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1 INTRODUCTIONTOPLANNED
MAINTENANCE
Commercialshippinginthemodernworldishighly
competitive, which coupled with high crewing and
fuelcosts,leadstohighoperationalcosts.Oneofthe
paramountfactorinvolvedinvessel operation is the
Maintenancecostandthereisadireneedtokeepthis
cost
to a minimum. Fortunately the earlier policy of
repaironlymaintenanceincommercialshippinghas
beendoneawaywith,andwasreplacedbythepolicy
of preventive maintenance. Planned Maintenance
System was introduced by ship management
companiesintheearly90’s.
2 WHYCONDITIONBASEDMAINTENANCE
PlannedMaintenanceofferedbenefits
overtherepair
only policy, but has its own demerits. Many a time
machinery equipment is opened up for routine
maintenanceafteraspecifiedtimeinterval,
irrespectiveoftheneed. This could lead to potential
failures,whichisexplainedbythefactthatpreventive
Using Reliability Block Diagrams and Fault Tree
circuits, to develop a Condition Based Maintenance
Model for a Vessel’s Main Propulsion System and
Related Subsystems
M.Anantharaman
A
ustralianMaritimeCollege(Utas),Tasmania,Australia
ABSTRACT: Merchant shipping has undergone a great transformation over the past three decades. The
shipping market is highly competitive, which coupled with high crewing and fuel costs, leads to high
operationalcosts.OneoftheparamountfactorinvolvedinvesseloperationistheMaintenance
costandthereis
a dire need to keep this cost to a minimum. Fortunately the earlier policy of repair only maintenance in
commercialshipping has beendone away with,and was replacedby thepolicyof preventive maintenance.
Planned Maintenance System was introduced by ship management companies in the early
90’s. Planned
Maintenanceofferedbenefitsover the repair only policy, buthasitsowndemerits. Many a timemachinery
equipmentisopenedupforroutinemaintenanceafteraspecifiedtimeinterval,irrespectiveoftheneed.This
could lead to potential failures, which is explained by the fact that preventive maintenance
resulted in
meddlingofawellsetpieceofmachineryequipment,leadingtoitssubsequentfailure.Thisiswhere Condition
based maintenance or CBM steps into prominence. CBM monitors the health of the machinery equipment,
analysestheconditionandhelpsyouindecisionmaking. The Main Propulsion system forms theheart
ofa
vesselandweneedtoensureitsreliability,togetherwiththereliabilityofitsassociatedsubsystems.Theentire
systemcanberepresentedbyreliabilityblockdiagrams,toshowtheinterdependence ofvariouscomponents
comprisingthesystem.ThishelpsinthedecisionmakingprocessofCBMwherebyship’s
engineermaydecide
tostoptherunningmachineryequipment, openandoverhaulthesame,elsepostponetheoverhaulforalater
safedate.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 7
Number 3
September 2013
DOI:10.12716/1001.07.03.13
410
maintenanceresultedinmeddlingofawellsetpiece
ofmachineryequipment,leadingtoits(Bhattacharjya
and Deleris, 2012)subsequent failure. The author
wouldliketociteanincidentexperiencedduring his
vastsailingcareer.Ageneralcargovesselwasonher
wayfromIndiatoEurope.Thispassageinvolvesthe
SuezCanaltransitwherelargenumbersofvesselsgo
on a convoy. A few days before transiting the Suez
canaltheNo.1Steeringgearmotorwasopenedupfor
routine overhaul, as specified in the Planned
MaintenanceScheduleforthevessel.Themotorwas
overhauledandreassembled.Thevesselthen
entered
theconvoyandallwent off well for an hour,under
theguidanceofthe Suez Canalpilotwhotravelson
board the ship during the transit. The pilot then
ordered a helm movement and the vessel failed to
steerasrequired.Thereasonwasoverloadtrippingof
the overhauled
motor, and investigations revealed
errorsduringthereassemblyofthemotor.Thevessel
was then tied alongside the canal with the help of
tugs, the motor had to be reopened again, and new
bearingswere fitted.Motor tried out and finally the
vesselmanagedtotransittheSuezCanal,though
she
hadtobelastontheconvoy.resultinginconsiderable
losses in terms of thousands of dollars to the
company. This is where Condition Based
Maintenance or CBM steps into prominence. CBM
monitors the health of the machinery equipment,
analyses the condition and helps you in decision
making.Accordinglya
ship’sengineermaydecideto
stop the running machinery equipment, open and
overhaul the same, else postpone the overhaul for a
latersafedate.
3 ISMCODEANDMAINTENANCE
When it comes to operation of ships, all shipping
companies need to abide by the ISM Code which is
the International
Safety Management Code, the
purpose of this Code is to provide an international
standard for the safe management and operation of
ships and for pollution prevention. The above
research will also address section 10.3 of the code
which states that, ‘The Company should identify
equipment and technical systems the sudden
operational
failureofwhichmayresultinhazardous
situations. The safety management system should
provideforspecificmea s ures aimedatpromotingthe
reliability of such equipment or systems. These
measuresshouldincludetheregulartestingofstand
byarrangementsandequipmentortechnicalsystems
thatarenotincontinuoususe.’(ISM,2002)
4 DIAGNOSISANDPROGNOSIS
TheconceptofCBMforship’smachineryisstillinits
infancy. Reproducing a recent finding which says
‘However,accordingto class records,onlyabout2%
of thw world fleet is operating using
CBM’.(MER,2012).Effectiveapplication of CBM
techniques will result in large savings to the vessel
owner
/operator.Aship’smachineryspaceisalarge
mainpropulsionsystemwithseveralsubsystems.All
thesesystemshaveafairlyhighdegreeofcorrelation;
failureofanyonesubsystemcouldresultinstoppage
of the vessel, which is a highly undesirable event.
CBMisatwosidedcoinwith
diagnosisononeside
and prognosis being the other side. For an efficient
vesseloperationboththesidesofthecoinarevital.
Prognosis is an important element of the CBM
program as it deals with the predication of failure
faults.Theaboveresearchshouldbeuseful topredict
the occurrence
and timing of a failure in a single
subsystem (example ship’s main propulsion and
power generation system) or in several different
subsystems (example ship’s main propulsion and
powergenerationsystemandcontrolairsystem)
5 GROSSMAINTENANCEDEFECIENCIES
A few instances of major shipping disasters on
accountofgrossmaintenance
deficiencieshave been
highlighted below. This information was gathered
fromleadingreputableMarineAccidentInvestigation
bodiesinthecommercialshippingworld.
‘The scavenge space inspection after the fire in
numberthreeunit,shortlyafterthefirstturbocharger
failure,apparentlyrevealeda high level ofscavenge
fouling.Similarlytheconditionofthe
scavengespaces
afterthesecondturbochargerfailurewaspooraround
number two cylinder, albeit as a result of piston
coolingoilleakingfromthedefectiveOring.Whether
or not the condition of the scavenges led to a fire,
which in turn caused the turbocharger failures,
cannotbeconcludedwith
anycertaintyhowevertheir
conditiondoesindicatethatthevessel’smainengine
maintenance regime in this respect may have been
deficient.’(ATSB,2006).
Figure1.Conditionofscavengeports onCylinderno.1of
MainpropulsionEngineSourcebasedonATSB(2006)
Figure2.Conditionofscavengeports onCylinderno.2of
MainpropulsionEngineSourcebasedonATSB(2006)