,.,.-

--.

SSC-348

CORROSION EXPERIENCE .DATA REQUIREMENTS

Thiscklmmtlmkcilajpvalforpublicml- ad salqiu

distciilicmiaUnlimitul

SHIP STRUCTURE COMMITTEE

1991

The SHIP STRUCTURE COMMllTEE is constitutedto prosecutea rsearch pqram to improvethe hullWucturee of shipsand othermarinestructuresby en extensionof knowtedgepertainingto design,matetials,and methodsof construction.

RADM J. D. Sims, USCG, (Chairman)Chief, Offioeof Marine Safety, security

and EnvironmentalProtectionU.S. Coast Guard

Mr. Alexandw MalaldmffDirector,Stwcturat Integrity

Su roup(SEA 55Y)YNava Sea SystemsCommaruf

Dr. DonafdLiuSenior~~e President~erican Bureauof Shipping

Mr. H. T. Hailer_ie Administratorfor Ship-

buildingand Ship OperationsMaritimeAdministration

Mr. ~omas W. AllenEngineeringOfiicer (N7)MMtatySealiit Command

CDR Miihael K Parrnelae,USCG,Secretary,Ship StructureCommitteeU.S. Coast Guard

CONTRACTING OFFICFR T=HNIP. REPRFSENTATWS

Mr. William J. SikierkaSEA 55Y3Naval Sea SystemsCommand

Mr. Greg D. WedsSEA56Y3Naval Sea SystemsCommand

SHIP ST17JCTURFWRCWMIIJEEThe SHIP STRUCTURE SUBCOMMlllEE acts for the Ship StructureCommitteeon technicalmattersbyprovidingtechnicalcoordinationfor determinatingthe goalsand objectivesof the programand byevaluatingand interpretingthe resultsin termsof structuraldesign,corrstrudlon,and oparatiin.

AMERICAN OUREAU OF SHIPPING

Mr. Stephen G. Arntson(Chairman)Mr. John F. ConIonDr. John S. E@noerMr. Glenn M. Aeha

Mr. Altwt J. AttermeyerMr. Michael W. ToumaMr. JefferyE, Baaoh

NAVALsEhSYSTEMS COMMANDMr. Rokt A SielskiMr. CharlesL NullMr. W. ~omae PaokardMr. Atlen H. Engle

CAPT T. E. ThompsonCAPT DonaldS. JensenCDR Mark E. tdoli

Mr. FrederickseiboldMr. NormanO. HammerMr. Chao H, LinDr. Walter M, M=l=n

SHIP SIEL!CTURF SURCQIMMITTFF I WSON MFldE!EKS

US. COMTE!JMD AmFMyLT BruceMuaWn

u.s.MERCHANT MARINE ACADEMY

Dr. C, B. Kim

u.s.NAVAIACADEMY

Dr. RamsWarBhattacharyya

Dr. W. R, Porter

WELDING RESEARCH COUNCIL

Dr.MartinPrager

-

Mr. AlexanderB. Stavovy

f4AT10W ACAD SC FN EIEMYOF I cs -S

Mr. Stanley G. Stii

SOCIEIYOF Iw VAL ARCHITECTS ANDMARINE ENGINEERS -

Dr. VMlliamSandbarg

mERKAN IRON AND STEEL INSTITUTE

Mr. Akrwtder D. Wilson.-

Membr Agencies:

UnitedStates@as? GuaudNaval &a Systems bmmand

Manlime AdministrationAmericanBureau of Sh@ping

MilitaySealifl~tnmand

ShipStructure

CommitteeAn InteragencyAdviemyCommittee

DedicatedtotheImprovementofMarineStniwtures

January 31, 1991

Address Corre~ndence to:

Secretary, ShipStructure CommilteeU.S.CoastGuard (G-MTH)2100 Semnd StreetS.W.Washington, D.C. 20593-0001PH: (202)267-03FAX (202)257-0025

SSC-348SR-1306

CORROSION EXPERIENCE DATA REQUIREMENTS

The detrimental effect of corrosion on marine structures is wellknown. Assessing the extent of corrosion damage and predictingcorrosion rates, however, can be difficult. The purpose of thisproject was to develop a corrosion survey methodology that couldbe used in assessing vessel structures. This report contains themethodology and data collection requirements that could be usedto assess corrosion rates, damage, and margins.

G!/%-Rear Admiral, U.S. Coast Guard

Chairman, Ship Structure Committee

TmchnicalReptirt DocumentationPage1. R-part w. 2. Government Acc-ssion No. 3. Rocipionts C*telog No.

SSC-348L Titlo ond $ubtitlo 5. Report Dot.

1 ~88 ICORROSION~PERIENCE DATA REQUIREMENTS 6. Pmrfoming Orgnnj;otion Cod.

I

8. Por{ormino Oreonization Ropott No.?. Author/s)

Karl A. $tambaugh, John C. Knecht SR-1306

y.Prtfoming Or#mizotion Namomd Address , 10. Work Unit No. (TRAIS)Giannotti & Associates, Inc.703 Giddings Avenue, Suite U-3 11.Contract or Gront No.Annapolis, Maryland 21401 DTCG23-86-C-20064

13.Typ*of Roportond Pm,iod Cow.r.d1

It. Sponsoring AgoncYNomc mdAddr*x8 Technical ReportU.S. Coast Guard 10/86 - 5/88Office of Merchant Marine SafetyWashington, DC 20593 Ii.Sponsoring Agency Ctid-

G-M15. Supplmm~ntory Not-g

The USCG acts as the contracting office for the Ship Structure Committee.

16. Abcfroci

A corrosion survey methodology is presented to obtain corrosion data from ships.The corrosion data will be used to develops rational method for assessing cor-rosion margins. The project included a survey of ship operators for corrosiondata to define data collection and analysis requirements to characterize the-corrosion rates that affect strtictural integrity of ships. The techniques usedto predict corrosion ratesand assess the strength of corroded structure werealso reviewed to determine data requirements. The methodology consists of adata collection procedure ,,with recommended instrumentation. Forms were developedfor documenting the measurements. Finally, an outline of the database was deve-loped that includes an Expert Syst,em -interface for data input, analysis andretrieval. ..

.

17. K-y Wotd~ 18. Di sfiibution Stotem.nt

corrosion This document is available to the U.S.ship structures public through the National Technicalcorrosion data Information Service, Springfield, VA,

22161

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TABLE OF CONTENTS

1.0

2.0

3.0

4.0

5.0

6.0

7.0

INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . .1

BACKGROUND AND PREVIOUS CORROSION SURVEY. . . . . . . . . 2

2.1 Corrosion Surveys of Ship Structure. . . . . . . . . 22.2 Corrosion Survey Results . . . . . . . . . . . . . . 3

CORROSION DATA REQUIREMENTS . . . . . . . . . . . . . . .13

3.1 Corrosion Margin Assessment. . . . . . . . . . . . .133.2 Corrosion Rate Prediction and Survey Techniques. . .163.3 Summary of Data Requirements and Recommended

Corrosion Rate Survey Technique. . . . . . . . . . .22

DATA COLLECTION REQUIREMENTS. . . . . . . . . . . . . . .25

4.1 Types of Corrosion to Survey . . . . . . . . . . . .254.2 Corrosion Locations. . . . . . . . . . . . . . . . .284.3 Correlation Parameters . . . . . . . . . . . . . . .324.4 Sample Size and Accuracy . . . . . . . . . . . . . .344.5 Instrumentation Requirements . . . . . . . . . . . .39

CORROSION SURVEY METHODOLOGY. . . . . . . . . . . . . . .40

5.1 Data Acquisition . . . . . . . . . . . . . . . . . .405.2 Data Recording. . . . . . . . . . . . . . . . . . .545.3 Data Analysis. . . . . . . . . . . . . . . . . .. .705.4 Program Implementation . . . . . . . . . . . . . . .84

CONCLUSIONS AND RECOMMENDATIONS . . . . . . . . . . . . .87

REFERENCES. . . . . . . . . . . . . . l - . . . . . . . .83

i

LIST OF FIGURES

2-1 Gauging Measurement History for the M/V CHESTERPoling Before & After Hull Failure . . . . . . . . . . . 7

2-2 Ultrasonic Measurements of a Transversely FramedContainership with Corrosion Fatigue Cracks. . . . . . . 9

2-3 Weld Detail Used on the Converted ContainershipShowing Corrosion Fatigue Cracks . . . . . . . . . . . .10

2-4 Result of Plate Thickness Gauging and CorrosionDistribution of Girder Web Slot. . . . . . . . . . . . .12

2-5 Number of Cases of Damage by Ship Type 13roken-Down into Types of Damage Between 1976-1984. . . . . . .12

4-1 Simplified Schematic of Uniform Corrosion. . . . . . . .26

4-2 Schematic of Formation of a Pit. . . . . . . . . . . . .27

4-3 Typical Bottom Shell Loss Patterns . . . . . . . . . . .29

4-4 Typical Bottom Structure Defects . . . . . . . . . . . .31

4-5 Normal Distribution Curve. . . . . . . . . . . . . . . .36

5-1 CRT Display Equipment. . . . . . . . . . . . . . . . . .44

5-2 Thickness Measurement Through a Coating Layer. . . . . .45

5-3 Digital Display Equipment. . . . . . . . . . . . . . . .46

5-4 Single Transducer Pulse EchoPattern . . . . . . . . . .49

5-5 Twin Transducer Pulse-Echo Pattern . . . . . . . . . . .50

5-6 Focused Transducer Concept Showing the DivergingBeam from the Point of Focus as it Enters Parallelinto the Steel Plate . . . . . . . . . . . . . . . .. . .51

5-7 Spurior Signal S1 Caused by a Sidelobe . . . . . . . . .52

5-8 Effect of Acoustic Mask. . . . . . . . . . . . . . . . .53

ii

LIST OF FIGURES (continued)

5-9

5-1o

5-11

5-12

5-13

5-14

5-15

5-16

5-17

5-18

5-19

5-20

5-21

5-22

Pitting Intensity Diagrams . . . . . . . . . . . . . . .55

Ship Designations. . . . . . . . . . . . . . . . . . . .53

List of Compartment Designations . . . . . . . . . . . .60

List of Ship Structures. . . . . .. . . . . . . . . . . .61

Ship Information Sheet . . . . . . . . . . . . . . . . .65

Compartment Correlation Parameter Sheet. . . . . . . . .66

PanelDataSheet. . . . . . . . . . . . . . . . . . . .69

Example Panel Gauging Patterns . . . . . . . . . . . . .71

Database Configuration . . . . . . . . . . . . . . . . .72

Data Input Sequence. . . . . . . . . . . . . . . . . . .74

Sample Data Input Sequence . . . . . . . . . . . . . . .76

Data Editing Sequence. . . . . . . . . . . . . . . . . .81

Sample Data Analysis Output. . . . . . . . . . . . . . .83

Data Analysis Sequence . . . . . . . . . . . . . . . . .85

LIST OF TABLES

2-1 Corrosion Rates. . . . . . . . . . . . . . . . . . . . .4

3-1 Correlation of Corrosion Data Requirements andFailure Modes Relevant to Structural Integrity . . . . .23

iii

1.0 INTRODUCTION

Ship structure design involves a thorough application ofscientific and engineering approaches. In many instances wherethere are no rational theories or methods, it becomes necessaryto develop this knowledge as an extrapolation of existingtechnology. Most often, new techniques and methods are developedfrom survey of structural systems to obtain empirical data. Thisapproach would improve the analysis of corrosion in shipstructures. Currently, corrosion margins are applied based onpast experience and most maintenance efforts are guided by trialand error experiences.

To improve on current practice and develop a rational method forassessing corrosion margins, it is necessary to survey corrosionin ship structures, develop a corrosion rate data base to predictcorrosion rates accurately and determine the time frame a givencorrosion margin will be depleted.

This report presents a corrosion survey methodology that willobtain corrosion data to develop rational methods for predictingcorrosion rates and assessing corrosion margins. The corrosionsurvey methodology is based on the review of corrosion data, dataanalysis requirements, data collection requirements forcharacterizing corrosion that affects structural integrity. Themethodology presented consists of a data collection procedurewith recommended instrumentation. The methodologysapplicability ranges from specific problem areas to ship hullgirderstructural systems. Data collection forms are presentedfor recording measurements. An outline database was developedthat uses an Expert system for data input, analysis, andretrieval. A list of recommended research is presented tosupport development of rationally based corrosion margins.

1

2.0 BACKGROUND AND PREVIOUS CORROSION SURVEYS

To develop the corrosion survey methodology it was necessary toreview corrosion data and determine the types of corrosionaffecting the integrity of ship structures, the locations wherecorrosion occurs, the parameters that apply, and the surveytechniques used. A number of ship owners, regulatory bodies, andindustry representatives were surveyed to obtain corrosion data.The following describes the corrosion data, as it impacts therequirements for a corrosion rate survey methodology.

2.1 Corrosion Surveys of Ship Structure

Traditionally, corrosion surveys fall into two categories: thoserequired by classification societies or regulatory bodies andthose conducted by ship owners to determine the generalcondition, effectiveness of corrosion prevention systems,corrosion rates, and repair assessments. Each survey hasspecific requirements, objectives, and survey resources.

Classification society and regulatory body surveys include annualand intermediate surveys, drydocking surveys, special periodicalsurveys, and occasional surveys. Special surveys are generallyrequired at four-year intervals. The scope of the special surveyvaries according to the age of the ship. Generally, the surveysare conducted during drydocking. Depending on the accessibilityof structural components and the extent of corrosion, surveys areconducted at sea to minimize time spent in drydock.

While the classification societies and regulatory bodies areconcerned with compliance to standards and for overall structuralstrength, the ship owners require information on structuralcondition that affects opdrating and repair costs . Thisinformation may be obtained at the time of annual or specialsurveys. Generally, the ship owner will require surveys of:

1. the present state and estimated corrosion rates of thevarious structural components;

2* the present condition and expected rate ofdeterioration of existing corrosion control systems;

3. the existence, severity, and potential for furtherdevelopment of structural defects due to expectedcorrosion patterns;

4. the potential for cargo contamination or pollutionincidents due to corrosion and structural problems.

As evidence by the previous discussions the type of surveyperformed depends on the information required. A corrosion ratesurvey is a derivative of the classification and owners survey.

2

2.2 Corrosion Survey Results

The results of previous surveys were reviewed to highlightcorrosion data collection techniques and requirements. A briefdescription of each survey is presented below.

During the time frame between 1981 and 1982, a tanker operator{2-1, 2-2) surveyed 32 VLCCS. The survey was conducted oninternal structure. Eighty-five (85) to ninety (90) percent ofthe internal tank structure was surveyed including underbellmouths and flume openings.

Inspections were conducted using ultrasonic instruments. Datawas recorded on forms for data analysis at a later time. Foreach structural member, information was collected on scale,pitting, visible thickness loss, fractures, and general wastage.Ultrasonic measurement patterns varied depending on the extentand location of corrosion. However, a detailed record of gauginglocations was a key part of the data acquisition process. Tankcharacteristics (i.e. contents, cathodic protection, coatingtype) were recorded for each set of data.

The majority of general wastage occurred on internal tankstructures subject to two-sided corrosion, including horizontalstringer platforms and webs and bottom plating, particularly inunprotected cargo/dirty ballast tanks. Generally main deck, sideshell, and bulkhead plating had much lower corrosion rates. Insegregated ballast tanks, wastage was most severe in the splashzone. Ships with flume tanks showed heavy wastage on stiffeningin way of flume openings and side shell stiffening opposite theflume openings. Heavy wastage was also found on horizontalsurfaces in cargo/clean and cargo/dirty tanks where tank washingmachines help remove protective wax or oil films.

Pitting and grooving on coal tar epoxy coated tank structure wasa common problem. Plating under bellmouths was vulnerable topitting in both coated and uncoated tanks due to added effects ofhigh fluid velocity. Several cases of bottom penetrationoccurred. The corrosion rat@ data derived from the survey ispresented in Table 2-1.

During 1980 to 1981 a tanker operator conducted internal tanksurveys (2-3). The surveys included visual checking for cracksand patterns of wastage and pitting. Periodic thickness gaugingwas conducted. In cargo-only tanks, uncoated surfaces showedonly modekate corrosion wastage of .1 - .15 mm (4-6 roils)peryear. Corrosion was noticeable primarily on structural membersadjacent to connections with the bulkhead and side shell plating.No problems existed in coated areas except minor deteriorationaround sharp edges. EPOXY coated tank bottoms in all cargo tanksdisplayed severe pitting, which was greater in tanks that were

3

TABLE 2- ICORROSION RATES

[*I BEGRE Gz~~ BALLASTTANKS [ballast hctor R ~}Lfnprorwt8d &olwmdwifhtis

Ull*ge - 1 sided 0.20 mmvr Not spplicabfe2 sided 0.30 Notlpplicable

Spkh - 1 sided 0.60 Nol#pDlicsble2 sided 0,s5 Not spplictble

Immersed - 1 sidecl 0.s0 O.lB mm.vr2 sided 0.B5 0.25 .

w CARGO CLEAN BALLASTTANKS [bsll#st factor = 45*JZone L@rotectcd PrOfcctedwirh lnodes

Ullage - 1 sided 0.10 mm,vr Not lpplicable2 sided 0.15 Not applicable

Splash - 1 sided 0.45 NoI applicable2 saded 0.65 Not applicable

Immersed - 1 sided 0.45 0.15 mmvr2 sided 0,65 0.20

k) CARGO DIRTY BALLASTTANKS [ballas! factor = 5%)Zone LJnprotecred Frorecfed wiIh anodes

Ullage - 1 sided 0.10 mm yr NOI lpplicable2 s,ded 0.15

SplashNot lpplicable

- 1 s!ded 0.15 Not applicable2 sided 0.20 NoI lpplicable

Immersed - 1 sided 0.15 0.15 mm vrb2 sided 0.20 0.20

l Excep! for bottom platinB of lft Iwo b~vs where corrosion rate islssumed 10 be 0.45 mm yt. (Water residue increases the ballas!factor J

u Anodes nol effeclwe due 10 low residence time

(dl CARGO ONLY TANKSCorrosion IS assumed to be extremely low unless uhrasonics indicateolherwse

4

cleaned with salt water washing. Several penetrations occurredin bottom shell plating under bellmouths.

In cargo/ballast tanks fitted with fixed tank washing machineswith no anodes, uncoated surfaces had higher corrosion rates thancargo-only tanks. Corrosion occurred extensively on horizontalgirder surfaces. The operator felt that a change from seawaterto crude oil for tank washing would reduce corrosion.

All significant hull corrosion occurred in permanent waterballast tanks. Corrosion problems on ships six to ten years oldconcentrated in the following areas:

1. oil tight bulkhead stiffeners;2. transverse web plating at bulkhead attachments;3. side shell longitudinal stiffeners;4. horizontal girders, plating, and supporting structure.

Corrosion rates were as high as 1.0 mm (40 roils) per year inupper sections and .5 to .6 mm (20-24 roils)per year in remainingparts of tanks. Higher corrosion rates were found in locallyhigh stressed areas. Zinc anodes did not provide necessaryprotection for uncoated ballast tank surfaces. The operators didnot see traditional grooving effects but rather large .amounts ofgeneral wastage.

Munger (2-4) separately reported results of a pitting corrosionsurvey of four VLCCS carrying sour crude. The pitting corrosionwas found primarily on horizontal surfaces of internal tankstructure. Visual inspections were conducted with gauging toobtain pit depth and diameter. Munger reported the surveyresults for each ship.

1. A Japanese tanker (250 KDWT), in service for one year,experienced extensive pitting in its oil/ballast tanks.In tanks fitted with zinc anodes, pits developed on allhorizontal surfaces from the highest stiffener to thebottom shell. Anodes had no effect on pitting pattern.The density of pits increased with increasing tankdepth. Pitting occurred on the horizontal surfaceswith the pitting density of four to 16 per square foot,diameter -of % to 1% inches, and depth of 80 to 160roils.

2. A European tanker (250 KDWT), in service for threeyears in Persian Gulf trade, experienced pitting in thecargo ballast tanks with no anodes or coating. Thepitting occurred on all upper horizontal surfaces,longitudinal, and upper flanges of the center verticalkeel. Pits were severe, actually growing into eachother with diameters ranging from one-half inch to six

5

inches. Pits increased in size from upper horizontalto the bottom.

3. A U.S.-owned tanker (265 KDWT) in Persian Gulf servicefor 28 months experienced pitting corrosion in cargo/ballast tanks. The tanks had no anodes and pitting waslocated on the bottom and underside of the deck.Pitting corrosion was observed on horizontal surfacesbetween upper and lower coated areas with an estimated25% of the horizontal surface corroded. Pit depthoccurred between 1/16 inch and 1/4 inch.

4. A U.S.-owned tanker (250 KDWT) in Mideast to Europetrade route for 18 months experienced pitting corrosionin its cargo/ballast tanks with no anodes. Allhorizontal surfaces were coated with one coat ofinorganic zinc primer. Vertical surfaces were in goodcondition with some corrosion starting. The two tophorizontal stiffeners showed pitting, of 3/16 inch toone inch in diameter and 1/16 inch in depth.Horizontal stiffeners showed pitting up to two inchesin diameter, depth 1/16 inch to 1/8 inch and frequencyof one to 10 per square foot.

The pitting action in all four tankers reported above wasaggravated by hydrogen sulfide in the sour crude oil. Sulphurdissolved in crude and available from hydrogen sulfide, oxygenfrom sprayed seawater used to clean tanks and from air enteringthe tanks, reducing environments existing during part of thecrude-seawater cycle and unfavorable area relationships betweenthe active pits (anodes) and the surrounding areas covered by thecorrosion products acting as the cathode, contributed to thepitting corrosion.

During a winter storm in 1977, a coastal tanker foundered andsank (2-5). Corrosion wastage was identified as a major cause ofthe casualty. Ultrasonic gauging of plates were compared for1968, 1972, 1976, and salvaged plates, as shown in Figure 2-1.When results were compared there were some discrepancies noted.Metal thickness readings were greater at later dates for manyreadings. However, general trends did show the hull thicknesswas reduced by corrosion and structural failure resulted.

A class of containerships (2-6.) sustained corrosion fatiguecracks and ultrasonic gauging was conducted to determine theextent of wastage in the shell plating. Figure 2-2 shows theresults of the gauging for the bottom and side shell. Theminimum thickness of the bottom shell shown in Figure 2-2 is 17.5mm (.70 inches). The original thickness was .8175 inches.Additionally, severe local pitting was identified inside thedouble-bottom tank. The observed corrosion fatigue cracking isshown in Figure 2-3.

6

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--- . . .

FIGURE2-1 .

GAUGINGMEASl,lREHENTHISTORYFOR THE

M/VCHESTER POLINGBEFORE&AFTER HULL FAILURE

7

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0.9

FIGURE 2-2ULTRASONIC MEASUREMENTS OF A TRANSVERSELY FRAMED

CONTAINERSHIP WITH CORROSION FATIGUE CRACKS.(MEASUREMENTS ARE IN MM.)

9

FIGURE 2-3

MELD DETAIL U$EDON THE CONVERTEDCONTAINERSHIPSHOHINGCORROSIONFATIGUECP4CKS

10

A Japanese organization (2-7) sponsored a research project tosurvey corrosion in ship structures. The surveys were conductedbetween 1976 and 1984. Corrosion was observed in areas of stressconcentration. They observed that corrosion product (rust) isnot as strong as paint coats and the product breaks down fasterin high stressed areas. The corrosion pattern around alongitudinal cutout shown in Figure 2-4 illustrates this finding.The report also presented the results of a survey of corrosionaggravated by (physical) wear. This corrosion/wear phenomenonwas ship type dependent as illustrated in Figure 2-5. Theinvestigators found the corrosion wear was also dependent on shipage. In general cargo ships, corrosion wear occurs up to .5 mm(20 mils)/year. Similar corrosion/wear was observed in lumbercarriers, bulk carriers, and ore carriers.

A containership operator (2-8) reported pitting corrosion in saltwater ballast tanks. In worst cases the ballast tanks werecoated, yet as the coating systems reached their respective lifeexpectancies, the corrosion commenced. The ballast tanks wereusually of the inner bottom type, therefore shell plating,longitudinal bulkheads, girders floors, shell and tank topstiffeners, and tank top plating were all affected by thecorrosion. These areas were all affected by pitting which in theoperators opinion creates the most detrimental effect oncontainership structural integrity. Another area whereaccelerated corrosion occurred was in the bottom of the containerholds . The hold plating was subjected to a somewhat hostileenvironment due to containerized tank leakage, difficulty ingaining access for maintenance, and the damp/wet environmentgenerally found. The corrosion principally affects the tank topplating and the boundary longitudinal and transverse bulkheads.Corrosion wastage in these areas tends to be compounded becausethe plating involved is affected by corrosion from salt waterballast as described above.

11

Residualplate thickness

1 T r

_ 2.3mm or fess- 3.0-3.9n 4.0-4.9m 5.0-5.9B 6.0mm or more

HtttttH

ResultofPlateThicknessGaugingandCorrosionDistributionofGirderWebSlot(3yearsafterconstructionoriginalplatethicknew.9mm)

FIGURE 2-4

0$ Ikti%.1,

NumberofCasesofDamagebyShipTypeBroken.downintoTypesofDamaEeBetween1976-1984

FIGURE 2-5

12

3.0 CORROSION DATA REQUIREMENTS

Corrosion data collected by therequirements of the structuralindividuals must be able

methodology must be based on theengineer or surveyor. These

to determine the geometriccharacteristics of the remaining, unwasted structure. Knowingthis information, they will be able to analyze the intactstructure and determine the margin of strength remaining. Thecorrosion rate data will permit them to determine when the marginof strength will be depleted.

3.1 Corrosion Margin Assessment

Existing corrosion margin parameters were examined to determinethe corrosion characteristics required by the structural engineerand the parameters needed to perform a structural analysis of theremaining plate. The parameters were identified considering thefailure modes most important to structural integrity: yielding,buckling, fatigue and fracturing.

3.1.1 Review of Existing Corrosion Margins

Classification societies and the regulatory bodies includecorrosion margins in design and inspection standards. Althoughthe ABS rules for building and classing steel vessels do notmention explicitly the allowances adopted, they have on severaloccasions made known its views on wastage allowances (3-1).Other corrosion allowances were inferred by inspection of therules as presented by Evans (3-2). Here the key point is thatmargins are directly additive to thickness requirements.

For example:

Strength decks on longitudinal beams t = .0069 Sb + .16

Where: t = the required minimum deck plate thicknessSb = spacing of deck beamsand .16 is presumably the corrosion allowance.

The extent of reduction in practice is treated as a percentage ofthe required plate thickness; the allowable reduction depends onseveral factors, such as ship type and age, frame spacing, andstructural component. The range-of allowable wastage ranges from15 to 30 percent.

Similarly, the U.S. Coast Guard wastage limits are a function ofplate thickness. The average corrosion limit of 20 percent isallowable. In practice, wastage allowances are evaluated frombelt gauging, defined as measuring plate thicknesses aroundseveral complete transverse sections of the hull, including deck,sides and bottom.

13

Plate and structural member thicknesses are obviously keyparameters in assessing corrosion margins. Other importantfactors include location, local extent and global extent.

3.1.2 Evaluating the Strength of Corroded Structure

Beyond existing corrosion margin assessments additionalinformation is required to rigorously assess the strength ofcorroded structure.

As presented by Evans (3-2) individual panel failure byinstability or plate stress (yielding) is approximately afUnCtiOn Of the square Of the thickness as illustrated in thefollowing relationships:

~2E t 2

(-)

b 2

Ucr = ------ -- Ka=()?4Kph -12(1-MZ) b t

Where: K and K are functions of the panel aspect ratio, P isthe unit weight of the loading medium and h is thepressure head. The panel dimension is given by b.

It is obvious that thickness (or predicted corrosion wastagesubtracted from a known thickness) is the dominant parameter.However, to perform a thorough structural analysis of a panelplate or stiffener member the extent of wastage must be known.For example, if the corrosion wastage is generally uniform andcovers the entire plate, an average thickness can be used toanalyze the plate strength. However, if the wastage covers apercentage of the plate (say 50%) then the plate bucklinganalysis is more complex and simplified techniques have not beendeveloped to dater for general wastage. This situation becomesmore complex for analysis of the reduction in panel strength dueto pitting, which occurs in a non-uniform manner.

The effects of pitting on panel strength have been investigatedand techniques developed for estimating the strength of a pittedplate (3-3). Again thickness of structure in way of pits isimportant and the percent of remaining structure must beestimated. The authors of reference 3-3 proposed a method todetermine an equivalent thickness of a panel by estimating anequivalent volume of wasted material and subtracting the volumeof material wasted from the panel. Using this technique anaverage pit depth and frequency must be obtained. Thus , pittingdata must include depth and frequency of pitting (representativeof panel area covered by a given depth of pit). This isdifficult in practice because pitting occurs at different rateswithin the same panel and an average pit depth must be derivedfrom numerous pit measurements.

14

Structural yielding in corroded panels is a function of remainingplate thickness. Similar to general wastage, the extent ofwastage must be determined to analyze the strength of the plate.Very localized general wastage or pitting does not reduce theoverall yielding strength of a plate. But again, the extent ofcorrosion must be known to determine when the plate is corrodedto a point where strength is sacrificed. To assess structuralintegrity, the key parameters are thickness (average) and extentof wastage.

Corrosion also affects the integrity of structure by forminglocations where fatigue cracks initiate and subsequentlyaccelerates fatigue crack growth. Corrosion wastage and pittinghave the effect of reducing plate thickness and decreasing panelor member strength for a given load. This decrease in strengthcan be determined by fatigue life estimates for each stressrange. Corrosion fatigue is a term that describes the behaviorwhen a material is subjected to fluctuating forces in a non-benign environment. The factors that contribute to this failuremode are characteristic of corrosive environments as described insection 2.0. From a structural strength view point, corrosionfatigue is characterized by the widely used crack- growth lawgiven by:

da = C@k)mz

Where: a is the characteristic dimension of the crack, itsdepth and width for example, and N is the number of cycles. Akis the stress intensity range at the tip of the crack. C and Mare related constants that depend on the material and theenvironment.

Bokalrad (3-4) presented an approach for assessing fatigue andcorrosion margins using ultrasonic inspection of ship structures.Bokalrad shows the effects of a corrosive environment on crackgrowth of ship steel in terms of the probability of failure. Theresults indicate that corrosion is a critical element to considerin assessment of corrosion effects on fatigue and structuralfailure.

The global structural response must be evaluated to assessstructural integrity. Globally, corrosion wastage reduces theships section modulus. The number of panels and stiffeners andgirders affected by corrosion must be determined and the overallhull girder section modulus reduction and net strength must beevaluated to assess corrosion margins.

According to the IACS Unified Requirement S.2 (3-5), the minimumsection modulus must generally be maintained throughout .4Lamidships. However, the section modulus may be reduced away from

15

the midships area providedvertical still water andof midships stress levels.

that the stresses due to combinedwave bending moments are not in excess

In ships where the longitudinal strength material in the deck orbottom area are forming boundaries of tanks for ballast or oilcargo, reductions in scantlings are permitted providing that aneffective corrosion protection system is used, certain reductionsin scantlings are allowed by classification societies. Howeverthe minimum hull girder section modulus reduction must not exceed10% depending on coating.

Section modulus requirements indicate additional key areas tosurvey. Corroded structures most important in assessingcorrosion margins are located in the deck and bottom areas at thegreatest distance from the neutral axis of the ship hull girder.

3.2 CORROSION RATE PREDICTION AND SURVEY TECHNIQUES

To assess corrosion margins, the engineer or structural surveyormust be able to predict the rate of corrosion or hence thetimeframe in which the margin will be depleted. Traditionally,corrosion rate predictions have been based on service monitoring,trial-and-error case studies or samp16 exposure tests. Eachmethod has an impact on user requirements and recommended surveytechnique.

3.2.1 Analytical Methods

Early efforts to predict corrosion rates analytically involvedsolving the LaPlace equation (the governing equation forpotential distributions in electrochemical cells). These effortswere successful but limited to cases of simple geometries andconstant material properties. However, simple geometries seldomappear in real-world structures, and the electrochemical andmechanical properties are not constant with changing potentialand current. Solutions can be applied to general geometriesusing numerical methods. These can accommodate varyinginhomogeneous, nonlinear properties for electrolyte andconstituent metals. Numerical methods have recently beenemployed in various levels of sophistication to solve thegalvanic potential distribution problem. These methods includethe finite element method, the finite difference method, and theboundary integral method.

3.2.1.1 Finite Element Method

The finite element method is a powerful tool for solving physicalproblems governed by a partial differential equation or an energytheorem using a numerical procedure. This method has beenapplied to a number of galvanic corrosion problems. Oneapplication was for the solution of the electric potential

16

distribution and current fluxes near a ~ultimetallic ~Y~temsubmerged in an electrolyte. The model could handle general andarbitrary geometries and the effects of nonlinear polarizationbehavior (3-4).

Another application of this method uses the principle of energyconservation to determine the strength and distribution of theenergy field within a finite element model. It calculates therequired current to maintain the minimum energy balance of eachelectrolyte element. The energy that enters the model at anodeelements must leave at cathode elements. The advantages of thisapplication are that shielding effects in nodes and othercritical areas can be detected and, moreover, time-dependentpolarization characteristics can be represented.

3.2.1.2 Finite Difference Method

The finite difference method is a numerical discretizationprocedure for the approximate analysis of complex boundary valueproblems. This method has been used for theoretical treatmentsof electrode systems, but lately is being used in offshorecathodic protection. Computerized finite difference analysis isuseful in simulation and design of cathodic protection systemsfor offshore structures. It is also useful in analyzing electricfield strengths, current density, and potential readings. Thismethod has also been used to solve the Poisson equation for theelectrochemical potential distribution in an electrolytecontaining an array of fixed-potential electrodes and electrodeswith activation, passivation, and diffusion-controlledpolarization kinetics. The results were presented as a displayof the potentials at selected coordinates or as a printed listingof the potentials at all nodal points in the electrolyte.

3.2.1.3 Boundary Integral Method

The boundary integral method is similar to the finite element andfinite difference methods in that it solves the LaPlace equationto obtain the potential distributions in electrochemical cells.However, this numerical method is more efficient than the othersbecause it does not require modeling the electrolyte bodies toobtain the potential distribution on the surface of thestructure. This sa,ves computer time. One application of thismethod utilizes nonlinear and dynamic cathodic boundaryconditions to simulate real polarization conditions during theformation of calcareous deposits. Applications includedetermining corrosion rates in offshore structures.

3.2.2 Empirical Rate Prediction Methods

Empirical techniques are also used to predict corrosion rates.They include correlating laboratory data to field measurementsand field surveys.

17

3.2.2.1 Polarization Potential Rate Prediction Methods

Another method used to calculate metal corrosion rates is basedon the use of the metal polarization curves depending on localpolarization of the surface. This method is commonly referred toas the polarization resistance technique. The term polarization,as it applies to corrosion studies, is defined by ASTM as thechange from the open-circuit electrode potential as the result ofthe passage of current (3-4). Simply stated, polarization isthe changing of a metals natural potential (voltage), as definedon the Galvanic Scale, in either a positive or negative directiondue to the fluctuation of corrosion current resulting from theintroduction of electrolytes, metals, or protective systems tothe base metal. Potentials referenced on a Galvanic Scale asshown in Figure 3-1 are based on a metal-water interaction.During a corrosion process, any deviation of a metals potentialfrom that referenced in the galvanic series is known aspolarization. Every corrosion process, (i.e. metal-electrolyteconnection) , has an associated corrosion potential (e) andcurrent (i) which are measurable quantities. The polarizationresistance technique involves the use of the developed i/e curvefor a given corrosion process. An example i/e curve, or Tafelcurve as often termed, is shown in Figure 3-2. The assumption isthat once the shape of the Tafel curve is known in a potentialrange such as + 50 mV around the corrosion potential of thesystem under study, the corrosion rate is equal to the inverseslope of the curve. The following relationship is generallyobserved:

i = i. [10 - P/Be - 10 P/Ba] (1)

Where: i is the applied current density;ic is the corrosion rate expressed as current density;Bc and Ba are the Cathodic and Anodic Tafel (or beta)constants;and P iS the overpotential equal to (Ec - E) , where Ecis the corrosion or open circuit potential and E is thepolarized potential.

At low values of P, Equation (1) may be approximated by:

R =AP= (Ba)(Bc) (2)Ai 2.303 ie (Ba + Bc)

Where: R is the slope obtained from a linear plot of E vs i.R has the units of resistance and is inversely proportional tothe corrosion rate, ie . However, there are some problemsassociated with this method. One lies in the way R is measured:one wants the slope over a very narrow potential interval (toensure reasonable linearity) but must compromise in order to getusable signals. The most serious disadvantage of this technique

18

PRACTICAL ULVAHIC SERIES

Hetal volts*

Commercially pure magnesium -1.75Magnesiumalloy (6z Al, 3% Zn, 0-152 Mn)Aluminum-Zinc- Indilun (a)

Aluminum-Zinc-Mercury (a)

Zinc

Conmnercially pure aluminum

Mild steel (clean and shiny)

Mild steel (rusted)

Cast iron (not graphitized)

Lead

Mild steel in concrete

Copper, brass, bronze

High silicon cast iron

Mill scale on steel

Carbon, graphfte, coke

-1.6

-1.16

-1.1

-1.1

-0.8

-0.5to -0.8

-0.2 to -0.5

-0.5

-O*5

-0.2

-O*2

-0.2

-0.2

+().3

*Typical potential normally observed in neutral soils and water, measured withrespectto coppersulfatereferenceelectrode.

(a) addedto originalreference

FIGURE 3-1

19

r542II 1

12

I

M* Uu

FIGURE 3-2EXAMPLE POLARIZATION (TAFEL) CURVE

20

is the Tafel constants that describe the shape of the medium-range i/e curve are assumed constant, they are not. Corrosiontakes place in a variety of uncontrolled solutions and is therebynotoriously unreproducible.

At large negative values of P, the second exponential in equation(1) approaches zero. Thus, a plot of E vs the logarithm of iyields a straight line under these conditions. The slope of thisline is Bc and its extrapolated value at E = o is equal to i..Ba can similarly be obtained for large positive values of E.This is known as the Tafel Extrapolation Method.

Both of the above methods have been successfully used todetermine corrosion rates in a variety of industrial systems, butnot without limitations. It is often difficult to obtain asufficiently long region of linearity to permit accurate Tafelextrapolation. Deviations from linearity are caused byresistance effects and concentration polarization, especially athigh values of overvoltage. Unfortunately, Tafel extrapolationis only valid at high overvoltages (+ 50 mV). Polarizationresistance is usually not affected by resistance or concentrationpolarization effects since it is performed at low overvoltage.However, equation (2) is an approximation which is valid atovervoltages of 10 mV or less. Experimental errors becomesignificant in this range since the sensitivity of electrodepotential measurements is +/0.5 mV at best. Also, accuratecalculations of the corrosion current density by equation (2)requires prior knowledge of the Tafel constants. These valuesare sometimes difficult to obtain for the reasons mentionedpreviously. Tafel extrapolation and polarization resistance haveadditional limitations. Both methods are only valid for alimited range. Tafel extrapolation cannot utilize data obtainedat overvoltages less than about 50 mV, while polarizationresistance is limited to the first 10 mV or less. Historically,both methods have used graphical calculations which are bothcumbersome and often inaccurate. The majority of corrosioncalculations carried out to date have been done in terms ofdirect problems of mathematical physics. Formulation of suchproblems have enabled, using a given distribution of the electro-chemical activity over the metal surface, the calculation of theelectric state of the medium near the corroding surface andestimation of the corrosion rates at different points on thesurface.

3.2.2.2 Statistical Rate Prediction Methods

Statistical methods provide an alternative to analytical andempirical methods for predicting corrosion rates. The use ofanalytical methods are very limited, such as for controllable,laboratory-reproducible corrosion processes. Statistical methodshave widespread use in virtually every industrial, commercial andeven laboratory process that is characterized by complex, ever-

21

changing corrosion reactions. Statistical methods are concernedonly with the end result of corrosion loss whereas the analyticaltechniques are concerned with the understanding and modeling ofthe corrosion reactions that produce an end result.

The American Society of Testing Materials (ASTM) has issuedguidelines for applying statistics to the analysis of corrosiondata (3-6). The guideline addresses the subjects of errors,sample sizes, confidence limits, mean and variance comparisons,and standard deviations as they pertain to a set of corrosionwastage data. Details associated with the application andadaptation of this guideline will be discussed in section 4-4.The general application of this guideline is aimed at thedevelopment of true means and standard deviations in addition tothe recognition of errors associated with a quantity ofmeasurable corrosion data. The corrosion rate is equal to theaveraged metal loss/time between measurements for a givenlocation or specimen. This method will produce a statisticaldatabase, for a structure that experiences many differentcorrosion reactions over a period of time, that includes theaverage corrosion rate and associated errors.

The statistical rate prediction method has been applied to shipstructures by a ship operator and the method refined by theTanker Structure Cooperative Forum (3-4).

3.3 SUMMARY OF DATA REQUIREMENTS AND RECOMMENDED CORROSION RATESURVEY TECHNIQUE

The preceding sections discuss the parameters that must beobtained to characterize corrosion wastage to assess corrosionmargins. The parameters are summarized in Table 3-1 for variousfailure modes. These characteristics must be determined for eachpanel surveyed and in specific belt and survey patterns todetermine the extent of hull girder wastage.

Additional work is required in this area. Specifically,development of simple methods for assessing strength of wasted orpitted plates other than conducting detailed finite elementanalysis.

A mathematical model may be developed which accurately describesthe contribution of each variable to the overall corrosion of aclosed system such as a pipeline. However, in the case ofinternal ship structures, which encounter many environments, itis virtually impossible to analytically model the corrosionprocess because of the irregular contribution of a large numberof variables. The interaction of variables constantly changesthe corrosive environment making it very difficult to separatethe true contribution of each variable.

22

TABLE!3-1

CORRELATION OF CORROSION DATA REQUIREMENTS ANDFAILURE MODES RELEVANT TO STRUCTURAL INTEGRITY

FAILURE MODE

TYPE OF CORROSION YIELDING BUCKLING FATIGUE

GENERAL WASTAGE T,A T,A T

PITTING N,D1,D2 N,D1,D2 D1 ,D2

GROOVING W,D1,L W,D1,L W,D1,L

~:

T= THICKNESSL = LENGTHD1 = DEPTHA = AREAD2 = DIAMETERN = NUMBER/UNIT AREAw = WIDTH

FRACTURE

T

D1,D2

W,D1,L

23

The statistical approach remains the only alternative for thequantitative treatment of corrosion allowances in ship structure.The usual procedure of introducing an(for

additional safety factorexample, in the determination of allowable stress) is

inadequate. The statistical approach will indicate the possibledeviation from an expected value, i.e., it indicates thedispersion about the mean of the distribution function.

24

4.0 DATA COLLECTION REQUIREMENTS

In addition to users requirements presented in previous sections,there are data collection requirements that must be met to ensurethat the required data is obtained. This section presents thedata collection requirements including types of corrosion,locations, supporting parameters, accuracy, and instrumentationrequired for the survey.

4.1 Types of Corrosion to survey

Traditionally there have been eight classifications of corrosion:

1. General (Uniform) 5. Intergranular2. Galvanic 6. Selective Leaching3. Crevice 7. Velocity Corrosion4. Pitting/Grooving 8. Stress Corrosion Cracking

A certain degree of overlap exists among them. As discussed inSection 2.0, two types of corrosion have been found to commonlyexist within ships: General and Pitting/Grooving.

General corrosion is the most common of the types of corrosion inship structures. The corrosion product appears as a non-protective rust which can uniformly occur on uncoated, internalsurfaces of a ship. The rust scale continually breaks-off,exposing fresh metal to corrosive attack. The rust scale alsoappears to have a constant depth and similar consistency over thesurface. The mechanism of general corrosion is illustrated inFigure 4-1.

There are micro cathodic and anodic areas caused by variations ingrain structure, impurities in the metal, alloying elements, andother inhomogeneities. For general corrosion, the cathodic andanodic areas constantly switch back and forth due to a differencein potential or degree of polarization, thus accounting for theuniform corrosion of the surface.

Pitting corrosion is often described as a cavity whose diameteris the same or less than its depth. Pitting is a localized formof corrosion and usually grows in the direction of gravity. Itis also self-generating, i.e. autocatalytic, starting fromimpurities in the metal, scale or other deposits, or someinhomogeneity in the metal. Figure 4-2 shows a progressive pitbeing formed.

A specialized form of pitting corrosion known as groovingcorrosion also occurs frequently within ships. This corrosion,sometimes referred to as in-line pitting attack, is a linearcorrosion occurring at structural intersections where watercollects or flows. Grooving can also occur on vertical membersand flush sides of bulkheads in way of flexing.

25

Figure 4-1

SIMPLIFIED SCHEMATIC OF UNIFORM CORROSION

26

~Metal

rtsCathodicareas 7

Anodic area~

Figure 4-2

SCHEMATIC OF FORMATION OF A PIT

27

4.2 Corrosion Locations

Generally stated, corrosion of structural steel will occurwherever salt water is present. However, corrosion also occursin areas that are not directly exposed to salt water. This isevidenced by the fact that many factors contribute to thecorrosion process in ships and often combine to create corrosiveenvironments within ships.

The majority of internal structure within a ship usuallyexperiences corrosion to a certain extent. However, it is thehorizontal structural members as mentioned herein that encounterthe greatest corrosive attack simply due to the ability tocollect and trap water and to facilitate pit growth. Thecorrosion patterns discussed have generally been descriptive ofthe results found for tanker surveys. However, it is importantto note that all cargo ships experience corrosion, the extent andseverity depending on such factors as cargo, temperature,humidity, and protection system. Ballast tanks in all ships willhave similar corrosive patterns but dry cargo compartments willnot suffer the same amount of corrosion wastage as liquid cargocompartments, The common finding from the review of data hasbeen that ballast tanks experience the highest corrosion rate.This is due to the fact that greater exposure of metal to saltwater increases the corrosion rate. The following are locationswhere corrosion is found and are important to structuralintegrity.

4.2,1 Bottom Plating

The bottom plating within a ship typically experiences thegreatest amount of corrosion wastage. As a result of watercollecting and settling on the bottom, pitting, grooving, andgeneral wastage occur frequently. For coated plating, wastagewill take the form of localized pitting and grooving in way ofcoating failure. For inorganic zinc coating, the wastage willtend to be patches of scaly areas with only minimal thicknessloss. For coal tar epoxy coated plating, wastage will tend to bedeep pits of limited area which present a definite risk of bottompenetration if not repaired.

For uncoated tanks, bottom wastage is more general, affecting thehigher velocity flow paths of the drainage patterns to a greaterextent than stagnant areas. Thus, wastage is highest in way ofcutouts in transverse web frames and bottom longitudinal, andlowest just forward and aft of web frames outside the line of thecutouts. Figure 4-3 illustrates an example of this loss pattern.Bottom wastage generally increases frem forward to aft, mostlikely due to water wedges caused by the normal trim patterns bythe stern, both in full load and ballast. However, this can bereversed on some ships where the tendency is to trim slightly bythe bow in the full load condition. The water wedges are a

28

LONGITUDINAL NOI Sh2wN

u.

FOR CL ARITV. _

AREAW MOOERATE SIEE~

EELAPEA OF HEAVY STEEL LOSS

FIGURE4-3

TYPI CAL BOTTOM SHELL LOSS PATTERNS

29

combination of unstrippable ballast water and water settling outfrom cargo within certain compartments. Thus , aft bays of liquidcargo and ballast tanks can experience corrosion almostcontinuously. Also occurring on bottom plating and often onother typical areas of bottom structure are grooving of the weldsof bilge longitudinal and thinning and cracking at the toes oflongitudinal girder brackets. These are shown in Figure 4-4.

The bottom structure is an important area to survey because it iswhere corrosion is most prevalent and a location that is criticalto structural integrity.

4.2.2 Side Shell and Bulkhead Stiffeners

Wastage patterns on the side shell and the stiffened sides ofbulkheads are usually limited to the horizontal webs of thestiffening. In coated tanks, wastage occurs in way of coatingfailures which generally start at welds, cutouts and sharp edges.In uncoated tanks, wastage is more general and usually increasestoward the bottom of the tank. Deep pitting is often found onlower stiffening, usually near web frames. On ships withfabricated longitudinal where the face flat extends above theweb, wastage can be rather severe due to the trapping of water onthe web.

4.2.3 Deckheads

For coated deckhead structure, general wastage usually occurs atconnections of deck longitudinal to deck plating in way ofcoating failure. Uncoated compartments suffer more uniformcorrosion both when empty or full of either liquid cargo orballast. When the compartment is empty, the area is subject to ahighly corrosive, moist, salt-laden atmosphere. Oxygen isreadily available high in the compartment from hatches, vents anddeck openings and contributes greatly to the uniform corrosionprocess. When a compartment is full of ballast or liquid cargo,general wastage results from the same causes in this ullage spacearea because the deckhead is not protected by an oil film.Deckheads are important structural locations to survey becausethey are strength decks that contribute to structural integrity.

4.2.4 Special Locations

There are other special locations that should be surveyed wherelocal corrosion is prevalent. Wastage can occur in high stressareas where coatings break down and corrosion attack begins.These locations include longitudinal cut-outs in frames. Theplating under bellmouths is vulnerable to general wastage in bothcoated and uncoated tanks due to the added effectsvelocity during

of highballast discharge. Other special locations

should be surveyed where structural integrity is reduced or areaswhere watertight integrity is reduced.

30

8* *M Loll@ mhd

DETAL

BLQE LONGITUDINALl WELD QROOVNQ

rt DETAL

Bottom Shell

~ FRACTUREQF WEB FRAMEu 6TFFENERS

?op of Brmcko~ThhnhgDownmd FracWrod

.

---7 ----

I 3ml nm5-L--- -

8ottom Shd OTB

2 FRACTURE QF LONGITUDINALOIRDER BRACKET ~

Fmcturo Across Toeof Brsckot

\\\

\ \

4 FRACTURE AT CONNECTON OFBOTTOM WtiSTO LONGLBES

1

FIGURE 4-4

TYPICAL BOTTOM STRIJCTIJfIE DEFECTS

31

4.3 Correlation Parameters

Similar compartments within the same ship and certainly amongdifferent ships often experience different and varying rates ofcorrosion. This can be attributed to different operating,climatic and protective conditions that exist within acompartment throughout the duration of a voyage. Theseconditions are called correlation parameters. Knowledge of theseconditions are important and direct decisions to analyzecombinations of compartment data. Nine correlation parametershave been identified as exerting the greatest influence oncorrosion rates:

1.2.3.4.5.6.7.8.9.

time in ballast;cargo content;coating system;anode system;vessel navigational routes;compartment humidity;tank washing medium (tankers only);tank washing frequency (tankers only) ;tank inerting medium (tankers only).

4.3.1 Time in Ballast

Typically, the longer the duration of salt water exposure, thegreater the corrosion rate of steel. If a compartment is notprotected by coatings or anodes, the time in ballast representsthe most corrosive condition. As a result, ballast tankstypically experience the highest corrosion rates.

4.3.2 Cargo Content

There are generally three types of cargo carried aboard vessels;bulk, containerized and liquid. Depending on whether the cargocompartments also function as ballast tanks, the highestcorrosion rates are usually associated with liquid cargo. Alimited amount of water or moisture may accumulate in bulk orcontainer holds which would lead to localized corrosion.Corrosion within liquid cargo tanks is generally widespread andis related to the type of cargo carried. Sour crude oil is morecorrosive than sweet crude oil. Acidic cargos and high-oxygencargos, such as gasoline, typically lead to high corrosion rates.Liquid cargos can also temporarily render anodic protectivesystems inert through the presence of residual films. Whereliquid cargo is involved, careful attention must be paid tocomposition and properties so as to avoid possible erroneousgroup analysis of data.

32

4.3.3 Coating System

Wellmaintained coating systems offer the best protection againstcorrosion. However, coating breakdown due to depletion,deterioration or damage can result in high corrosion rates andpitting in way of the breakdown. It is important to know theextent and type of coating protection provided so as to developan understanding of the protection system.

From a corrosion margin assessment standpoint coatingeffectiveness is an important parameter. Effective coatings canprolong corrosion initiation and hence minimize the marginrequired. The white coating condition assessment is notspecifically addressed by the survey methodology, it is a by-product of the surveys. The absence of corrosion should bedocumented for each panel inspected and the coating breakdownrate determined. The time frame between re-coating must also bedetermined. A re-coated area becomes a new set of corrosiondata.

4.3*4 Anode System

Next to coatings, anodes provide the best protection againstcorrosion in seawater. However, anodes only function whenimmersed in an electrolytic solution. Therefore, onlycompartments containing electrolytes such as seawater ballasttanks benefit from anode protection. The location and density ofanodes play a major role in the deterrence of corrosion. Certainlocations, such as underdeck structure, do not benefit from anodeprotection. High current densities generally afford greaterprotection against corrosion but can damage coatings.

4.3.5 Ship Navigational Routes

Navigational routes can have an effect on corrosion rates dueprimarily to two factors; temperature and voyage length.Preferential solar heating of one side of a ship due to thenavigational route can lead to increased corrosion of affectedwing tanks. In addition, voyages of short duration canlead toincreased corrosion of anode-protected compartments due toinsufficient anode activation period.

4.3.6 Compartment Humidity

High humidity within a compartment may lead to the accumulationof moisture which in turn can lead to increased atmosphericgeneral corrosion. The level of humidity can be closely tied tothe navigational route.

33

4.3.7 Tank Washing Medium

Compartments containing petroleum cargos can exhibit increasedcorrosion rates based on the washing medium used. Typicalmediums used are: hot seawater, ambient seawater, and crude oil.Seawater washings introduce corrosive seawater which can lead toincreased corrosion rates. Hot seawater is more damaging thanambient seawater. All washing mediums can deteriorate coatingsand remove protective oily films residing from crude oilcarriage.

4.3.8 Tank Washing Frequency

Increased frequency has been found to increase the corrosion rateof liquid-cargo compartments (see 4.3.7).

4.3.9 Tank Ine5ting Medium

Gas inerti.ngof liquid-cargo tanks can help to increase or reducecorrosion rates. Sulfuric oxides resulting from flue gasinerting can lead to accelerated corrosion due to the formationof sulfuric acid. Gas inerting also may reduce corrosion ratesof ullage areas due to a reduction in oxygen content. However,air (oxygen) leakage into a tank via deck openings can lead toincreased corrosion of surfaces adjacent to the leakage site.

4.4 Sample Size and Accuracy

The number of data points required for measurement must bedetermined during the planning stage. The size of the data setfor a given location is very important, as it is directlyproportional to the resulting accuracy associated with that dataset. The procedure used to determine the required data size isspecified in ASTM guidelines(41) . According to ASTM, the samplesize is dependent on two parameters: Standard deviation andlevel of accuracy. The following relationship is used:

N= (zo)2/Az

Where: N= Number of samples,

z = Level of confidence statistic ( = 2, for 95% ofthe normal distribution) ,

a = Standard deviation, which represents the errorassociated with individual measurements,

A= Level of accuracy associated with the mean value -of a set of N data points.

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In order to determine N, we must know A, Z, and a. For normaldistributive systems, the quantity (Za) represents deviation fromthe expected value, or mean, and corresponds to the area underthe normal bell curve. Statistical theory reveals that & 2a isequal to 95% of the area under the normal curve as shown inFigure 4-5, as applied to corrosion data analysis. The quantity2a is equal to the maximum expected error associated with eachindividual data measurement. Therefore, 2a is equal to theassociated instrument/operator error. The instrument/operatorerror is composed of all possible errors contributing to a singlemeasurement. These include, expected instrument error andoperator systematic error. The instrument error is usuallyspecified by the manufacturer. The operator systematic error istechnique related and is influenced by gauging environment,experience and surface condition. The error value will bedifferent for different instrument/operator combinations and mustbe determined prior to survey. The TSCF conducted a series oftests aimed at determining instrument/operator error and foundthat accuracy varied from kO.5mm (20 roils) to ~3.~mm (120 roils)(4-2). The best possible accuracy attainable for a givenmeasurement was AO.5mm (20 roils). Continuous increases ininstrument technology and operator training ultimately willprovide for better accuracy levels however, for illustrativepurposes a value of 20 roilswill be used herein. In addition toindividual measurement error, there is also an error associatedwith the mean or average value of a data set. This value,~ ,will be less than 20 and is dependent on the sample size, N.Therefore, sample size is determined based on a desired level ofaccuracy associated with the average corrosion rate of a dataset. A large sample size will afford a small error value, whilea small sample size will have a larger error value approaching,but never exceeding, the instrument/operator error.Understanding of the relationship that exists between thesevariables is best provided through an example.

Example

Given: 2U = 0.5mm N= (2u)Z/Az

~ & A1 20 roils 20 roils10 20 roils 6.4 roils25 20 roils 4 roils50 20 roils 2.8 roils100 20 roils 2 roils

Notice the relationship that exists between A and N. As thenumber of samples increaser the accuracy of the average valuealso increases.

35

Valuo

-3 -2 -1 0 1, 2 3.Average

Thickness

FIGURE 4-5

NORMAL DISTRIBUTION CURVE

36

An example thatsurvey methodology

illustrates thisis as follows:

application to the corrosion

Example

Expected corrosion loss for a given location for one year =15 roils

Assume 3 year gap between surveys

At = 3 x 15 = 45 roilsAt=F.-FQ to = thickness at year zero

t3 = thickness at year 3

- Assume 20 = *2 roils

For At = 45 roils, assume desired level of accuracy of 95% (20 =2 roils). But , A=A. +A3

where

A. = error associated with year zero average,Az = error associated with year 3 average

Assuming desired accuracy levels are constant, Ao =A2 = 1 roil.

Therefore: N = (2a)z/AzN3 = (2a)z/Asz = (2)2/(1)2 = 4N3=4

This shows that a sample size of four in year three is requiredto ensure a level of accuracy of A1 mil for the year threeaverage thickness value, with the individual measurement errorequaling *2 roils. The error associated with the difference inthicknesses (corrosion rate) is 2 xAa = 2 roils. Note that sincethe operator/instrument error (20) is small, a small sample sizeis needed for 95% accuracy. This example demonstrates severalimportant points:

1. Corrosion rate is the difference of two calculated means;2. Associated error of the corrosion rate is the sum of the

associated errors of the individual means (~a - ~b ) ;3. The value of the actual average thickness (t) is not

important, rather it is the value of the difference betweenaverage thicknesses that governs the selection of A, thusAa andAb;

4. Errors are additive when comparing differences;5. Error a of year i mean is equal to one-half the desired

error of a corrosion rate (i - datum) ;6. Error of a mean (A) cannot exceed the error of a measurement

(29).

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One final example is included taking a slightly differentapproach to determining sample size and error.

Example: First implementation of surveyt given prior data.

Statement: If the average thickness of side shell platingmeasured four years ago is 200 roils,how manysamples should I measure and what is the bestpractical accuracy I can achieve for the corrosionrate?

Given: Datum Year Year Four

= 200 roils t4=?;: = *1O rnils 2a = 4 roils (assume known)20 = unknown A4=?N. = unknown N~=?

Solution: Assume & - ZI = 30-40 roilsmagnitude of corrosion losstrial measurements) Say To

= A. + A4 = 10 + A4

IJ4

1102550100200

Best Accuracy = AI (t.

& A4.

4 44 1.264 .804 .564 .404 .28

(must determine approximatebased on historical data or- 74 = 40 roils

-T4) -1

A

1411.2610.810.5610.410.28

= (10 +A4L _l40

Best Accuracy

65.0%71.8%73.0%73.6%74.0%74.3%

Results of best accuracy vs. practical sample size are determinedfor several values of Na . For this case, the accuracy isdirectly related to the associated accuracy at the datum year.Recent advances in instrumentation allow for errors as low as 1-2roils which correspond to mean accuracies (A) less than 1-2 roils.Therefore, it is highly recommended that this survey be initiallyimplemented to establish a datum year with a minimum accuracy of95% and all parameters recorded. Thereafter, practical samplesizes can be determined that correspond to 5% error values.Depending on instrument/operator error, mean accuracies may begreater than 95% for relatively small sample sizes.

When reviewing corrosion data, careful consideration must begiven to the analysis due to the large and varying number ofinfluential factors contributing to corrosion. Corrosion data inits raw form, is a massive compilation of ultrasonic thicknessreadings generally expressed in mm or roils. Usual practiceconsists of averaging a group of readings for a given location

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and then converting the corrosion loss into a rate based on thetime between surveys.

4.5 Instrumentation Requirements

Corrosion surveys in ships are conducted in holds, compartmentsor tanks that are accessed through hatches or manholes. Thesurvey team must climb structure to measure thickness andvisually inspect the structure. This type of survey requiresinstrumentation that is portable, easy to read accurate in fieldapplications and operable by qualified users. This means lightweight instruments typically carried by one person. Theinstruments must have internal power supplies or operate fromships power with light cabling. The instruments must havedisplays that are easy to obtain data without extensive fineadjustments. The display should be bright enough so the operatorcan read the display in dimly lit spaces. The instrumentationmust be rugged and not affected by occasional impacts. Theinstrument must operate in humid, damp, environments wheretemperature varies between 30 to 100 degrees F.

Skilled operators are required to ensure accurate results. Thelevel of experience and the degree of training of the survey teamhas a significant influence on the accuracy of the survey data.The survey technicians should be qualified in the operation ofthe ultrasonic measuring equipment. Initial training can be on-the-job learning from a more experienced operator or a formaltraining program offered by a non-destructive testing society. Aformal program is recommended because the operator learns theconcept as well as the skills and is certified to a specificlevel of skill and experience. An operator can be certified as alevel I, 11, or III technician as quoted in The American SocietyOf Non-Destructive Testing (ASNT) standard, with level I beingthe initial certification. The gauging team should have at leastone of the operators qualified to level II to ensure that theequipment will be operated by an experientied technician. Inaddition, the survey team should be familiar with the shipboardgauging environment. An operator experienced only in land-basedenvironments may find it difficult to adjust to shipboardsurveys.

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5.0 CORROSION SURVEY METHODOLOGY

This section details the methodology developed to surveycorrosion loss and ultimately to predict corrosion rates of shipstructures. The method is intended for use on any ship wherecorrosion exists. Efforts are aimed at establishing a highlystandardized procedure covering theanalysis of data

acquisition, recording, andthat will ensure acceptable accuracy.

Accordingly, this section is broken down into four mainsubsections: Data Acquisition, Data Recording, Data Analysis,and Program Implementation.

5.1 Data Acquisition

Data Acquisition consists of defining, measuring and recordingdata parameters that will accurately describe the corrosionpattern within a given ship. The intent of the survey iscollecting information on corrosion rates in specific shipstructural components and general information on corrosion ratesof the entire ship. The extent of the survey is determined bythe needs of the user and may vary from ship to ship. The needfor accuracy and practical results is of paramount importance andthus requires a thoroughly standardized procedure encompassingthe entire data acquisition process. Aspects of this processinclude:

l Planning and Coordinationl Safety and Access. Instrumentation. Gauging patterns.

Prior to surveying a ship and before actual data can be obtained,a thorough planning strategy must be developed and followed toensure consistency, completeness and accuracy.

5.1.1 Planning

Prior to beginning a survey it is necessary to ensure that thescope of work is fully defined. This involves carefulidentification of all structural components to be surveyedthroughout the ship so as to expedite the survey and obtain arepresentative assessment of a ships corrosion rates. A NavalArchitect should meet with the steelinspector that will lead thesurvey team to review any past history and data available for thevessel to determine the ships structural arrangement, corrosioncontrol systems and potential problem areas. A completedetermination of the types of structure and identification ofexact locations consisting of panels, stiffeners, etc., should benoted on the structural plans of the ship. A detailed discussionof locations to survey is contained in section 5.1.3.4. Afterthe survey locations have been determined, the survey team mustcoordinate with the ship operator. The time between surveys will

40

generally be determined by the ships owner and for commercialships will typically fall between the four/five year drydockingsurvey required by regulatory bodies. Therefore, the corrosionsurvey will typically be accomplished while the ship is underwayor at pierside so as to minimize interference with the shipsoperating schedule. If the time allotted for the survey will notbe sufficient to enable completion of the entire ship, a prioritylist should be established indicating the locations that shouldbe surveyed first. Coordination with the master of the ship isnecessary to develop the timeframe, inspection route, andpriority list that the survey team will follow. In addition, themaster of the ship should be responsible for ensuring that allnecessary safety precautions and access requirements arefulfilled.

5.1.2 Safety and Access.

During the planning stage, considerations must be given tolocation safety and access. The considerations must involve thepreparation and acceptance of safety procedures and agreement ofmeans to access the various structural locations to be surveyed.

Safety procedure and standards vary among owners and ships andthe survey team must be aware of the practices. Typical items ofimportance to survey personnel may include:

. Suitable Atmosphere (Oxygen Content, Hazardous Gases...)

. Temperature Extremesl Lightingl Climbingl Equipment. Rescue Procedures.

The International Safety Guide for Oil Tankers and Terminals(ISGOTT) contains basic requirements and sets minimum standardsregarding tank entry of oil tankers (5-2). Depending on theowner and vessel, these safety guidelines may or may not beapplicable. However, safety is an important issue of any surveyand a set of procedures and practices should be recognized andadopted prior to commencement of the survey.

In addition to safety awareness, and indeed an integral part of.

safety procedures, is the consideration of accessing internalstructural locations. The easiest and most straight-forwardapproach is to simply climb about the existing structural membersusing ladders, walkways, stiffener platforms, etc. However, themajority of internal locations that will be surveyed cannot bereached via the permanent structure. Safety precautions willgenerally restrict the height above bottom or height above waterthat survey personnel may climb. Therefore, additional means ofmobility are required to access vertical members and deckheads.

41

During at-sea surveys, rafting has become the most commontechnique for allowing surveyors to move about within a tank.

In tank ships, the use of inflatable or rigid rafts maneuveringover a ballasted surface within a tank permits close-upinspection of bulkheads and deckheads. Adjustments to the heightof ballast levels allows surveyors access to virtually the entireinternal surface of a tank. Mobility within compartments thatcannot accommodate ballast may be accomplished via temporarystaging or mobile platforms. The use of temporary staging isoften restrictive to repair yard or pierside surveys andgenerally does not facilitate an at-sea survey. Mobile platformsare a form of temporary staging but differ from conventionalscaffolding in that they have freedom of movement. The mostcommon type of mobile platform consists of a portable, self-elevating platform suspended from wires through holes drilled inthe upper deck which allows access to deckhead areas. Othertypes consist of articulated or telescopic arms that can positionplatforms throughout a compartment. Mobile platforms are highlysusceptible to the motions of the vessel and therefore, are oftenused in drydock or sheltered conditions as opposed to at-sea. Asevidenced, there are several methods that can be employed toaccess locations in addition to numerous safety considerationsthat may apply under given conditions.

It is the responsibility of the survey team, ship owner andmaster to determine the exact procedures that will be used toensure safety and allow access to survey locations. Uponaccessing the proper locations, the survey team is ready to beginmeasuring.

5.1.3 Instrumentation

Ultrasonic devices are currently the most common type ofinstrument used to measure structural steel thickness in ships.A complete ultrasonic instrument is composed of a display andtransducer (probe). The method ultrasonic instruments use tomeasure thickness is commonly termed the pulse-echo technique.In this technique, the instrument generates an ultrasonic signalwhich is transmitted to the structure via a connecting coaxialcable and special probe which is placed in contact with thestructural surface. The pulse (sound waves) travels through thestructure to the far side and then reflects back to theinstrument via the probe and cable. Thus , thickness is obtainedby measuring the elapsed time between signal entrance and exitfrom the structure.

42

5.1.3.1 Displays

There are three types of displays available that allow anoperator to acquire thickness readings:

l CRT DISPLAYS. DIGITAL DISPLAYl COMBINATION CRT-DIGITAL DISPLAY

The CRT display resembles an oscilloscope in that the ultrasonicsignal generated is displayed on a screen. An example CRTdisplay is shown in Figure 5-1. Thickness is measured on thescreen as the distance between leading edges of peaks, as shownin Figure 52. This feature permits interpretation of back wallscatter and coating thickness effects. The accuracy associatedwith a CRT unit typically ranges from ~ 0.010 to ~ 0.020.

The digital display is a compact, hand-held unit that recordsthickness directly in the form of a numerical LCD display. Anexample digital unit is shown in Figure 5-3. The accuracyassociated with a digital display unit typically ranges from ~0.005 to f 0.020. The main advantage is the digital unit is inits compactness. The main disadvantage is that impurities in thesteel or surface coatings or scale that reflect sound energy alsocreate misleading echoes and influence the sound wave pattern andcannot be discriminated as such by digital units. The CRT unitcan discriminate between these echoes and true backwall echoesbecause the operator can observe the echo pattern and pick outfalse echoes.

Combination units are larger and more expensive than the othertypes (42). For these reasons they are not commonly used inhull survey work. CRT display and digital display units are themost commonly used by inspection and non-destructive testing(NDT) companies for structural inspection. It is generallyagreed upon in the inspection industry that digital display unitsare rapidly becoming the favorite for measurement ease andaccuracy. However, CRT units are still widely used because theypermit interpretation of back wall scatter. Marked differencesexist between the display units and must be addressedaccordingly.

l

While the CRT and digital units exhibit a difference in operationand versatility, the performance and accuracy are similar. CRTunits require a greater skill level to operate than the digitalunits but operators typically spend an equal amount of time toobtain a clear thickness reading. If the surface has beenprepared prior to gauging (i.e., removal of coating,scale. .etc.) a reading can be taken every 10-20 seconds. If theinspector must prepare the surface on the spot, readings can takethree to four times longer. Generally, surfaces with intactcoating do not require survey. Surfaces with scale or

43

.. .

FIGURE 5-1CRT DISPLAY EQIJIPHENT

44

/n SINGLE CRYSTAL INDIRECT CONTACT WITHCOATING LAYER

1 I

IAMPLITUDE I

I ITRANSMITTER i I

PuLSE

L

IAINT + I,TEELBACKWALL:C+m

I

I

1

COATING

BARE

STEEL

p mm

tmm

I O* STEEL ~OUIVALENTOF COATINGTHICKNESS

4 BACKWAU 3r~STEELBACKWALL1 1 I 1 1 1 I J 1 I I

1 I I

FIGURE 5-2THICKNESS MEASUREMENT THROUGH A

CORTINB LdYER

45

FIGURE 5-3DIGITAL DISPLGY EQUIPIIENT

46

failed coating should be prepared prior to gauging, otherwise thecoating thickness must be determined and often the coating is notof constant thickness. If a digital display is used, surfacesmust be prepared since the operator cannot observe the sound wavepattern and distinguish coating effects. Recent advances indigital equipment offer the ability to read multiple echoes, thusenabling the distinction between steel and coating thicknesses.However, regardless of the equipment used, inspectors prefer tomeasure bare surfaces. This practice helps to reduce possibleerrors due to coating and scale. In addition, the presence ofscale or roughened surfaces may cause a scattering of soundenergy and adversely affect the echo pattern resulting inerroneous measurements.

Current indications from the inspection industry reveal that bothCRT and digital units are used with equal preference. However,digital units are rapidly becoming the favorite as accuracy andflexibility increase, thus overcoming the advantage of actualwave pattern visualization as afforded by CRT units. Astechnology advances, so does the reliability of digital units.Flexibility increases through the use of several different probescoupled with the ability of recent digital units to read multipleechoes. Multiple echo processing allows digital units todistinguish faulty readings and thus improve accuracy beyond theCRT level without requiring wave visualization. The most recentadvances in digital units include:

1 Microprocessor-based design,2 Internal Datalogger allowing the storing and

sorting of up to 1000 readings,3 Ability to off-load stored readings directly to a

computer or printer via a two-way communicationsport,

4 Interfacing ability with a host computer to runmost statistical processing control softwarepackages.

The ability of digital units to interface directly with computersis a tremendous advantage which could ultimately eliminate theneed to hand-record every reading. Digital units will clearlybecome the choice of the future however, CRT units are not to beneglected. The corrosion survey methodology warrants the needfor consistency and standardization and equal success can beachieved with several display-probe combinations. Once adisplayprobe combination is selected, inspectors should completea survey using the same instruments and vary probe types onlywhere special conditions necessitate this practice. In addition,subsequent surveys of the same vessel should be conducted withthe same instruments (make and model). Regardless of whetherinstrument consistency is indeed adhered to, careful calibrationand instrument error must be established. Instrument error isthe single largest source of error associated with statistical

47

sampling and greatly influences the amount of data required for acorrosion survey.

5.1.3.2 Transducers

In addition to the display units, a probe must also be chosen.There are two types of thickness-measuring probes; single andtwin. The single probe uses the same crystal for bothtransmitting and receiving while the twin probe has thetransmitting signal electrically and acoustically separated fromthe receiving signal. Figures 5-4 and 5-5 illustrate the methodused by both probes. Two characteristics of transducers mustalso be addressed: frequency and diameter. Frequency affects thesound transmission character