‘SHIPSTRICTURE CO}vIMImEESSC-293

UNDERWATERNONDESTRUCTIVETESTING OF SHIP

HULL WELDS

Thisdocumenthasbeenapprovedforpublicralaaseandsale;itsdistributionisunlimit~d,

SHIP STRUCTURE COMMllTEE

1979

Mm R, GIANGERELLTMm J, GRE’WRY

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#truuEytu*An InteragencyAdvkry Committee

Ddicated toImprovingtheStructureofShips

SR-1243

September 1979

The Ship Structure Committee has sponsored astate-of-the-art study of underwater nondestructiveinspection techniques to determine whether or notadditional research is necessary to provide adequatemethods for inspection of large vessels and mobiledrilling units whilebelow the waterline.

This reportsurvey.

afloat and of offshore platforms

documents the results of this

Rear Admiral,-U.S. Coast Guard

Chairman, Ship Structure Committee

METRIC CONVERSION FACTORS

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UNCLASSIFIEDSECURITY CLASSIFICATION -F Thlls *ACE lWh.n D.t* Enl.r.dl—- r--------- ,.--— .—... ..

REPORT DOCUMENTATION PAGE1. REPORT NUMBER 2. GOVT ACCESSION NO.

SSC-293

4. TITLE (.nd Subtitlej

UNDERWATER NONDESTRUCTIVE TESTING OF SHIPHULL WELDS

7. ALltn OR(@

R. Youshaw and C, Dyer

~. PERFORM!UG ORGANIZATICIN NAME AND ADDRESS

Naval Surface Weapons CenterSilver’Spring, MD 20910

II. CONTROLLING OFFICE UAME AND ADDRESS

Department of the NavyNaval Ship Engineering Center

AD’DRESS(lldlIf,rMtl,e~.CO”tmlll~~Olllc.)

Ship Structure CommitteeU.S. Coast Guard HeadquartersWashington, D.C. 20590

!6. DISTRIBUTION STATEMENT (d/hliRtp@

READ 1NSTRUCTIONSBEFORE COMPLETING FORM

3. RIECIPIEUT,S CATALOG NUMBER

5. 7YPE OF REPORT k ● ERIOD COVEREO

FINAL REPORTt.● ERFORMINGoRG. REPORT NUMBER

0. C0NTRAC70R GRANT NUMBER(U)

270099-671375

10. PROGRAM ELEI,IENT,PROJECT. TASKAREAb WORK UNIT NUMBERS

!2. RE*oRT DATE

September 197913, MIJMmEROF PAGES

231$. SECURITY CLASS. (ofthhr.pofl)

UNCLASSIFIED15m. DECLASSIFICATION/DOWNGRADING

SCHEOULE

UNLIMITED

17. DISTRIBUTION STATEMENT (af fhs ●blwscr ●nrwmdln #lack 20, /fd/ffsrmt Ir=m R.p.+

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1S. SUPPLEMENTARY NOTES

Non-destructive TestingRadiographyMagnetic particleUltrasonicUnderwater Testing

10, ABSTRACT (Conihw= on rmvmrmm ●Id. Ifn. c.*. aw rndld.ntlfy by block nuwb=r)

Techniques are presented whereby nondestructive-testing of hulTEU’Wwelds canbe accomplished underwater: Radiography, ultrasonic inspection, and magneticparticle testing are discussed including the modifications necessary forunderwater applications. In all cases, trained divers are required.

DD ,;;:?, 1473 EDITION OF 1 NOVS51SOBSOLETC

S/k 0102-014-6601 UNCLASSIFIEDSECURITY CLASSIFICATION OF THIS ●AGC(Wh*n Dcfs BniSr@

INTRODUCTION

OBJECTIVE AND SCOPE

NONDESTRUCTIVE TESTING OF HULL WELDS

DIVING EQUIPMENT

UNDERWATER CLEANING

ENVIRONMENTAL LIMITATIONS

NONDESTRUCTIVE TESTING METHODS

ADDITIONAL METHODS OF NONDESTRUCTIVE TESTING

COST CONSIDERATIONS

CONCLUSIONS

REFERENCES

BIBLIOGRAPHY

Page

1

1

1

3

3

5

5

17

19

20

21

22

LIST OF FIGURES

Figur+

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

Table

1

2

An example of a Steel Weld and Adjacent Area Cleane&of Marine Growth to Bare Metal

A Gauge for Measuring Weld Reinforcement

Recommended Positioning of Electrical Prods WhenInspecting Butt Welds

An Example of Crack Detection Using FluorescentMagnetic Particle Inspection

Watertight and Neutral Buoyancy Film CassettePackage for Underwater Radiography

Schematic Arrangement of the Isotope Holder forUnderwater Radiography

Maximum Voltage or Radioactive Energy forMinimum Steel Thickness

Radiographic Technique Curves for Steel at 250 KVPwith Various Thickness of Water MadeBetween the Source and Object

Radiographic Technique Curves for Steel AT 2 MeVwith Various Thickness of Water InterposedBetween the Source and Object

Increase in Radiographic Exposure at 250 KVPMade Necessary by Interposing Water Betweenthe Source and Object

Increase in Radiographic Exposure at 2 MeV MadeNecessary by Interposing Water Between theSource and Object

Degradation of Radiographic Sensitivity for Steelat 250 KVP due to Interposition of WaterBetween the Source and Object

Degradation of Radiographic Sensitivity for Steelat 2 MeV due to Interposition of Water Betweenthe Source and Object

Positioning and Manipulation of the Probe forUltrasonic Shear Wave Inspection of Butt Welds

Typical Test Block for Calibration of theUltrasonic Instrument

Diver Using Acoustical Holography Probe

LIST OF TABLES

Present NDT Techniques: Advantages and Limitations inUnderwater Application

Electrical.Current Requirements for Magnetic Particle Inspection

vi

!?aE’

4

7

7

7

.10

10

12

13

13

13

13

15

15

16

16

18

2

8

INTRODUCTION

At present, steel ships are nondestructively inspected atthe time of fabrication and thereafter only in drydock. Theexpense of drydocking a modern vessel is such that this is doneonly .~t the time of scheduled hull maintenance or if structuraldaqage has been incurred. Until recently, there were no otheroptions. However, in a related industry, offshore drilling~nondestructive testing is being done onsite, including thatportion of the structure positioned underwater.. Consideringthe lost operating time and sizable expense involved in drydockinga modern steel vessel, it may be desirable to do underwaternondestructive testing on some occasions to provide assurance ofhull integrity. In addition, as underwater welding techniques areinproved and further developed, it is conceivable that hullrepairs may be done underwater, thus obviating the need for dry-docking. Such repairs might not be acceptable to underwritingassociations or code bodies unless proof of adequate weld qualitycan be verified by nondestructive testing.

The Ship Structure Committee has recognized the advancementin technology and has requested the Naval Surface Weapons Centerto prepare a state-of-the-art report on Underwater NondestructiveTesting.

OBJECTIVE AND SCOPE

The objective of this task is to provide the maritimeindustry with nondestructive testing (NDT) techniques suitable forthe underwater inspection of steel welds. This is to be donewithin the framework of existing methods of nondestructive testing.

In addition, to a state-of-the-art survey, the methods of NDTwhich are suited to underwater work will be analyzed in regard totechnical capabilities and limitations and the modificationsdesirable or necessary for their application to underwater weldinspection.

NONDESTRUCTIVE TESTING OF HULL WELDS

Traditionally, steel hull welds are nondestructively testedwith one of five methods of nondestructive testing: visual,magnetic particlef qadio~raphy~ ultrasonics and L$-quid penetran.t.With the exception of liquid penetrant, all of these methods havebeen adapted to underwater steel weld inspection. The advantagesand disadvantages of each method as applied to underwater hullweld inspection are presented in Table I.

METHCID

Visual

Magnetic

Particle

Ultra-

sonics

Radio-graphy

TABLE 1

PRESENT NDT TECHNIQUES: ADVANTAGES AND LIMITATIONS IN UNDERWATER APPLICATIONS

DEFECTS ADVANTAGES

Surface cracks, Easy to interpret,

Impact damage= can be photographedFindings Limitedor Surface

LIMITATIONS

to surface defects,must be cleaned for

transmitted to topside by detailed observation.television and video recorded.

Surface cracks, Indications can be photographed

laps, seams, and or televised topside for

some near-surface evaluation and video recording.

flaws.

Cracks, Inclusionsr Especially sensitive to cracks.

Lack of fus%on and Can be used to evaluate

incomplete subsurface integrity.penetrameter in Equipment is lightweight.

welds. Results can be transmittedtopside and video recorded.

Internal defects Provides a permanent record.

such as shrfnks, Standards have been establishedinclusions ~ and are accepted by codes andporosity, lack of industry.

fusion and incompletepenetration in welds.

Requires thorough cleaning.

Weather dependent in splash

zone. Limited to surface and

near surface defects. Presenequipment limited to diver usCumbersome to perform underwat

Thorough cleaning required.Operator skill is required.

Surface roughness can’affectPresent equipment limited todiver use,

Potential health hazard. Wateshould be displaced between s

and subject. Requires accessboth sides. With available iit is difficult to obtain 2%sensitivity,

DIVING EQUIPMENT

Underwater NDT requires a diver. Because the hulls are notvery deep, the diver can use scuba equipment. This system has thediver carrying his own air supply and gives maximum freedom ofmovement. Alternatively and preferably,the diver can be suppliedbreathing media (air or mixed gas) from the surface by a flexiblehose. Although the umbilical cord of this latter system inhibitsfreedom of movement somewhat, the umbilical cord is necessary forother reasons: First, voice communication is very valuable.Second, in most cases, video transmission topside is worthwhile.Third, the diver needs electricity for a number of reasons,ranging from lights to power equipment. Considering theserequirements, the surface-supplied system seems preferable. Itcan be used to depths of 50 meters using air and to 90 meters withbreathing gas. 1 These depths exceed any depth of hull immersionin commercial shipping.

The diver may have a dry suit and a heavy helmet or a facesealing mask and a wet suit which can be filled with warm water.The heavy helmet seems to be advantageous in that it readilyaccommodates voice communication equipment and a source of light;an be mounted on the side of the helmet to illuminate the workarea which frees the divers hands for other tasks. A televisiontransmitter can also be mounted on the helmet which permitstopside personnel to view the work and the work area.

The diver equipment described above represents the currentstate of the art and is commercially available.

UNDERWATER CLEANING

With the exception of radiography, every method of NDT now inuse for underwater work requires that the surface to be inspectedbe cleaned to bare metal as illustrated in Figure 1. Mostly, thismeans removing marine organisms such as barnacles~ but scalel loosepaint and rust must also be removed. This requirement often ismore time consuming and difficult then the inspection itself.

The cleaning can be done by hand with a scraper or brush, butmanual methods are not practical for jobs larger than two or threelinear feet of weld. For the bigger jobs, power tools are availableof which water jetting is most often used. The system consists ofa surface pump and a high-pressure hose. At the work site, the divermanipulates the jet with gun-like controls. Pressures on the orderof 15,000 pounds/square inch are possible. TO keep the diver frombeing pushed off site, the nozzle also has a lesser pressure flowin the opposite direction - thus counterbalancing the force ofreaction. Cleaning rates of two - three square feet per minuteare claimed possible by skilled and experienced operators.

-3-

FIGURE I AN EXAMPLE OF ASTEELWELD AND ADJACENT AREACLEANED OF MARINE GROWTH TO BARE METAL

-4-

Note: The water jet is very dangerous and if inadvertentlydirected toward the diver can inflict physical damage as severeas limb amputation.

Other power tools for underwater cleaning include hydraulicgrinders and needle guns. In regard to the needle gun, it shouldbe noted that this tool delivers impact blows to the work area and~therefore, to a degree peens the metal. This makes suspect thepossibility of concealing defects otherwi~~ open tb the. surface. ~This has caused one insurer (Lloyds Register) to state a preferencethat2the weld and heat-affected zone not be cleaned with a needlegun.

ENVIRONMENTAL LIMITATIONS

The diver may be hampered in his work by murky water. Thiscondition can be improved by flooding the work area with cleanwater pumped from topside.

Extreme cold may severely limit the diver’s stay time. ASmentioned before, diving suits are available which can accommodatean injection of warm water.

Turbulent water is the most severe limitation likely to beimposed on the diver. This is especially true near the waterline where wave action is most pronounced. Solutions would haveto be determined on a per case basis, but scaffolding loweredfrom topside may be useful.

NONDESTRUCTIVE TESTING METHODS

Visual Inspection. Visual inspection requires water clarityand adequate illumination. Previously discussed, equipment andtechniques can be used to achieve this. Because the face plateof the diver acts as a lens~.with slight magnification.,, which isusually helpful in visual inspection, it does require thatmeasurements be made against a standard (Rule) rather than beestimated.

If the nature of the inspection is surveillance, visualinspection should also be done prior to cleaning as there issometimes a perceptible c lor change in the marine growthimmediately over a crack. 9 The detection of such a conditionprior to other NDT would be very meaningful in further planning orwork. After cleaning, the weld should again be visually inspectedin-as-much as cracks found this way can reduce the need for moresophisticated NDT or enable an improved scope of inspection.

-5-

Using visual inspection on new welds, the diver can measure aweld profile using commercially available gaugesl Figure 2, whileundercut can be measured with a depth gauge.

Underwater photography is a well established art and canbeused to provide a record and for more extensive evaluation topside.

Magnetic Particle Inspection. Magnetic particle testing (MT)can be applied to ferromagnetic materials such as ordinary carbonsteels. Of the sophisticated methods of NDT, magnetic particletesting is the most widely used for the inspection of offshoredrilling rigs and is readily adaptable to underwater hull weldinspection.

The test consists of three basic operations:

1. Establishing a suitable magnetic field in the object,where the magnetism must be in the correct direction, and thefield strength must be sufficiently strong.

2. Applying magnetic particles to the surface of’ the objectin the magnetized area.

3. Examining locations where the particles accumulate.

The method uses a pair of electrical prods positionedalongside the weld as shown in Figure 3. It is recommended thatthe prods be used in conjunction with lead shoes or other lowmelting point materials to suppress arcing and thus prevent burnmarks on the steel surface. The electrical current required forproper magnetization underwater is the same as that used whenworking in dry air, Table 2.4

Alternating current yokes and permanent magnets can also beused to induce the magnetic field but the surface should be groundsmooth for good contact.

Assuming the weld area has been cleaned of marine growth andthe diver has adequate visibility (Underwater lighting) , ordinarymagnetic particles in a slurry in a squeezable container can beused to complete the inspection. The suspension is directed at theinspection area and the results obtained are essentially the sameas when done topside using dust.

Superior results are obtained by using fluorescent particlesand illuminating the work area with ultraviolet light. Cracks arereadily detected, Figure 4. The ultraviolet lamps for use in thistype work have been designed with adequate electrical insulationand resistance to hydrostatic pressure and are commerciallyavailable.

If a record of the magnetic particle inspection is desired,the diver’s equipment can include a television transmitter andvideo recording can be done topside. Alternativelyj photographscan be taken.

-6-

/-

FIGURE 2 - A GAUGE FOR MEASURING WELD REINFORCEMENT

MAGNETIZING

FIGURE 3 - RECOMMENDED POSITIONING OF ELECTRICALPRODS WHEN INSPECTING BUTT MELDS

FIGURE 4 - AN EXAMPLE OF CRACK DETECTION USINGFLUORESCENT MAGNETIC PARTICLEINSPECTION

-7-

TABLE Z ELECTRICAL CURRENT REQUIREMENTS FOR MAGNETIC PARTICLE INSPECTION

Pf3013 SPACING [INCHES) AMPERESSECTION THICKNESS

3

4

5

6

7

8

9

10

11

12

UNDER 114”

300–400

400–500

500–625

600–750

700–875

800-1000

900– 1100

1000-1200

1100–1300

1200– 1400

3/4’’ANDOVER {AL’IPERES)

375–500

500–625

625-775

750–900

875–1100

1000– T2OO

1100–1300

1200–1400

1300–1500

1400–1600

Radiographic Inspection. Radiography requires that the filmcassette and radiation source be positioned on opposite sides ofthe hull. The film cassette can be placed on the outside of thehull with the source of radiation inside, or this arrangement canbe 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 radiationsource inside the hull but there is room to position a cassette,then radiography can still be done using the other arrangement.

Radiography of steel welds of ordinary hull thicknesses isusually done with positive pressure cassettes containing leadintensifying screens. For the film cassette outside the hull,there is a need for watertight integrity and a polyethyleneenvelope will suffice. 5 The cassette package can be firmly fixedin place using permanent magnets with attached springs.

Because the water behind the cassette is a back-scatteringmedia, a sheet of lead, approximately 1/8” thick, should be fixedbehind the cassette to minimize film fogging. If the package istoo heavy for underwater work, neutral buoyancy can be achievedby placing a low–density material, such as styrofoam sheet, behindthe lead and within the watertight envelope, Figure 5. Simpleexperiments should enable a close approximation to neutralbuoyancy. Alternatively, the cassette package can be attached toa rope supported from topside and maneuvered into position by thediver using voice communication to coordinate the work.

Aligning the film cassette and the radiation source isdifficult to accomplish by coordinate measurements, Pinging isreported6to be of considerable help in locating the approximateposition , but precise positioning can be done with ultrasonictransducers. For this, an Ultrasonic probe is fixed in positionat the desired location within the hull and the ultrasonicinstrument is set to receive with the entire range displayed.The diver then manipulates a second probe of the same frequency,but powered separately. When the two probes are aligned, a signalwill be received on the oscilloscope and, using voice communication,the diver is instructed to mark that location.

The thickness of the hull at the weld site can be determinedprecisely using an ultrasonic thickness gauge. This information,in conjunction with radiographic technique curves, affords theradiographer a means for correct exposure to obtain the desiredfilm density.

Should circumstances dictate that the source be placed out–side the hull, neutral buoyancy becomes more important. Theunderwater source of radiation will be an isotope rather than anx-ray machine as no commercial x-ray equipment has been modifiedto this purpose whereas this has been done with isotopes.’ Theisotope is invariably encased in lead of substantial thickness.In addition, rigid structure must be used to maintain the sourceto object distance, Figure 6. This structure can be either conicalor pyramidal in shape. If made watertight, either air or water canbe allowed to fill the space. Whichever system is used,

-9-

POLYETHYENVELOPE

STYSHE

FILM /.CASSETTE

FIGURE 6 - SCHEMATICARRANGEMENTTHE ISOTYPEHOLDER FORUNDERWATERRADIOGRAPHY

FIGURE 5 -

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WATERTIGHT AND NEUTRAL BUOYANCYFILM CASSETTE PACKAGE FOR UNDER-MATER RADIOGRAPHY

\ HU1.LI

HULL

PIATE

L-%-’*A%“

.-4 MRHMRADIATIONLEAKAGE

-1o”

consideration must be given to the diver’s limitation to manipulatethe structure into position, even if assisted by ropes maneuveredfrom topside.

Once the source is properly positioned, permanent magnets canbe used to grip flanges and fix the structure in place. As before,the correct exposure time can be calculated, but it is difficultto control in-as-much as it depends upon the diver’s promptaction in regard to cutting off the source or, having the cassettemoved from the field of continuing radiation.

Where the radiation source is inside the hull, an x-raymachine is preferable to an isotope, because it affords theradiographer a means of selecting a kilovoltage suited to theobject thickness whereas isotopes operate at specific energies andonly C060 and Ir192 are commonly available. If an x-ray machineis used, the selection of kilovoltage for a specific thicknessshould not exceed that shown in Figure 7. Also shown in thisfigure are the ordinary thickness range of C060 and Ir192 and theirpossible extension beyond this range with marginal radiographicsensitivity.

If the radiation source is outside the hull and the filmcassette inside and the water is displaced,then the exposure timeis unchanged. If, however, the water is not displaced and theradiation must penetrate the water before reaching the weld~ thenthe exposure time will be lengthened and the radiographicsensitivity will be degraded. The extent of these effects weredetermined experimentally using x-rays of 250 KVP and 2 MeV whichapproximate the isotopes Ir192 and C060. This was done by makingradiographs of steel plates of various thicknesses with and withoutcolumns of water between the source and the plate.

From measurements of the film densities, radiographictechnique curves were constructed for steel and specific watercolumns, Figures 8 and 9 and also for water and specific thick-nesses of steel, Figures 10 and 11. These curves enable adetermination of the half-value-layer of water for each energy,e.g., for 250 KVP this is 2.4” and for 2 MeV this is 5.2”.

The extent of degradation was determined by placing an arrayof penetrameters. on top of the steel plates and then calculatingthe sensitivity according to the smallest visible penetrameterholes using the equation:

SIX”N1 = S2XN2 where:

S1 = equivalent penetrameter sensitivity

‘1=2

S2 = contrast sensitivity

N2 = ratio of minimum detectable hole diameters to penetrameterthickness.

“11-

0,1 0.150.2 0.3 0.4 0.6 0.81.0 1.5 2 34567810 1520

30

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200

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●

FIGURE 7

STEEL WECIMEN THICKNESS (INCHES)

BROKEN LINE INDICATES MARGINAL SENSITIVITY

MAXIMUM VOLTAGE OR RADIOACTIVE ENERGY FORMINIMUM STEEL THICKNESS

-12-

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l-mm IFILM WKSITY. 2.0, TFU. 37; AA FILM,DPIELOIWIG TIMPEHATLIHC k SI° Fi

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I0.25 Q,50 0.75 1,00 1,25 1,S0 1.75

.lHIcUNESS OF SILEL - lNcl{F$

FIG. 8 - RADIOGRAPHIC TECHNIQUE CURVESFOR STEEL AT 250 KVP WITH VARIOUSTHICKNESSES OF WATER INTERPOSED BETWEENTHE SOURCE AND OBJECT

L-l ~~,,. STEEL, TFD. 32,.. o,* 2.0, AA FILM,DEVELOPING TEf,!PkilATUHE $lOIV

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2 4 G a 10 12 14

TH!cKNE$$oF WATER - INcHES

FIG. 10 - INCREASE IN RADIOGRAPHICEXPOSURE AT 250 KVP MADE NECESSARYBY INTERPOSING WATER BETWEEN THESOURCE AND OBJECT

-13-

,---_5.5(FILM IJENSIIY -7 n, TFD. 6,”!/r LEADFILTER AT Tu13E,1/8 LEAD FILTEII A’1 61LM,AA FILM, DEvFLDP!NG TEMPERA1U17E- El” f’)

D 0,5 1,0 1,5THICKNESSOF STEEL - lNmlES

FIG. 9 - RADIOGRAPHIC TECHNIQUE CURVESFOR STEEL AT 2 MeV WITH VARIOUS THICK-NESSES OF WATER INTERPOSED BETWEEN THESOURCE AND OBJECT

10 I 1 I I I I 1

04 8 12 16 20 24 2s

THICKNEWOF wATER - lNchiE$

FIG. 11 - INCREASE IN RADIOGRAPHICEXPOSURE AT 2MeV MADE NECESSARY BY INTER-POSING WATER BETWEEN THE SOURCE ANDOBJECT

The results of this determination presented in Figures 12and 13 show a progressive degradation of radiographic sensitivitywith increased thicknesses of water between the source and object.

Ultrasonic Inspection. The adaption of ultrasonic inspectionto underwater work is simple in principle. The water serves as acouplant; and except for the transducer being made watertight,the technique is the same as th~t,topside, Figure 14. Battery-powered ultrasonic inspection equipment is commercially availableand if made watertight, can be carried by an inspector–diver.The technique is restricted to work near the surface unless theequipment housing is designed to withstand hydrostatic pressure.

A more practical approach is for the instrument td remaintopside while the probe is taken below by the diver. Throughvoice communication the diver can be instructed regarding theposition and manipulation of the probe. A television transmitterattached to the side of the diver’s helmet with transmissiontopside permits the ultrasonic technician to see the probe move-ments as well as scope presentation and comprises a reasonableultrasonic inspection by remote control.

Before doing ultrasonic inspection, the area of probemanipulation must be cleaned of marine growth-to bare metal.

When ultrasonic inspection is done in air, cracks have anair interface with a reflection coefficient of almost 100%.In water, assuming the cracks come to the surface of the plateand water enters, the acoustic impedance mismatch is changed andsome of the ultrasonic energy is transmitted into the water.Acc rding to theory,

5’the reflectivity coefficient is reduced to

88% . Laboratory experiments confirm this approximate reduction.This difference is not considered cause for concernr because evensmall cracks are very efficient reflectors of ultrasound readilyfound by ultrasonic inspection.

The use of a very long cable increases the capacitance loadon the instrument pulser and necessitates a higher gain settingon the instrument to achieve the same sensitivity level used inultrasonic 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, Figure15, should be sealed (epoxy cement is suggested~ to maintain theair-steel reflectivity coefficient upon which the present weldinspection sensitivity level is based.

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

-14-

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FIGURE 13 - DEGRADATION OF RADIOGRAPHIC SENSITIVITY FOR STEEL AT 214eVDUE TO INTERPOSITION OF WATER BETWEEN THE SOURCE AND OBJECT

-15-

‘IGURE 14 - POSITIONING AND MANIPULATION OF THE PROBEFOR ULTRASONIC SHEAR WAVE INSPECTION OFBUTT WELDS

‘-r ‘T “T ‘T “T “t38 mm 44 mm 51mm 58 mm 64mm 70mm(1.5 in. ) (1.75in.) (2in.) (2.25in. ) (2.5in. ) (2.75 in.

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FIGURE 15 - TYPICAL TEST BLOCK FOR CALIBRATION OF THEULTRASONIC INSTRUMENT

-16-

Commercially available video-tape recorders can record theultrasonic oscilloscope presentation as well as an accompanyingvoice description. The video recorder can also record the diver’sfield of view as transmitted from the camera mounted on his helmet.These can be combined on one tape with playback on a split screen.

ADD~TIONAL METHODS OF NONDESTRUCTIVE TESTING

~. Acoustical holography is analogous tooptical holography with the exception that the object is on focusin a plane. The system uses a matrix of ultrasonic transducers,focused to inspect each point of the weld volumelO, and act bothto send and receive. The returning signal is received separatelyat several transducers where signal amplitude, time and phase aremonitored in conjunction with an electronic gate. Then, thephased signals corresponding to the gate time are electronicallyprocessed to obtain a focused acoustic hologram.

As developed for underwater weld inspection, the diverpositions a probe, Figure 16, adjacent to the weld and the data hretransmitted to electronic equipment elsewhere (A lockout submers-ible at present, but could also be topside for hull weld inspection) ,There, the data are processed into an image on a plane; and bycombining this with a reference plane, it can be viewed as anoblique projection similar to three-dimensional viewing. The baseof the probe contains a television presentationof the constructedimage which helps the diver to manipulate the probe for bestposition.

The system appears capable of detecting cracks but has notbeen fully evaluated. While acoustical holography may be ofconsiderable use in surveillance work, especially in murky water,it seems unlikely that it could be used as a primary inspectiontool for evaluating welds.

Magnetographic Method. The magnetographic method usesmagnetic tape instead of a powder or slurry of particles.11

The tape is placed on top of the weld by a diver and then amagnetic field is induced in the work piece. Leakage flux atdiscontinuity sites are detected and recorded on the tapes.Evaluation is done topside with electronic processing to produceeither an oscilloscope display or a strip chart. The tape can bestored as a permanent record. This method has not gained wide–spread recognition and is not generally used.

A Harness of Ultrasonic Transducers. Currents or wave action,particularly near the surface may make it difficult or impossiblefor the diver to stay in a position which would make it difficultor impossible for him to do either ultrasonic or magnetic particleinspection. Recognizing the difficulties caused by turbulence,a British Corporation (BIX) has developed an ultrasonic inspectiondevice consisting of a linear array of ultrasonic shear wavetransducers which is incorporated into a flexible and magnetic pack.

-17-

FIGURE16 DIVER USING ACOUSTICAL HOLOGRAPHY PROBE

-18-

The diver places this device atop the weld, slides it into properposition and magnets fix the pack firmly against the work surface.Topside, signals obtained from all transducers are displayedsimultaneously on an oscilloscope. If no signals are obtained,the weld is evaluated as free of cracks. If a signal is obtained,then the transducers are separately turned on and off to establishthe location and length of crack. The diver then moves the packinto a new position.

While this system provides reasonable assurance of crackdetection, no provision is made for to-and-fro motion of thetransducers and it, therefore, cannot be considered a suitabletool for primary weld inspection.

Television from Topside to Diver. A patented diver’s helmetcontains a television receiver and a system of moveable opticalprisms whicch enables visual information from topside to be trans-mitted to the diver.12 This can be blueprints of the work area orthe ultrasonic oscilloscope presentation. For the person mafiip-ulating the probe, it is very helpful to see the oscilloscopescreen and, in most cases, will result in better ultrasonicinspection.

COST CONSIDERATIONS

The cost of performing nondestructive testing underwater isdifficult to determine because of the many considerations involved;such as cleaning of marine growth, depth and temperature of water,visibility, tidal currents, and the type of inspection. Theexperience of those working on offshore platforms in the North Seamay not be typical of what can be expected for ship hull inspectionin harbor, but little else is available for purposes of comparison.Det Norske Veritas reports that an inspection vessel with eightcrew members, sixteen cleaning divers and sixteen inspection diverscan test two one–meter welds per day.1~

Although these figures undoubtedly represent a situation ofextreme difficulty, nonetheless, they suggest a very high cost forunderwater inspection.

After inspection, areas cleaned to bare metal will require arestoration of the protective coatin Epoxy paints can beapplied underwater by brush rolling 12”and other means, but theadditional expense of doing this must be considered a part of thecost of inspection.

-19-

CONCLUSIONS

With the exception of liquid penetrant, the ordinary methodsof nondestructive testing (“visual, magnetic particle~ ultrasonicsand radiography) used topside to inspect steel butt welds can beextended to underwater applications.

Performing NDT underwater will be expensive - far more sothen the cost of such work topside - but, as has been demonstratedin the related. industry of offshore drilling, it can be done.

-20-

REFERENCES

1.

2.

3.

4.

5.

6.

7.

8,

9.

10.

11.

12.

13.

14.

Busby, R. Frank, Underwater Inspection/Testing/Monitorin9of Offshore Structure, Feb 1978, pp. 65

Busby, R. Frank, Underwater Ifispection/Testing/Monitoringof Offshore Structure, Feb 1978, pp. 75

Routine Inspection of Subsea Structures is the Diver’sBread and Eutter; Offshore Engineer, May 1978.

Recommended Practice for Underwater Inspection (IIWDocument V-624-78, Johannes Hatlo, Det Norske Veritas.

Man Taylor, Underwater Testing, Marine Technology(Germany) Dec 1971, pp. 251-2,

Ship Underwater Maintenance, Evaluation and Repair,NAVSEC Report 6136-77-9, Feb 19h7,

E. L. Criscuolo, D. P. Case and D, Polansky, “The

Investigation of Radio Isotopes for the Inspection of ShipWelds,” SSC–11O, Feb 1958, pp. 17

J. Walter, R. Sletten, “Underwater Inspection of OffshoreStructures ,“ Det Norske Veritas, Eighth World Conference

on Nondestructive Testing, pp. 6

Nondestructive Testing Handbook, 1959, Volume 2, pp. 43.13

Alain G. Stankoff, and Dale H. Collins, “Application ofAcoustical Holography to Inspection of Offshore Platforms,”Offshore Technology Conference, Houston, TX, 1978

The Underwater Inspection of Fixed Offshore Platforms,AERE Harewell, Jul 1975, pp, 51

Sylvester Underseas Inspection, Rockland, MA.

R. Sletten, “Underwater Inspection of Offshore Structures,”Eighth World Conference on Nondestructive Testing,

Ship Underwater Maintenance Evaluation and Repair, NAVSECReport 6136-77-9, Feb 1977, pp, E-25

-21-

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

BIBLIOGRAPHY

Criscuolo, E. L., et al., “Manual of IsotopeShip Structure Committee, SSC-121, May 1960.

Criscuolo, E. L., et al., “The Investigation

Radiography ,“

ofRadioisotopes for the Inspection of Ship Welds,” ShipStructure Committee, SSC-11O, Feb 28, 1958.

Bainton, K. F., et al., “The Underwater Inspection of FixedOffshore Platforms, A Review and Assessment of Techniques,”AERE-R8067, RR-sMT/R7403 , Materials Physics Division AEREHarwell, Oxforshire, England, Harwell United KingdomAtomic Energy Authority, Jul 1975.

Silk, M. G., et al., “The Continuous Monitoring of FixedOffshore Platforms for Structural Failure, A Review andAssessment of Techniques,” AERE-R8055, Materials PhysicsDivision AERE Harwell, Oxfordshire England, Harwell UnitedKingdom Atomic Energy Authority, Sep 1975.

The Ronald Press’ Company, Nondestructive Testing Handbook,New York, NY, 1959.

American Bureau of Shipping, Rules for NondestructiveInspection of Hull Welds, New York, NY, 1975.

American Welding Society, Inc., Welding Inspection,New York, NY, 1968.

Hirschfield, J. J., and O’Connor D. T., “Development ofUnderwater Cobalt Camera,” U. S. Naval Ordnance Laboratory,White Oak, MD, Nov 5, 1951.

Youshaw, R. A., “A Guide for Ultrasonic Testing andEvaluation of Weld Flaws,” Ship Structure Committee,SSC–213, 1970.

Cook, W. V., “Using Ultrasonic To Test Offshore Structures,”Ocean Industry, Ott 1970, V. 5, No. 10, Page 15 & 16.

Peters, V. “NDT for Offshore Drilling Platform Structure,”Welding and Metal Fabrication, Jul 1973~ V. 41–7.

Taylor, A. , “Underwater Nondestructive Testing,” MarineTechnology (Germany), Dec 1971, V. 2-6, Page 251 & 252.

Goodfellow, R. and Bourne, M. J. , “Underwater Inspectionof Deepwater Facilities,” Petroleum Engineer~ Jun 1976,V. 48-7, Page 11.

-22”

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

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 theNondestructive Testing of Non-Butt Welds in CommercialShips Part l,” Naval Ordnance Lab., White Oak, MD,Dec 31, 1974.

Busby, Frank, and R. Associates, “Underwater Inspection/Testing Monitoring of Offshore Structures,” NOAA/Officeof Engineering, Rockville, MD,,Feb 1978.

Stankoff, Alain G., and Collins, Dale H. , “Application ofAcoustical Holography to the Inspection of OffshorePlatforms, “ 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 UnderwaterInspection,” Document V, 1978.

?dages, D. Michael, “Underwater Inspection of OffshoreStructures - Methods and Results,” Offshore TechnologyConference OTC 1565, 1972.

Sletten, R., et al., “Underwater Inspection tif OffshoreStructures ,“ Det Norske Veritas, Eighth World Conferenceon Nondestructive Testing.

Tamehiro, Masaki, et al., “Development of UnderwaterWelding 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 OFFICE: 1’379-311-586/249

-23-

$W RESEARCHCOMMITTEEMaritimeTransportationRmemh Board

NatiQ!IalAcadmyof Sciences-NationalResearchCouncil

Tha ShiPinteragencyShip

********

ResearchConmdtteehas technicalcognizanceof theStructureCmnrnittw’sresearchproqrarn~

W, 0. H.Pk. M. D.

Dr. J. N.m. n. P.Mr. Id.J,Mr. A. C.Clr,w,R,

Oakley,Chairman,CQnauZtmti, Jfa&mzn, VA

The-SfLipMatertals,Fabrtcaticm,& Inspextlon AdvisoryGroupprepared”the projectpmqmctus, evaluatedthe proposalsfor thts project,providwlthe liaisontechnicalguidance,and reviewedthe projmt reportswith the investigate:

SHIP STRUCTURECOMMIllEEPUBLICATIONS

SSC-279,

SSC-280,

SSC-281,

SSC-282,

SSC-283,

SSC-284,

SX-285,

SSC-286,

SSC-287,

SSC=288,

SSC-289,

SSC-290,

SSC-291.,

SSC-292,

...

Thee documents ape die+pibu+ed by the Natwnal TechnieaZInformation Semite. SpringfieZd3 VA 22161. The8& bc-wments have been announced in the CZeax4nghouee JownuZU. S., Govemmen+ l?eaeareh & Development Reporte (USGRDR)undeP the indioa+ed ADnuinb.ere.

Modij%dRadzr and Stanckrd%ker Wavemeter SL-7 ~ontaine~ehip Databy J. F. Dalzell. 1978. AD:A062393.

RemiZts and Evacuation ofbha SL-7’ Containemhip Ra&P and Tuoke~ Wave- “meb.ep~ta by J. F, Dalzell. 1978. AD-A062392.

Compcmison of Stre8se6 CaZcuZa%ed U8ing the DAISY Syatiem to Tho~eMecm.wed on the SL-7-Continex@ip fiogrm by H. Y. Jan, K. T. Chang,and M. E. Idojnarowski.1979. AD-A069031,

A Li~ture Survey on the CoZZis<im and GPounding EYotiection ofSh<psby N, Jones. 1979. AD-A069032,

Ctitieal Evaluai%on of Low-~ePgy Ship CoZliebAlamage Theoz%e8 andDe~ignMetho&Zogies -’VoZwne I . lWaZuabion and R@onmendzi3ions byP. R. Van Mater,Jr., and J. G. Giannotti. 1979. AD-A070567.

CriticaZ Evahatih of Low-Energy Ship CoZZision-Damage Theories andDeeigniWetihodoZogies - VOZUW 11- Literature Search and Revieza byP. R. Van Mater,Jr., and J,-G. Giannotti. 1979. AD=A070568.

Results of the FiPGb F4w “Data YaP8° of Extnme Stiess Se~ateh Gauge~z Collecti<m Abomd Sea-ti’~ SL-?’e by R. A. Fain and E. T. Booth.

● AD-A072945.—

Esxzminatwn of Serviee and Stress Dati ofl’Jmee Ships for’ DevelopmentofHuZZ GiPdSZJLwd Cz%tati by ~. F. lla~zell, ti. l!. Maniar,andM. M. nSU. 1979. AD-AO7291O.

A lfebhodfop Eoonomie “~ade-Offe of Alternate Ship StPL&UMZ MateriaZs~~z:. R. Jordan,J. B. Montgomery,R. P. Krumpen,and D. J. Woodley.

.

i%gn%f%oawe ad ContiZ ofkmeZZaP Teaz%ng #StaeZ PZate in theSh@UiW~ Indus~ by J. Somel I a. 1979.

A De@,@ Rcocediuw fo~Minimii&zgtiopeZ>I*d. V<bmfiim in HuZZStitiZ Eti+3 by 0. H. Burnside,D. D. Kana.and F. E. Reed.1979.

Report on Ship V{bration 08i~ ‘~~ j9 Shemton iFationaZ Bo*@Z,qAPZing~, VA. by E. SCOtt fllon= *

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