuel gas operation is stopped if

one of the systems detects a failure. As

parameters vital for fuel gas operation

are monitored, both by the Plant Control/

Fuel Control and the Safety Control

System, these systems will provide mu-tual backup.

Fig. 18: Fuel gas operation state model

Start of auxiliary

equipment

Start of fuel

gas supply

Running on

fuel gasSafe condition

Stop to safecondition

Purging

Safe condition/

purged system

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Summary

The twostroke engine technol-

ogy is a most widely used and

stateoftheartsolution for optimum

utilisation of the fuel when burning HFO

and gas.

The technology selected for the

twostroke solutions, such as gas

compres sors, is wellproven from the

LNG and power generation industries.

The control and safety system for the

MEGI system is based on the experi-

ence obtained from working gas plants,

including the 12K80MCGIS in Japan,

and cooperation with the Classication

Societies.

The twostroke diesel engine of today is

superior to the traditional steam turbine

solution with regard to the operating

economy, when the MEGI engine is

chosen

REFERENCES

BOG Boiloff gas

CIMAC Congress International des, Machines a Combustion

CNG Compressed natural gas

ELGIvalve Electronic gas injection

ESD Emergency shutdown

FIVAvalve Fuel injection valve actuator

GACU Gas auxiliary control unit

GCCU Gas cylinder control unit

GCSU Gas control safety unit

GECU Gas engine control unit

GSSU Gas system safety unit

HFO Heavy fuel oil

LNG Liquied natural gasMCR Maximum continuous rating

MEGI ME engine with gas injection

PMI Pressure mean indicator

TR Turndown ratio

Abbreviations

[1] “LNG Carriers with Low Speed

Diesel Propulsion”, Ole Grøne,

The SNAME Texas Section14th

Annual Offshore Symposium,

November 10, 2004, Houston, Texas

[2] “Basic Principles of Ship Propulsion”,

p.254 – 01.04, January 2004,

MAN B&W Diesel A/S

[3] “MEGI Engines for LNG Application”

System Control and Safety Feb. 2005

Ole Grøne, Kjeld Aabo,

Rene Sejer Laursen,

MAN B&W Diesel A/S

Steve Broadbent,Flotech

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