29 MEl 19B0 tab. y. Scheepsbouwbn ARCHiEF SSC-293 Technische Hogeschool Deift

UN DERWATER NON DESTRUCTIVE TESTING OF SHIP HULL WELDS

This document has been approved for public release and sale; its distribution is unlimited.

SHIP STRUCTURE COMMITTEE 1979 SHIP STRUCTURE COMMITTEE The SHIP STRUCTURE COMMITTEE is constituted to prosecute a research program to improve the hull structures of ships and other marine structures by an extension of kno.dedge pertaining to design, materials and methods of construction.

RADM H. B. BELL (Chairman) Mr. M. PITYJN Chief, Office of Merchant Marine Assistant Adminis,rator for Safety Convnercial Development U. S. Coast Guard Maritime Adminittration

Mr. P. M. PALERMO Mr. R.B. KRÄHE Director Chief, Branch of Marine Oil Hull Integrity Division and Gas Operations Naval Sea Systems Corrricozd U. S. Geological Survey

N. HANNAH Mr. C. J. WHITESTONE 'eri dent Chief Ar'can Bureau of Shipping Military Sealift Coramzd LCDR T. H. ROBINSON, U.S.Coast Guard(Secretary)

SHIP STRUCTURE SUBCOMMITTEE

The SHIP STRUCTURE SUBCOMMITTEE acts for the Ship Structure Committee on technical matters by providing technical coordination for the determination of goals and objectives of the program, and by evaluating and interpreting the results in terms of structural design, construction and operation.

U.S. COAST GUARD MILITARY SEALIFT COMMAND

CAPTR.L.BROWN Mr. T. W.CHAPMAN CDR J. C. CARD Mr. A. B. STA VOTI(Chairman) LCDR J. A. SAHIAL, JR. Mr. D. STEIN CDR W. M. SIft'SON,JR. AMERICAN BUREAU OF SHIPPING NAVAL SEA SYSTEMS COMMAND Dr. H.-Y.JAN tr. R.C'BlU Dr.D. LIV Mr. R. JOHNSON Mr. I.L. STERN Mr.J. B. OBRrEN Mr.G. SORYJN MARITIME ADMINI STRATI ON

U. S. GEOLOGICAL SURVEY Mr. F. J. DASHNAW Mr.N. O.HAMMER Mr. R. GIAÍGERELLI Mr.F. SEIBOLD Mr. J. GRE'GORY Mr. M. TOLIMA

NATIONAL ACADEMY OF SCIENCES INTERNATIONAL SHIP STRUCTURES CONGRESS SHIP RESEARCH COMMITTEE Mr. S. G.STIAWSEN -Liaison Mr.O.H. OAKLEY -Liaison Mr. R.W. RU

THE SOCIETY OF NAVAL ARCHITECTS Mr.R. H.STERNE - Liaison & MARINE STATE UNIVERSITY OF NEW YORK MARITIME COLLEGE Mr. N. O. HAMR -Liaison Dr. W.R.PORTER - Liaison RESEARCH COUNCIL ¡J.S. COAST GUARDACADEMY Mr. K.H. KOOPMA]q -Liaison CAPT W.C. NOLAN - Liaison U. S. MERCHANT MARINE ACADEMY U. S. NAVAL ACADEMY Dr. C.-B. KIM - Liaison Dr.R. BHATTA CHARYYA -Liaison Addre Corrponden to: Member .Agenci: United Skit Cct Guoth Naval Sez Systems Command US. Ct Guard Hdquarters, (G-M/82) Mlikry Seft Command Whington, D. C. 20590 Mantxme AdminLsäon United Stritem Geologicxil Survey Amena2n Bure'.yu of Sh,pping ScspStructure Committee An Interagency Advisory Committee Dedicated to Improving the Structureof Ships SR- 1243 September 1979

The Ship Structure Committeehas sponsored a state-of-the-art study of underwaternondestructive inspection techniques to determinewhether or not additional research is necessary toprovide adequate methods for inspection of largevessels and mobile drilling units while afloat andof offshore platforms below the waterline.

This report documents the resultsof this survey. // Hen ;ell Rear Admiral, U.S. Coast Guard Chairman, Ship Structure Committee Approximate Conversions to Metric Measures METRIC CONVERSION FACTORS Appr.ximale Conversiono from Metric Measures S ym b ci When Voit know Mitiliply by o Find Symbol e Symbol Whan You know LENGTHMultiply by Te Find Symbul iO LENGTH nrcnreIn nrihllnleloqsCenlimelerS 0.40.04 lIchesedres in reiydft milesyardsPeelinches 30 0.92.51.6 kilonieleisnrelerscenhrmetersceillinrelors o.cmkm kmnr kilonrelersmelaremeters 0.63.31.1 mulesyardsbIel OrIFI square inches A R EA 6.5 square cenrImelors cm2 or re2Cr152 squareSquare centirrrelersmoIns AREA 1.20.16 square riChes n2 ¡ nil9d2 merossquaresipilara yardsmilesbel 0.80.42.60.99 llirCtilflS541lIresquare541lIre kuloiuietirs lIre11.rirelr.ss s hakm2nr2 halorI2 hectaressquare 10.000 hiloirroters 2.50.4 squali,squire yalItsm,les ari2yd o, ounces () 28 e. MASS (weiqht ib poliuidsshun (2000 ib) loris VOLUME 0.90.45 1015145kilogramsgrams kg9 e 9 tkg Imaneskilogramsgrunts (1000 kg( 2.20.0351.1 slisrlpor.ndsounce, liais IbOr Iltbsptsp os huidlablespoonsteaspoons maneen 3016 5 nullilibeiCitrilliliber$ w hieromill (hIers VOLUME 2.10.03 purInllii.d milices piIl o! gPpic goonspinIecups 0.470,950.24 Pipersilrjllililer$(1ers ta m3nu3 cubicliters calero 35 0.261.06 gallolisguarI;Cubic feel galqt 543ii1gal cubicgalloise yardsbet 0,760,033.8 CubiCcsIr,c melees nlelert(1ers ris cubic melero 1.3 cubiC yards gd3ll (exact) Celsius TEMPERATURE (exact) Op Fahrenheil lemperalure 321oubirectiog5/9 (oHne CelsiuS leinperalore OC IS Ieelrperaliire add9/5 (Iban32) Fahrenheit lempCraluure n. 40 0 32 40 O so 986 f IZO f60 200 "2 Unite ol WoulInite miel Measorie. PIio 52.25,00i, r Calaloil No. Cl3.l0286. 2.54 i..s. riel. Fa oSrel erad Couurnrsnl,us seul rune rIa,lrmt 54,11,5. u.s. NilS Meo. Pohl. ?iai. ue -40 b -20u.J °l° i 0 f co 3? I.140 so I ljSU 'CIOU UNCLASSIFIED SECURITY CLASSIFICATION OF THIS DAIGE I4,.,, O.tEnr.r.d) REPORT DOCUMENTATIOP'l PAGE BIFORECORM I. REPORT NUMBER 2. GOVT ACCESSION NO. 3.RECIPIENT'S CATALOG NUMBER SSC- 293

4 TITLE (.nd SIrIì 5 TYPE OF REPORT & PERIOD COVERED

UNDERWATER NONDESTRUCTIVE TESTING OF SHIP FINAL REPORT 6. PERFORMING ORO. REPORT NUMBER

7.AiJT.4OR(s) I. CONTRACT OR GRANT NUMBER(I)

R. Voushaw and C. Dyer Z70099-671375

9.PERFORMING ORGANIZATION NAME ANO ADDRESS 10. PROGRAM ELEMENT, PROJECT, TASK AREA & WORK UNIT NUMBERS Naval Surface Weapons Center Silver Spring, MD 20910

II. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT GATE September 1979 Department of the Navy Naval Ship Center IlNUMIEmopAGEs Washingthn, n r 2fl3S2 14 MONITORING AGENCY NAME & ADDRESS(II d/ff.r..,f (ro,,, Controlllná Ot(Ic.) IS. SECURITY CLASS. (of 1h11 :.porf) Ship Structure Committee U.S. Coast Guard Headquarters UNCLASSIFIED Washington, D.C. 20590 15& DECLASSIFICATION/DOWNGRAOING SCN EDU LE

15. DISTRI8UTION STATEMENT (of IhIs RIpoff)

UNLIMITED

¶7. GIST RI 6UTION ST AT EM ENT (of 1h. aS.rracI nt.r.d iI'Dlock 20, IidIf(.r,f fr.., R.p.rI)

UNLIMITED

II. SUPPLEMENTARY NOTES

19. KEY WORDS (Cor,tInu. or, rIvIrII id. if n.c...,y Id.r,1!f>' by block Non-destructive Testing Magnetic particle Ultrasonic Underwater Testing 20. AUSTRACT (ContInu. o, rIf.,.. .!d. If n.c....ry did.ntify by block nub.r) Techniques are presented wherebynondestructive testingof huit butt welds can be accomplished underwater. Radiography,ultrasonicinspection, and magnetic particle testing are discussed includingthe modificationsnecessary for underwater applications. In allcases, traineddivers are required.

FORM DDI JAN 731473 EDITION OF I NOV5 IS OISOLtTE S/N Ot02-014 6601 UNCLASSIGIED SICURITY CLASSIFICATION OF THIS PAGC (,.n D.f. Snf.r.d) CONT ENTS

Page

i INTRODUCTION

i OBJECTIVE AND SCOPE

NONDESTRUCTIVE TESTING OF HULLWELDS i

3

3 UNDERWATER CLEANING

5 ENVIRONMENTAL LIMITATIONS

NONDESTRUCTIVE TESTING METHODS 5

17 ADDITIONAL METHODS OFNONDESTRUCTIVE TESTING

19 COST CONSIDERATIONS

20 CONCLUSIONS

21 REFERENCES

22 BIBLIOGRAPHY

V LIST OF FIGURES

Figure Paqe

1 An example of a Steel Weld andAdjacent Area Cleaned of Marine Growth to Bare Metal 4

2 A Gauge for Measuring Weld Reinforcement 7 3 Recommended Positioning of Electrical ProdsWhen Inspecting Butt Welds 7 4 An Example of Crack Detection Using Fluorescent Magnetic Particle Inspection 7 5 Watertight and Neutral Film Cassette Package for Underwater Radiography 10 6 Schematic Arrangement of the Isotope Holder for Underwater Radiography 10 7 Maximum Voltage or Radioactive Energy for Minimum Steel Thickness 12 8 Radiographic Technique Curves for Steel at 250 KVP with Various Thickness of Water Made Between the Source and Object 13 9 Radiographic Technique Curves for Steel AT 2 MeV with Various Thickness of Water Interposed Between the Source and Object 13 10 Increase in Radiographic Exposure at 250 KVP Made Necessary by InterposingWater Between the Source and Object 13 11 Increase in Radiographic Exposure at 2 MeV Made Necessary by Interposing WaterBetween the Source and Object 13 12 Degradation of Radiographic Sensitivity for Steel at 250 KVP due to Interposition of Water Between the Source and Object 15 13 Degradation of Radiographic Sensitivity for Steel at 2 MeV due to Interposition of Water Between the Source and Object 15 14 Positioning and Manipulation of the Probe for Ultrasonic Shear Wave Inspection of Butt Welds 16 15 Typical Test Block for of the Ultrasonic Instrument 16

16 Diver Using Acoustical Holography Probe 18

LIST OF TABLES

Tabi e

i Present NOT Techniques: Advantages and Limitations in Underwater Application 2

2 Electrical Requirements for Magnetic Particle Inspection 8

vi INTRODUCTION

At present, steel ships arenondestructively inspected at the time of fabrication and thereafter onlyin drydock. The expense of drydocking a modernvessel is such that this is done only at the time of scheduled hullmaintenance or if structural damage has been incurred. Until recently, there were no other options. However, in a related industry,offshore drilling, nondestructive testing is being done onsite,including that portion of the structure positionedunderwater. Considering the lost operating time and sizable expenseinvolved in drydocking a modern steel vessel,it may be desirable to do underwater nondestructive testing on some occasions toprovide assurance of hull integrity. In addition, as underwater weldingtechniques are inproved and further developed, it is conceivablethat hull repairs may be done underwater, thusobviating the need for dry- docking. Such repairs might not be acceptable tounderwriting associations or code bodies unless proof ofadequate weld quality can be verified by nondestructivetesting. The Ship Structure Committee hasrecognized the advancement in technology and has requested the NavalSurface Weapons Center to prepare a state-of-the-art report onUnderwater Nondestructive Testing.

OBJECTIVE AND SCOPE

The objective of this task is to providethe maritime industry with nondestructive testing (NDT)techniques suitable for the underwater inspection of steel welds. This is to be done within the framework of existing methodsof nondestructive testing. In addition, to a state-of-the-art survey,the methods of NDT which are suited to will beanalyzed in regard to technical capabilities and limitations and themodifications desirable or necessary for theirapplication to underwater weld inspection.

NONDESTRUCTIVE TESTING OF HULL WELDS

Traditionally, steel hull welds arenondestructively tested with one of five methods of nondestructivetesting: visual, magnetic particle, radiography, ultrasonicsand liquid penetrant. With the exception of liquid penetrant, allof these methods have been adapted to underwater steel weldinspection. The advantages and disadvantages of each method as applied tounderwater hull weld inspection are presented in Table I. TABLE 1 METHOD DEFECTSPRESENT NDT TECHNIÇUES: ADVANTAGES AND LIMITATIONS IN UNDERWATER ADVANTAGES APPLICATIONS LIMITATIONS Visual SurfaceImpact cracks,damage. televisiontransmittedcanEasy beto photographedinterpret. and to videotopside orrecorded. by Findings detailedSurfaceLimited musttoobservation. surface be cleaned defects. for ParticleMagnetic flaws.somelaps,Surface near-surface seams, cracks, and evaluationorIndications televised and topsidecan video be recording.forphotographed Weathernearzone.Requires surface dependent thorough defects. in splashcleaning. Limited to surface and Present Ultra- Cracks, Inclusions, Especially sensitive to cracks. Cumbersomeequipment limited to perform to diver underwater. use.. sonics welds.penetrameterincompleteLack of fusion in and ResultsEquipmentsubsurfaceCan be canused isbeintegrity. to lightweight.transmitted evaluate diverPresentSurfaceOperatorThorough use. roughness equipmentskill cleaning is canrequired.limited required. affect to test. graphyRadio- suchInternal as shrinks, defects StandardsProvidestopside anda havepermanent video been recorded. establishedrecord. Potential health . Water penetrationporosity,fusioninclusions, and lack inincomplete welds.of industry.and are accepted by codes and sensitivity.itbothandshould is subject.sides. difficult be displaced to obtain between 2% source With availableRequires isotopes, access to DIVING EQUIPMENT

Underwater NDT requires a diver. Because the hulls are not very deep, can usescuba equipment. This system has the diver carrying his own air supply andgives maximum freedom of be supplied movement. Alternatively and preferably,the diver can media (air or mixed gas) fromthe surface by a flexible Although the umbilical cord ofthis latter system inhibits hose. for freedom of movement somewhat, theumbilical cord is necessary other reasons: First, voice communication is veryvaluable. Second, in most cases, videotransmission topside isworthwhile. Third, the diver needs electricityfor a number of reasons, ranging from lights to power equipment. Considering these requirements, the surface-supplied system seemspreferable. It meters with can be used to depths of50 meters using air and to 90 »- These depths exceed any depthof hull immersion in commercial shipping. The diver may have a dry suitand a heavy helmet or a face sealing mask and a wet suit which canbe filled with warm water. The heavy helmet seems to beadvantageous in that itreadily light accomodates voice communicationequipment, and a source of the work :an be mounted on theside of the helmet to illuminate area which frees thedivers hands for other tasks. A television transmitter can also be mounted onthe helmet which permits topside personnel to view the workand the work area.

The diver equipment describedabove represents the current state of the art and is commerciallyavailable.

UNDERWATER CLEANING

With the exception of radiography, everymethod ofNDT now in use for underwater workrequires that the surface tobe inspected be cleaned to bare metal asillustrated in Figure 1. Mostly, this scale, loose means removing marineorganisms such as barnacles, but paint and rust must also be removed. This requirement often is more time consuming anddifficult then the inspectionitself. The cleaning can be done byhand with a scraper or brush,but manual methods are not practicalfor jobs larger than two orthree linear feet of weld. For the bigger jobs, powertools are available of which water jetting is mostoften used. The system consists of diver a surface pump and ahigh- hose. At the work site, the manipulates the jet with gun-likecontrols. on the order of 15,000 pounds/square inch arepossible. To keep the diver from being pushed off site, the nozzlealso has a lesser pressureflow in the opposite direction - thuscounterbalancing the of feet per minute reaction. Cleaning rates of two - three square are claimed possible byskilled and experienced operators.

-3- FIGURE 1 AN EXAMPLE OF A STEELWELD AND ADJACENT AREA CLEANED OF MARINE GROWTH TO BAREMETAL

-4- Note: The water jet is very dangerous and if inadvertently directed toward the diver can inflict physical damage as severe as limb amputation. Other power tools for underwater cleaning include hydraulic grinders and needle guns. In regard to the needle gun, it should be noted that this tool delivers impact blows to the work areaand, therefore, to a degree peens the metal. This makes suspect the possibility of concealing defects otherwise opento the surface. This has caused one insurer (Lloyds Register) to state apreference that the weld and heat-affected zone not be cleaned with aneedle gun.2

ENVIRONMENTAL LIMITATIONS

The diver may be hampered in his work by murky water. This condition can be improved by flooding the work area with clean water pumped from topside. Extreme cold may severely limit the diver's staytime. As mentioned before, diving suits are available which canaccomodate an injection of warm water. Turbulent water is the most severe limitationlikely to be imposed on the diver. This is especially true near the water line where wave action is most pronounced. would have to be determined on a per case basis,but scaffolding lowered from topside may be useful.

NONDESTRUCTIVE TESTING METHODS

Visual Inspection. Visual inspection requires water clarity and adequate illumination. Previously discussed, equipment and techniques can be used to achieve this. Because the face plate of the diver acts as a lens:wIth slight magnification,which is usually helpful in visual inspection, it does requirethat measurements be made against a standard (Rule)rather than be estimated. If the nature of the inspection is surveillance,visual inspection should also be done prior to cleaning as thereis sometimes a perceptible cqlor change in the marine growth immediately over a crack. The detection of such a condition prior to other NDT would be very meaningful in furtherplanning or work. After cleaning, the weld should again be visuallyinspected in-as-much as cracks found this way can reduce the needfor more sophisticated NDT or enable an improved scope ofinspection.

-5- Using visual inspectionon new welds, the diver can measure a weld profile using commerciallyavailable gauges, Figure 2, while undercut can be measured witha .

Underwater photography is a wellestablished art and can be used to provide a record and formore extensive evaluation topside.

Magnetic Particle Inspection. Magnetic particle testing (MT) can be applied to ferromagnetic materialssuch as ordinary carbon steels. Of the sophisticated methods ofNDT, magnetic particle testing is the most widely used forthe inspection of offshore drilling rigs and is readily adaptableto underwater hull weld inspection.

The test consists of three basicoperations:

Establishing a suitable magnetic fieldin the object, where the magnetism must be inthe correct direction, and the field strength must be sufficientlystrong.

Applying magnetic particles to the surface ofthe object in the magnetizedarea.

Examining locations where the particlesaccumulate. The method uses a pair of electricalprods positioned alongside the weld as shown in Figure 3. It is recommended that the prods be used in conjunction with leadshoes or other low melting point materials tosuppress arcing and thus prevent burn marks on the steel surface. The electrical current required for proper magnetization underwater is the same as that used when working in dry air, Table 2.

Alternating current yokes and permanent magnetscan also be used to induce the magnetic field but the surfaceshould be ground smooth for good contact.

Assuming the weld area has been cleaned of marinegrowth and the diver has adequate visibility (Underwater lighting),ordinary magnetic particles in a slurry ina squeezable container can be used to complete the inspection. The suspension is directed at the inspection area and the results obtainedare essentially the same as when done topside using dust.

Superior results are obtained by using fluorescentparticles and illuminating the work area with ultravioletlight. Cracks are readily detected, Figure 4. The lamps for use in this type work have been designed with adequate electricalinsulation and resistance to hydrostaticpressure and are commercially available.

If a record of the magnetic particle inspection isdesired, the diver's equipmentcan include a television transmitter and video recording can be done topside. Alternatively, photographs can be taken.

-6- GAUGE

FIGURE 2 - A GAUGE FOR MEASURING WELDREINFORCEMENT

MAGNETIZING CURRENT

MAGNETIC LINES OF FORCE WELD

FIGURE 3 - RECOMMENDED POSITIONING OF ELECTRICAL PRODS WHEN INSPECTING BUTT WELDS

FIGURE 4 - AN EXAMPLE OF CRACK DETECTION USING FLUORESCENT MAGNETIC PARTICLE INSPECTION

-7- TABLE 2 ELECTRICAL CURRENT REQUIREMENTS FOR MAGNETIC PARTICLE INSPECTION PROD SPACING (INCHES) UNDER 1/4" SECTION THICKNESS AMPERES 3/4"AND OVER (AMPERES) 43 400-500300- 400 500-625 375- 500 65 600-5 00-625 750 750-900625- 775 87 800-1000700-875 1000-1200875-1100 lo 9 1000- 1200900-1100 1200-14001100-1300 1112 1200-14001100-1300 1400-16001300- 1500 Radiographic Inspection. Radiography requires that the film cassette and radiation source be positioned on opposite sidesof the hull. The film cassette can be placed on the outside of the hull with the source of radiation inside, or this arrangement can be reversed. There are advantages using the first arrangement and, if possible, hull radiography should be done this way. However, if a physical obstruction prevents placement of the radiation source inside the hull but there is room toposition a cassette, then radiography can still be done using the other arrangement. Radiography of steel welds of ordinary hull thicknesses is usually done with positive pressure cassettes containing lead intensifying screens. For the film cassette outside the hull, there is a need for watertight integrity and a polyethylene envelope will suffice.5 The cassette package can be firmly fixed in place using permanent magnets with attached springs. Because the water behind the cassette is a back-scattering media, a sheet of lead, approximately 1/8" thick, should befixed behind the cassette to minimize film fogging. If the package is too heavy for underwater work, canbe achieved by placing a low-density material, such as styrofoamsheet, behind the lead and within the watertight envelope, Figure 5. Simple experiments should enable a close approximation to neutral buoyancy. Alternatively, the cassette package can be attached to a rope supported from topside and maneuveredinto position by the diver using voice communication to coordinate the work. Aligning the film cassette and the radiation source is difficult to accomplish by coordinate measurements. Pinging is reported6to be of considerable help in locating the approximate

position , but precise positioning can be done with ultrasonic transducers. For this, an Ultrasonic probe is fixed in position at the desired location within the hull and theultrasonic instrument is set to receive with the entire range displayed. The diver then manipulates a second probe of the samefrequency, but powered separately. When the two probes are aligned, a signal will be received on the oscilloscope and, using voicecommunication, the diver is instructed to mark that location. The thickness of the hull at the weld site canbe determined precisely using an ultrasonic thickness gauge. This information, in conjunction with radiographic technique curves,affords the radiographer a means for correct exposure to obtain thedesired film density. Should circumstances dictate that the source beplaced out- side the hull, neutral buoyancy becomes more important. The underwater source of radiation will be an isotoperather than an x-ray machine as no commercial x-rayequipment has been modified to this purpose whereas this has been donewith isotopes.7 The isotope is invariably encased in lead of substantialthickness. In addition, rigid structure must be used tomaintain the source to object distance, Figure 6. This structure can be either conical or pyramidal in shape. If made watertight, either air or water can be allowed to fill the space.Whichever system is used,

-9- SEAL FIGURE 5 - WATERTIGHT AND NEUTRAL BUOYANCY FILM CASSETTE PACKAGE FOR UNDER- WATER RADIOGRAPHY

RADIATION SOURCE

POLYETHYLENE/ ENVELOPE

STY RAFOAM SHEET HULL' LEAD SHEET FILM / CASSETTE HULL PLATE

FILM LEAD HOOD

FIGURE 6 - SCHEMATIC LEAD SHIELDING ARRANGEMENT OF THE ISOTYPE p \ HOLDER FOR UNDERWATER RAD 10G RA PHY I

4 MRHM RADIATON LEAKAGE

-10- consideration must be givento the diver's limitationto manipulate the structure into position,even if assisted by ropes maneuvered from topside.

Once the source is properlypositioned, permanent magnets be used to grip flanges can and fix the structure inplace. As before, the correct exposure timecan be calculated, but it is difficult to control in-as-muchas it depends upon the diver's prompt action in regard to cuttingoff the sourceor, having the cassette moved from the field of continuingradiation.

Where the radiationsource is inside the hull,an x-ray machine is preferableto an isotope because it affordsthe radiographer a means of selectinga kilovoltage suited to the object thickness whereasisotopes operate at specificenergies and only Co60 and Ir'92are commonly available. If an x-ray machine is used, the selectionof kilovoltage fora specific thickness should not exceed that shownin Figure 7. Also shown in this figure are the ordinarythickness range of Co6° and Ir192and their possible extension beyond thisrange with marginal radiographic sensitivity.

If the radiationsource is outside the hull and the film cassette inside and the water isdisplaced, then theexposure time is unchanged. If, however, the water isnot displaced and the radiation must penetrate thewater before reaching the weld, then the exposure time will belengthened and the radiographic sensitivity will be degraded. The extent of these effectswere determined experimentally usingx-rays of 250 KVP and 2 MeV which approximate the isotopes Ir'92and Co60. This was done by making radiographs of steel platesof various thicknesses with andwithout columns of water between thesource and the plate.

From measurements of the filmdensities, radiographic technique curves were constructedfor steel and specific water columns, Figures 8 and 9 andalso for water and specific thick- nesses of steel, Figures lO and 11. These curves enable a determination of the half-value-layerof water for eachenergy, e.g., for 250 KVP this is 2.4" and for2 MeV this is 5.2".

The extent of degradationwas determined by placing an array of penetrarneterson top of the steel plates and thencalculating the sensitivity according to thesmallest visible penetrameter holes using the equation: S1XN1 = S2xN2 where: S1 equivalent penetrameter sensitivity N1 = 2 S2 = contrast sensitivity N2 = ratio of minimum detectablehole diameters to penetrameter thickness.

-11- I i I COBALT 60 ow 4 Ir-192 e p. 1 III 0.1 0.15 0.20.30.4 0.6 0.8 1.0 1.52 67810 1520 30

20

15

10 > g w 8 7 6 5 4 3

2 MAXIMUMPERMI SSIBLE VOLTAG E

1 MEV 900 800 700 600 500 40o /1 PTABLE REGI o ACC E ON 300

200

150

100 0.1 0.15 0.2 0.3 0.40.6 0.8 1 0 1 52 3 45 6 7 8 10 15 20 STEEL SPECIMEN THICKNESS (INCHES) BROKEN LINE INDICATES MARGINAL SENSITIVITY

FIGURE 7 MAXIMUM VOLTAGE OR RADIOACTIVE ENERGY FOR MINIMUM STEEL THICKNESS

-12- 7000 10,000

1000

z 100 to

IC D o 'C

Q X 4 'p o o 15 IC

10

(FILM OENSITY7O. TFOR' 1/2" LEAD (FIL M DENSITY .2.0, TFO32' AA FII,M, FILTER AT TUSE, 1/8" LEAl) FILTER AT FILM, DEVELOPINC TEMPEFIATEIRE" 0I"F) AA FILM, OEVELOPINO TEMPERATURE - 01°F) I I I I I J I I I IO 1.50_I1.75 0,5 1.0 1.5 2.0 2.5 3.0 3.5 't1.05 7.25 0.20 0.50 0.70 .1HICKNESSOF STIEL - INCHES THICKNESS OP STEEL .INEHES FIG. 8 - RADIOGRAPHIC TECHNIQUE CURVES FIG. 9 - RADIOGRAPHIC TECHNIQUE CURVES FOR STEEL AT 250 KVP WITH VARIOUS FOR STEEL AT 2 MeV WITH VARIOUS THICK- THICKNESSES OF WATER INTERPOSED BETWEEN NESSES OF WATER INTERPOSED BETWEEN THE THE SOURCE AND OBJECT SOURCE AND OBJECT

10.000 - 10.000-

a z 8 1000 1000

t.. a

1< Q X 4 a o 1. a 100 700 4

IS 114" STEEL, TFDG FEET. D-2,01/2" LEAD FILTER AT SOURCE. 1(8" LEAD FILTER AT FILM. AA FILM. DEVELOPING TEMPERATURE - 81°EI

(1" STELL, TFD - 32", 0" 2.0. AA FILM. DEVElOPING TEMPERATURE 87°F) t t I I 10 t' 10 8 JJ10 12 14 24 28 4 6 4 0 12 (6 20 T(-I'CKNESS or WAÌ ER - INCHES THICKNESS OP WATER ..INCHES

FIG. 10 - INCREASE IN RADIOGRAPHIC FIG. 11 - INCREASE IN RADIOGRAPHIC EXPOSURE AT 250 KVP MADE NECESSARY EXPOSURE AT 2MeV MADE NECESSARY BY INTER- BY INTERPOSING WATER BETWEEN THE POSING WATER BETWEEN THE SOURCE AND SOURCE AND OBJECT OBJECT -13- The results of this determination presented in Figures 12 and 13 show a progressive degradation of radiographic sensitivity with increased thicknesses of water between the source and object.

Ultrasonic Inspection. The adaption of ultrasonic inspection to underwater work is simple in principle. The water serves as a couplant; and except for the transducer being made watertight8, the technique is the same as that topside, Figure 14. Battery- powered ultrasonic inspection equipment is commercially available; and if made watertight, can be carried by an inspector-diver. The technique is restricted to work near the surface unless the equipment housing is designed to withstand hydrostatic pressure. A more practical approach is for the instrument to remain topside while the probe is taken below by the diver. Through voice communication the diver can be instructed regarding the position and manipulation of the probe. A television transmitter attached to the side of the diver's helmet with transmission topside permits the ultrasonic technician to see the probe move- ments as well as scope presentation and comprises a reasonable ultrasonic inspection by remote control.

Before doing ultrasonic inspection, the area of probe manipulation must be cleaned of marine growth-to bare metal. When ultrasonic inspection is done in air, cracks have an air interface with a reflection coefficient of almost 100%. In water, assuming the cracks come to the surface of the plate and water enters, the acoustic impedance mismatch is changed and some of the ultrasonic energy is transmitted into the water. Accrding to theory, the reflectivity coefficient is reduced to

88% . Laboratory experiments confirm this approximate reduction. This difference is not considered cause for concern, because even small cracks are very efficient reflectors of readily found by ultrasonic inspection.

The use of a very long cable increases the capacitance load on the instrument pulser and necessitates a higher gain setting on the instrument to achieve the same sensitivity level used in ultrasonic inspection topside with shorter cables. In addition, instrument calibration should be performed in a salt-water bath. However, the ends of the drilled holes in the test block, Figure 15, should be sealed (epoxy cement is suggested) to maintain the air-steel reflectivity coefficient upon which the present weld inspection sensitivity level is based.

A word of caution: Present procedure for inspecting butt welds with ultrasonics assumes a flat surface (250 RMS or better) adjacent to the weld. Corrosion pits may be sufficiently numerous such that beam directivity is weakened by scatter of the waves at the interfaces of the pits. The scatter may be severe to the point where an expected signal is not obtained even though the test is otherwise done correctly. There are no quantitative data available in this regard.

-14- 2.2 - 2.0 --- -

1.8

1.6

1.4 ------1.2 ------

'l.O

)D 2.0, TFD = 32", AA FILM) 0.8 i I ) ) I O 1 2 3 4 5 6 7 8 9 10 THICKNESSOFWATER-)NCHES FIGURE 12 DEGRiDATION OF RAD'IOGRAPHIC SENSITIVITYFOR STEEL AT 250 KVP DUE TO INTERPOSITION OF WATERBETWEEN THE SOURCE AND OBJECT 3.8-

3.6

3.4'-

3.2-

3.0-

2.8 \1:'- 2.6 ) 2.4

2.2

2.0

1.8 _,_-'

1.6..

1.4

1.2 (D = 2.0, TED = 6.0', AA FILM)

1.00 8 10 12 14 16 16 20 22 24 26 22 30 32 34 36 38 40 THIÒKNESS OF WATER - INCHES

FIGURE 13 - DEGRADATION OF RADIOGRAPHIC SENSITIVITY FOR STEEL AT 2MeV DUE TO INTERPOSITION 0F WATER BETWEEN THE SOURCE AND OBJECT

-15- H- -

9GURE 14 - POSITIONING AND MANIPULATION OF THE PROBE FOR ULTRASONIC SHEAR WAVE INSPECTION OF BUTT WELDS

6 1.2mm (3/64 n.) THROUGH HOLES -j- i, .-1 il i; ii II Ii il 32 mm Ii Ii Ii II ii ii II ii (1.25 in.) ,l iI I Il I li 125.5 mm.±51 mm mm mm 51 mm 25.5 mmH51 (2 ¡n.)H(2in.)+(2in.> (2 in.) 1mm (lin.) (1 in.)

V.

UT 01 76 mm 0F o-r (3 in.) 44mm 51 mm 58 n m 64mm 70mm (1.75 in.) (2 in.) (2.25 ¡rL) (2.5ri.)(2.75 ir,.) T t 306 mm (12 in.>

MATERIAL - LOW CARBON STEEL - SURFACE FINISH 6.3 x 10 RMS MICROMETERS (250 RMS MICROINCF4ES)

FIGURE 15 - TYPICAL TEST BLOCK FOR CALIBRATION OF THE ULTRASONIC INSTRUMENT

-16- Commercially available video-taperecorders can record the ultrasonic oscilloscope presentationas well as an accompanying voice description. The video recorder can also record thediver's field of view as transmitted fromthe camera mounted on his helmet. These can be combined onone tape with playback on a split screen.

ADDITIONAL METHODS OF NONDESTRUCTIVETESTING

Acoustical Holography. Acoustical holography is analogousto optical holography with the exceptionthat the object is on focus in a plane. The system uses a matrix of ultrasonictransducers, focused to inspect each point of theweld volume10, and act both to send and receive. The returning signal is received separately at several transducers where signalamplitude, time and phase are monitored in conjunction withan electronic gate. Then, the phased signals corresponding to thegate time are electronically processed to obtain a focused acoustichologram.

As developed for underwater weld inspection,the diver positions a probe, Figure 16, adjacentto the weld and the data àre transmitted to electronic equipment elsewhere(A lockout submers- ible at present, but could also betopside for hull weld inspection). There, the data are processed intoan image on a plane; and by combining this with a referenceplane, it can be viewed as an oblique projection similar to three-dimensionalviewing. The base of the probe containsa television presentation of the constructed image which helps the diver tomanipulate the probe for best position.

The system appears capable of detectingcracks but has not been fully evaluated. While acoustical holographymay be of considerable use in surveillance work,especially in murky water, it seems unlikely that it couldbe used as a primary inspection tool for evaluating welds.

Magnetographic Method. The magnetographic method uses magnetic tape instead ofa powder or slurry of particles.11

The tape is placed on top of the weld bya diver and then a magnetic field is induced in the work piece. Leakage flux at discontinuity sites are detected and recordedon the tapes. Evaluation is done topside with electronicprocessing to produce either an oscilloscope displayor a strip chart. The tape can be stored as a permanent record. This method has not gained wide- spread recognition and is not generallyused.

A Harness of Ultrasonic Transducers. Currents or wave action, particularly near the surfacemay make it difficult or impossible for the diver to stay ina position which would make it difficult or impossible for him to do either ultrasonicor magnetic particle inspection. Recognizing the difficulties caused by turbulence, a British Corporation (BIX) has developedan ultrasonic inspection device consisting ofa linear array of ultrasonic shear wave transducers which is incorporated intoa flexible and magnetic pack.

-17- ACOUSTIC/V IDEO IMAGING GUN

MONITORING OF ThE IMAGE

VIDICON TUBE

LIGHT SOURCE

ELECTRONICALLY FOCUSEDISCANF4ED 160 ELEMENT FLEXIBLE ACOUSTIC ARRAY

FIGURE 16 DIVER USING ACOUSTICAL HOLOGRAPHY PROBE

-18- The diver places this deviceatop the weld, slides it intoproper position and magnets fix the packfirmly against the work surface. Topside, signals obtained fromall transducers are displayed simultaneously on an oscilloscope. If no signals are obtained, the weld is evaluated as free ofcracks. If a signal is obtained, then the transducers areseparately turned on and off to establish the location and length of crack. The diver then moves the pack into a new position.

While this system provides reasonableassurance of crack detection, no provision is made forto-and-fro motion of the transducers and it, therefore,cannot be considered a suitable tool for primary weld inspection.

Television from Topside to Diver. A patented diver's helmet contains a television receiver anda system of moveable optical prisms which enables visual informationfrom topside to be trans- mitted to the diver.12 This can be blueprints of the workarea or the ultrasonic oscilloscopepresentation. For the person mañip- ulating the probe, it isvery helpful to see the oscilloscope screen and, in most cases, will result in betterultrasonic inspection.

COST CONSIDERATIONS

The cost of performing nondestructivetesting underwater is difficult to determine because ofthe many considerations involved; such as cleaning of marine growth,depth and temperature of water, visibility, tidal currents, andthe type of inspection. The experience of those workingon offshore platforms in the North Sea may not be typical of what can be expectedfor ship hull inspection in harbor, but little else isavailable for purposes of comparison. Det Norske Ventas reports thatan inspection vessel with eight crew members,sixteen cleaning divers and sixteen inspectiondivers can test two one-meter weldsper day.1-3

Although these figures undoubtedlyrepresent a situation of extreme difficulty, nonetheless, theysuggest a very high cost for underwater inspection.

After inspection, areas cleaned tobare metal will require a restoration of the protective coatin9. Epoxy paints can be applied underwater by brush rolling1- and other means, but the additional expense of doing thismust be considered a part of the cost of inspection.

- 19- CONCLUS IONS

With the exception of liquid penetrant, theordinary methods of nondestructive testing (visual, magneticparticle, ultrasonics and radiography) used topside to inspect steel buttwelds can be extended to underwater applications. Performing NDT underwater will be expensive -far more so then the cost of such work topside - but, as hasbeen demonstrated in the related industry of offshore drilling, it canbe done.

-20- REFERENCES

Busby, R. Frank,Underwater of Offshore Inspection/Testing/Monitoring Structure, Feb 1978,PP. 65 Busby, R. Frank,Underwater of Offshore Inspection/Testing/Monitoring Structure, Feb 1978,pp. 75 Routine Inspectionof Subsea Structures Bread and Butter; is the Diver's Offshore Engineer,May 1978.

Recommended Practice forUnderwater Inspection (11W Document V-624-78,Johannes Hatlo, DetNorske Ventas. Alan Taylor, Underwater Testing, MarineTechnology (Germany) Dec 1971,pp. 251-2. Ship Underwater Maintenance, Evaluationand Repair, NAVSEC Report 6136-77-9,Feb 1977. E. L. Criscuolo, D. P. Case and D.Polansky, "The Investigation of Radio Isotopes for the Inspectionof Ship Welds," SSC-llO, Feb1958, pp. 17

J. Walter, R. Sletten,"Underwater Inspection Structures," Det Norske of Offshore Ventas, Eighth WorldConferencL on Nondestructive Testing,pp. 6

Nondestructive TestingHandbook, 1959, Volume2, pp. 4313 Alain G. Stankoff, and Dale H. Collins,"Application of Acoustical Holographyto Inspection of Offshore Technology Offshore Platforms," Conference, Houston,TX, 1978 The Underwater Inspection of FixedOffshore Platforms, AERE Harewell, Jul1975, pp. 51

Sylvester UnderseasInspection, Rockland,MA. R. Sletten, "Underwater Inspection of OffshoreStructures," Eighth World Conferenceon Nondestructive Testing. Ship Underwater Maintenance Evaluationand Repair, NAVSEC Report 6136-77-9, Feb1977,pp. E-25

-21- BIBLIOGRAPHY

Criscuolo, E. L., et al.,"Manual of IsotopeRadiography," Ship Structure Committee,SSC-121, May 1960. Criscuolo, E. L., et al., "TheInvestigation of Radiolsotopes for the Inspectionof Ship Welds," Ship Structure Committee,SSC-hO, Feb 28, 1958. Fixed Bainton, K. F., et al., "TheUnderwater Inspection of Offshore Platforms, A Reviewand Assessment ofTechniques," AERE AERE-R8067, RR-SMT/R7403,Materials Physics Division Harwell, Oxforshire, England,Harwell United Kingdom Atomic Energy Authority, Jul1975. Continuous Monitoring ofFixed 4 Silk, M. G., et al., "The Offshore Platforms for StructuralFailure, A Review and Assessment of Techniques," AERE-R8055,Materials Physics United Division AERE Harwell,Oxfordshire England, Harwell Kingdom Atomic Energy Authority,Sep 1975.

The Ronald Press Company,Nondestructive TestingHandbook, New York, NY, 1959. American Bureau of Shipping,Rules for Nondestructive Inspection of Hull Welds, NewYork, NY, 1975.

American Welding Society, Inc.,Welding Inspection, New York, NY, 1968. Hirschfield, J. J., and O'ConnorD. T., "Developmentof Underwater Cobalt Camera," U.S. Naval OrdnanceLaboratory, White Oak, MD, Nov 5, 1951.

Youshaw, R. A., "A Guidefor Ultrasonic Testing and Evaluation of Weld Flaws," ShipStructure Committee, SSC-2l3, 1970. Structures," 10 Cook, W. V., "Using UltrasonicTo Test Offshore Ocean Industry, Oct 1970, V.5, No. 10, Page 15 &16. Drilling Platform Structure," 11. Peters, V. "NDT for Offshore Welding and Metal Fabrication,Jul 1973, V. 41-7. Nondestructive Testing," Marine 12 Taylor, A., "Underwater Technology (Germany) ,Dec 1971, V. 2-6, Page251 & 252. Goodfellow, R. and Boume, M. J.,"Underwater Inspection 13. 1976, of Deepwater Facilities,"Petroleum Engineer, Jun V. 48-7, Page 11.

-22- Irish, J., "Plugging the Holes in Subsea Checkups," Offshore Engineer, Jul 1976, page 23.

Attfield, K., "Putting Test Onus Where It Belongs," Offshore Engineer, Jul 1976, page 25.

Youshaw, R. A., and Criscuolo, E. L., "A Guide for the Nondestructive Testing of Non-Butt Welds in Commercial Ships Part 1," Naval Ordnance Lab., White Oak, MD, Dec 31, 1974.

Busby, Frank, and R. Associates, "Underwater Inspection/ Testing Monitoring of Offshore Structures," NOAA/Of f ice of Engineering, Rockville, MD, Feb 1978.

Stankoff, Alain G.,, and Collins, Dale H., "Application of Acoustical Holography to the Inspection of Offshore Platforms," Offshore Technology Conference, Houston, TX, May 1978.

"Ship Underwater Maintenance, Evaluation & Repair, (Sumer) Master Plan," Naval Ship Engineering Center, Report 6136-77-9. Feb 1977.

Hatlo, Johannes, "Recommended Practice for Underwater Inspection," Document V, 1978. Hages, D. Michael, "Underwater Inspection of Offshore Structures - Methods and Results," Offshore Technology Conference OTC 1565, 1972.

Sletten, R., et al., "Underwater Inspection of Offshore Structures," Det Norske Ventas, Eighth World Conference on Nondestructive Testing. Tamehiro, Masaki, et al., "Development of Underwater Welding Technique," Eighth Meeting of the Marine Facilities, Panel of the US - Japan Natural Resources, Sep 9, 1978. Thornton, D., "A Case Study of the Forties Field," Northern Offshore, No. 7, 1976.

U.S. GOVERNMENT PRINTING OFFICES ¶979-3t1586/249

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