LNG Storage Tanks Advancements in Weld Inspections
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Transcript of LNG Storage Tanks Advancements in Weld Inspections
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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
Select 85
at
www. Hydrocarbon Processing,com/RS
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