243
1 INTRODUCTION
Thereductionoffuelcosthasalwaysbeenoneofthe
key strategic business goals for ship owners and
operators.Inthecurrentclimateofhighoilprices,the
reduction of fuel costs becomes essential; and
furthermore a variety of recent legislations require
ownersandoperatorstomovetowa
rdsthereduction
ofemissionsfromshipsofSOx,NOxandCO.
Hence the pressure on designers to achieve both
reduced fuel costs and reduced emissions by
optimising the hull and propeller has never been
higher.Inparalleltotheperformanceimprovementof
newbuiltvessels,therehasbeengreatinterestinthe
potentia
l to enhance the performance of existing
vesselsthroughretrofitofdevicestothehull.Awide
rangeofconceptshasbeenproposed,manyofwhich
involve modification or control of the flow in the
vicinity of the propeller.The interest in these
devicesariseswithincreasingoilprice.Thesedevices
are commonly called “energy saving devices (ESD)”
and sometimes “
retrofitting technologies” although
manycanbeconsideredfornewdesignsaswell.
Thesavingslookveryattractivetoshipoperators,
for instance a saving of 6‐7% from installation of a
wakeequalisingduct(Schneekluth,1986,Schneekluth
Numeric Wake Equalizing Duct Geometry
Optimization for a Given Ship
G.Martinas&O.S.Cupsa
M
aritimeUniversityofConstanta,Romania
ABSTRACT:Thereductionoffuelcosthasalwaysbeenoneofthekeystrategicbusinessgoalsforshipowners
and operators. In the current climate of high oil prices, the reduction of fuel costs becomes essential; and
furthermoreavarietyofrecentlegislationsrequire owners and operators to move towa
rds the reductionof
emissionsfromshipsofSOx,NOxandCO.
Hencethepressureondesignerstoachievebothreducedfuelcostsandreducedemissionsbyoptimisingthe
hullandpropellerhas never beenhigher.Inparallel to the performance improvement of new built vessels,
therehasbeengreatint
erestinthepotentialtoenhancetheperformanceofexistingvesselsthroughretrofitof
devicestothehull.InanycaseforinstancetheWEDdevicemustbecustomizedtofittotheafterbodyofthe
shipintermsofperformingitssupposedfunction. TheDesigneristhereforepla
cedinthe frontofmultiple
geometricsolutionsfrombetweenhehastomakeachoice.ThispaperisintendedtohelptheDesignerstohave
arationalchoosingapproachbyinvolvingthenumericoptimizationofthegeometryoftheWEDinorderto
select the best fitted WED to perform the best in order to achieve some predefined parameters. In thi
s
paperworkagivengeometryofaWEDdeviceistakenandviaDesignOptimizationthegeometryoftheduct
wasrefinedsothatbetterresultsareachievedwithasmallerandmorecompactWED.Indoingso,theDesigner
is assisted by numeric op
timization methods to choose from only three final candidates instead of several
thousandsinordertoprovidethebestfittedWEDgeometryforagivenshipafterbody.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 9
Number 2
June 2015
DOI:10.12716/1001.09.02.12
244
(WED) and Bertram, 1998) or 7‐9% from a
combination of wake equalising duct and pre‐swirl
fins (Mewis, 2008, Mewis, 2009). In general, the
negative aspects of the devices include the
considerablecostofinstallation,andalsothereported
reluctanceofmanufacturerstoguaranteetheclaimed
savings.
The claims
may not give details as to the
conditions under which the savings have been
achieved and/or how the savings have been
calculated and/or measured. Furthermore, the
magnitude of the savings may well be within the
rangeofuncertaintiesandmeasurementerrorsonthe
full‐scale vessel. Consequently, cautious operators
may
wellbeskepticalaboutthevalidityofthefigures
being presented to the market, and it is absolutely
reasonable and necessary for a buyer to verify
independently the amount of savings before any
investmentonanESDorESDs.
InanycaseforinstancetheWED devicemust be
customizedto
fittotheafterbodyoftheshipinterms
ofperformingitssupposedfunction.TheDesigneris
therefore placed in the front of multiple geometric
solutionsfrombetweenhehastomakeachoice.This
paper is intended to help the Designers to have a
rationalchoosingapproachbyinvolving
thenumeric
optimizationofthegeometryoftheWEDinorderto
selectthebestfittedWEDtoperformthebestinorder
toachievesomepredefinedparameters.
2 CADANDFINITEVOLUMEANALISYS(FVA)
MODELOFTHESHIP
The goal of this paper is to calculate via software
Ansys 13
TM
the best geometric solution for a WED
device.ItisknownthatapoordesignoftheWEDis
notonlyimprovingtheoverallefficiencyofthevessel
butmayhaveanadverseimpactfailingtoachieveits
purpose.
The model has as departure point a real
portcontainer as seen below,
with the following
parameters(Fig.1):
LengthL‐[m]‐173
BreadthB‐[m]‐25
DraughtT‐[m]‐9.50
DiameterD‐[m]‐5
NumberofbladesZ‐6
PropellerRPM‐120
AverageSpeed‐16knots(7m/s)
Figure1.Port‐Container
Inordertohaveastartingpointforthesimulation,
first of all the afterbody was firstly CAD generated
withtheWEDdeviceattached,andalltheparameters
for fluid flow were calculated accordingly.From
thatpoint,theAnsysDesignExplorerwasinvolvedin
ordertooptimizethegeometry(Fig.2).
Figure2.CADgeometrywithWED
Inordertoprovidemoredetailsonthegeometry
of starting model of the WED device and the
optimization input parameters, the below figure is
shown,withdimensionsin[mm](Fig.3):
Figure3.WEDdevicegeometry
5 input geometric parameters were defined as
follows(Table1):
Table1.Inputgeometricparameters
_______________________________________________
Name TypeLowerlimit Upperlimit
_______________________________________________
P1 Angle Continuos 14[grade] 22[grade]
Minimize
P2 DuctLength Continuos 1980[mm] 2420[mm]
Minimize
P3 Smallradius Continuos 1600[mm] 2250[mm]
Minimize
P4 BiggerconeContinuos 2250[mm] 2750[mm]
radius
Minimize
P5 Distancefromthepropeller
Continuos 1800[mm] 2200[mm]
Minimize
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