LNG Propulsion 7

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LNG CARRIER

ALTERNATIVE PROPULSION SYSTEMS

AUTHORS:Richard GilmoreStavros HatzigrigorisSteve MavrakisAndreas SpertosAntonis Vordonis

Presented to: SNAME GREEK SECTION in 17 February 2005

Maran Gas Maritime Inc.February 17, 2005


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LNG CARRIER ALTERNATIVE PROPULSION SYSTEMS SNAME - Greek Section

Maran Gas Maritime Inc.

February 17, 2005 Page 2 of 36

A. ABSTRACT

The LNG industry is heading into a new and sustained period of growth. This growth isdriving a re-examination of the propulsion system for LNG vessels. This paper will review

technical and economic aspects of the most promising propulsion alternatives of future LNGvessels. The initial cost, operating cost, reliability, maintenance and manning parameters thatwill affect the management of ships provided are presented and the pros and cons of eachoption are highlighted.

Recent developments and trends in the LNG newbuilding market are reviewed.

B. INTRODUCTION

The realisation of the reduced long-term availability of oil combined with the environmentalconcern for greener fuel has driven the rapid expansion of the gas industry. As aconsequence, the LNG shipping industry, being the most efficient transportation vehiclebetween the remote reliquefication plants and end users, is in a phase of rapid growth andgaining momentum. A record 72 LNG tanker newbuildings were ordered in 2004, farsurpassing the previous annual record of 24 in 2001 (see Appendix for LNG NB orderbook). The total order book for LNG ships stands at 107 confirmed orders and can go up to140 if all options are exercized.

Over the last 30 years the LNG industry has seen steady average growth of about 2.7% perannum. Today LNG represents about 6% of world primary energy consumption. Recent

forecasts show this growth pattern changing dramatically with the industry set to double insize in the next 5 years.

LNG has been a commercial cargo for over 40 years with a remarkable record of safety andreliability. One of the factors that enhanced this record was the choice of the propulsionsystem which was dominantly the steam turbine. Apart from its reliability, one of the majoradvantages of using steam propulsion is that the unavoidable boil-off gas, created by thermalconductivity through the tank walls and the movement of the liquid, can be directly burned inthe boiler.

Why then, should the LNG industry now be exploiting changing this proven propulsion

system?

As environmental, economic and technical expectations increase, the apparent drawbacks ofthe steam power plant make it a less attractive option. Among the drawbacks are thecomparative low efficiency of the plant, its high fuel consumption, which in turn translatesinto high exhaust emissions. In addition, the comparatively large engine room spacerequirement, the limited propulsion redundancy, the high initial cost to install and the limitedsupply of experienced engineers, further encourage the transition to alternative propulsionplants.

Advances in the design of dual fuel diesel engines, shipboard LNG reliquefaction plants and

marine gas turbines, provide meaningfull alternatives to the traditional steam power plant forLNG vessels.


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C. PROPULSION SELECTION CONSIDERATIONS

Before presenting and discussing the propulsion system alternatives, it is essential to reviewwhat are the requirements of an LNG vessel according to the current market demands. Allconsequences from choosing a specific propulsion plant type should be considered.

1) Operating Efficiency

a) Plant efficiency

The propulsion plant efficiency is one of the critical factors as for a given displacement, itdetermines the weight of fuel needed and hence the remaining cargo containment space butmore importantly, it affects the leading element of operating cost of the ship, the fuel cost,i.ethe cost required to transport a specific quantity of cargo per mile at sea .

b) Best utilisation of the boil-off gas(BOG)

The specified quantity of evaporated LNG from the containment systems is 0.15% per day asa maximum of the liquid tank volume on the loaded voyage of the vessel. This quantity ofcargo is left to evaporate in order to control temperature and pressure in cargo tanks.

LNG is a concentrated fuel (high Calorific value / unit weight) and hence using LNG in agiven plant, can save up to 25% of fuel weight. (Typical Gas HHV: 13000 kcal/kg, TypicalHFO HHV: 10280 kcal/kg)

The easiest way to handle the ever-present boil off gas (BOG) is to use it as fuel. Until now

this was done by simply burning it in a simple steam boiler. More recently, emergingtechnologies allow the BOG to be used as fuel directly in diesel engines. Alternatively,reliquefaction plants have been developed to return the BOG back to the cargo tanks.

2) Environmental Concern

Environmental friendliness and Ecology is nowadays a major concern. New technology andengineering applications in the propulsion plants have achieved significant results minimizingCO2, SOx and NOx emissions.

3) Safety

The LNG industry has an excellent safety record. This is the result of several factors, such ashigh standards of design and construction ,high levels of redundancy and high standards oftraining and operation.

4) Reliability and redundancy

A key factor for ship reliability is the choice of propulsion. For more conventional vessels, abreakdown results in off hire at the daily charter rate with the expected financial costs. Incontrast, the LNG trade, any outage during service could mean that the liquefaction plantexporting the cargo may have to shut down if there is no storage available until the next

vessel arrives. So the out of service cost of an LNG ship has multiple consequences for thewhole LNG project compared to out of service consequences for a conventional carrier. Inthe past,


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there were few LNG ships available on short notice to fill in for a ship that was out of servicemaking this issue very important.

5) Maintenability

The propulsion plant should be designed to require minimum maintenance. Maintenancerequirements should be covered easily during port calls and scheduled dry-dockings withoutaffecting ships sailing schedule and operations.

6) Flexibility

Although at the moment the LNG trade is mostly a project-specific oriented market, withvessels built for long term time charters operating on standard routes, the spot market and theshort term trade (currently limited to 5-10 % of the transported volume) is expected toincrease in the near future. Single voyage based charters will require more flexible vessels interms of vessel size ,fuel requirements and equipment, allowing trading between alternativeports and facilities (including offshore terminals).Many LNG terminals have limited or nobunkering facilities,therefore ,bunkering is often done by barges ,possibly at the loading-discharge port,or by stopping the ship enroute.

LNG vessels must also be prepared to halt cargo operations and depart the loading ordischarge port at any time in case of an emergency. This requirement impacts propulsionsystem design and maintenance plans.

7) Crewability

The rapid increase of the LNG fleet, resulted in a shortage of experienced crew for theexpanded fleet and has already become a major industry concern. The problem is enhancedwhen considering vessels equipped with steam power plants.Since LNG ships are the onlyremaining commercial vessels still using steam power ,this has become a small specialisedniche in the marine industry.Many suppliers of steam equipment have been driven out ofbusiness and the opportunities for obtaining steam training are few. This is another criticalfactor in deciding towards the optimum power plant.

8) Reduced Engine room dimensions

As in all types of vessel, maximisation of cargo spaces is the ultimate objective of ownersand charterers. The propulsion technologies available have significant differences in engineroom dimension requirements, hence this parameter should not be overlooked.


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D. PROPULSION ALTERNATIVES

A significant number of propulsion alternatives have been considered over the recent years aspossible candidates for the replacement of the traditional steam plant on the LNG vessels,

these involve different prime movers, different power generating sets, or combinations ofthem, different transmission systems, different fuel configurations together with differentmeans for dealing with the unavoidable BOG.Here we will attempt to describe and compare the most attractive and promising alternativesbased on the feed back currently being received from the industry.

Table 1, presents the alternatives we will deal with in the remaining part of this paper. Anattempt has been made to group the alternatives on the basis of the prime mover as this isconsidered the leading parameter, other groupings are also possible, eg by the type of fuel tobe used (see following chart), to suit a particular study.

PRIMEMOVER

CONFIGURATION FUEL USED -BOG HANDLING-BACK UP BOGHANDLING

TRANSMI SSION ELECTRIC POWER

STEAMTURBINE

TWO BOILERSWITH HP &LPSTEAM TURBINE

HFO AND /ORGASALMOST INANYCOMBINATION

-BURNING ONBOILERS-STEAM DUMPING

MECHANICALDRIVE THROUGHREDUCTION GEARS

USUALLY TWOTURBOGENERATORS AND ONE OR TWODIESEL GENERATOR

HFO - 2X100%RELIQUEFACTIONPLANTS

-NOT REQUIRED

USUALLY FOURDIESELGENERATORS

SLOWSPEEDDIESEL

ONE OR TWOSLOW SPEEDDIESELS

HFO AND/OR

GAS

-BURNING ON

THE ENGINE-OXIDIZER*2

DIRECT DRIVE

USUALLY FOUR

DIESELGENERATORS

COMBINATION OFHFO BURNINGDIESELS ANDDUAL FUELDIESELS

-HFO

-GAS OR MDO

-BURNING IN THEDUAL FUELENGINES-OXIDISER*2

ELECTRIC POWERAVAILABLE FROMMAIN GENERATORENGINES

GAS OR MDO -BURNING IN THEENGINES-OXIDIZER*2


MEDIUMSPEEDDIESEL

4*

DUAL FUELDIESELS

GAS OR HFO*1

-BURNING IN THEENGINES-OXIDIZER*2

ELECTRIC DRIVETHROUGH SLOWSPEED PROPULSIONMOTOROR MEDIUM SPEEDPROPULSIONMOTOR ANDREDUCTION GEAROR ELECTRIC WITHAZIPOD*3


SIMPLE CYCLEGAS TURBINEUSUALLY ONEPROPULSIONTURBINE ANDONE AUXILIARYTURBINE

GAS OR MGO -BURNING IN THEPROPULSION ANDAUXILIARYTURBINE-OXIDIZER

GASTURBINE

4* COGES(COMBINED GASTURBINE ANDSTEAM)

GAS OR MGO -BURNING ONTHE PROPULSIONAND AUXILIARYTURBINE-OXIDIZER

ELECTRIC DRIVETHROUGH SLOWSPEED PROPULSIONMOTOROR MEDIUM SPEEDPROPULSIONMOTOR ANDREDUCTION GEAROR ELECTRIC WITHAZIPOD*3

ELECTRIC POWERAVAILABLE EITHERFROM MAIN GASTURBINE ORAUXILIARY GASTURBINEONE DIESELGENERATOR ISALSO PROVIDED ASBACK UP

Notes:

1. This option is not available in the market yet2. For these options the oxidizer is shown as the back up means of handling BOG.Separately adding reliquefaction plant is discussed as an alternative design concept.

Table1. Pro ulsion alternativesaccordin toPrimeMover


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3. For these options direct mechanical drive is a rarely discussed alternative.

4 .A possible alternative resulting form a combination of medium speed diesels and gasturbine is the CODAG (combined diesel and gas turbine).

Table 2.presents the alternative propulsion plants available, based on the type of fuel eachalternative utilizes.

TYPE OFFUEL

CONFIGURATION TRANSMISSION

SLOW SPEED DIESEL +RELIQUIFACTION

DIRECT DRIVE

ELECTRIC THROUGH REDUCTION GEAR

ELECTRIC DIRECT

HFO

MEDIUM SPEED DIESEL +RELIQUEFICATION

AZIPOD

STEAM TURBINE MECHANICAL THROUGH REDUCTION GEAR

SLOW SPEED DIESELDUAL FUEL

DIRECT DRIVE


ELECTRIC DIRECT

MEDIUM SPEED DIESELDUAL FUEL

AZIPOD


ELECTRIC DIRECT

GAS +HFO

MEDIUM SPEED DIESELDUAL FUEL +MEDIUMSPEED DIESEL HFO

AZIPOD


ELECTRIC DIRECT

GAS +MDO MEDIUM SPEED DIESELDUAL FUEL

AZIPOD

MECHANICAL THROUGH REDUCTION GEAR


ELECTRIC DIRECT

GAS TURBINE SIMPLECYCLE

AZIPOD

MECHANICAL THROUGH REDUCTION GEAR


ELECTRIC DIRECT

GAS +MGO

GAS TURBINECOMBINED CYCLE

AZIPOD

Table 2. Propulsion alternatives according to type of Fuel


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1. Steam Turbine.

The steam propulsion plant used in modern LNGs is very similar in outline to those used onearlier vessels. The plant usually comprises of two boilers supplying steam to high and lowpressure turbines, which in turn drive a single screw via a gearbox. The steam also drives theelectrical generators as well as powering many auxiliaries and provides the heat source to fueltanks, air-conditioning etc. The vessels are equipped with one or two diesel generators, whichare only for backup when manoeuvring, in port and for cold starting purposes.

a) Advantages

i. Very easy and reliable method to utilise the BOG. The power requirementsof a vessel in service, exceed the energy available from the BOG, enablingcomplete utilisation.

ii. High reliabilityiii. Low maintenanceiv. Low vibration levelsv. Very low lubricating oil consumption

Figure 1. Diagram of a typical Steam Plant configuration


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b) Disadvantages

i. Low efficiency of the turbine plant with the inevitable high fuel consumption.

ii. The need to continue developing experienced crew, familiar with the operation andmaintenance of a steam plant.

iii. Long delivery time for turbines and reduction gears and very limited productionversus demand. Hence in case of failure, major delays and off-hire may beencountered, unless depot spares of the major components are maintained whichincreases considerably the ships capital cost, this becomes more pronounced as thenumber of sister ships in the fleet is reducing.

iv. Excessive CO2 emissions due to high exhaust gas volumes.v. Larger engine room space requirements than for a motor shipvi. The layout offers limited propulsion redundancy.vii. At low speeds or at anchor, the power generated by continuing to burn the BOG is

much lower than the energy available from the BOG. The excess steam is dumpedinto the main condenser resulting in the loss of economic value of the boil off.

2. Slow Speed Diesel .

The slow speed diesel is a proven prime mover with about 80% of the commercial vessels inservice today propelled by slow speed diesels.For the LNG carriers however, the need to find an acceptable means to dispose of BOG, hasprevented the slow speed diesel from being a feasible propulsion alternative until recently.Slow speed diesel propulsion is now a realised solution mainly because of the following twooffered alternatives:

a) Slow speed diesel with reliquefaction plantb) Slow speed diesel with dual gas and heavy fuel oil burning.

The typical configuration of theEngine room for a vessel fitted witha steam turbine is presented onfigure 2

Figure 2. Engine room configuration


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The first solution has emerged from the fact that shipboard reliquefaction technology hasmatured considerably in recent years, tapping wide experience from land-based installations,

LPG carrier installations and an experimental plant on one LNG carrier. The plant offers asolution for pumping the reliquefied BOG back to the cargo tanks and hence the opportunityto deliver more cargo to the buyers.

The principle of the reliquefanction plant, is based on the closed Brayton cycle. The BOG isremoved from the tanks, compressed, cooled and condensed into LNG in a cryogenic heatexchanger by means of a nitrogen compression-expansion cycle. Any non-condensibles,mainly nitrogen, are removed in a separator and released to the atmosphere. For a 149,000 m3carrier, the plant requires about 3.5 MW of electrical power (This converts to about 20% ofthe energy available in the recycled BOG being expended during the reliquefaction process orabout 20 tonnes of fuel per day is consumed to cover the electrical demand for thisreliquefaction plant).

Figures below show a typical layout of the reliquefaction plant and that of a slow speed dieselinstallation with reliquefaction plant.

Figure 3. Diagram of a typical Reliquefaction plant


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The second and possible slow speed diesel alternative for LNG carrier propulsion is the slowspeed diesel with dual fuel (HFO-Gas) burning capability. This alternative has not yet been

thoroughly tested. There is one land based installation on a power plant having a slow speeddiesel in operation on natural gas only. While this tests the ability of the engine to operateonly on gas, it does not test the ability to operate with a mix of two fuels, HFO and gas .

As with the dual fuel medium speed diesel offered alternatives, which will be described later,the higher efficiency of the slow speed diesel, reduces the amount of energy required forpropulsion and brings it much closer to the amount of energy available from the boil off gas

Therefore the supplementary fuel oil requirement is drastically reduced, or even eliminated,compared to that of a steam turbine installation.

Fuel burning options available with this type of engine are, the dual fuel mode with

minimum pilot oil amount, the specified gas mode with the injection of fixed gas amountand the fuel oil only mode.Slow speed engine manufacturers claim that with the recent introduction of electronic controlon slow speed diesels (as opposed to the camshaft controlled fuel injection/exhaust valve),the gas burning capability option is further technically enhanced.

A potential drawback of this option is that the gas supply must be compressed to about 300bar, to facilitate injection into the cylinder. This requires considerable energy and expensiveand maintenance intensive compressors, it also raises safety concerns.

Figure 4. Typical slow speed diesel system with reliquefaction plant


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a) Advantages of the slow speed diesel with reliquefaction plant

i. High overall fuel efficiency up to 50 %- (about 60% higher than for the steamplants) resulting in lower energy consumption and thus lower operating costcompared to steam plants.

ii. Smaller Engine room required hence more cargo space for a given vessel compared tosteam propulsion.(For a 138,000m3 vessel, the increase in cargo carrying capacity is of the order of6,000m3 when compared with a steam turbine vessel).

iii. The amount of LNG delivered is higher as the BOG is reliquified.iv. The amount of CO2 released can be reduced by approximately 60,000 mt/ship/year

compared to a steam ship for a 150,000 m3 LNG carrier.v. In the case of a design using twin diesel engines, with separate engine rooms, there is

full propulsion redundancy and added safety margins against floods and fires in theengine room.

vi. The reliquefanction plant ensures that all cargo handling takes place on the deck,avoiding gas entering the engine room. This makes cargo and engine room operationssimpler and safer.

vii. The reliquefanction plant and the separation between the cargo and the engine room,reduces the constraints on the propulsion plant design and the type of fuel.

viii. Availability of engineers experienced with this type of propulsion system.

b) Disadvantages

i. The readily available and clean BOG is not utilised for the propulsion of the vesselii. Higher NOx and SOx emissions compared to alternatives burning LNG instead of

HFOiii. Less redundancy than existing steam systems in the single engine layout.iv. Diesel engines require more maintenance on a routine basis than steam turbinesv. Higher lub oil consumption compared to steam turbine which adds to the operating

cost.

Figure 5. GI Engine Configuration (source: MAN B&W)


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c) Investment cost:

The capital cost of an LNG carrier with slow speed diesel and reliquefaction plant for a130,000-150,000 m3 LNG is expected to increase by about 0-1% when compared with asteam turbine driven vessel, twin engine installation gives obviously the highest difference.

Likewise the capital cost of a vessel with dual fuel slow speed installation is about 3% lowerwhen compared to a steam turbine driven vessel.

3) Medium speed diesel (Diesel electric).

All options with medium speed engines are almost exclusively considered in combinationwith electric propulsion. The engines operate as generator sets and deliver the propulsionpower to the propeller through either medium speed electric propulsion motor(s) withreduction gear or directly through low speed electric propulsion motor(s). Single and twinscrew configurations as well as azipod options are also available to suit the particular shipdesign depending on the level of propulsion redundancy, maneuvrability and draftcharacteristics required.

Fig 6. below shows a typical medium speed diesel electric propulsion layout.

Medium speed dual fuel engines have been installed in both offshore and onshore powerplant applications for many years. In the case of onshore installations, natural gas is utilizedand for offshore installations generally process gas is used as fuel for power generation.

Dual fuel engines proposed for LNG vessels are developed upon these same principles. Theengine is capable of burning either gas with marine diesel oil as pilot fuel for injection, ormarine diesel oil. It is capable of changing over instantly between the two modes of operationwhenever required with stepless power output.

Figure 6. Typical configuration of a Medium speed dual fuel diesel electric


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Gas-mode Diesel-mode

During the gas operation mode, the engines operate on Otto cycle, gas is introduced into thecylinder during the air suction cycle at low pressure. Gas injection sub-system is normallylocated directly on the engine and its basic function is to provide timely and accurate deliveryof the gas fuel into the cylinder, gas is delivered through an electronically actuated controlvalve leading to the engine air inlet ducting. Knocking sensors adjust combustion timingbased on the gas quality.

When the engine is switched to the MDO or HFO mode then it operates under the normalDiesel cycle.

It should be mentioned that large bore medium speed engines operating on gas have not beentested in service yet.

The information we receive from a leading medium speed engine manufacturer is that thedual fuel medium speed engine is going to become a more attractive option for LNG vesselsby the introduction of a gas and HFO burning engine. This will eliminate the need to operate

the engine on the relatively expensive diesel oil when gas is not available. Authorsunderstand that this type of engine is currently undergoing shop tests and manufacturing ofthe first engine may start this coming summer.

The most common configuration options available with medium speed diesel electricpropulsion therefore are:

DUAL FUEL ,(BOG/MDO) ENGINES ONLY

A suitable number of dual fuel gas and MDO burning engines only (typicallyfour).This option is suitable for trades where gas(natural and forced boil off) is

considered as the primary fuel in all normal modes, except when no gas isavailable(first voyage or after drydock), in which case the engines are operated onMDO.

DUAL FUEL ,(BOG/MDO) ENGINES AND HFO ENGINES

A combination of dual fuel (gas and MDO) burning engines and HFO burning engines(typically two and two). This layout is suitable when gas cannot be predicted withconfidence to be the primary fuel over the vessels service life. In this case when gas isnot available or the price difference between gas and HFO trigers the choice of HFO,the installation can produce the propulsion and auxiliary power requirement byoperating only the HFO burning engines. Obviously higher installed power is required

Figure 7. Modes of operation for a Medium speed dual fuel diesel electric engine


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if the normal power demand needs to be available when either only gas or only HFO isavailable.

DUAL FUEL ,(BOG/HFO) ENGINES ONLY

A suitable number (again usually four) of dual fuel (gas and HFO) burning engines.Although no such engine is available in the market so far, its introduction in the nearfuture ,as promised by a leading manufacturer, will offer the most economicallyattractive layout among the three, since the plant will be able to produce the requiredpower by utilising either gas only or fuel only or a combination of gas and fuel withoutinstalling excess power.

In the three alternative configuartions described above, the need to have a back up means ofdealing with the boil off gas is normally handled by the installation of an oxidiser .Theoxidiser burns the amount of BOG which exceeds the propulsion requirements. Separately,the reliquefaction plant can be installed as a primary means of dealing with the BOG with thethree alternatives described above. This option is discussed at the end of this section.

It should be mentioned here that a possible but less promising alternative is to combinemedium speed diesels with gas turbine on a CODAG (Combined Diesel and Gas Turbine)configuration.

A schematic representation of the fuel system of a dual fuel medium speed engine can beobserved on the following figure.

Dual Fuel Medium Speed Engine Fuel Supply

Figure 10. Secondary fuel Figure 11. Twin needle injection valve

Figure 9. Pilot fuelFigure 8. Gas fuel


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a) Advantages

i. Higher efficiency compared to steam propulsion (but lower compared to slowspeed diesel options)

ii. High propulsion redundancy, especially on a twin motor twin skeg configuartioniii. High flexibility on the available fuel and engine load, especially on the third

optioniv. Reduced emissionsv. Increased cargo carrying capacity (compared to the steam propulsion option for a

given ship size), because of the shorter engine room length and possibly becauseof the reduced size of bunker tanks. The later is more pronounced on the firstconfiguration alternative (The relative increase in cargo carrying capacity for agiven ship size is comparable to that achieved by the slow speed diesel option,when both are compared to the steam turbine option). For a 138,000m3 vessel, theincrease in cargo carrying capacity is of the order of 6,000m3 compared to asteam turbine vessel.

vi. Higher availability of experienced crew comparing to steam turbine.vii. Auxiliary Electric power demand is covered by the propulsion engines eliminating

the need for additional auxiliary gen sets.viii. The provision of usually four or even more prime movers facilitates voyage

maintenance with no or little available power reduction.

b) Disadvantages

i. Added complication due to the handling of gas in the engine room, however lowpressure gas is supplied into the engine room similar to the existing steam turbinedesign.

ii. Higher capital costiii. Higher maintenance as a result of more moving parts, higher speed and more

cylinders in a particular installation.iv. Limitations on satisfactory engine operation while burning gas based on the gas

composition (max 22% Nitrogen and minimum 78% Methane)v. About 4% efficiency loss in the electric power generation process.vi. At low speeds or at anchor, the power requirement is much lower than the energy

available from the BOG. Excess BOG is then sent to the oxidiser which results in

the loss of economic value of the boil off.

c) Investment cost:

The capital cost of an LNG carrier with medium speed diesel electric propulsion for a130,000-150,000 m3 LNG is expected to increase by about 3-5% when compared with asteam turbine driven vessel. The installation of a reliquefaction plant will increase the capitalcost by another 5%.


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Figure 12. Medium Speed Diesel Configuration Alternatives (scource : Wartsila)

Fig 12a. Single-scew, direct drive Fig12b.. Single-scew, medium speed reductiongear

Fig 12d. PodsFig 12c. Twin-scew, direct drive


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Figure 13. Medium Speed Diesel Fuel Alternatives

Fig 13a. Dual Fuel Engines running on natural and forced BOG (or MDO if no BOG)

Fig 13b. Dual Fuel Engines: 2 engines running on natural BOG (or MDO) & 2 DieselEngines running on HFO only

Fig 13c. Dual Fuel Engines Running on either BOG or HFO (Not available yet)(scource : Wartsila)


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4) Medium Speed Diesel with Reliquefaction Plant

A possible alternative which is being discussed within the industry and is considered to boostconsiderably the flexibility of the preceeding alternatives with medium speed engine dieselelectric propulsion, is the addition of a reliquefaction plant. Obviously the highest flexibility

is obtained if the reliquefaction plant is installed in combination with the third diesel electricoption-the dual gas/HFO burning engines-Such a configuartion will give complete freedomon the boil off gas handling (use as propulsion fuel or reliquefy) and consequently on the fuelused for propulsion. The configuration will allow optimum adoption to the relative prices ofgas and HFO and to different vessel operating profiles and speeds. It seems therefore idealfor spot trade ships.

The draw back on this alternative is obviously the added capital for the cost of thereliquefaction plant which is of the order of USD 10mil for 2x100% units.

It should be mentioned here that the slow speed diesel dual fuel engine with a reliquefactionplant is an equally feasible solution.

The reliquefaction plant viability depends also on the vessels speed as indicated on figure14.

The curve indicates the LNG consumption for the diesel-electric design at various speeds.The boil-off-rate is assumed to be more or less constant at 0.15% / day which is about 100 mt/ day for a 142,000 m3 vessel. The shaded area above the curve indicates the excess boil-off.Hence, if for example a vessel without a reliquification plant is sailing at 18 knots, then 25 tof excessive boil-off would be lost every day. If the vessel was sailing at lower speed, the

amount lost would be greater and vice versa. Hence the reliquification plant becomes morebeneficial as the service speed of the vessel is reduced.

Figure14. LNG consumption vs. speed (SourceC. Clucas, Dorchester Maritime )


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5. Gas turbine

Even though the Gas Turbine (GT) propulsion system has many advantages in power toweight ratio, emission level, flexible machinery arrangement, efficiency and consequential

cargo volume increase, it has not been adopted as a new propulsion system in an LNG carrierso far. As the GT propulsion system has some unique features and limitations compared withconventional marine propulsion systems, detailed technical and economic issues have to besolved in order to implement this power plant in an actual LNG carrier.

The GTs proposed for LNG propulsion are usually marinized aeroengines of the latestgeneration with lower ratings compared to those used on aircraft. These changes promise toenhance reliability in the marine environment.

The primary fuel considered for the alternatives associated with the GT is gas, with MGObeing considered only as a back-up fuel in case of emergency. The alternatives therefore aresuitable for projects where the use of gas (boil off and forced) has been established by overalleconomic considerations, similar to the medium speed dual fuel (gas and MDO burning)electric alternative .

Further, the configuration options available with gas turbine are mainly based on electricpropulsion, although mechanical drive through reduction gear is possible, it is not considereda likely candidate for LNGs because it removes some of the advantages achieved with the gasturbine, like flexibility of installation, elimination of auxiliary electric power generators etc.

Electricity generated by the gas turbine-driven alternators is delivered to the distribution

network on to high-voltage main busbars. Power for the propulsion motor - or motors - istaken directly from these busbars and converted to provide a variable speed drive.

The alternatives are:

Simple cycle, usually with one main turbine and one auxiliary turbine Combined cycle (COGES), usually with the same configuration as the simple cycle as

far as number of gas turbines is concerned but with the addition of a heat recoverysystem which utilises the energy in the main turbine exhaust gasses to add steamturbine driven electric generation into the propulsion/auxiliary power system. Thisconfiguration is promised to offer a 10% increase in overall efficiency compared to

simple cycle one but it increases the propulsion system capital by 25% and addsconsiderably to plant complexity.

The figures below show typical arrangement of the three propulsion configurations associatedwith gas turbine as prime mover.


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Figure 15. Simple Cycle

Figure 16. Combined cycle (COGES)


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Figure 17. Direct Drive

a) Advantages:

i. Increased cargo carrying capacity (for a 138,000m3 vessel, the increase in cargo

carrying capacity is of the order of 8,500m3 compared to a steam turbine vessel )

ii. Low machinery weight and volume

iii. Reduced installation costsiv. Low engine noise and vibration

v. Low equipment routine maintenance

vi. Increased thermal efficiency compared to steam turbines

vii. Design flexibility

viii. Lower fuel consumption than steam plant (but not as good as diesel options)

b) Disadvantages:

i. Expensive back up fuel (MGO)

ii. Higher Capital cost

iii. Lower redundancy compared to alternatives

iv. Relatively untried technology for the commercial ships

v. Combined cycle plant has the complexity of Gas Turbine plus the steam plant.

vi. Specialised training of engineers is required

c) Investment cost:

The capital cost of an LNG carrier with gas turbine simple cycle plant for a 130,000-150,000m3 LNG is expected to increase by about 3% when compared with a steam turbine powered

vessel, and that of a combined cycle (COGES) powered vessel by about 5%.


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6) Azimuth Thrusters

A promising alternative for the propulsion and thrust power transmission, apart fromcommon tailshaft propeller arrangement, is the Azimuth thrusters.

The alternative is not an exclusive option for LNGs but for all ships with electric propulsion.

Recently they have become a popular option for the latest generation of Cruise carriers. Thefeedback we get from such applications indicates that there are still teething problems torectify.

Azimuth Thrusters are electric podded drive systems mounted on a 360 degree rotating shaftunder the ship. The absence of rudder improves the underwater dynamics and increasesmanoeuvrability. By allowing flexible machinery arrangements, azimuth thrusters makepossible a smaller vessel with the same cargo space.

The main component in the underwater unit is the electric motor, in this case of synchronoustype with brushless excitation and with a stator shrink fit in the pod housing. In most cases,the motor is equipped with double windings in order to permit a continuous operation >50%load, even in case of winding failure. There is also a shaft brake, locking device andequipment to slowly turn the shaft in order to assist when undertaking maintenance work.

a) Owner/Operator benefits:i. Increased cargo capacity or reduced vessel sizeii. Increased propulsion system efficiency (energy saving up to 10%)iii. Increased propulsion system redundancy and power availabilityiv. Reduced total installed power generationv. Reduced total fuel consumption & exhaust emissionsvi. Reduced noise & vibration levels

vii. Reduced vessel turning circle

b) Shipyard & Construction benefits:i. Flexible machinery arrangementii. Modularised designiii. Simpler vessel machinery installationiv. Simpler hull form and structurev. Reduced installation time and costvi. Fewer componentsvii. Reduced shipyard/sub-supplier co-ordination work

Figure 18. Azipod Drive Figure 19. CRP Azipod Drive


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E. COMPARISON OF ALTERNATIVES

The steam turbine plant is highly reliable and widely adopted on LNG carriers. The ability touse both BOG and HFO ensures clean combustion. However, these plants suffer from inferior

fuel efficiency and therefore higher fuel consumption. Furthermore, these propulsion systemsare heavy and expensive to install, consume valuable cargo space and require experiencedseagoing engineers.

The diesel engine with a reliquefaction plant enables complete separation of the engine andthe BOG handling system. The fuel efficiency is better, the amount of cargo delivered isincreased but at the cost of eliminating the flexibility of consuming gas as propulsion fuel,higher maintenance and higher levels of NOX and SOX emissions.

The slow speed dual fuel diesel eliminates some of the drawbacks associated with the slowspeed/reliquefaction plant alternative and at the same time maintains the high thermalefficiency of the slow speed diesel.

The medium speed dual fuel electric propulsion concept for LNG carriers offers a way tocontinue to use the boil-off for ship propulsion while achieving the higher thermal efficiencyof a diesel engine. It is one of the cleanest power plants and offers multiple levels ofredundancy and flexibility using multiple diesel engines producing electricity for propulsion,services and cargo transfer.

Dual fuel diesel engines is one of the favored alternatives of the new generation LNGcarriers. Special attention though should be paid on the safety requirements governing the

construction and operation.

The gas turbine combined cycle plant offers better fuel efficiency and clean emissions ascompared to the conventional steam turbine. However it requires a high quality petroleumfuel (MGO-as back up-) and is a complicated system combining the high technology Gas

Turbine with the traditional steam plant and associated auxiliary systems.

The table below indicates the effect each propulsion alternative has on the main elementswhich should be considered when attempting to evaluate propulsion alternatives different tothe basic steam propulsion. Further, figure 20 represents the effect different propulsion

alternatives have on engine room dimensions and cargo carrying capacity.

Obviously the decision on a particular project will have to be based on a thorough techno-economic analysis which would incorporate the whole transportation chain.


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CONFIGURATION

EFFECTON

CAPIT

ALCOST

PLANT

EFFIC

IENCY

BOG

UTILI

SATION

CAPABILITIES

ENVIR

OMENTA

L FRIEN

DLINESS

RELIA

BILITY

REDU

NDANCY

CREW

ABILI

TY

EFFECTON

CARG

O

CAPACITY

STEAM PLANT B B B B B B B BSSD/RELIQUEFACTION

CO2SOXNOX

(2X)

(1X)

DUAL FUELBURNING SSD

(2X)

(1X)

MEDIUM SPEEDDUAL FUEL GAS /

MDO

(RE)

MEDIUM SPEEDCOMBINED DUALAND HFO ENGINES

(RE)

MEDIUM SPEEDDUAL FUEL GAS/HFO

(RE)

GAS TURBINESIMPLE CYCLE

GAS TURBINECOMBINED CYCLE

Indicates positive effect, Indicates negative effect, Indicates no appreciable effect

COMPARISON OF ENGINE ROOM LENGTH AND CARGO CARRY ING CAPACITYWITH DIFFERENT PROPULSION ALTERNATIVES

Table3. Com arison of alternatives

Figure20.Engine room arrangements


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F. CONCLUSION

The LNG industry has entered a period of renewed growth which is driving technologicalchange. The traditional method of powering LNG ships, the steam turbine, is beingquestioned in light of developments in alternative propulsion systems and reliquefaction

systems for the boil-off gas.

There appear to be significant benefits in changing to diesel or gas turbine power plants, interms of fuel economy, cargo carrying capacity and reduced emissions. Also, moving awayfrom steam to diesel has the additional benefit of easing the difficulty of bringing Engineers,experienced with the power plant, into the LNG industry.

Each alternative system has its advantages and disadvantages. Individual Owners, Chaterers,or LNG Projects might weigh these differently in terms of importance when selecting theirdesign. Forecasts of the relative price of LNG versus fuel oil (heavy fuel oil or marine dieseloil) will also greatly influence the selection.

Work continues on each of these designs, further improving their capabilities and betterintegrating each into the hull and cargo containment designs, as well as the ship/shoreinterface. As a result, there is no best design that has been agreed upon by the engineers orthe market. Whether one finally emerges, or there develops a number of excellent choicesthat become proven and available to the market; either way, the LNG industry will benefitfrom this exciting period of development.


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G.RECENT NEWBUILDING TRENDS ON ALTERNATIVE PROPULSION

The LNG carrier projects with propulsion other than the standard steam turbine which have

recently been contracted are:

1. The LNGs under construction at Chantiers deLAtlantique on behalf of Gaz de Francewere the first non steam turbine propelled LNG carriers to be contracted in recentyears.

The characteristics of the first vessel of the series which was due to be delivered lastNovember but delayed due to problems encountered with the cargo tank insulation areas follows:

Cargo capacity 74,130 m3Main engines WARTSILA ,4x6L50DFInstalled Power 4x5,700kW=22800kW

Type of transmission Electric with two motorsthrough reduction gear on asingle shaft to a singlepropeller

Service speed 17.5 knots

The second and third vessels of the series have double cargo carrying capacitycomparing to the first one and have the following characteristics:

Cargo capacity 153,500 m3Main engines WARTSILA ,3x12V50DF

+1x6L50DFInstalled Power 3x11,400+1x5,700kW

=39,900kW



The layout of the plant for the above ships is similar to the one shown on figure 6

Table4.

Table5.


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2. Eight very large LNG carriers destined for transport of gas from Qatar to the UK.Four have been ordered at DSME of Korea, and two each from HHI and SHI.

Also for the same project, each two LNG carriers of approximately the same capacityhave been ordered at HHI and SHI respectively, the characteristics of these eightvessels are:

Cargo capacity Approximately210,000 m3Main engines 2x MAN B&W

6S70ME-CGenerator engines 4x MAN B&W Holeby

7L32/40Installed Power 2x16500kW=33,000kW

for propulsion

4x3600kW=14400kWfor Electric power

Type of transmission Direct drive ,twin skegReliquefaction plant 2x100% unitsService speed 19.5 knots


3. Four (plus four options) LNG carriers ordered by BP at HHI with the followingcharacteristics:

Cargo capacity 155,000m3Main engines WARTSILA ,2x12V50DF


=39,900kW




Table7.

Table6.


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4. Four (plus two options) LNG carriers ordered at SHI for AP-Moller, and also two(plus one option) LNG carriers ordered by K-Line at the same yard, all with thefollowing characteristics:

Cargo capacity 153,000 m3Main engines WARTSILA ,3x12V50DF


=39,900kW



Again the layout of the plant for the above ships is similar to the one shown on figure 6.

From the foregoing we get a total of 13 firm and 7 optionally contracted LNGs withmedium speed diesel electric and 8 firm contracts with slow speed diesel andreliquefaction.

In addition to the above, there are a number of LNG carrier projects under discussionwhich are considered to be equipped with propulsion systems other than steam turbine,these are:

The Ras Gas II (train 6) project with either twelve 210,000m3 ships or ten250,000m3 ships, with twin slow speed diesels and reliquefaction plant (sameas item 2 above).

Tangguh, Indonesia-Tendering for 7 LNG vessels, 135,000-160,000m3capacity with either steam turbine or dual fuel diesel electric.


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H.APPENDICES

Figure 21. Typical Thermal Efficiencies of Prime Movers (Source MAN B&W)


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Figure 21.Predicted Brake Power requirements vs. speed (SourceChris Clucas Paper)


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Figure 22. Predicted Brake Power requirements vs. size (Source MAN B&W)

Figure 23. Emissions for alternative configurations (Source Wartsila)


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Growing Global LNG Demand

Pipeline

74%

LNG

26%

Source: BP Statistical R eview of World Energy J une 2003

Natural Gas Trade Movement2002

Pipeline

74%

LNG

26%

Source: BP Statistical R eview of World Energy J une 2003

Natural Gas Trade Movement2002

7% per year growth (1992-2002)

Growth in LNG Demand

-

1,000

2,000

3,000

4,000

5,000

6,000

1970 1980 1990 1992 1994 1996 1998 2000 2002

bcf

J apan South Korea Taiwan France Spain USAItaly Belgium Turkey Greece Portugal UK

Source: Cedigaz, BP Statistical Review of World Energy June 2003

LNG is about 6% of worldwide natural gas consumption

and about 94% of natural gas consumption in Japan.


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Development of LNG Infrastructure

1. LNG TERMINALS EXISTIN

G

PLANNED % INCREASE

EXPORT 18 28 155 %

RECEIVING 46 72 150%

2. LNG FLEET 160 100 ~200 100%

DEBOTTLE NECK LNG PLANTS - INCREASES CAPACITY AND NEED FOR SHIPPING

POSSIBLE FUTURE OPPORTUNITIES FOR SPOT TRADING


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Factors To Increasing LNG Shipping Productivity

Increase throughout and / or lower cost

1.Increasesizeof shi

Size / cubicmeter

30,000 75,000 125,000 138,000 145,000 220,000

2020

?

2. Change propulsion fromSteam to?

Dual fuel Diesel Electric

Gas Turbine and Diesel or Steam

Slow speed Diesel with re-liquefaction

3. Lower Ship Cost

Year 1960 1970 1980 1990 2000 2010


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References

Propulsion Alternatives for LNG Tankers David Furnival & Chris Clucas,Dorchester Maritime Ltd. Isle of Man, UK.

Evaluation of Propulsion options for LNG Carriers Gastech 2002, Dr. ManfredKuver, Chris Clucas, Nils Fuhrmann.

Dual-Fuel-Electric LNG carrier PropulsionBarend Thijssen, Wartsila, Finland. Fueling the Future-Powering the Carriers (LNG) Dominic Welch, Rolls-Royce

Marine Systems, UK. Dual Fuel Operation of MAN B&W Two Stroke, ME-GI Engines-Design Concepts

Rene S.Laursen, MAN B&W Diesel A/S, Copenhagen, Denmark. Guide for design and installation of Dual Fuel Engines ABS, January 2003 LNG Carrier Propulsion by ME Engines and Reliquefaction MAN B&W. LNG Safety and Security Institute for Energy, Law & Enterprise, University of

Houston, October 2003.

LNG World Shipping newsletter, December 2004. Competitive pressure rises on steam propulsion for LNG TankersThe Naval

Architect, March 2004. L NG Carrier Propulsion by ME-GI Engines and/or Reliquefaction MAN B&W

Diesel A/S, Copenhagen, Denmark. Dual-fuel-electric LNG Carriers Barend Thijssen, Wartsila, Finland. Electric Propulsion for new generation LNG Carriers The FrontRunner, Sept 2004,

Published by ABB Marine. Wartsila 50DF Engine HFO Development October 2004. Diesel Engines for LNG Carrier Propulsion Ole Grone & Peter Skjoldager MAN

B&W Diesel A/S, Wemer Oehlers & Dirk Fabry-MAN B&W AG.August 2002. Two-Stroke Diesel Engines and Reliquefaction System for LNG Carriers PeterSkjoldager, Tore Lunde & Eirik Melaaen.

Websites

http://www.rolls-royce.com/marine/product http://www.manbw.com/files/news/filesof3856/P9027-04-04.pdf

http://research.dnv.com/marmil/gasprop/info.htm http://www.wartsila.com/?_nfpb=true&_pageLabel=shippower_en

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