ME_GI engines for LNG vessels

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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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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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



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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14

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.

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ME_GI engines for LNG vessels

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