641
1 INTRODUCTION
Whenstudying,professionalpublicationsconcerning
LNG marine propulsion green ships technology (i.e.
[1], [3], [6], [10]) and after analysing some
environmental aspects regarding marine transport
efficiency ([2], [4]) we can draw the following
conclusions:
Currentlywehavewiderangeofmodernmarine
green ships technologies available on the market
used to enhance performance and sustainability
for oceangoing vessels. These technologies range
fromsimplewithlowcapabilitysuchastraditional
sailsorsmallscalerenewableswindpowerplants
or solar modules to very capable and highly
complex systems such as hybrid diesel‐electric,
wasteheatrecovery,va
riablespeed generatorsor
LNG&dualfuelpowerplants.
Today, mostmerchantshipsareusingHeavyFuel
Oil (HFO), Marine Gas Oil (MGO) or Marine
Diesel Oil (MDO) as a main fuel for ship
propulsion system. These fuels are cost effective
but unfortunately they produce significant
amountsofnoxiousemissionssuchasCO2,NOX,
SOX & PM (Particulates Matter). The largest
am
ount of noxious emissions is observed
especially in the costal and harbour areas where
the marine traffic density is also much higher
compare with traffic density observed at open
ocean[2],[6].
Ontheotherhand,itmustbealsonotedthatthe
seatransportationisoneofthemostenvironment‐
friendlymeanswhenwecomparetransportingthe
samecargoonthesamedistancebyusingdifferent
means of transportation such as rail, road and
marine[2].
To comply with IMO and MARPOL
environmentalregulations,LiquefiedNaturalGas
(LNG) is becoming an interesting option for
shippingindustry.TheuseofLNGasfuelallows
reducing emissions, complying emissions rules
and reducing operating cost [6]. Several energy
Study of New Generation LNG Duel Fuel Marine
Propulsion Green Technologies
G.Rutkowski
GdyniaMaritimeUniversity,Gdynia,Poland
ABSTRACT: Nowadays, most merchant vessels use Heavy Fuel Oils (HFOs) for the ship propulsion. These
fuels are cost effective but they produce significant amounts of noxious emissions. To comply with IMO &
MARPOLenvironmentalregulations,LiquefiedNaturalGas(LNG)isbecominganinterestingoptionforthe
merchantships.Thepurposeandscopeofthi
spaperistodescribethefactorstoconsiderwhendetermining
LNG & duel fuel new generation marine propulsion technologies implemented in the shipping industry to
promotegreenshipsconceptandchangetheviewofseatransportationtoamoreecologicalandenvironment‐
friendlysystem.Theaimof the researchpresentedin thi
spaper istoanalyse the economicupturnthat can
resultfromtheuse ofLNGasfuel formerchant shipsand toassess theeffects ofits utilization in termsof
environmentalimpact.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 10
Number 4
December 2016
DOI:10.12716/1001.10.04.14
642
recoverysolutionshavebeenconsideredonaLNG
tanker[3]toimproveefficiency(+15%)andreduce
costs(−20%).Someeconomicincentiveshavebeen
shownalsotobeadvantageoustorunninganLNG
propulsion system on other ships than LNG
tankers.ThebestcandidatesforLNGusearesmall
and
medium product tankers, LNG & LPG
carriers, cruise vessels, ferries, offshore vessels,
rollonrolloff,feedercontainersandtugboats[6],
[10].
SinceitisestablishedthatLNGforshippropulsion
reduces CO2 and other pollutants compared to
common heavy fuel oils, LNG implementation
depends on the following key factors: gas
availability, demand for ships, emission limits in
ECA (emission controlled areas), LNG tank
installationandsafetyrequirements.
Another challenge is hazards associated with the
LNG being stored at very low temperatures.
Insulation of the tank is critical, and structural
brittlenessandpersonnelfrostbiteinjuriesarevery
likelytohappen.Inadditions,thechallengessuch
as the lack of infrastructure in most commercial
ports, crew’s limited experience running
engines
withgasfuels,thefuturepriceofgas,therequired
safetymeasuresetc.Thereareallcriticalpointsto
be considered when we want to use LNG in
shippingindustry.
Regardingnatural gas,itmustbe alsonoted that
there is an environmental issue called methane
slip. Methane slip happens when gas leaks
unburned through the engine. Methane has a
GWP100 (100‐year global warming potential),
whichis25xhigherthanCO2.Ifthemethaneslip
isn’t controlled, environmental benefits
to using
naturalgasarereduced[6].
Thepurposeandscopeofthispaperistodescribe
factors to consider when determining LNG & duel
fuel new generation marine propulsion technologies
implemented in shipping industry to promote green
ships concept and change the view of sea
transportation tomoreecological and
environmental
friendlysystem.Theaimoftheresearchpresentedin
thispaperistoanalysetheeconomicupturnthatcan
resultfromtheuseofLNGasfuelformerchantships
andtoassesstheeffectsofitsutilizationinterms of
safetyissuesandenvironmentalimpact.
2 MARINELNG
DUELFUELENGINE
A marine LNG (Liquefied Natural Gas) engine is a
dualfuelenginethatusesnatural gas(NG)andliquid
bunkerfuel(HFO,MGO, MDO)to convert chemical
energyintomechanicalenergy.
The advantage of transporting LNG is clear: a
defined volume of LNG contains approximately 600
times more
energy thanthe same volume of natural
gas (NG). The natural gas NG that fuels dual fuel
engines is carried on ships as a boiling liquid, and
transported at slightly higher than atmospheric
pressureinheavilyinsulatedtankstokeeptheLNG
inliquidstateataround‐160°Cwhere
fromtheboil‐
off gas (BOG) is routed to and burned in dual fuel
engines. When tank insulation is penetrated by any
influx in heat, it will cause the temperature of the
liquefied natural gas to rise, which allows for
vaporizationfromliquidtogas.Whenheatpenetrates
thetank,the
tank’spressureincreasesduetoboil‐off
gas(BOG)effect.
The insulation of the tanks is designed with the
most advanced technology. However even now, on
LNG carrier the insulation of the tanks is still
penetrated by heat and the boil‐off process occurs
during the ships voyages. During a
storm for the
example,theLNGcargomovesandsloshesaroundin
thetanks.Theboil‐offgasrepresents0.1%to0.25%of
the ships capacity per day. Tanks need to be
maintained at a steady pressure. If the pressure in
tanks is not controlled relief or safety valves are
forced
to open, venting the boil‐off into the
atmosphereuntilpressureisrelieved.Atthispoint,it
hasbeenproventhaton‐boardLNGreliqueficationis
uneconomicalformostships([3],[6]).Instead,thegas
producedbythisboil‐offeffectisroutedtotheship’s
propulsionsystemandused
asfuel forpowerplants
such as steam boilers or i.e. dual fuel marine diesel
engines([5],[8],[10]).Thisreducestheuseofbunker
fuel,reducingfuel costsandequipmentmaintenance
costs.In addition, whennatural gas cleanerburning
properties are compared to HFO, MDO or MGO
burningproperties,the
useofLNGtechnologyinthe
shipping industry is becoming an interesting option
tocomplywithIMOandMARPOLinternationalrules
®ulations[6],[10].
Nowadaysinshippingindustrywecanfindwide
rangeofdifferentpropulsionsystemsuchasdual‐fuel
steam turbine mechanical (DFSM), dual‐fuel diesel
electric
(DFDE),dual‐fuelgasturbineelectric(DFGE),
dual‐fuel diesel mechanical (DFDM) or i.e. diesel
mechanicalpropulsionwithreliquefication(SFDM+R)
([4],[5],[6],[10]).
Figure1.Comparisonofsteamplant(ontheleft)withslowspeeddieselenginewithare‐liquefactionplant(ontheright)as
exampleoftypicalpropulsionplantusedonLNGcarrierof150,000m3.Source:adaptedfrom[4].Nov.2016.
643
Figure2.Comparisonofdualfueldieselengineplant(ontheleft)withadvancedcyclemarinegasturbine(ontheright)as
exampleofpropulsionplantforLNGcarriers.Source:adaptedfrom[4]. Nov.2016.
Today steam turbine (ST) installation has
dominatedinLNGshippingsofar.Thisisbecauseit
caneasilyhandletheevaporatedBOG,inadditionto
itshighreliabilityandmaintainability.Although,this
installationhassuccessfullymetmarketrequirements,
thetotal plantefficiencyis verylow (basedon [4]it
has
beencalculatedandfoundtobe30%),whichled
tohighrunningcosts.AnotherdisadvantageofSTis
the shortage of qualified crew and low system
redundancy.
Another solution is slow speed diesel engine,
whichhasbeendominatedthepropulsionandelectric
power generation in all segments of merchant
shipping, except
LNG carriers. Experience gained
from thousands of diesel engine installations in
service [6] has resulted in the development of high
efficiency(about50%),reliableandsafeoperation.It
commonlyburnsliquidfuelasHFO.However,BOG
in such case must be re‐liquefied onboard by re‐
liquefactionplantandfed
backto thecargotank. In
caseofre‐liquefactionplantfailure,agascombustion
unit (GCU) must be also installed. These re‐
liquefaction plants require a substantial amount of
electricpower (≈3600kW) tooperate, representing a
considerableaddedcostforinstallation(basedon[4]
itcanbeabout
6millionUS$extracost)andweight.
However,thisinstallationcangiveusopportunityto
delivermorecargotoLNGterminalduetosavingthe
BOG.
Anothersolutionisto usedualfuel powerplant,
which can work as mechanical or electrical drive.
Usingelectricdrivearrangement,theenginescan
be
installed on a higher deck and hence, a great
reduction in engine room could be provided. The
dualfuelenginesutilizenaturalboil‐offgas(N‐BOG)
in addition to the MDO for their pilot injection. In
case of no NG available the engine will burn HFO,
MDOorMGO.
Themaindisadvantageofthissystem
is the slightly higher cost for alternators and
transformers,thelowthermalefficiencyoftheplant,
(basedon[4]thecalculatedefficiencywasreducedto
42%)andtheincompatibilityoflubeoilwhenshifting
betweengasandliquidmode.
Another feasible alternative option for
LNG
carriers is gas turbine propulsion systems presented
onFigure2ontheright[2].Theadvancedmarinegas
turbinecyclesuggestedi.e.byDaewooShipbuilding
& Marine Engineering (DSME) and Gaztransport &
Technigaz (GTT) is employed in two gas
turbine/electric generators in father and son
arrangement. The larger generator is
based on the
converted simple cycle and provides all power
neededforseagoingservice.Thesmallunitprovides
powerof5000kW,forcargopumpingandportduty.
Additionally, the small unit provides get‐home
service in case of non‐availability of the large unit.
Theprimaryfuelused
forgasturbines isnaturalboil‐
off gas (N‐BOG) and forced boil‐off gas (F‐BOG).
Marinedieseloil (MDO)iscarriedonlytoprovidean
emergency secondary fuel source to/from dry dock
when gas is not available. The suggested plant
efficiency has been calculated by [4] and recognized
as
approximately40%.
Nowadays commercial ship propulsion system
manufacturers such as Finland’s Wärtsilä [10],
Germany’s MAN Diesel &Turbo [5] or Siemens [9],
Japan’s Mitsubishi [7] or British’s Roll Royce [8],
produce large bore dual‐fuel diesel engines that
comply with all modern emission legislation when
sailinginenvironmentallysensitiveareas
andwhich
meetthestrictsafetyrequirementsthatLNGcarriers
operate under. All above manufacturers can deliver
LNG systems for propulsion and power generation
foranyapplicabletypesofshiporengine[6].
Whether newbuilt or retrofitted, LNG ships are
clearly the way of the future. Per MAN Diesel &
Turbo [5] website’s subfolder dated December 2014
theMANB&WDualFuelEnginesstartinganewera
in shipping industry having a total of 116 ordered
LNGengineprojects.Ontopofthis,moreandmore
new‐buildings has been constructed as LNG‐ready,
which means that they can relatively easy
be
retrofittedwithdualfuelenginesatalaterpoint.The
MAN B&W ME‐GI Engines [5] offer extremely
flexiblefuelmodesthatrangefrom95%naturalgasto
100%HFOandanywhereinbetween.Aminimumof
5% HFO for pilot oil is required as these are
compression ignition
engines and natural gas is not
self‐combustible.
Wärtsilä [10] is also recognized as a leader in
propulsionsolutionsfor gas fuelledvessels,and has
ledthewayindevelopingacompletevaluechainof
systems,solutionsandbunkeringarrangements,both
on‐board and shore‐based, to accelerate the use of
environmentally sustainable and economically
competitive LNG fuel. Since 2000, Wärtsilä engines
have been selected for more than 200 LNG fuelled
vessels either in operation or under construction. In
644
addition,attheendof2014,Wärtsilähadperformed
14LNGPacinstallationsandtheoldestofwhichhad
beenoperatingsuccessfullyformorethanthreeyears
withoutanyproblem[10].
The LNGPac system has been specified with
varioustypesofships.Theseincludeproducttankers,
cruisevessels,offshorevessels,roll
androlloff,feeder
container vessels, LPG and ethylene carriers and,
obviously, ferries, such as the ‘Viking Grace’
operating between Turku, Finland and Stockholm,
Sweden [10]. This is the largest passenger vessel in
theworldtodayequippedwithtwo200m3Wärtsilä
LNGPacs, including a Wärtsilä patented system
utilising the
latent heat from the LNG evaporation
process for the vessel’s heating, ventilation and air
conditioning system (HVAC). In the new cold
recovery system Wärtsilä can directly connect the
ship’s HVAC (or other refrigeration systems) to the
tank connection space and thus remove a complete
heating media circuit consisting of heat exchangers,
valves and pumps. This system provides significant
energy savings for the whole ship by increasing its
totalefficiency.
There is also possibility of improving energy
efficiency on board by considering that combustion
gases, produced by LNG, are cleaner, thus
simplifying the introduction of exhaust gas heat
recovery: simple heat recovery
and heat recovery to
drive a turbine (ORC). The results show (based on
[10])thatitispossibletoachieveareductioninfuel
consumptionofupto15%.
In addition, when certain systems such as waste
heat recovery WHR [5], [9] (using waste heat to do
work rather than dissipate)
are added to the power
plant,significantsavingscanbeobserved.Onestudy
[3] shows that an LNG engine with a WHR system
savesmoneycomparedtoadieselenginewithWHR.
There is a higher initial investment cost but it is a
cost‐efficientmethodandenvironmentallysoundone
[6].
The Wärtsilä LNGPac system is based on IMO
typeCLNGstoragetankwithdesignpressure6‐9bar
and either double walled vacuum or single walled
polyurethaneinsulation. Bunkeringtakes place from
the bunkering station to the LNG tank via an
insulatedpipe.Allnecessaryprocessequipment, the
heating
media skid, and the control and monitoring
systemare installed in aseparate unit whichcan be
either mounted directly to the LNG tank or placed
remotely from the LNG tank. The main process
equipment ensures correct gas temperature and
pressure for the engines and other gas consumers.
TheLNGPac
systemcanbe customised tothe needs
of each project on a case to case basis. Dedicated
engineering is conducted from the beginning of the
projecttomatchthespecificoperationalrequirements,
safety and classification society requirements. The
LNG fuel system can be offered as a standalone
product, as well as
a part of a complete propulsion
system [10]. By upgrading the system into a more
compactandtechnicallyadvancedversion,safetyand
reliability will be enhanced, while the capital and
operating expenditures (CAPEX & OPEX) will be
reduced.Thenewsystemhasfewermovingpartsand
therefore less maintenance is
required. Furthermore,
the compact design and increased integration of
componentsmakesinstallationattheshipyardfaster
andeasier[10].
The Wärtsilä gas valve unit (GVU) is a module
located between the LNG storage system and the
dual‐fuel (DF) engine. It is used to regulate the gas
pressure and ensure a
safe disconnect of the gas
system should that be necessary. By combining the
LNGPacand theGVU into a single, fully integrated
system, considerable space can be saved and a
relatively simple ‘plug and play’ solution will save
installationtimeandcostsfortheyard.
In addition, the airlock and
control cabinet has
been integrated into the tank connection space. This
innovation results in a dramatic reduction of
interfacessincetheamountofelectricalcabling from
the tank connection space to the external
switchboardscanbesignificantlyreduced.
Development of the LNGPac is the result of
Wärtsilä’s extensive experience and technical
leadership in gas propulsion, as well as its
comprehensive in‐house know‐how concerning all
aspects of the vessel’s machinery, fuel gas handling
system, and ship design. By removing the
intermediateheatingmediaskidanditspumps,and
by directly utilising the engine’s cooling water, less
interfacesandinstallationwork
isrequired.Withless
electrical consumers they are making the ship even
moreenvironmentallyfriendly.
Unfortunately, even now, in the marine industry
liquefiednaturalgas(LNG)asafuelisstillsometimes
mentioned as a novel technology although platform
supply vessels have long been using it. The ‘Viking
Energy’forthe
example(IMO9258442,Lengthoverall
94.90m,breath20.4m,tonnage5073GT)wasthefirst
new generation PSV LNG‐powered vessel, built in
Norway in 2003 by Kleven Verft AS for Eidesvik
ShippingASconsolidatedgroupasDPclassshipfor
delivering supplies to oil and gas platforms in
the
North Sea [8]. The ‘Viking Energy’ was fitted with
four6‐cylinderWärtsilä32DFdual‐fuelengines,each
with an output of 2010 kW at 720 rpm, driving the
main generating sets. The engines run on liquefied
naturalgas(LNG)toreduceNOxemissions,butcan
alsorunondiesel
oilasabackupfuel.Thisaddsfuel
flexibility into the mix. The switch between fuels is
conductedautomatically,withoutanylossofpower.
Whenrunning in gasmode, which is the normal
operational mode, the Viking Energy’s emissions of
nitrogen oxides (NOX) are 85% lower than in diesel
operation.
Thedramaticreductionispossiblebecause
the Wärtsilä engines operate on the lean burn
principle: the mixture of air and gas in the cylinder
contains more air than is needed for complete
combustion. Lean combustion reduces peak
temperatures,sotheformationofNOxdropsagreat
deal. Sulphur oxide (SOX) emissions
are eliminated
becauseLNGcontainsnoSulphur.Andsincenatural
gascontainslesscarbonperunitofenergythanliquid
fuels, emissions of CO2 are also lowered by
approximately30%.Naturalgashasnoresiduals,and
the production of particulates is practically zero.
Clean combustion has also a positive impact
on
maintenanceoftheenginecylinderliners/covers.The
maintenance interval can be longer than for liquid
fueloperation[6],[10].
645
Ingasmode,Wärtsilämedium‐speedDF engines
are already compliant with the IMO’s Tier III
regulations without the need of any secondary
exhaust gas purification systems. Furthermore, in
liquidfueloilmode,allWärtsiläDFenginesarefully
compliant with the IMO’s Tier II exhaust emission
regulationssetout
inAnnexVIoftheMARPOL73/78
convention.
TheenginesperformverywellongasduringDP
(DynamicPositioning)operationsandsevereweather
conditions in the North Sea. Diesel mode is only
required during LNG refuelling or certain minor
preventivemaintenanceoperations.Anotherstrength
ofthe‘VikingEnergy’ishow
smoothlysherunsfora
completelynewsystem.In13yearsofoperationnota
single hour of technical off‐hire has been caused by
thegassystem[10].
LookingbackatthefirstLNGdecadeforshipping
industry, one thing that has greatly improved is
managing methane slip. In 2003,
when the ‘Viking
Energy’ was launched, methane slip was not yet on
theagenda.Createdbygasengines thatallowasmall
fractionofthefueltogothroughunburned,methane
slipisagraveconcern.Unfortunately,theproblemof
methaneslipispresentinallOtto‐cycleengines,both
dualfuelandspark‐ignited.Tocombatthis,Wärtsilä
hasexecutedaresearchanddevelopmentprogramme
that has shown good results.All the latest dual‐fuel
enginesarenowoptimizedtokeepmethanesliptoan
absoluteminimum[10].
Other challenges have also been faced and
overcomeinthepastten
years.InitiallyLNGvessels
suffered from close to non‐existent LNG bunkering
infrastructure.Todaywecanobserveasignificantrise
inLNGinfrastructure.Terminalsarebeingbuilt,and
the supply of gas can also be organized with trucks
and small‐scale storage facilities or LNG bunker
barges.
3 CONCLUSION
Whether
newbuilt or retrofitted, LNG ships are
clearly the way of the future. Short‐sea routers, as
well as ferries constantly operating between defined
ports, are the main applications where a propulsion
train based on gas‐propulsion offers the biggest
advantages. This consideration becomes even
strongerwhenthe operationis influenced by
sailing
periods spent in Emission Control Areas (ECA).
Containers, product tankers, cruise vessels, offshore
vessels,rollandrolloff,LPG,LNG,ethylene carriers
and passenger ferries represent typical vessel fleets
thatcouldbeconvertedtogasoperation.Gasoffersa
simpler solution for new built ships and for
retrofittingvessels
foragreenertomorrow.Takingall
of this into our consideration today we can predict
thatduringthenexttenyears,LNGwillfind itsway
toathousandnewships.
Nowadays,whenmarineLNGDFenginesrunin
natural gas as the primary source of energy, the
followingtargetsare
achieved:
CO2emissionsarereducedbyapproximately15%
to30%,thankstoalowercarboncontentinnatural
gascomparedtoliquidfuels[4],[6],[10].
NOX emissions are reduced by approximately
85%,thanks to the leanburn combustion process
implementedinDFengines[6],[10].
SOXemissionsarealmosteliminated,sincenatural
gasdoesnotcontainanySulphur[10].
Particleproductionispracticallynon‐existent,due
to the efficient combustion of natural gas, a fuel
withalmostnoresiduals[5],[9],[10].
TheuseofLNGasfuelallowsreducingemissions,
complyingemissionsrulesandreducingoperating
cost up to 20% (based on [4]) to 35% (based on
[10]).
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