7/25/2019 LNG Storage Tanks Advancements in Weld Inspections

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L N G A N D G TL D E V E LO P M E N T S

S P E C I A L R E P O R T

LNG storage tanks: advancements

in weld inspections

New inspection method improves jobsite examinations

and ve rification

of

welds

R. KRUZIC,

CB&I,

T he W oodlands, Texas

T

he inter t ia t ional l iquefied naturalgas( I . N G ) i n du s -

try continues togrow, atid tnoves stranded natu ral

gas

(NG)

fro tn coun tries th at have surplus supply

to

expanding markets. Inorder tofu lf i l l g lobal detnat id, the

nunibetoffacil i t ies being bu ilt toexpo rt, shipand i m p o r t

L N G hasalso increased tte tnen dotisly . LNG storage tanks

(Fig. 1)playa prom inent roleinthis process and are typica llyon

the critical pathforconstructing L N G terminal projects.

T hree L N G storage tank designs are available and selections

are made based on specific project needs: single, double and full

containment tanks.Allthree are designedtostoreLNG .safely

and contain any spillsinthe unlikely eventof leakinthe pri-

mary' liqu id container. D ueto the stored fuel's nature,allthree

tank types are designed with an inner and otiter wall separated

by insulation materials.Theinner w all musthedesignedfor

L N G 's cryogenic temperature (-2 60F ). T he m aterial used most

extensivelyis 9%nickel steel asthe material remains ductileat

cryogenic temperatures.

To construct theinner l iqu id co ntain ing tank, large plates

of 9 nickel steel are welded together (Fig.2),W eldingpro-

cedures require using appropriate weld filler toachievethe

required mechanical properties androughness. A pplicable

design codes and standards governtheriondestrtictive ex ami-

nation(NDE)techniques requiredtoensure that weldsare

acceptable.

Radiography vs ultrasonic technologies

Tradition-

ally, tul l tt isi iiii weld.s on L N G storage tanks have been inspected

using radiographic exam ination (R T ). T echnicians take weld

radiographs using ei tht r x-rayorgamma ray siiutces, developthe

filtn atid then reviewitto determine if the weld is acceptable.K'l'

is know nforits sensitivity to volumetric flows and clearly identi-

fies slag and porosity flaws that indicate a problem ineitherthe

weld itselforwelding pniLedure.

Ultrasonic examination (UT) has heen used primarilyto

inspect weldsonstructures madeofcarbonor lowalloy steels

(ferritic metal). Becau.se9%nick el steel fille r metalsarea

high nickel alloy,iisstructtire is atistenitic rather than ferritic .

A ustenitic m aterials produce weld deposit that has a coarse,

grainy elongated (dendritic) structure. T his tendstoscatter

sound waves, thus causing distortions and interference with

results.

Furthermore, considerable equipment costs, high skill level

of technicians,andlackof integrated systems performingthe

procedureshas made usingUT forLNG storage tank weld

inspections rare. However, recent technological advances have

leveled

rhe

playing field between

UT

and

Rl '

technologies.

As

a result.UThas emerged

as

an inspection method equally

well-

suitedforjobsite examination and,insome cases,itprovidesa

better alternative.

Austenitic weld examinations Recent developments

in ultrasonic technology have made

it

practical

to

develop

an

examination methodology designed specifically forausten-

itic welds. T his process combines m ultip leUT techniquesto

effectively examine the weld joint and detect detrimental flaws

wi th i nit.

C'ommercial U ]' hardware and software has become more

sophisticatedinitsabilityto handle multiple transducersand

to process and d isplay data.

For

ultrasonic ferritic weld inspec-

t ion,

transducer arrays have been mounted on rolling carts tied

to comp uterized data collection systems. T his same concept

was applied

in

developing asystem

for

austenitic welds.

How-

ever, highly specialized traiisdticersarecarefu lly selectedand

assembledonthe carriage.

T he teqtiired specialized transducers are determined from

weld schematics. P roduction weld mock-tips are mandatoryto

determine appropriate gain settitigs atid

to

con firm adequate

coverage. Transducers are then mountedtoacarriage andare

readyforthe Inspection proce.ss.Apump isincludedon the

apparatusto constantly spray water ontothe examinationsur-

face,ensuring that high-frequenc y sound waves are successfully

transferred.

Semi-automatic UT examination system. A recent

developmentin methodology and apparatus combines technol-

ogy advancements into

a

single process designed s pecifica llyfor

U T inspectionofiuistcnitic weld joints . The semi-automated

apparatus, which has been patentedin theU as well as an u m -

ber o f other countries, includes hardware (transducers, carriage,

computer equipment,etc.) and software systems neededto

perform examinations and capture theresultsinthefield.It

is automatic inthatthe conipurtr software captures ultrasonic

data asitexamines the weld joint whilethecarriageismoving

along the weld seam.

The computerismon itored as data is collected, enstiring the

systemisfunctionin g correctly. The operator, whois

a

qualified

Level

11

U'l'N D E technician, can also screen data

as

itisheing

HYDROCA RBON PROCESSING JULY 2004jS3

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SPECIALREPORT

LNG AND GTL DEVELOPMENTS

T A B L E 1 RT vs UT nondestructive examination

collectedthus, proaccively addressing any welding problems

detected immediarely, rather than waiting for final reports to

be reviewed.

Computer hardware, which has become snKiller and more

portable, is incorporated into the system and can be used easily

in the field without cumbersome restrictions. Operator training

for the equipment and proLedures is provided in a laboratory

environment utilizing tank mock-ups. The equipment is then

transported to the construction site and put into production .

U T b e n e fi ts f o r L N G s t o r a g e ta n k s There

are a

number

of advantages in u.singUI' to inspect 9 nickel steel weld joints

for LNG storage tanks. These include safety considerations,

quality factors, time and cost savings, and space allocations

{Table 1).

Safely considerations Safety is the primary consideration

on every LNG project. By elimin ating tadiatio n usage in the

ND E p rocess, U T inspections remove radiation hazards as a

safety concern.

Th e U T process also eliminates the need to handle and

dispose of chemicals associated with RT technology. Anothet

important safety consideration is the elimination of the NDE

operators working during off-hours to perform inspections,

Lsolating inspectors to perform RT examinations is necessary

to protect other workers in the area, which poses risks for those

performing the examinations.

Quality factors While RT is normally better at detecting

three-dime nsional volum etric flaws, such as slag and potositi,',

U T is normally better at detecting two-dimensional planar flaws,

such as cracks and non-fusion occurrences. In welding, these

two-dimensional flaws are those that cause the greatest concern.

Detecting and correcting detrimenral two-dimensional flaws

LS

essential to ensu ring the LNG storage tank struc tute integrity',

Fot R'J' examinations, acceptance criteria are based on con-

sensus standards, which inspectors are ttained to read. For U T

examinations, engineering calculations derived from fracture

mechanics are used to set the acceptance standard. This provides

for a much more precise methodology for interpreting results

and providing a sound structure.

Othe r ways that U T conttihu tes to the inspection qu ality

process is by providing a permanent electronic examination

tecord that can be easily copied ot ttan smitted . Thus, test results

Item

S ensitivity for

vofumetric flaws

(slag and porosity)

Sensitivity for pianar flaws

(cracks and rod-fusion)

Prouide permanent record

(baseline)

M ultiple copies of results

Data on flaw length

Data on flaw depth

(ferritic material)

Data on flaw depth

(austenitic material)

Thin matefial

Safety

Schedule

O perator skill level

Acceptance basis

API approved

RT S e mi-a u to ma te d UT

wi th co mp u te r-b a se d

d a ta a cq u is i t io n me th o d s

Strengtii

M oderate

Yes

No

Yes

No

No

No l imitation

Isotope/radiation hazard

M oderate to high

High

Yes

Yes

Yes

Yes

Yes, within limits

Limited to 9 mm

No hazard

Work restricted to Work can be done

nonproduction work in unison wit h

time unless heavy shielding production wo ik

M oderate

Workmanship

Yes

High

Fracture mechanics

Yes

can be displayed and analyzed on a computer monitor.

Th e U r test outpu t is more consistent than a correspond-

ing RT examinarion. Output qtiality with RT, like other

photographic techniques, can vary depending on the rime rhe

shot is taken, film placement, chemical quality used in fiim

processing and film development techniques, U T provides

a consistent view that is not dependent on these variations.

Furtherm ore, the semi-atitomated U T examination sysrem,

with its advanced data acquisition systems, places less reli-

ance on the individual technician's skill since other trained

inspectors can easily review records.

Time andcost savings. U T inspections can provide time and

cost savings that can be significant over the cotirse of

a

project.

Th e U T proced ure is a linear scannin g process, as opposed to

raster scan techniques employed in manual procedures. The

process allows both sides of the weld to be examined at the same

time. Once welding is completed jnd cooled, U T inspection

can follow behind the welders and inspect joints immediately.

The inspection time for rhe L'T process is less than thar spent

performing RT examination.

In contrast, RT examinations cannot occur during production

hours unless shielding or distance is provided which allows work-

ers in the area to work safely. Normally, a second shift comes in

at night or on weekends to take radiographs. The film mtist then

he developed and results analyzed. This can cause considerable

delay in resolving any identified weld issues. KI also constrains

the contractor when faced with adding extra shifrs because time

is already devoted to taking weld radiograpbs.

Wirh U T, there is no delay; the results can be viewed and

immediate action taken if necessary. While UT equipment

is more expensive ro purchase, rhe time savings and schedule

improvements may well offset the higher equipmenr costs.

54 JULY 2004 HYDROCARBO N PROCESSING

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Around The

Clock Valve

Safety

Babbitt Safety

eel provides reliable

ive actuation 24

ursa day in refining,

xessing and power

mts around

the

Space all oca tion s. I^ss area is needed to srore the U T inspec-

tion results than that needed

for

storing radiograpbs produced

by RT examinations. Once

RT

film

is

developed,itmust

be

stored onsitc

for

reference, U I' results must also be stored. How-

ever, since rhe data

is

captured electtonicallj', the results can

bt

downloaded onto a CD or computer, Ftom there, the results are

reviewed for

afinal

report and can be easily stored, backed up and

transmitred from a desktop comp uter.

Testing and implementing UT inspections

After

developing thesemi-automatic U l methodology andequip-

ment, the new procedure w.is rested extensivelyin awelding

lahoratory. Third-party process validation was ,sought, as well as

API acceptance of U T

in

lieu of

RT

based on fracture mechan-

ics criteria.

An independenr thitd parry has validated the procedure and

the API .Standards C^ oinmittee has approved using U T inlieu of

RT based on fracture mechanics acceptance criteria.U willbe

permittedasan alternative to RT in Appendix U of Addendum

1, API 620 Tenth edition (publication pending ).

Mo v i n g f o r w a r d As LNG projects proliferate globally, new

construction technologiesareemerging that will help m ake

these projects safer, technically supctior and more cost-effec-

tive tobuild. Companies continuetodevelop and evolve these

technologies, bringing them

to the

held

as

soon

as

they

are

adequately tested and approved foruse.

Th e development of a patented U T methodology and appa-

ratus

to

inspect austenitic welds

for

I.NCi storage tanksis

one example

of

how research, creativity

and

experience

can

help improve safety, shorten thewotk schedule and produce

a higher-quality examination processforLNG projectsin the

field,HP

R o n K r u z i c is a senior weldin g engineer, Corp ordle N DT Level III, and has been

vi i th ceS I

lor

33 years. A fter dev eloping the process for austenilit LIT weld in ipec -

t ion,hepatentedthemethod and apparatus. He earnedhisBS degreeinmetal-

lurgical engineering and an MS degreeinscience fromtheUniversityof Illinois,

Champa ign-Urbana He currently

is

an ASNT ACCP Professional Level

lit in the

magnetic pa rticle (MT ), liquid pene trant (PT). radiography IRT). ultrasonic (UT) and

visual (VT)examination methods Mr Kruzicis also an NDT L evellll in leak testing (LT)

and a memher

of

AS ME 's Section V Code S ubcommittee on nondestructive testing.

Babbitt

S.

S p e c i a l t y C 6 . . t

P.O,

Box

5 I Z 0 8 , [ ^ .

N e w Be d f o r d , M i n O 2 7 4 S A

Tel: 508.995-9533 Fax; 508-993^

E-mail: sprocket 'ttebbittsteam.com

b:

www.babbinsteain,ccim

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