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Transcript of Introduction to MAN ME-GI engines - ME-GI Engines for LNG Application System Control and Safety
8/8/2019 Introduction to MAN ME-GI engines - ME-GI Engines for LNG Application System Control and Safety
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ME-GI Engines for LNG ApplicationSystem Control and Safety
Introduction .......................................................................................................... 3
Propulsion Power Requirements for LNG Carriers ..................................... 3
Boil-off Gas from LNG Cargo ........................................................................... 4
Design of the Dual Fuel ME-GI 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 on-line ............................................................................................... 17
– Safety remarks .................................................................................................... 17
Summary ............................................................................................................... 17
References ............................................................................................................. 17
Abbreviations ........................................................................................................ 18
Content Page
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3
ME-GI Engines for LNG ApplicationSystem Control and Safety
Introduction
Until the end of 2004 there was still one
market for ocean-going cargo ships to
which the two-stroke engine had not yet
been introduced: i.e. the LNG market.
This market has so far been dominated
by steam turbines, but the first orders
for two-stroke 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 areto be built in Korea, Ref. [1].
For these plants, the boil-off gas is retur-
ned to the LNG tanks in liquefied form via
a reliquefaction plant installed on board.
Some operators are considering an alter-
native two-stroke solution, which is the
ME-GI (Gas Injection) engine operating
at a 250-300 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 se-
cured as profit per year when using
two-stroke diesel engines, irrespective
of whether the HFO or the dual fuel
engine type is chosen. When it comes
to first cost, the HFO diesel engine com-
bined with a reliquefaction plant has the
same cost level as the steam turbine
solution, whereas the dual fuel ME-GI
engine with a compressor is a cheaper
solution.
This paper will describe the application
of ME-GI 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 two-stroke dieselengine is winning in this market.
Fig. 1: Typical propulsion power requirements for LNG carriers
20.000
30.000
40.000
50.000
1 25 .0 00 15 0.0 00 17 5.0 00 2 00 .00 0 2 25.00 0 2 50 .0 00
(m3)
Engine Power
(kW)
21.0 knots
20.0 knots
19.0 knots
Fig. 2: Typical thermal efficiencies of prime movers
35
30
40
25
50
45
Medium speeddiesel engine
20
Capacity ( MW)501 10
55
Thermal efficiencies %
Gas turbine
Combined cyclegas turbine
Steam turbine
Low speed diesel engine
5
LNG carrier
Propulsion power requi-rements for LNG carriers
Traditionally, LNG carriers have been
sized to carry 130,000 – 140,000 m3
liquefied natural gas, i.e. with a carrying
capacity of some 70-80,000 tons, which
resembles that of a panamax bulk carrier.
The speed has been around 20 knots,
whereas that of the panamax bulk carriersis around 15. Now, even larger LNG
carriers are in project up to a capacity
of some 250,000 m3 LNG. Such shipswill be comparable in size to a capesize
bulk 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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4
Fig. 3: Propulsion alternative – energy need for propulsion Fig. 4: Fuel Type Modes – MAN B&W two-stroke dual fuel low speed diesel
Boil-off Gas from LNGCargo
The reason for having a continuousevaporated rate of boil-off gas is that itis generated by heat transferred fromthe ambient temperature through theLNG tanks and into to cold LNG. Theboil-off gas is the consequence if theLNG cargo should be staying liquid atatmospheric pressure and at a tempera-ture of some minus 160 degrees Celsius. To keep the evaporated rate of boil-off at a minimised level, the cargo is keptin 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, theboil-off gas is conveniently sent to twinboilers to produce steam for the propul-sion turbine.
As mentioned, diesels are now being
seen as an alternative to steam, first of
all because of the significant difference
in thermal efficiency reflected also in the
system efficiency, as illustrated in Fig. 2.
With a power requirement of the mentio-
ned magnitude, the illustrated efficiency
difference of up to 20 percentage points
amounts to significant 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 diesel-electric
drive of one or two propellers.
The choice depends on economical and
operational factors.
Over time, the evaluation of these factors
for the options of propulsion technology,
for ordinary larger cargo vessels (viz.
container vessels, bulk carriers and
tankers), has led to the selection of a
single, heavy-fuel-burning, 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 feasible
for LNG carriers.
Due to the proper insulation, the boil-off is usually not enough to provide the energyneeded for propulsion, so the evaporatedgas is supplemented by either forced boiloff of gas or heavy fuel oil to produce therequired steam amount.
In a diesel engine driven LNG carrier,the energy requirement is less thanksto the higher thermal efficiency, so thesupplementary energy by forced boil off or heavy fuel oil can be reduced signifi-
cantly, as shown in Fig. 3
100%
60%
50%
Steam
NBO
Gas
FBO
Gas
orFuel
NBOGas
or
Fuel
DieselFuel
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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5
Design of the Dual FuelME-GI Engine
In terms of engine performance (i.e.:
output, speed, thermal efficiency, exhaust
gas amount and temperature, etc.) the
ME-GI engine series is generally identical
to the well-established and type approved
ME engine series. This means that the
application potential for the ME-engine
series applies to the ME-GI engine seriesas well – provided that gas is available
as a main fuel. All ME engines can be
offered as ME-GI engines.
Consequently, the following description
of the ME-GI engine design only deals
with new or modified engine compo-
nents 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.
General Description
Fig. 5 shows the cross-section of a
S70ME-GI, with the new modified parts
of the ME-GI engine pointed out, com-
prising gas supply piping, large-volume
accumulator on the (slightly modified)
cylinder cover with gas injection valves,
and HCU with ELGI valve for control of the injected gas amount. Further to
this, there are small modifications to the
exhaust 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 modified parts on the ME-GI engine
Fig.6: General arrangement of double-wall piping system for gas
Exhaust receiver Cylinder cover with gas valves
LargeVolume accumulator
Gas supply piping
HCU with
ELGI valve
1. High pressure pipe from 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
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Fig. 7: ME-GI fuel injection system
• High-pressure gas compressor supply
system, including a cooler, to raise the
pressure to 250-300 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. includes a
hydrocarbon analyser for checkingthe hydro-carbon content of the air in
the compressor room and in the
double-wall gas pipes.
• Ventilation system, which ventilates
the outer pipe of the double-wall 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 high-
pressure gas from the compressor-unit
flows through the main pipe via narrow
and flexible branch pipes to each cylinder’s
gas valve block and large-volume accu-mulator. The narrow and flexible branch
pipes perform two important tasks:
• They separate each cylinder unit from
the rest in terms of gas dynamics, utili-
sing the well-proven design philosophy
of the ME engine’s fuel oil system.
• They act as flexible connections be-
tween the stiff main pipe system and
the engine structure, safeguarding
against extra-stresses in the mainand branch pipes caused by the in-
evitable differences in thermal expan-
sion of the gas pipe system and the
engine structure.
The large-volume accumulator, con-
taining about 20 times the injection
amount per stroke at MCR, also per-forms 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
safety 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 prin-
ciple, consists of the ME hydraulic con-
trol (servo) oil system and an ELGI
valve, supplying high-pressure control
oil to the gas injection valve, thereby
control-ling 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 can
be operated in the various relevant
modes: normal “dual-fuel mode” withminimum pilot oil amount, “specified
gas mode” with injection of a fixed gas
amount, and the “fuel-oil-only mode”.
The ME-GI control and safety system is
built as an add-on 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 fuel-gas (or of pilot
oil) is either increased or decreased.
If there is any variation over time in the
calorific value of the fuel-gas it can be
measured on the rpm of the crankshaft.
Depending on the value measured, the
amount of fuel-gas is either increasedor decreased.
The change in the calorific value overtime is slow in relation to the rpm of the
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 device
pressure 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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7
engine. 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 set-point signals.
System description
Compared with a standard engine forheavy fuel operation, the adaptation tohigh-pressure 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 for dual-fuel 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 thesystems/ components on the engine, aspointed out in Fig. 5, will be described.
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 boththe injection of pilot fuel and gas fuel.
The valves are of separate types, and
two are fitted for gas injection and two
for pilot fuel. The media required for
both fuel and gas operation is shown
below:
• High-pressure 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 design andthe use of mainly rotational symmetrical
parts. The design is based on the principle
used for an early version of a combined
Fig. 8: Engine control system diagram
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 leakage through
the gas sealing rings will be led through
bores in the gas injection valve and the
cylinder cover to the double-wall gas
piping system, where any such leak-
ages will be detected by HC sensors.
The gas acts continuously on the valve
spindle at a pressure of about 250-300
bar. In order to prevent the gas from
entering the control oil activating systemvia the clearance around the spindle,
the spindle is sealed by means of sealing
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 +supplementaryfuel oil between
5-100%
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 up to 250 bar
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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8
pressure higher than the gas pressure
(25-50 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 oiloperation. In this case, it may be nec-
essary 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 burning,
the cylinder cover is designed with a
welded-on 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 providedwith two sets of bores, one set for sup-
plying gas from the valve block to each
gas injection valve, and one set for lead-
ing any leakage of gas to the sub-atmo-
Fig. 9: Internal and external systems for dual fuel operation
spheric pressure, ventilated part of the
double-wall piping system.
Hydraulic Cylinder Unit (HCU)
To reduce the number of additional hy-draulic pipes and connections, theELGI valve as well as the control oil pipeconnections to the gas valves will beincorporated in the design of the HCU.
Valve block
The valve block consists of a square
steel block, bolted to the HCU side of the cylinder cover.
The valve block incorporates a largevolume accumulator, and is provided
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with a shutdown valve and two purgevalves on the top of the block. All high-pressure gas sealings lead into spacesthat are connected to the double-wallpipe system, for leakage detection.
The gas is supplied to the accumulatorvia a non-return valve placed in the ac-cumulator inlet cover.
To ensure that the rate of gas flow doesnot drop too much during the injection
period, the relative pressure drop in theaccumulator is measured. The pressuredrop should not exceed about 20-30 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 theshut down valve, which in the gas mode,is kept open by compressed air. From theshutdown valve (V4 in Fig. 9), the gas isled to the gas injection valve via boresin the valve block and in the cylinder cover. A blow-off valve (V3 in Fig. 9), placed ontop of the valve block, is designed toempty the gas bores when needed.
A purge valve (V5 shown in Fig. 9), whichis 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 fitted for high-pressure gas
distribution to each valve block.
Gas pipes are designed with double walls,
with the outer shielding pipe designed
so as to prevent gas outflow to the ma-
chinery spaces in the event of rupture
of the inner gas pipe. The intervening
space, including also the space aroundvalves, flanges, etc., is equipped with
separate mechanical ventilation with a
capacity of approx. 10 – 30 air changes
per hour. The pressure in the intervening
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 from spark-free
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
double-wall 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 double-wall piping system is desig-
ned so that every part is ventilated. how-
ever, minute volumes around the gasinjection valves in the cylinder cover are
not ventilated by flowing 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 leakage
gas will be led to the ventilated part of
the double-wall piping system and be
detected by the HC sensors.
The gas pipes on the engine are designedfor 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 all-welded as far as practicable,
with flange connections only to the nec-
essary extent for servicing purposes.
The branch piping to the individual cylin-
ders must be flexible 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
fluctuations 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 in-
jection 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
amount of pilot oil is monitored by the
position sensor.
The injected gas amount is controlled bythe duration of control oil delivery from
the ELGI valve. The operating medium
is the same servo oi l as is used for the
fuel oil pressure booster.
Miscellaneous
Other engine modifications will, basically,
be limited to a changed position of
pipes, platform cut-outs, drains, etc.
Fig. 10: Gas injection valve
Cylinder
cover
Gas inlet
Gas spindle
Sealing oilControl oil
Connection to
the ventilatedpipe system
Sealing oil inlet
Control oil inlet
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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 flows
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, and it is likely thatthe gas will burn with a diffusion type
flame (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 above-mentioned
situations will be prevented by the de-
tection of defective gas valves, which
are arranged as follows:
The gas flow to each cylinder during one
cycle will be detected by measuring the
pressure drop in the accumulator. This
is to ensure that the injected gas amountdoes not exceed the amount correspon-
ding to the MCR value.
It is necessary to ensure that the pres-
sure in the accumulator is sufficient for
gas operation, so the accumulator wil l
be equipped with a pressure switch and
a differential pressure switch. An in-
crease of the gas flow to the cylinder
which is greater than corresponding to
the actual load, but smaller than corre-
sponding to the MCR value, will only giverise to the above-mentioned exhaust gas
temperature alarm, and is not harmful.
By this system, any abnormal gas flow,
whether due to seized gas injection valves
Safety aspects
The normal safety systems incorporated
in the fuel oil systems are fully retained
also during dual fuel operation. However,
additional safety devices will be incorpo-
rated in order to prevent situations which
might otherwise lead to failures.
Safety devices – External systems
Leaky valves and fractured pipes aresources of faults that may be harmful.
Such faults can be easily and quickly de-
tected by a hydro-carbon (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 elimi-
nate such risks are double-wall 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 leak-
age will be led to the HC-sensors 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 flow
into the control oil activation system and,
thereby, create gas pockets and preventthe 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 double-wall pip-
ing system prevents the safety function
of the HC sensors, so the system is to
be equipped with a set of flow switches.If the switches indicate no flow, or nearly
no flow, an alarm is given. If no correc-
tion 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 detection of a
malfunctioning switch, even in case of
an electric power failure.
• In case of malfunctioning valves (not
leaky) resulting in insufficient 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 pressurecould be monitored by either one
pressure pick-up, or by a pressure
switch and a differential pressure
switch (see later for explanation).
As natural gas is lighter than air, non-re-
turn 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 ME-GI
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 gasinjection 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.
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11
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 cyl-
inder concerned will increase. This will
be detected when the exhaust gas
temperature 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 high-pressure
pipes, making the fuel oil booster in-
operative.
• Seized plunger in the fuel oil booster.
• Other faults on the engine, forcing the
fuel oil booster to “O-index”.
• 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 first failure to inject the
pilot oil.
In extremely rare cases, pilot fuel can beinjected 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 sufficient to ensure ignition of
the pilot oil. Consequently, gas and pilot
fuel from that cylinder will be supplied to
the exhaust gas receiver in a fully un-
burned condition, which might result in
violent burning in the receiver. However,
burning of an exhaust valve is a rather
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 misfiring.
A seized spindle in the pilot oil valve is
another very rare fault, which might in-
fluence the safety of the engine in dual
fuel operation. However, the still operat-
ing valve will inject pilot oil, which will ig-
nite 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 condition.
As will appear from the above discussion,
which has included a number of veryunlikely 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, the-
refore, not subject to the type approval
of the engine. The external systems de-
Fig. 11: Gas system branching
scribed here include the sealing oil system,
the ventilation system, and the gas sup-
ply 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 moving
parts.
The sealing oil pump has a separate
drive and is started before commencing
gas operation of the engine. It uses the
200 bar servo oil, or one bar fuel oil, and
pres- surises it additionally to the oper-
ating pressure, which is 25-50 bar higher
than the gas pressure. The consumption
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
double-wall gas pipe system is ventilated
with air, and it acts as a separation
between the engine room and the
high-pressure gas system, see Fig 11.
Ventilation is achieved by means of an
electrically driven mechanical fan or
extractor fan. If an electrically driven
fan is chosen, the motor must be placed
outside the ventilation duct. The capac-
ity must ensure approx. 10 – 30 air
changes per hour. More ventilation gives
quicker detection of any gas leakage.
Fig. 12: Gas supply system – natural BOG only
The gas CompressorSystem
The gas supply system is based on
Flotech™ packaged compressors:
• Low-pressure GE Oil & Gas RoFlo™
type gas compressors with lubricated
vanes and oil buffered mechanical seals,
which compress the cold boil-off gas
from the LNG tanks at the temperatureof -140oC to -160oC. The boil-off gas
pressure in the LNG tanks should nor-
mally be kept between 1.06-1.20
bar(a). Under normal running conditions,
cooling is not necessary, but during
start up, the temperature of the boil-off
gas may have risen to atmospheric
temperature, hence pre-heating and
after-cooling is included, to ensure
stabilisation of the cold inlet and inter-
mediate gas. temperature
• The high-pressure GE Oil & GasNuovo Pignone™ SHMB type gas
compressor; 4 throw, 4-stage hori-
zontally opposed and fully balanced
crosshead type with pressure lubri-
cated and water-cooled cylinders &
packings, compresses the gas to ap-
proximately 250-300 bar, which is the
pressure required at the engine inlet
at full load. Only reciprocating piston
compressors are suitable for this high-
pressure duty; however the unique GE
fully balanced frame layout addresses
concerns about transmitted vibrations
and also eliminates the need for heavy
installation structure, as is required with
vertical or V-form unbalanced compres-
sor designs. The discharge temperature
is kept at approx. 45oC by the coolers.
• Buffer tank/accumulators are installed
to provide smoothing of minor gas
pressure fluctuations in the fuel sup-
ply; ± 2 bar is required.
• Gas inlet filter/separator with strainer
for protection against debris.
• Discharge separator after the final stage
gas cooler for oil/condensate removal.
• Compressor capacity control system
ensures that the required gas pressure
is in accordance with the engine load,
and that the boil-off gas amount is
regulated for cargo tank pressure
control (as described later).
• The compressor safety system handles
normal start/stop, shutdown and
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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13
emergency shutdown commands.
The compressor unit includes a pro-
cess monitoring and fault indication
system. The compressor control sys-
tem exchanges signals with the ME-
GI control system.
• The compressor system evaluates
the amount of available BOG and
reports to the ME-GI 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 recomendation
that reduces the costs and the require-
ment for space while ensuring a fully op-
erational ME-GI engine. The dual fuel en-
gine concept, in its nature, includes redu-
nancy. 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 x7S65ME-GI. For other sizes of LNG car-
riers the setup will be the same but the
% will be changed. Figs. 12 and 13 showour recommendations for a gas supply
system to be used on LNG carriers, and
figure 15 shows the compressor system
in more detail. Depending on whether the
ship owner wishes to run on natural BOGonly, Fig. 12, or run on both natural BOG
and forced BOG, Fig. 13 is relevant.
Both systems comprise a double (2 x
100%) set of Low Pressure compressors
each with the capacity to handle 100%
of the natural BOG if one falls out (alter-
natively 3 x 50% may be chosen). Each
of these LP compressors can individually
feed both the High Pressure Compressor
and the Gas Combustion UUUUUnit. All com-
pressors can run simultaneously, which
Fig. 13: Gas supply system– natural and forced BOG
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 fal ls out then
the ME-GI 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 compressors
on natural gas (LNG) service is 98%,consequently, an extra HP compressor
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 ME-GI engine is
a turndown ratio of 3.33 which equals a
regulation down to 30% of the maximum
flow (For a twin engine system, the TR
is 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 flex-
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
of compressor speed variation and gas
Fig. 14: Typical HP fuel oil gas compressor
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.
NoAdd up with HFOExpect from pilot oil
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discharge to recycle. The system is re-
sponsible for maintaining the BOG pres-
sure set tank pressure point within the
range of 1,06 – 1,20 bar(a) through 0
=> 100% compressor capacity.
At full load of the ME-GI engine on gas,
the HP compressor delivers approxi-
mately 265 bar whereas at 50% load,
the pressure is reduced to 130-180 bar.The discharge pressure set points are
controlled within ±5%. Compressor
speed variation controls the capacity
range of approximately 100 => 50% of
volumetric flow. Speed control is the pri-
mary variation; speed control logic is in-
tegrated with recycle to reduce speed/
capacity when the system is recycling
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 ME-GI 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 API-11P standard
(skid-packaged compressors) and are
designed and certified in accordancewith relevant classification 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 engineer.
External systems
External safety systems should include
a gas analyser for checking the hydro-
carbon content of the air, inside the
compressor room and fire 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 conditionsare detected by the local control system.
Pressure safety valves vented to a safearea guard against uncontrolled over-pressure of the fuel gas supply system.
Inert gas system
After running in the gas mode, the gas
system 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 dual-fuel operation when
the engine is operating on compressed
gaseous fuels. See fig. 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 enginerun in gas mode.
As mentioned earlier the system is desig-
ned as an add-on system to the original
ME control system. The consequence is
that the Bridge panel, the Main Operating
Panel (MOP) & the Local Operating 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)
• Dual-Fuel State
The plant control can operate all the fuel
gas equipment shown in fig. 10. For the
plant control to operate it is required that
the Safety Control allows it to work oth-
erwise 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 differ-
ent modes shown in fig.4.
Safety control
The task of the safety system is to monitor:
• All fuel gas equipment and the related
auxiliary 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 releasesthe fuel gas Shut Down sequence below:
The Shut down valve V4 and the master
valve V1 will be closed. The ELGI valves
will be disabled. The fuel gas will be blow
out by opening valve V2 and finally the
gas pipe system will be purged with inert
gas. See also fig. 9
Architecture of the Dual Fuel Control
System
Dual Fuel running is not essential for the
manoeuvrability of the ship as the engine
will continue to run on fuel oil if an unin-
tended 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 equipmentmust cause stop of gas operation and
change over to Gas Safe Condition.
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 purgingof high pressure fuel gas pipes which
releases all gas from the entire gas sup-
ply system.
If the failure relates to the purging system
it may be necessary to carry out purging
manually before an engine repair is car-
ried out. (This will be explained later).
The Dual Fuel Control system is a single
system without manual back-up control.
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 failure
which 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 Unitsand the GCSU , both developed byMAN B&W Diesel A/S.
The Multi Purpose Controller Units areused for the following units: GECU, GACU,GCCU, and the GSSU see also fig. 17.In the following a functionality description
for each units shown in fig. 17
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Gas Main Operating Panel (GMOP )
For the GI control system an extrapanel called GMOP is introduced. From
here all manually operations can be initi-
ated. For example the change betweenthe different running modes can bedone and the operator has the possibil-
ity to manually initiate the purging of thegas pipes 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 Control
and 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 start-up of
the sealing oil pump. When the engine
condition for Dual Fuel running, which is
monitored by the GECU, is confirmed
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
fulfilled.
The Plant Control monitors the condition
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 automatical ly 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.
Fig. 17: ME-GI 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
MEE CU - Engine Control UnitEICU - Engine Interface Control Unit
A CU - Auxiliary Control UnitC CU - 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 Auxil iary Contro l UnitGCCU - Gas Cylinder Control Unit per 6 cyl inder
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17
Based on these data and including in-
formation about the fuel gas pressure,
the Fuel Control calculates the start
and duration time of the injection, then
sends the signal to GCCU which effec-
tuates 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 purging
with 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 the
fuel 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
overrules 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 on-line
The purpose of the GCSUs is to monitor
the cylinders for being in condition forinjection 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 the
ELGI 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 per-
sonnel. 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 operationare monitored, both by the Plant Control/
Fuel Control and the Safety Control
System, these systems will provide mu-
tual back-up.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 two-stroke engine technology is a
most widely used and state-of-the-art-
solution for optimum utilisation of the
fuel when burning HFO and gas.
The technology selected for the two-
stroke solutions, such as gas compres-
sors, is well-proven from the LNG and
power generation industries. The control
and safety system for the ME-GI systemis based on the experience obtained
from working gas plants, including the
12K80MC-GI-S in Japan, and coopera-
tion with the Classification Societies.
The two-stroke diesel engine of today is
superior to the traditional steam turbine
solution with regard to the operating
economy, when the ME-GI engine is
chosen
REFERENCES
BOG Boil-off gas
CIMAC Congress International des, Machines a Combustion
CNG Compressed natural gas
ELGI-valve Electronic gas injection
ESD Emergency shut-down
FIVA-valve 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 Liquified natural gas
MCR Maximum continuous rating
ME-GI 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] “ME-GI 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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