LNG as a Fuel

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LNG Infrastructure

ME
-
G
I retrofit Description

for L
NG

Operation

Summary

The purpose of this document is to provide the necessary basic information to the shipowner

and shipyard for
retrofit of a main engine to the dual fuel ME
-
GI engine. The ME
-
GI engine is capable of operating on conventional
fuel oil or gas fuel, depending on the operator’s fuel preference. Thus providing a hitherto unseen degree of
flexibility f
or the operator, accommodating the economical advantages of choice of fuel type, environmental
benefits of gas fuel operation, and the accessibility of fuel oil.

Contents

Summary

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........

1

LNG as a Fuel

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..

2

Applic
ation of ME
-
GI

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3

Engine Operating Modes

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3

Safety Aspects

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4

Safety Devices



External Systems

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.........................

4

Safety Devices


Internal Systems

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..........................

5

Engine modifications for ME
-
GI

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.....

6

External Systems

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............................

7

Ventilation System

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.....................

7

LNG Fuel
Gas Supply system

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......

7

Block and Bleed Valve Arrangement

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..........................

8

Gas Piping

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................................
...

8

Inert Gas System
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9

Sealing Oil System

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9

Appendix I


ME
-
GI system diagram

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................................
............................

10





L
N
G
as a Fuel


Figure

1:

Requirement of

LN
G
properties

for ME
-
GI operation

Further emission benefits appear when making a direct comparison between L
N
G
operations

in comparison with
HFO operation. It is seen that gas gives a much cleaner exhaust. Having very low or no su
lphur, SOx
-
sulphur oxides
are negligible in the exhaust gas. Particulates will be reduced considerably as well as the emission of NOx where a
reduction is found in the range of 12%, also for a Tier II engine.


All typical NO
x
reduction techniques such as
E
xhaust
G
as
R
ecirculation

and S
elective
C
atalytic
R
eduction
, can be
used on an ME
-
GI engine, except water emulsification. When combining the EGR system with gas operation, it is
expected that the EGR process can be significantly simplified since the exhaus
t gas coming from gas operation
holds only a small amount of SOx and particulate matter. The cleaning of the exhaust gas in the EGR scrubber will
become less comprehensive since the sludge amount coming from the
cleaned

water will be negligible, and it is
very likely that the EGR scrubber can eventually be by
-
passed. From an EGR test, using HFO, it has been seen that
quite promising results can be
accomplished

and MAN Diesel are close to fulfilling Tier III simply by using the EGR
system alone. It is theref
ore foreseen that by using the EGR in combination with
LNG as

fuel, that no other
reduction techniques are needed to
fulfil

Tier III.



Application of ME
-
GI

In general, the ME
-
GI engine series, in terms of engine performance (output,

speed, thermal effici
ency, exhaust gas
amount and temperature, etc.), is identical

with the well
-
established, type approved ME engine series. This means
that the

application potential for the ME
-
GI engine applies to the entire ME engine catalogue. In addition to the
required e
ngine mod
ifications
, additional auxiliary systems are required for gas fuel operation:




Fuel Gas Supply (FGS) system for delivery of gas fuel to the main engine



Block and bleed valve arrangement



Ventilation system for venting the space between the inner
and outer pipe of the double
-
wall piping,
including HC leakage monitoring, and silencer



Inert gas system, which enables purging of the gas syst
em on the engine with inert gas



Sealing oil system, delivering sealing oil to the gas valves separating control o
il and gas



Control oil supply for actuation of gas injection valves

Please refer to Appendix I for ME
-
GI Gas System diagram
(ref. dwg. No.
0787402
-
6
)

Engine Operating Modes

O
ne of the advantages of the ME
-
GI engine is its fuel flexibility. In addition,
not only L
N
G carriers, but also other

types of

ships can benefit from burning L
N
G.

The
LN
G may come from differ
ent sources and be of different

compos
itions,
which means that the heat value is a
variable. The diesel working principle is well known to be ca
pable of coping with such differences in the heat value.
A two
-
stroke, high
-
pressure gas injection engine can burn different fuels without lowering the thermal efficiency of
the engine.

The control concept of the ME
-
GI comprises two different fuel modes:

1.

The
fuel
-
oil
-
only mode
is well known from the ME engine. In this mode, the engine operates on fuel oil
only, and the engine is considered to be “gas safe”. If a failure occurs in the gas system, this will result in a
gas shutdown and a return to the fuel
-
o
il only mode.

2.

The
minimum
-
fuel mode
has been developed for gas operation, and it can only be started manually by an
operator from the Gas Main Operating Panel (GMOP) located in the control room. In this mode, the control
system controls the amount of gas
fuel, combined with use of a minimum preset amount of fuel oil .The
preset minimum amount of fuel oil (pilot oil) to be used is set at 5% on small and medium bore two
-
stroke
engines. Both heavy fuel oil and marine diesel oil can be used as pilot oil. The m
in. pilot oil percentage is
c
alculated from 100% engine load.

When the engine passes the lower load limit, the engine returns to fuel
-
oil
-
only mode.

Gas fuels correspond to low
-
sulphur fuels, and for this type of fuel we recommend the cylinder lube oil TB
N40 to
be used. Very good cylinder condition with this lube oil was achieved from the gas engine on the Chiba power
plant. A heavy fuel oil with high sulphur content requires the cylinder lube oil TBN70. Operators intending to run
their engine both on high
-
sulphur fuels as well as on low sulphur fuel for longer periods of time are recommended
to install two lube oil tanks. Our general recommendation is that when changing from fuel oil to gas operation, or
vice versa, for a period of two weeks or longer, it
is also recommended to change the lube oil TBN type as well.

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 easily and quickly be
detected by a hydrocarbon (HC) analyser wit
h an alarm function. An alarm is sounded when the gas concentration
reaches max. 30% of the Lower Explosion Limit (LEL) in the vented duct, and a shutdown signal is given at 60% of
the LEL. The safety devices that will virtually eliminate such risks are do
uble
-
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 malfunctioning sealing oil supply system. If the sealing oil pressure becomes too
low in the gas injection valve, gas will flow into the control oil actuation system, thereby, creating gas pockets,
ev
entually preventing the ELGI valve from operating the gas injection valve at the correct time. Therefore, the
sealing oil pressure is m
onitored
by a set of pressure sensors and, in the event of a too low pressure, a second
sealing oil pump will start. The
engine will shut down the gas mode and return to the fuel oil mode if it is still not
possible to generate an acceptable sealing oil pressure.

Lack of ventilation in the double
-
wall piping system prevents the safety function of the HC sensors, so the syst
em is
to be equipped with a set of flow measurement sensors. If the flow measurements indicate no flow, or nearly no
flow, an alarm is given. If no correction is carried out, the engine will be shut down on gas mode.

In case of malfunctioning valves (not
leaky) 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.



Safety Devices


Intern
al 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



Ignition failure of injected
gas

These aspects will be discussed in detail in the following together with the suitable counter
-
me
asures:

Defective Gas Injection Valves

In case of sluggish operation or even seizure of the gas valve spindle in the open position, larger gas
quantities m
ay 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 temperature of the mixture after the valve will increase consid
erably, and it is likely that the 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
e
xhaust 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. The
refore, 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 described below:

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. As it is necessary to ensure that t
he pressure in the accumulator is sufficient for gas
operation, 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 gas injection valves
or fractured gas pipes, will b
e 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 exhau
st 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, ho
wever,
are the result of failure to inject pilot oil in a cylinder:



Leaky joints or fractured high
-
pressure pipes, making th
e fuel oil booster inoperative



Seized p
lunger 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 immediately from the first
failure to inject the pilot oil. In extremely rare cases, pilot fuel can be injected without being igni
ted,
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 temperatu
re 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 operating va
lve will still inject pilot oil, which will ignite the
corresponding gas injection,
and also the gas injected by the other gas valve, but knocking cannot be ruled
out in this case. The cylinder pressure 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 ma
rgin is obtained.

Engine modifications

for ME
-
GI


For conversion from M
E

to ME
-
GI

engine:




Supplier

Installation

Hours

New c
ylinder cover

6

MDT

MDT


Gas block

6

MDT

MDT


High pressure gas branch pipes

6

MDT

MDT


Gas injectors

12

MDT

MDT


Gas dummy
injectors

12

MDT

MDT


Control oil high pressure pipes

6

MDT

MDT


Gas Control System & Instrumentation

1

MDT

MDT


Fuel oil high pressure pipes

6

MDT

MDT


Exhaust valve high pressure pipes

6

MDT

MDT


Gas main pipe

1

MDT

MDT


Various pipes & cabling

1

MDT

MDT


Engine Control System &
Instrumentation

1

MDT

MDT


Tools

1

MDT

MDT


PMI online

1

MDT

MDT


Engineering cost

1

MDT

MDT


Commissioning

1

MDT

MDT






External Systems

The detailed design of the external systems will normally be carried out by the individual shipyard and
includes the inert gas system, the ventilation system, and the LPG gas supply system.

Ventilation System

The purpose of the ventilation system is to e
nsure 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
.

Ventilation is achieved by means of an electrically driven mechanical fan or extracto
r fan. If an electrically
driven fan is chosen, the motor must be placed outside the ventilation duct. The capacity must ensure
approx. 30 air changes per hour. More ventilation gives quicker detection of a gas leakage, but at the same
time dilutes the gas

leakage. Inert Gas System Both before and after running in the gas mode, the gas
system on the engine should be purged with inert gas to avoid any mixing of air and gas. An inert gas supply
of either N2 or CO
2
needs to be established.

LN
G
Fuel Gas Supply

system


The gas supply system for LNG carriers comprises:

• A gas compressor or compressor system, which compresses the cold boil

off

gas from the LNG tanks at
the temperature of −140°C to −160°C. The

boil
-
off gas pressure in the LNG tanks should normally

be kept
within

1.03
-
1.25 bar.

Several compressor types can be

used to compress the

cold gasses. But for the high
pressure, only the reciprocating compressors

are suitable to generate the required inlet engine pressure at
full load,

which is 250
-
300 bar. A
t 30% load, the pressure is reduced to 130
-
180

bar. The discharge pressure
set points must vary within ±5%, and coolers

keep the discharge temperature at approx. 45°C.

• A buffer tank/accumulator must be installed. Their purpose is to provide

smoothing of
minor gas pressure
fluctuations in the fuel supply, ± 2 bar is

required.

• An inlet filtration system to remove particle matter and entrained liquids

from the gas stream. A two
-
stage filter solution comprising a pre
-
filter and

a coagulating filter.

• A com
pressor control system ensuring that the required gas pressure is in

accordance with the engine
load, and that the amount is acceptable for

constant pressure control. The minimum requirement for the
regulation is

down to 30% of max. flow, or the shipowner’
s requirement. The controlsystem should be able
to operate on normal start/stop, shutdown and

emergency shutdown commands. The compressor unit
should include a

process monitoring and fault indication system. The control system unit

should be able to
exchan
ge signals with the ME
-
GI control system.

• A cargo monitoring system keeping track of the amount of available boil

off

gas. The unit should be able
to report to the ME
-
GI control system, this

could be made as a part of the compressor control system.

Redun
dancy 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,

obviously, includes redundancy. If the gas supply
system falls out, the engine will

run on heavy fuel oil only.

Several solutions to the redundancy iss
ues are
being evaluated, and we

recommend the reader to seek additional information from
makers
.

In addition,
the external systems comprise safety systems, which should include

a hydrocarbon analyser for checking
the hydrocarbon content of the air, both

Block and Bleed Valve Arrangement


The valve block incorporates a large
-
volume accumulator, and is provided with a

shutdown valve 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 dro
p in the accumulator is measured. The pressure

drop should not exceed about 20
-
30 bar at
100% load.

Any larger pressure drop would indicate a severe leakage in the gas injection

valve seats or a fractured gas
pipe. The safety system will detect this as wel
l as

no pressure drop and shut down the gas injection. Any
malfunction of the purge

valve and blow off valve are also being monitored by this pressure drop

m
easurement

f
rom the accumulator, the gas passes through a bore in the valve block to the

shutdown
v
alve which, in the gas mode, is kept open by compressed air
. From the shutdown valve,

the

gas is led to
the gas injection valve via bores in the valve block and in the

cylinder cover. A blow
-
off valve is designed to
empty the gas bores

when needed.

A purge

valve
, is designed to empty the accumulator when

the engine is no longer to operate in the gas
mode.

To limit the number of connections that has to be disassembled during an

overhaul of the
combustion chamber component, the ELGI valve as well as the

seali
ng oil pipes connection is placed in the
valve block. The result is that only

two gas pipe connections, one control oil connection and one sealing oil

connection has to be dismantled in connection with an overhaul.

Gas Piping


Double wall pipe system

Gas
pipes

from

engin
e to deck

will be double wall pipe where the space between the inner and outer pipe
is ventilated.

As this ventilation air could contain gas during abnormal operation, and only in a short period
until gas shut down, the ventilation outlet m
ust be in gas hazard zone 2 (in front of the accommodation)

The air intake for this ventilation must be gas free, and could be anywhere suitable behind the
accommodation front.

To prevent the possible condensation in the ventilation air, which could lead t
o
corrosion of the high pressure inner pipe, there are two options:

1.

Installation of an air dryer at the air intake

2.

Pipe material in duplex steel




Single wall pipe system

Gas piping from deck to block & bleed valve arrangement, and all gas piping on deck is

single wall piping

Inert Gas System

Both before and after running in the gas mode, the gas system on the engine should be purged with inert gas to avoid
any mixing of air and gas. An inert gas supply of either N2 or CO
2
needs to be established.

Sealing Oi
l System

The sealing oil system supplies oil, via a double wall piping system, to the gas injection valves, thereby providing 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 available system oil, at one bar supply
pressure and pressurises it additionally to the operating pressure, which is 25
-
50 bar higher than the gas pressure. The
consumption of sealing oi
l is small and when compared to the amount of fuel used the amount is insignificant.




Appendix I


ME
-
GI
s
ystem
d
iagram