415
1
INTRODUCTION
Theshipʹslifecycleisacomplexprocessconsistingof
different stages and processes similar to those in
humanlife.Themainmilestonesintheshipʹslifecycle
can be highlighted in design, building, exploitation,
and recycling. The exploitation period continues the
longest. Normally, this period is about
20–25 years,
but nowadays, taking into account economic
conditions, COVID‐19, wars, etc., the exploitation
period is increased by up to 30 years. Through the
shipʹslifecycle,sheissubjectedtodifferentprocesses,
forces,andloads.Thecorrosionprocessoftheshipʹs
hull is unavoidable. The corrosion
process reduces
shipgirderstrengthandworkingcapacity.Therefore,
the study of its impact on ship characteristics is
important.
Acomputersimulationofultimategirderstrength
degradation by corrosion and fatigue is done in [3].
There is a developed system for solving practical
corrosion and fatigue problems. The system was
tested
on a double bottom tanker. Results shown
indicatefatiguecrackandcorrosionrepair.
Thereviewoftheultimatestrengthassessmentof
aginganddamagedshipstructuresispresentedin[7].
Inthiswork,attentionispaidtotheultimatestrength
ofplates,girders,andtheentireshipʹshullsubjected
to corrosion, fatigue, cracking, and damage. The
ultimate load capacity of the shipʹs hull is also
studied.
Theauthorsof[6]studiedpittingcorrosionwaste
assessment on the hull girder ultimate strength by
incremental‐iterative methods. They compared the
hullgirderʹsultimatestrengthcombinedwithpitting
corrosion and relevant
values from the ruleʹs net
scantling. The final result is that the incremental‐
iterativemethodisusefulfor new vessel designand
monitoringofagingships.Anincrementallyiterative
method for determining ultimate strength is used in
[8].
Insomecases,corrosionwasteappearsasaresult
oferosion.
Thiseffectiscommonundersuctionpipes
Corrosion Waste Impact on Ship Girder Strength
Through Life Cycle
Y.Denev
TechnicalUniversityofVarna,Varna,Bulgaria
ABSTRACT: The paper deals with analyzing corrosion waste impact on ship girder strength through the life cycle.
Evaluationofcorrosionimpactonshipstrengthis importantbecause in someloading cases theships areinoverloaded
condition.Itisassumedthatcorrosionwearvariesfrom0to60%.Reductionoftheirtransversestrengthincombinationof
bad weather or suddenstormsleadstocargo lost,shipdamage and loss ofhuman lives.Strength andplating elements
bendingcapacityareevaluatedbyshipmidsectionmodeldevelopmentandcomparedwiththesameforanewbuilding
ship. Helped by the relation between permissible and actual bending moment, plate bending efficiency is evaluated.
Ultimateshipgirderstrengthincaseswithcorrosionwasteofmorethan30%isevaluatedtoo.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 18
Number 2
June 2024
DOI:10.12716/1001.18.02.20
416
in ballast tanks after cavitation impact. The pitting
corrosion is clearly expressed and affects bottom
platesinthevicinity.Thisproblemisstudiedin[4].
Modern methodology for 3D model strength
analysis of corrosion impact on ship hull
characteristics is presented in [2]. In this work,
authors identify the corrosion
impact on the VLCC
hull.Thestrengthanalysiswasdoneoverthreecargo
holds. They investigated that after 15 years after
corrosion,wasteplatethicknessisreducedabout10%.
Based on problem study so far, it is necessary to
analyze ship girder strength bending and ultimate
strengththroughstudies.
Corrosion
wasteimpactstheweldjointsofashipʹs
hull. In some cases, the corrosion in welding seams
reaches 1‐2mm per year. The main welding seam
corrosion is near to the heat‐affected zone and the
fusion line [5]. The welding seam and joint
degradation and wearness leads to the
reduction of
ship girder integrity and caused the loss of ships,
human lives and cargoes. Corrosion impact on
welding joints turns them into a stress concentrator.
The stress concentrator is the primary crack and
deformation source which impacts hull girder
strengthnegatively.
2
SHIPMIDSHIPMODELDESCRIPTION
The analysis of a shipʹs hull corrosion waste during
herlifecycleisdonebymodeldevelopmentofamid
ship section of a 6000 t DW general cargo ship, one
box hold with BV software, MARS 2000. The shipʹs
maindimensionsareshownin
table1.
Table1.Modelmaindimension
________________________________________________
ItemDimension
________________________________________________
Length,m106.60
Breadth,m16.00
Depth,m9.17
Blockcoefficient0.721
Sailingareaunrestricted
________________________________________________
Midshiptopologyisshownonfig.1.
Figure1.Midshipsectiontopology
The ship has a double bottom and aninner side.
Double bottom height is 1450mm, the inner side
breath is 1500mm., cargo hold is designed for
containercarry.Thetopologyofthemidshipsection
isdesignedforproductionanddesignedforrepair.
Inthedesignstagesofaship,structural
elements
are envisaged for corrosion addition. Corrosion, in
additiontoeverystructuralelement,isdifferent.The
values vary from 1.00 for elements exposed in
seawater to 2.00 for the hopper wells for dredging
ships[1].
3
ANALYZEOFCORROSIONWASTEONSHIP
HULLPLATING
Corrosion waste is simulated by reducing the shell
plating, double bottom, inner side and main deck
plating thickness from different percent from 0after
shipdeliverytoshipownerto60%aftermorethan25
yearsofexploitation.
Helped by developing models, the
loading
capacity of hull girder strengthand its elementsare
evaluated. Changing of loading efficiency of every
plate element from mid ship section is evaluated by
permissible and actualσ
x. The gross thickness of a
plate element is a study object because they
participate in hull girder strength evaluation. The
platenamesareshownintable2.
Table2.Plateslocationinshiphull
________________________________________________
Item Location
________________________________________________
s1 horizontalkeel
s6 bilgeplating
s9 shellplatingat3500mmab.BL
s10 shellplatingat5000mmab.BL
s11 shellplatingat6500mmab.BL
s12 shearstrake
s13 doublebottomplating
s20 innersideat4500mmab.BL
s21 innersideat6000mmab.BL
s22 innerside
at7500mmab.BL
________________________________________________
Intheanalysis,itistracedwhatishappeningwith
to shipʹs hull subjected to corrosion waste. It is
assumed that the corrosion waste in early life cycle
periodsisnotsointensive,whileafter25yearsoldits
actionsareincreased.Tostudytheprocess,itisused
the
relation between permissibleσx and the actual
same.
permissible
r
actual
x
x
Thepermissibleσxis175N/mm
2
forallsections.It
isselectedbythematerialpropertiesoftheshipʹssteel
hull. For all sections, St 2530 is used, with Young
modulus 206000.00 N/mm
2
which is most used in
shipbuilding and ship repair. The actualσ
x depends
on plate location in the ship hull, thickness and
applied load in areas. Bending moments for the
originalsectionareshownontable3.
417
Table3.Bendingmoments
________________________________________________
Hogging,kNm Sagging,kNm
________________________________________________
SWBM,d 163886‐135563
VWBM,d 201019‐229369
________________________________________________
SWBM,d‐designstillwaterbendingmoment,kNm;
VWBM,d‐designverticalbendingmoment,kNm;
Horizontal keel on the bottom, bilge and side
plating up to 5000mm ab. base line are subjected
mainlytohydrostaticpressure,whileshearstrakeof
dynamicforcesarepermanent.Doublebottomplating
and inner side plating are subjected to cargo and
ballast water forces. Intensive corrosion of waste on
the
inner side plates is a danger to cargo water
tightness.Whenthewingballasttankisfullandthe
pumpscontinuetopumpinballast,innersideplates
are subjected to over pressure andlead to the crack
appearance. The same situation with double bottom
plating with intensive corrosion waste. In
fig.2, the
relationshipisshownbetweenthepermissibleandthe
actualσ
xoftheshipʹshullgirder.
Figure2. Relation between permissible and actual σx via
corrosionwaste
Thevalueoftherelationbetweenpermissibleand
actualσ
xvariesfrom1.00toabout4.00.Themaximum
value is 10% corrosion waste in all cases. Clearly
sharplyoutlinedthreeplategroups.Firstisthedouble
bottom, inner side and shell plating group with a
relation between the permissible and actualσ
x
between2.80and4.20. This plating groupisnear to
the base line and the neutral axe. The neutral axe
positionisat3.7mabovebaseline.Thesecondplating
group is double bottom, horizontal keel and shell
platingupto3500mmabovebaseline.Inthisgroup,
σ
ris inthe rangefrom1.50to2.5.Interestinginthis
groupisthattheelementsthatarenearandfaraway
form a neutral axe and base line. The value of the
originalconditionoftheshiphull,withoutcorrosion
wastesinshellplatingat5000mmabovebaseline,
is
higherthanotherelements.Thisisbecauseinthisare
is located void space, the ballast tank is not up to
main deck. The same is the situation wit h the same
plate at 6500mm above base line. The third plating
groupconsistsofshellplatingat6500mmabovebase
line
and the inner side at 7500mm above base line.
Theirσ
rvaryfrom1.00to1.50.
Continuingintime,corrosionwastereducesplate
thicknesstovalueswhicharenecessarytobereplaced
withanew onewiththicknessequaltoaoriginalor
greater. Corrosion waste at plate elements is highly
expressedinplatingelementsincomparisontoprofile
elementslike
bulbprofile,L‐profile,etc.Reduceplate
thickness,reducemechanicalpropertiesofhullgirder
strength, and she has become a danger for the
environment from one side and for crew and cargo
fromtheotherside.
Figure3.HGSbendingviaplatethickness
Hull girder strength bending of gross plate
thicknessisshowninfig.3.Theresultsareshownthat
intheoriginalplatethickness,whichpointislocated
ontherightinfig.3,isthenominalvalueofbending.
After that, a lead point with a lower value a which
correspondsto10%
corrosionwaste.Fromthispoint,
upwardbendingvaluesincreaseyourvalues.
In cases with maximum plate waste hull girder
strengthbendingvalueshavethehighestvalues.This
is an argument for section modulus and ultimate
strength check‐ out. Section modulus on deck with
corrosion waste is equal and more than
50% is
reduced by about 6‐7%. The ultimate strength in
navigation conditionin a sagging condition is about
13% higher than without corrosion waste, fig.4 and
fig.5.
Figure4.Ultimateshipstrengthat50%corrosionwaste
418
Figure5.Ultimatestrengthat60%corrosionwaste
4 CONCLUSIONS
The paper analyzes corrosion waste on ship hull
plating during its life cycle. Based on developed
models, corrosion waste is simulated with a
percentagevaluefrom0to60%.Corrosionimpacton
plateelementbendingisevaluatedbytherelationship
betweenpermissibleandactualbendingrelationships.
Ultimatehullgirderstrength
isevaluatedtoo.
Asforthebendingcapacityofplateelementsina
shipʹs hull, it has been investigated that with
corrosion impact, increasing bending capacity is
reduced, but in 10% corrosion, waste bending
capacity is heightened than without corrosion, in
originalhullcondition.Intheanalyzedcase,the
value
ofσ
risfrom1.00to4.50,basedonthefactthatthere
arethreegroupsofplateelements.
Thesituationwithhullgirderbendingisdifferent.
At 10%, corrosion waste obtained a lower value of
gross thickness bending in compared than without
corrosion waste. After that, upward bending values
extremelyincreased
yourvalues.
Corrosion waste impact on section modulus and
ultimategirderstrength.Intensivecorrosionwasteof
morethan25%reducessection modulus on deck by
about6% andultimate strength is about13% higher
thannormalhullcondition.
Futureworkofthestudyistoevaluatecorrosion
impact on profile elements
subjected to passive
corrosion and bottom plating strength in docking
periodswithfloatingdocksandcradles.
ACKNOWLEDGEMENT
The present study was developed according National
project, Young scientists and Postdoctoral”, financed by
MinistryofEducation,RepublicofBulgaria.
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