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8/13/2019 ME_GI engines for LNG vessels
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Introduction ..................................................................................................................................... 2
Propulsion power requirements for LNG carriers ........................................................................... 2
BOIL-OFF GAS FROM LNG CARGO.......................................................................................... 3
DESIGN OF THE DUAL FUEL ME-GI ENGINE ....................................................................... 4General Description..................................................................................................................... 4
System Description ..................................................................................................................... 6
Engine Systems ........................................................................................................................... 7
Exhaust receiver...................................................................................................................... 7
Fuel injection valves ............................................................................................................... 7
Cylinder cover......................................................................................................................... 8
Hydraulic Cylinder Unit (HCU) ............................................................................................. 8
Valve block ............................................................................................................................. 8
Gas pipes................................................................................................................................. 9
Fuel oil booster system ........................................................................................................... 9
Miscellaneous ....................................................................................................................... 10
Safety Aspects ........................................................................................................................... 10Safety Devices External systems ....................................................................................... 10
Safety Devices Internal systems ........................................................................................ 10
Defective gas injection valves............................................................................................... 10
Ignition failure of injected gas .............................................................................................. 11
External Systems................................................................................................................... 12
Sealing oil system ................................................................................................................. 12
Ventilation system ................................................................................................................ 12
THE GAS COMPRESSOR SYSTEM .......................................................................................... 12
Gas supply system capacity management.......................................................................... 14
Safety aspects........................................................................................................................ 15
Maintenance.......................................................................................................................... 15External systems ................................................................................................................... 15
Safety devices Internal systems ......................................................................................... 15
Inert gas system..................................................................................................................... 15
DUAL FUEL CONTROL SYSTEM............................................................................................. 16
General.................................................................................................................................. 16
Plant control .......................................................................................................................... 16
Fuel control ........................................................................................................................... 17
Safety control ........................................................................................................................ 17
Architecture of the Dual Fuel Control System...................................................................... 17
Control Unit Hardware ......................................................................................................... 18
Gas Main Operating Panel (GMOP). .................................................................................... 18
GECU, Plants control............................................................................................................ 18GACU, Auxiliary Control..................................................................................................... 18
GCCU, ELGI control ............................................................................................................ 18
The GSSU, fuel gas System Monitoring and Control........................................................... 19
GCSU, PMI on-line .............................................................................................................. 19
Safety remarks ...................................................................................................................... 19
SUMMARY................................................................................................................................... 20
REFERENCES .............................................................................................................................. 20
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2
Ole Grne, Senior Vice President
Kjeld Aabo, Senior Manager
Ren Laursen, Master of Science
MAN B&W Diesel A/S, Copenhagen, Denmark
J. Stephen Broadbent, Managing Director
FLOTECH Limited, Auckland, New Zealand
ME-GI Engines for LNG Application
System 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
ordered for eight LNG carriers, which are to be
built in Korea.
For these plants, the boil-off gas is returned to
the LNG tanks in liquefied form via a
reliquefaction plant installed on board.
Some operators are considering an alternative
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 given project
depends primarily on the price of HFO and the
price of the natural gas when sold.
Calculations carried out by the authors
company show that about USD 3 million is
saved in operational costs 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 HFOdiesel engine combined 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 supply 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 diesel engine is
winning in this market.
PROPULSION POWER
REQUIREMENTS FOR LNG
CARRIERS
Traditionally, LNG carriers have been sized to
carry 130,000 140,000 m3
liquefied naturalgas, 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 carriers is around 15. Now, even larger
LNG carriers are in project up to a capacity of
some 250,000 m3LNG. Such ships will be
comparable in size to a capesize bulk carrier
and an aframax tanker but, again, with a speed
higher than these.
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General DescriptionIn a traditional steam turbine vessel, the boil-off gas is conveniently sent to twin boilers to
produce steam for the propulsion turbine.Fig. 5 shows the cross-section of a S70ME-GI,
with the new modified parts of the ME-GI
engine pointed out, comprising 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.
Due to the proper insulation, the boil-off is
usually not enough to provide the energyneeded for propulsion, so the evaporated gas is
supplemented by either forced boil off of gas
or heavy fuel oil to produce the required steam
amount.
In a diesel engine driven LNG carrier, the
energy requirement is less thanks to the higher
thermal efficiency, so the supplementary
energy by forced boil off or heavy fuel oil can
be reduced significantly, as shown in Fig. 3
Fuel-oil-only modeFuel-oil-only mode
FIGURE 4: Fuel Type Modes MAN B&W two-
stroke dual fuel low speed diesel
DESIGN OF THE DUAL FUEL
ME-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 engineseries as 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 components 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.
FIGURE 5: New modified parts on the ME-GI engine
Apart from these systems on the engine, the
engine auxiliaries will comprise some new
units, the most important ones being:
High-pressure gas compressor supplysystem, including a cooler, to raise the
pressure to 250-300 bar, which is the
pressure required at the engine inlet.
Pulsation/buffer tank including acondensate separator.
Compressor control system.
Safety systems, which ex. includes ahydrocarbon analyser for checking the
hydro-carbon content of the air in the
compressor room and in the double-wall
gas pipes.
GasGas
100% load100% load30 - 40%30 - 40%
Fuel
Fuel
FuelFuel100%100%
8%8%
Minimum fuel modeMinimum fuel modeFuelFuel100%100%
FuelFuel
100% load100% load
100% load100% load
FuelFuel
GasGas
FuelFuel100%100%
Specified gas modeSpecified gas mode
8%8%
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5
The large-volume accumulator, containing
about 20 times the injection amount per stroke
at MCR, also performs two important tasks:
Ventilation system, which ventilates theouter pipe of the double-wall piping
completely.
It supplies the gas amount for injectionat only a slight, but predetermined,pressure drop.
Sealing oil system, delivering sealing oil tothe gas valves separating the control oil
and the gas. It forms an important part of the safety
system (as described later). Inert gas system, which enables purging
of the gas system on the engine with
inert gas. Since the gas supply system is a common 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 high-pressure control
oil to the gas injection valve, thereby control-ling the timing and opening of the gas valve.
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 accumulator. The narrow and flexible
branch pipes perform two important tasks:As can also be seen in Fig. 7, the normal fuel
oil pressure booster, which supplies pilot oil in
the dual fuel operation mode, is connected to
the ELGI valve by a pressure gauge and an
on/off valve incorporated in the ELGI valve.
They separate each cylinder unit from therest in terms of gas dynamics, utilising the
well-proven design philosophy of the ME
engine's fuel oil system.
They act as flexible connections betweenthe stiff main pipe system and the engine
structure, safeguarding against extra-stresses in the main and branch pipes
caused by the inevitable differences in
thermal expansion of the gas pipe system
and the engine structure.
1. High pressure pipe from gas compressor1. High pressure pipe from gas compressor
FIGURE 7: ME-GI fuel injection systemBy the control system, the engine can be
operated in the various relevant modes: normal
dual-fuel mode with minimum pilot oil
amount, specified gas mode with injectionof 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 requires any changes
to the ME system, and it is consequently very
simple to implement.
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 room2. 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)
FIGURE 6: General arrangement of double-wall
piping system for gas
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Emergencystop engine
BOG evaporatedEngine on morethan 30% load
Not enoughBOG for full
Dual fuel
operation
TBOG amountevaporated
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 less
than 200 bar (Gas pipes andvalves are flushed with N )
2
Gas led tooxidiser when
too much BOG
is available
Excess BOGburned inoxidiser
Gas led tooxidiser
Gas burned inoxidiser
Compressorinternal bypass
of remaining gas
Compressorup to 250 bar
Compressorup to 250 bar
Compressorup to 250 bar
Compressor
LP compressor
Compressorstarts up
Recirculationof gas
to buffertank
Compressor
100%BOF
100%BOF
100%BOF
100%BOF
100%BOF
AvailableBOG
FIGURE 8: Engine control system diagram
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 discharge 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
increased or decreased.
The change in the calorific value over time is
slow in relation to the rpm of the engine.
Therefore the required change of gas amount
between injections is relatively small.
To make the engine easy to integrate withdifferent suppliers of external gas delivering
systems, the fuel gas control system 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 for heavy
fuel operation, the adaptation to high-pressure
gas injection requires that the design of the
engine and the pertaining external systems will
comprise a number of special external
components and changes on the engine.
Fig. 9 shows the principal layout of the gas
system on the engine and some of the external
systems needed for dual-fuel operation.
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Fuel injection valvesFIGURE 9: Internal and external systems for dualfuel operation Dual fuel operation requires valves for both the
injection of pilot fuel and gas fuel.In general, all systems and components
described in the following are to be made "fail
safe", meaning that components and systems
will react to the safe side if anything goes
wrong.
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:
Engine Systems High-pressure gas supply
Fuel oil supply (pilot oil)In the following, the changes of the systems/
components on the engine, as pointed out inFig. 5, will be described.
Control oil supply for activation
of gas injection valves Sealing oil supply.
Exhaust receiver The gas injection valve design is shown in
Fig. 10.The exhaust gas receiver is designed to
withstand the pressure in the event of ignition
failure of one cylinder followed by ignition of
the unburned gas in the receiver (around 15
bars).
The receiver is furthermore designed with
special transverse stays to withstand such gas
explosions.
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FIGURE 10: Gas injection valve
This valve complies with our traditional design
principles of compact design and the use of
mainly rotational symmetrical parts. The
design is based on the principle used for an
early version of a combined fuel oil/gas
injection valve as well as experience gained
with our normal fuel valves.
Gas is admitted to the gas injection valve
through bores in the cylinder cover. To preventgas 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 leakages will be
detected by HC sensors.
The gas acts continuously on the valve spindle
at a pressure of about 250-300 bar. In order toprevent the gas from entering the control oil
activating system via the clearance around the
spindle, the spindle is sealed by means of
sealing oil led to the spindle clearance at a
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 thecustomer'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
equipment, the fuel valve nozzle holes will be
as the standard type for normal fuel oil
operation. In this case, it may be necessary touse a somewhat larger amount of pilot fuel in
order to assure a good injection 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 provided with
two sets of bores, one set for supplying gas
from the valve block to each gas injection
valve, or to each combined fuel oil/gas valve,
and one set for leading any leakage of gas to
the sub-atmospheric pressure, ventilated part
of the double-wall piping system.
Hydraulic Cylinder Unit (HCU)
To reduce the number of additional hydraulic
pipes and connections, the ELGI valve as well as
the control oil pipe connections to the gas valves
will be incorporated 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 large volume
accumulator, and is provided with a shutdownvalve and two purge valves on the top of the
block. All high-pressure gas sealings lead into
spaces that are connected to the double-wall
pipe system, for leakage detection.
The gas is supplied to the accumulator via a
non-return valve placed in the accumulator
inlet cover.
To ensure that the rate of gas flow does not
drop too much during the injection period, the
relative pressure drop in the accumulator is
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measured. The pressure drop should not
exceed about 20-30 bar.
Any larger pressure drop would indicate a
severe leakage in the gas injection valve seatsor a fractured gas pipe. The safety system will
detect this and shut down the gas injection.
From the accumulator, the gas passes through a
bore in the valve block to the shut down valve,
which in the gas mode, is kept open by
compressed air. From the shutdown valve (V4
in Fig. 9), the gas is led to the gas injection
valve via bores in the valve block and in the
cylinder cover. A blow-off valve (V3 in Fig.
9), placed on top of the valve block, is
designed to empty the gas bores when needed.
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) system 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 machinery spaces in
the event of rupture of the inner gas pipe. The
intervening space, including also the space
around valves, flanges, etc., is equipped with
separate mechanical ventilation with a capacity
of approx. 10 air changes per hour. The pressure
in the intervening space is to be below that ofthe 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 cylinderblocks, 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 designed so
that every part is ventilated. However, minutevolumes around the gas injection 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 designed for
50 % higher pressure than the normal workingpressure, 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 necessary extent
for servicing purposes.
The branch piping to the individual cylinders
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 designed 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 tocontrol 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 amount of pilot oil is monitored
by the position sensor.
The injected gas amount is controlled by theduration of control oil delivery from the ELGI
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valve. The operating medium is the same servo
oil as is used for the fuel oil pressure booster.
Miscellaneous
Other engine modifications will, basically, belimited to a changed position of pipes, platform
cut-outs, drains, etc.
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 incorporated in order to prevent
situations 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 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 eliminatesuch 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 leakage will be led to
the HC-sensors placed in the outer pipe.
Another source of fault could be a malfunctio-
ning 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 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 double-wall piping
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 noflow, or nearly no flow, 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 detection of a malfunctioning switch,
even in case of an electric power failure.
In case of malfunctioning valves (notleaky) resulting in insufficient gas supply
to the engine, the gas pressure will be too
low for gas operation. This is dealt with
by monitoring the pressure in the
accumulator in the valve block on each
cylinder. The pressure could 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-return
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 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
flows out and into the exhaust pipe and further
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on to the exhaust receiver. The temperature of
the mixture after the valve will increase
considerably, and it is likely that the gas will
burn with a diffusion type flame (without
exploding) immediately after the valve whereit 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 receiver without starting
to burn immediately, 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 detection 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 amount does not exceed the
amount corresponding to the MCR value.
It is necessary to ensure that the pressure in the
accumulator is sufficient for gas operation, so
the accumulator will be equipped with a pressure
switch and a differential pressure switch.
An increase of the gas flow to the cylinder
which is greater than corresponding to the
actual load, but smaller than corresponding to
the MCR value, will only give rise to the
above-mentioned exhaust gas temperature
alarm, and is not harmful. By this system, any
abnormal gas flow, whether due to seized gasinjection valves 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 cylinder
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-pressurepipes, making the fuel oil booster
inoperative.
Seized plunger in the fuel oil booster.
Other faults on the engine, forcing the fueloil 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 immediately fromthe first 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 burned 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 unburned 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 influence
the safety of the engine in dual fuel operation.
However, the still operating valve will inject
pilot oil, which will ignite the corresponding
gas injection, and also the gas injected by theother gas valve, but knocking cannot be ruled
out in this case. The cylinder pressure
monitoring system will detect this condition.
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 countermeasures, a most satisfactory
service reliability and safety margin is
obtained.
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External Systems
The detailed design of the external systems
will normally be carried out by the individual
shipyard/contractor, and is, therefore, not
subject to the type approval of the engine. Theexternal systems 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 providing a sealing
between the gas and the control oil, and
lubrication of the moving parts.
FIGURE 11: Gas system branchingThe 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 pressurises it additionally to
the operating 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.
Low-pressure GE Oil & Gas RoFlo typegas compressors with lubricated vanes and
oil buffered mechanical seals, which
compress the cold boil-off gas from the
LNG tanks at the temperature of 140oC to
160oC. The boil-off gas pressure in the
LNG tanks should normally 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
intermediate gas. temperature
Ventilation system
The purpose of the ventilation system is toensure 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 capacity must ensure approx. 10 air
changes per hour. More ventilation gives
quicker detection of any gas leakage.
The high-pressure GE Oil & Gas NuovoPignone SHMB type gas compressor; 4
throw, 4-stage horizontally opposed and
fully balanced crosshead type with
pressure lubricated and water-cooled
cylinders & packings, compresses the gasto approximately 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 compressor
designs. The discharge temperature is kept
at approx. 45oC by the coolers.
THE GAS COMPRESSOR
SYSTEM
The gas supply system is based on Flotech
packaged compressors:
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Buffer tank/accumulators are installed toprovide smoothing of minor gas pressure
fluctuations in the fuel supply; 2 bar is
required.
Gas inlet filter/separator with strainer forprotection against debris.
Discharge separator after the final stagegas cooler for oil/condensate removal, with
1 coalescer element limits oil carry-over.
Compressor capacity control systemensures 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 handlesnormal start/stop, shutdown and
emergency shutdown commands. The
compressor unit includes a process
monitoring and fault indication system.
The compressor control system exchanges
signals with the ME-GI control system.
The compressor system evaluates theamount 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 components,
but the costs are heavy and a lot of space is
required on board the ship. We have worked
out a recommendation that reduces the costs
and the requirement for space while ensuring a
fully operational ME-GI engine. The dual fuel
engine concept, in its nature, includesredundancy. If the gas supply system falls out,
the engine will run on heavy fuel oil only.
The gas supply system illustrated in Fig. 13
and 14 are based on a 210,000 M3LNG carrier,
a boil off rate of 0.12 and equipped with 2 dual
fuel engines: 2 x 7S65ME-GI. For other sizes
of LNG carriers the setup will be the same but
the % will be changed. Figs. 12 and 13 show
our 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 BOG only, Fig. 12, or run on
both natural BOG and forced BOG, Fig. 13 is
relevant.
FIGURE 12: Gas supply system natural BOG only
FIGURE 13: Gas supply system natural and forced BOG
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
(alternatively 3 x 50% may be chosen). Each of
these LP compressors can individually feed both
the High Pressure Compressor and the Gas
Combustion Unit. All compressors can run
simultaneously, 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 ME-GI
engine can run on Heavy Fuel Oil, and one of
the LP compressors can feed the GCU.
Typical availability of these electrically driven
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.
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Gas supply system capacity management At full load of the ME-GI engine on gas, the
HP compressor delivers approximately 265 bar
whereas at 50% load, the pressure is reduced to
130-180 bar. The discharge pressure set points
are controlled within 5%. Compressor speedvariation controls the capacity range of
approximately 100 => 50% of volumetric flow.
Speed control is the primary variation; speed
control logic is integrated with recycle to
reduce speed/capacity when the system is
recycling under standby (0% capacity) or part
load conditions.
The minimum requirement for the regulation
of supply to the ME-GI engine is turndown of
100 => 30% maximum flow, or according to
the shipowners requirement.
Both the LP and HP compressor packages have
0 => 100% capacity variation systems, which
allows enormous flexibility and control.
Stable control of cargo tank pressure is the
primary function of the LP compressor control
system. Dynamic capacity variation is
achieved by a combination 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.
LP & HP compressor systems are coordinated
such that BOG pressure is safely controlled,
whilst however delivering all available gas at
the correct pressure to the ME-GI engine. Loadand availability signals are exchanged between
compressor and engine control systems for this
purpose.
FIGURE 14: Typical HP fuel gas compressor
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FIGURE 16: Gas compressor system indicating capacity control & cooling systems
Safety aspectscompressor if fault conditions are detected by
the local control system.The compressors are delivered generally inaccordance with the API-11P standard (skid-
packaged compressors) and are designed and
certified in accordance with relevant
classification society rules.
Pressure safety valves vented to a safe area
guard against uncontrolled over-pressure of the
fuel gas supply system.
MaintenanceInert gas system
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.
After running in the gas mode, the gas system
on the engine should be emptied of gas by
purging the gas system with inert gas (N2,
CO2),
External systems
External safety systems should include a gas
analyser for checking the hydrocarbon content of
the air, inside the compressor room and fire
warning and protection systems.
Safety devices Internal systems
The compressors are protected by a series of
Pressure High, Pressure Low, Temperature High,
Vibration High, Liquid Level High/Low,Compressor RPM High/Low and Oil Low Flow
trips, which will automatically shut down the
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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 otherwise the Safety
Control will overrule and return to a Gas SafeCondition.
A fault in the Dual Fuel equipmentmust cause stop of gas operation andchange 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 running and
failures of the control system itself will result
in afuel gas Stop / Shut Down and changeover to fuel operation. Followed by blow out
and purging of high pressure fuel gas pipes
which releases all gas from the entire gas
supply system.
Fuel control
The task of the fuel control is to determine the
fuel gas index and the pilot oil index when
running in the three different modes shown in
fig.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 theME safety system.
the cylinder condition for being in acondition allowing fuel gas to be
injected.
If one of the above mentioned failures is
detected then the Safety Control releases thefuel gas Shut Down sequence below:
Figure 18:Fuel Gas Operation State Model
If the failure relates to the purging system it
may be necessary to carry out purging
manually before an engine repair is carried
out. (This will be explained later).
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.The Dual Fuel Control system is a single
system without manual back-up control.See also fig. 9
Architecture of the Dual Fuel Control
SystemHowever, the following equipment is made
redundant to secure that a single fault will notcause fuel gas stop:Dual Fuel running is not essential for the
manoeuvrability of the ship as the engine will
continue to run on fuel oil if an unintended fuel
gas stop occurs. The two fundamental
architectural and design demands of the fuel gas
Equipment are, in order of priority:
The communication network isdoubled in order to minimize the
risk of interrupting the
communication between the
control units.
Safety to personnel must be at least onthe same level as for a conventional
diesel engine
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Vital sensors are doubled and one setof 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 noconsequence for safe fuel gas
operation.
Control Unit Hardware
For the Dual Fuel Control System two different
types of hardware are used: the Multi Purpose
Controller Units and the GCSU , both
developed by MAN B&W Diesel A/S.
The Multi Purpose Controller Units are used for
the following units: GCEU, GACU, GCCU, and
the GSSU see also fig. 17.
In the following a functionality description for
each units shown in fig. 17
Gas Main Operating Panel (GMOP).
For the GI control system an extra panel called
GMOP is introduced. From here all manually
operations can be initiated. For example the
change between the different running modes
can be done and the operator has the possibility
to manually initiate the purging of the gas pipessystem 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 thefollowing:
HC "Sensors"
Gas Supply System
Sealing Oil System
Pipe Ventilation
Inert Gas System
Network connection to otherunits of the Dual Fuel System
and, if a failure occur, the Plant Control will
automatically interruptfuel gasstart operationand return the plant to Gas Safe Condition.
The GECU also contains the Fuel Control
which includes all facilities required for
calculating the fuel gas index and the Pilot Oil
index based on the command from the
conventional governor and the actual active
mode.
Based on these data and including
information about the fuel gas pressure, the
Fuel Control calculates the start and duration
time of the injection, then sends the signal to
GCCU which effectuates 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 gassupply system.
The sealing oil pressure set points
The pressure set points for the gas supplysystem.
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 GCCUreceive a signal ready from the safety system
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and GCCU observes no abnormalities then the
injection of fuel gas will starts at the relevant
crankshaft position.
The GSSU, fuel gas System Monitoring andControl
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 doublewall 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 by the PMI on-line system for being
in condition for injection of fuel gas. The
following events are monitored:
Fuel gas accumulator pressuredrop 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 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. Additionally, 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 mutual
back-up.
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Ole Grne holds an M.Sc. in Chemical
Engineering from the Technical University of
Denmark. He joined the Operation Dept. of
Burmeister & Wain in 1976, and in 1994 he
was appointed Vice President of MAN B&WDiesel A/S for Marketing and Sales of two-
stroke low speed engines.
Kjeld Aaboholds a B.Sc. in mechanical
engineering and a special diploma in market-
ing. He joined MAN B&W Diesel in the
Stationary Installation Department in 1983. In
2002, Kjeld Aabo was appointed manager of
the Engineering Services department. Kjeld
Aabo is also Chairman of the CIMAC Fuel Oil
Group, and a member of the lube oil and
emissions work group.
Ren Sejer Laursenholds a M.Sc. in
Mechanical .Engineering from the Technical
Institute of Denmark in 1989. Until 1992 he
was employed at Ris National Laboratory
where he worked with super-critical oxidation
technology. Until 1994 he worked with waste
incineration boilers at Aalborg Industries and
until 1998 he worked with drilling equipment
for the Greenland Ice Core Investigations
Project and research equipment at the Niels
Bohr Institute of Copenhagen. He joined MANB&W Diesel in 1998, and in early 2004 he
started in the ME-GI project group.
Steve Broadbent qualified as an aeronautical
engineer in 1982. After completing business
studies, he founded Flotech in 1986 to
specialise in high-pressure gas compressors for
the then burgeoning NZ market for CNG fuel
systems. As CNG declined in the late 1980s,
Flotech turned to heavy industrial applications
and since 1995 has delivered most of the high-
pressure gas-diesel fuel delivery systems thatare currently installed in marine and power
generation, worldwide. Steves current role is
Group Managing Director of Flotech, which
today has operations in Sweden, Australia and
New Zealand.