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





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.



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%



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



6

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



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

LP compressor

Compressor starts up

Recirculationof gas

to buffertank

Compressor

100%BOG

100%BOG

100%BOG

100%BOG

100%BOG

AvailableBOG



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



9

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



10

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.



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 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 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



12

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



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



14

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



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



16

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



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



18

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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Introduction to MAN ME-GI engines - ME-GI Engines for LNG Application System Control and Safety

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