271
1 INTRODUCTION
Thedemandforlargecapacityvesselsincommercial
shipping has increased over the last decade. These
largevesselsarepropelledbypowerfulmarinediesel
engines,Itisimperitivethatthemainengineshould
havehigh reliabilityfor safeopeartion ofthe vessel.
Thereliabilityofa vessel’smain
propulsionengineis
dependent on a number of essential sub systems,
including fuel oil system, lubricating oil system,
coolingwater systemand scavengeair system.Each
of this subsystem has its own individual system
components,thereliabilityofthemwoulddictatethe
reliability of the corresponding subsystem, (EPSMA
2005; Mollenhauer & Tschöke 2010). Turbochargers form
a very important part of the scavenge system and it is
essential that the turbochargers have high reliability to
ensure reliability of the main engine, (Takashi 194). Failure
of turbochragers could lead to disastrous consequences and
imobilisation of the main engine. To
determine the
reliabilityofthevarioussystemcomponentsoneneed
to look at the failure pattern depicted by these
components.Previousstudies haveshownthat most
of the system components in commercial vessels,
propelled by largetwo‐stroke engine will fall in the
secondandthirdphaseofthebath
tubcurve(shown
inFig1),whichisaconstantfailureratefollowedby
anincreasingfailurerate,(Hashemian&Bean2011).
Thereliabilityofvariouscomponentsofothersystems
suchasgearpumpsina fueloilsystemorfiltersina
lubricating oil system, exhibits constant failure rate
(random failure) independent of their history of
operation, therefore they could be analysed using
Markovmodelling.Othermajorcomponentssuchas
turbochargersexhibitstimedependentfailurerate.
Reliability Assessment of Main Engine Subsystems
Considering Turbocharger Failure as a Case Study
M.Anantharaman,F.Khan,V.Garaniya&B.Lewarn
UniversityofTasmania,AustralianMaritimeCollege,Launceston,Australia
ABSTRACT:Safeoperationofamerchantvesselisdependentonthereliabilityofthevessel’smainpropulsion
engine. Reliability of the main propulsion engine is interdependent on the reliability of several subsystems
includinglubricatingoilsystem,fueloilsystem,coolingwatersystemand
scavengeairsystem.Turbochargers
form part of the scavenge sub system and play a vital role in the operation of the main engine. Failure of
turbochargers can lead to disastrous consequences and immobilisation of the main engine. Hence due
consideration need to be given to the reliability assessment of the
scavenge systemwhile assessing the
reliability of the main engine. This paper presents integration of Markov model (for constant failure
components)andWeibullfailuremodel(forwearingoutcomponents)toestimatethereliabilityofthemain
propulsionengine.Thisintegratedmodelwillprovidemorerealisticandpracticalanalysis.Itwillserve
asa
usefultooltoestimatethereliabilityofthevessel’smainpropulsionengineandmakeefficientandeffective
maintenance decisions. A case study of turbocharger failure and its impact on the main engine is also
discussed.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 12
Number 2
June 2018
DOI:10.12716/1001.12.02.06
272
Phase 1 Infant Mortality, decreasing failure rate (Weibull
Analysis)
Phase2Usefullife,constantfailurerate(Markovanalysis)
Phase 3 Wear out, increasing (time dependent Weibull
analysis)
Figure1.Bathtubcurveforfailurerate
Thewearingoutfailureratecanbeanalysedusing
Weibull analysis. This paper presents integration of
Markovmodel(forconstantfailurecomponents)and
Weibullfailuremodel(for wearingout components)
to estimate the reliability of the main propulsion
engine. This integrated model will provide more
realisticandpracticalanalysis.Itwillserveasauseful
tooltoestimatethereliabilityandmakeefficientand
effectivemaintenancedecisions.ReliabilityofFuelOil
system
Fig.2 below shows a reliability block diagram
(RBD) for a main engine fuel oil sysytem. QC
representstheQuickClosingvalve,FSrepresentsthe
FuelSupplypumps,FListheDischargefilters,FMis
the Flowmeter, BT is the Buffer tank, BP represents
the Booster pumps, HT represents the steam
heater
andVIStheViscotherm.Thenextstepistheanalysis
ofevaluating thereliabilityofthema inenginefueloil
sysytem,byusingMarkovanalysis(Gowid,Dixon&
Ghani2014).
Thefollowingpointsaretakenintoconsideration.
1 Each block represents the maximum number of
componentsinorderto
simplifythediqgram.
2 Thefunctionofeachblockiseasilyidentified
3 Blocksaremutuallyindependentinthatfailureof
oneshouldnot affecttheprobabilityoffailure of
another, (Anantharaman 2013; Xu 2008),
(Bhattacharjya&Deleris2012).
Figure2.RBDforMainEngineFuelOilSystem
1.1 ReliabilityoftheQuickClosingValve
The quick closing valve is the main tank outlet valve,
which can be operated remotely in case of an emergency. If
we assume a constant failure rate λ,(PCAG 2012), then the
reliability of this component may be expressed as
,
t
QC
R
te
where the mean time to failure MTTF 1/ λ.
1.2 ReliabilityoftheFuelOilSupplypumpFS
ThefueloilsupplypumpsFSareofthegeartypeand
identical in design and construction. The state
diagram for the pumps is shown in figure 4 The
reliabilityfunctionisanexponentialfunctionoftimet
andthe
failurerateλexpressedasnumberoffailures
perrunninghours,(Bhattacharjya&Deleris2012).
Table1.StateofFueloilsupplypump
_______________________________________________
State Pump1Pump2
_______________________________________________
1Operating Standby
2FailedOperating
3FailedFailed
_______________________________________________
FromTable1itisclearthatthereare3states.In
thiscasethetwofueloilsupplypumpsareidentical
units,Liberacki),oneofwhichisonlineandtheother
standby.Thereliabilityofthetwoidenticalsystemsis
derivedas,
1
0
!
i
t
s
i
t
Rt e
i
In this case
1
t
s
R
te t
and MTTF
(Meantimetofailure)=2/λ
Markovanalysisisusedtocomputethereliability
oftheothercomponenntsinthefueloilsystem.The
reliabilityofmainenginefueloilsystemwillbegiven
by
..F O QC FS FL FM BT
BP HT VIS
R
t R tR tR tR tR t
RtRtRt
(1)
2 RELIABILITYOFLUBRICATINGOILSYSTEM
The next step in the analysis of evaluating the
reliabilityofthemainenginelubricatingoilsysytem
isshownbelow:
Thefollowingfive(5)casessareanalysed:
1 FailureofsuctionstrainerS
2 FailreofpumpsP
3 Failureofdiscahrge
filterF
4 FailureofTemperatureControlValveTCV
5 FailureofcoolerCLR
Each block represents the maximum number of
components in order to simplify the diqgram.The
functionofeachblockiseasilyidentified.Blocksare
mutually independent in that failure of one should
not affect the probability of failure
of
another.(Anantharaman2013;Xu2008).
273
Figure3. Detailed RBDfor M.E. LubeOil system, with all
systemcomponents
2.1 StatediagramfortheMainEngineLubeOilStrainer
S
ThefirstcomponentsuctionstrainerSisabaskettype
strainer, located before the lubricating oil pumps,
(Khonsari & Booser 2008).This is a duplex type of
filter with a changeover cock for isolation of filters.
One of the filters is
in use, the second one being a
standby.Cloggingofthestrainercanresultinpump’s
inabilitytodrawsuctionfromthesump,whichmay
sound a low‐pressure alarm. This provides time for
changingovertothestandbystrainer.Failureofthis
changeoverwillresultinpump’sinability
tosupply
lubricating oil to the engine, finally resulting in an
enginefailure,(Cicek&Celik2013).Thesefilterswill
beidenticalasshowninFig.4.Thestatediagramfor
thefiltersisshowninFig.5.Thereliabilityfunctions
anexponentialfunctionoftimetandthefailurerateλ
expressed
asnumber offailures perrunning hours,
(Brandowski2009;Navy1994).
Figure4. Lube oil suction strainers for the Main Engine
Lubeoilsystem
Figure5.Markov Model analysis for the M.E. Lube oil
StrainerS
Table2.StateofLubeoIlstrainerS
_______________________________________________
State Strainer1Strainer2
_______________________________________________
1CleanCllean
2CloogedClean
3Clogged(Failed) Clogged(Failed)
_______________________________________________
As shown in Table 2, there are 3 states. In this case the
two main engine lube oil pump strainers are identical
standby units, one of which is on line and the other
standby.The reliability of the two identical systems
isderived as,
1
0
!
i
t
s
i
t
Rt e
i
In this case
1
t
s
R
te t
and MTTF
(Meantimetofailure)=2/λ
2.2 ReliabilityoftheMainEngineLubeOilSystem
The state diagrams for all other components of the
systemareanalysedonthesamelines,asdoneforthe
suction strainer S. Markov analysis(Smith 2011;
Troyer2006), carried outto determine
the reliability
ofthesystemcomponents.Finallytherelaiilityofthe
lubricatingoilsystemisdetermined,(Liberacki2007)
..L O s p F TCV CLR
R
t R tR tR tR tR t (2)
where
p
R
t isthereliabilityofthePumps
F
R
t istherelaibilityoftheFilter
TCV
R
t isthereliabilityof thetemperature control
valve
CLR
R
t isthereliabilityofthecooler.
3 RELIABILITYOFSCAVENGEAIRSYSTEM
Reliability of a scavenge air system for a large
propulsionengineconsistsmainlyofan exhaustgas
turbocharger,(TakashiandSusumu,1994),(Conglin
Dong2013).Theheatenergyoftheexhaustgasdrives
the exhaust gas turbine
coupled to a rotary air
compressor,which draws air from the engineroom.
Thecompressorcompressestheairandtheniscooled
in an air cooler before being sent to the engine
cylinder.OnesuchturbochargerisshowninFig.6.In
shorttheturbochargerandthecoolerformthemain
elements
of the scavenge air system, failure of any
oneofthecomponentcouldleadtofailureofthemain
engine,asshowninthefaulttree, (Zhu 2011),diagram
inFig.7.
274
Figure6.Turbochargerforalargetwostrokeengineattest
bedinQMD,Qungdao,China.
3.1 FaulttreeforMainEnginefailure
Figure7.FaulttreeforaMainEngineScavengesystem
FailureofeithertheTurbocharger,(ATSB,2006),or
AirCoolerwouldresultinfailureoftheMainEngine
Scavengesystem,(Laskowski2015).
3.2 RBDforScavengeairsystem
A reliability block diagram for the scavenge air
systemisshownbelow
Figure8.RBDforMainEngineScavengesystem
Theturbochargerandaircoolerareinseries,hence
the reliability of the scavenge air system could be
computed.Thesetwocomponentsformaveryrobust
partofthescavengeairsystem.Dependinguponthe
engine capacity there could be one or more
turbochargers or air coolers fitted to the main
propulsionengine.Thisarrangementhasmoretodo
with the engine capacity and not based on a
redundancyfactor.
4.3 ReliabilityoftheTurbocharger
The turbocharger assembly consists of air filter,
blower casing, turbine casing, rotor and
bearings,(Schieman 1992‐1996). Modern
turbochargers are manufactured with sleeve type
bearingswhichhave
veryhighoperatingliferanging
up to 50,000 running hours, (SE)Hence while
determining the reliability of the turbocharger we
need to look into the phase 3 of the bath tub curve
wherethe endof life wearout couldbe considered,
rather than the phase 1 or phase 2 of the
bath tub
curve. In the phase 3 the reliability of the
Turbocharger may be computed using Weibull
distribution, (Dhillon, 2002).The Reliability of the
Turbochargercouldbeexpressedasafunctionoftime
t.
t
Airclr
Rte
andthehazardratefunctionwillbegivenby
1
θ
Airclr
t
t
whereθisthescaleparameterthatinfluenceboththe
meanandthespreadordispersionofthedistribution
andisthecharacteristiclifeandhasunitstothoseof
time t,in this case hours,θ> 0.βisreferred as the
shapeparameterandβ>0.TheWeibullhazardrate
function can be increasing or decreasing depending
on the value ofβ. If
β1 ,
Turbo
t
is constant and
equal to
1
,the distribution being identical to the
exponential.
3.3 ReliabilityoftheAircooler
Theaircoolerplaysaveryvitalroleinthescavenging
system. The high temperature air discharged by the
turbochargerneedstobe cooledbeforesendingit to
the engine cylinders. These air coolers are generally
seawater
cooled,theseawaterbeingpassedthrough
bronzealloytubes,bymeansofatwopasscooling
arrangement, to provide effective cooling of the
chargeair.Theairflowwillbeonepassthroughthe
aluminiumfinswhicharesolderedtothebrassalloy
tubes,toavoidexcessivepressuredrop.
Considering
the reliability of the air cooler we should again
consider the aging factor, hence we will be
considering the phase 3 of the bath tub curve. On
similar lines to detecting reliability of the
turbocharger, thereliability of theair cooler may be
computed using Weibull distribution, (Kiriya 2001).
The
reliabilityoftheaircoolercouldbeexpressedasa
functionoftimet.
t
Airclr
Rte
andthehazardratefunctionwillbegivenby
1
θ
Airclr
t
t
where θ is the scale parameter that influence both
the mean and the spread or dispersion of the
distribution and is the characteristic life and has
275
units to those of time t, in this case hours, θ > 0. β
is referred as the shape parameter and β > 0.
3.4 ReliabilityoftheScavengeairsystem
The turbocharger and air cooler being in a serial
configuration,bothneeds tofunctionforthescavenge
air system to function. Both the components are
critical and if either one of them fails the scavenge
system will fail. The combined Weibull system
reliabilitycan
becomputedasbelow
/
1,2
i
i
t
scavenege air
i
Re
(3)
where i=1 is the Turbocharger and i= 2 is the Air
cooler.
4 RELIABILITYOFTHEMAINPROPULSION
ENGINE
We have determined the Reliability of the Fuel oil
systembyMarkovanalysis,LubricatingOilsystemby
Markov analysis which are both modelled using
constant failure rate principle, and also determined
the Reliability of Scavenge air system as a time
dependent failure model, we
are now positioned to
determine the Reliability of the Main propulsion
engineasfollows:
1,2,3, MainEngine i I
RR
(4)
i=1 is the fuel oil system from Equation1 (Markov
modelling)
i=2 is the lubricating oil system from Equation2
(Markovmodelling)
i=3 is the scavenge air system from Equation3
(Weibullmodelling)
5 IMPROVINGRELIABILITY
Reliability of the main engine can be improved by
improvingtheindividualsystemreliabilityasseen
in
theaboveEquation4above,Forinstanceinthecase
of the scavenge air system, a modern high
performanceturbochargerwillimprovethereliability
of the turbocharger.This cost for improvement of
reliabilityneedtobeassessedandthecostbenefitfor
the incremental reliability to be determined. If the
original
value of reliability
O
R
at cost x is
improved to Reliability
I
R
at cost y, then the
incremntal reliability for the differential cost
I
O
R
R
y
x
shouldbe compared with the base relaibilityto cost
ratiowhichinthiscaseis
O
R
x
.
Forcostbenefit
I
OO
R
RR
y
xx
.
This could be a feasible proposition for some
components, but not for all components. Also we
couldlookatanappropriatemaintenanceprogramto
strike the right balance between reliability required
andthecostpenaltylikelytobeincurred.Iwouldlike
to draw attention to theair cooler in
the scaveneg
air system as an example. All modern air coolers
manufacturedbymajorenginemanufacturereshavea
cleaning in place system incorporatedfor
maintenance of air coolers, which involves no
dismantling of the air cooler whilst carrying out
maintenance,(Dhillon 2002). Accordingly the
maintenanceintervalsforaircoolerscouldbe
shorter,
atthesametimeincreasingthemaintenanceintervals
ofturbochargersandstilprovideamoreeffcientand
reliablemainengine.
6 CASESTUDYOFTURBOCHARGERFAILURE
ONAMERCHANTVESSEL
We shall study an intertsing case study of a main
engine turbocahrger failure of which could lead to
disastrous consequenes,
resulting in stoppage of a
mainengineatsea.Turbochargersplayagreatrolein
the opeartionof the main engine,Hence reliaility of
themainengineisdepenedenttoalargeextentonthe
reliability of turbochargers,(Heim 2002). An
importantfactortobetakenintoconsiderationisthe
matching
oftheturbocahrgertothemainpropulssion
engine, (Hountalas2000). Since the mainpropulsion
slow speed engine and the turbochargers are
normally manufactured by two different
manufacturers,whoareexpertintheirownfield,itis
inevitable that there could be an issue on the
conceptual thinking between the two parties.
However any matching discrepancies need to be
sortedoutduringtheship’ssea trial.Any mismatch
could be corrcetd by replacment of the diffuser or
nozzlering,(Kimetal.2009)
Thecasestudyreferstoavesseloutatseaandthe
inestigation as carried out by the Australian
Transport
safetyBureau,(AustralianTransportSafety
Bureau 2006,). The investigation refers to a bulk
carrierpoweredbyalargetwosstrokeenginewitha
ratedpowerof6400kW,propellingat14.5knts.The
vessel suffered srious damge due to failue of the
turbocharger, on two ocasions within a span of less
than5months.Theexactcauseofthedamagewasnot
available,butin bothth casesthe failurefollowed a
largeenginescavengefire.Thefigure9belowshow
theextentoftheseriousdamagetotheturbocharger
rotor,resultinginimmobilisationofthemainengine,
(Takashi194).
Figure9. Damaged turbocharer rotor shaft ( Courtesy :
ATSBInvestgation186and191)
276
7 CONCLUSION
In this paper we have looked at methods of
determining the reliability of three subsystems of a
vessel’s main engine which includes the fuel oil
system, lubricating oi system and the scavenge air
system. The fuel oil and lubricating oil sytem was
modeleld by Markov analsis and the
scaveneg air
system was analysed using Weilbull distribution
which is time dependent failure model. An attempt
has been made to make reliability assessment of
vessel’s main engine by combining Markov analysis
integratedwithtimedependentfailures.Wehavealso
discussedtheincreamentalreliabilitytoincreamental
costratioforthemainenginewhichshouldalwaysbe
greater than the original reliability to original cost
ratio, for cost benefits in the long run. Finally we
looked at some examples of effectively altering the
maintenanceintervalsofcertainsystemcomponents,
whwerbytheoverallreliabilityofthesystemcouldbe
improved.
Acasestudyofturbochargerfailureonamercahnt
vessel was studied and it was shown that the
turbochargerfailurecanhave amajor impactonthe
mainengineoperation,leadingtoimobilisationofthe
mainengine.Hencematchingoftheturbochargerand
main engine is very critical for safe and reliable
operationofthemainengine.
ACKNOWLDAGEMENTS
Authors thanfully acknladge the support provided
National Centre for Port and Shipping (NCPS),
AustralianMaritimeCollege,UniversityofTasmania
toenablethisstudy.
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