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7/28/2019 4. TankInspectionTechniques
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Tank Inspection
Techniques
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Ultrasonic Thickness of the
Shell
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Raw Shell Thickness Data
Averaged Thickness
Data over Lc
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Hammer Test
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z Magnetic Flux Leakage
z Coil Sensors
z Hall Effect Sensors
z Technology adapted from Smart Pig Technology
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MFL Tank Inspection
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Distribution of Flux in a Plate
With a Soil Side Defect
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Types of Sensorsz Coils
z Passive Devices
z Faradays Law
z Measures Change in the Flux Field
z Speed of Scanning is Important
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Types of Sensorsz Hall Effect Sensors
z Solid State Device
z Absolute Magnitude of Flux Density
z More Sensitive
z More Noise
z Temperature Sensitive
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Calibrationz Manufacturers Recommendations
z Simulate Tank Conditionsz Coating
z
Plate Thicknessz Temperature
z Material Properties
z API 653 Appendix G
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Why so critical?What an inspector cannot see cant be
effectively inspected.
Additionally, items such as vacuum box
inspections are severely compromised if tankbottom and weld seams arent properlycleaned.
Tank Surface Preparation
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Tank Surface Preparation
The visual portion of the API-653 inspection is
critical.
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The following are typical examples of what we
encounter:
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Wish Everyone WouldHere are some examples of good surfacepreparation using either brush blast or ultra-
high pressure water:
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Limitations of MFL Scanners
z Bottom Plate Lap Welds
z Weld Tacks
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Limitations of MFL Scanners
Shell
Surface Condition
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Limitations of MFL Scanners
z Plate Curvature
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Change in Plate ThicknessA thinner plate causes more flux lines to appear above the plate
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Defect Orientation
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Defect Orientation
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Keys to a Good Inspectionz Well trained, experienced
inspectors/operatorsz Proper cleanliness of tank floor
z
Understand limitations and plan your optionsz Proper equipment
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Late 1994 1995 Industry Conducted a PERF Study
on MFL/MFE Floor Scanning Equipment
Results
1. Identified a few obviously inferior pieces of equipment,butthe key finding was the Major Factor in a QualityMFL/MFE Inspection was the Inspector/Operatorby a widemargin.
2. A couple of key major oil companies began testing qualifyinginspector/operators.
3. API has now moved toward testing and will offer a certificationprogram based upon Appendix G.
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Conclusion
z MFL has physical limitations
z Successfully used for inspections every day
z Scanner Operator should understand
limitations
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U.T. Prove-Up of Tank Bottom
z Use Ultrasonic Thicknessto determine remainingthickness of bottomindications.
z Flaw dectector to size
defects and otheranomalies.
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SLOFEC(Saturated LOw Frequency Eddy
Current)
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SLOFECz Corrosion testing of material thicknesses up
to 35 mm and morez Much higher detectability than conventional
MFL techniques, especially on thicker walls
zTesting through surface coatings of 8 mmand more
z No physical coupling of the sensors
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Leak Testingz Vacuum Box Test
z
Bottom Lap Weldsz Bottom Repairs
z Pressure Test
z Reinforcement Pads
z False Bottom Sump
z Tank Hydro Test
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Helium Leak Detector
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Helium Leak DetectorWand
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Alternative Internal Inspectionz In-Service Robotic Inspection
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In-Service Internal Inspectionsz Sampling of tank bottom thickness
z 2 to 15 percent of the bottom inspected
z Utilize Statistical Methods
z Extreme Value Analysis
z Not a substitute for an out-of-serviceinspection
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Risk Assessment
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Risk Assessment
Considers both the Likelihood andthe Consequences of Failure
Risk Assessment is not a new idea, but
has recently become more formalized
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RBI Background
z 1985 ASME was commissioned to create a
guidance document published 1991z 1993 - API began development on RBI
z
2000 - API-581 - RBI(Base Resource Document)
z 2002 - API-581 Appendix O Tank RBI
Ad t f RBI
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Advantages of RBI (0.2.5.1)
z Introduces a another scheduling option
z Consistent approach for calculatingremaining life
z Consistent approach for determiningConsequence of Failure
z Focus inspection effort to reduce Risk
z Improved record keeping
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Risk Assessment
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Relative Risk Is Equal to:
Relative ProbabilityTimes
Perceived Consequences
Risk Assessment
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z Calculate relative probabilities of failure
z Calculate perceived consequences
z Calculate relative risk & rank segments
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Relative Risk Assessment
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Relative Risk Assessment
Components:Components:
AlgorithmAlgorithm
DataData
SoftwareSoftware
Risk Assessment
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z Identify Possible Failure Modesz Service History, DOT Incident Data, Experience
z Identify Possible Consequences
z Public, Environment, Business
Risk Assessment Algorithms
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Risk Assessment Algorithms
z Failures causes can typically be
classified
z Failures often result from the interactionof several factors
z Group the variables by failure mode andorganize into an algorithm
Develop Algorithms
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z Data (Variables)
z What data are available
z What data are relevant
z What data need to be collected
Relative Risk Assessment
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Relative Risk Assessment
z Create variables that describe pipeline
attributes
z
Organize variables into an algorithmcorresponding to failure modes
z Assign weighting factors
z
Perform sensitivity analysis
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Ranking by Relative Risk
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zThe risk is never zero
zThe more you do, the lower your
relative risk
Basic Tank RBI Methodology
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Basic Tank RBI Methodology
zThree basic components:
z Roofz Shell
z Bottom
z Looking for INDIVIDUAL Risk andCOMBINED Risk
Example
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Example
z Roof = High
z Shell = Lowz Bottom = Low
z
Combined Risk = Highz What can be done to reduce RISK?
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Risk Matrix
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ProbabilityC
ategory
Consequence Category
Summary Tank Data Example
2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014
T-1 Btm 24 24 24 24 24 24 21 21 17 17 13 13
Sh ll 19 19 19 19 19 19 19 19 19 19 19 19
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11 7 4 2 1
16 13 8 6 3
20 17 14 9 5
23 21 18 15 10
25 24 22 19 12
Shell 19 19 19 19 19 19 19 19 19 19 19 19
Roof 14 14 14 8 8 8 8 4 4 4 4 4
T-2 Btm 24 24 24 24 24 24 21 21 17 17 13 13
Shell 22 22 22 22 22 22 22 22 22 22 22 22
Roof 19 19 15 15 15 9 9 6 6 2 2 2
T-3 Btm 24 24 24 24 24 24 21 21 17 17 13 13
Shell 19 19 19 19 19 19 19 19 19 19 19 19
Roof 25 25 25 25 25 23 23 23 20 20 20 16
T-4 Btm 25 25 25 25 25 25 23 23 23 23 23 23
Shell 22 22 22 22 22 22 22 22 22 22 22 22
Roof 12 12 12 12 12 10 10 10 5 5 5 3
T-5 Btm 22 18 18 14 14 8 8 8 4 4 4 4
Shell 19 19 19 19 19 19 19 19 19 19 19 19
Roof 22 22 22 22 22 22 18 18 18 18 18 18
Required Data
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Required Data
zDesign:z Service, size, type, courses, year, new bottom,
roof, years in service, diked,
Required Data
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Required Data
zConsequence:z Fluid type, detection time, ignition source,
environmental risk, spill area, inventory (lbs), datein service, effective age per component, pressure,
potential release, bottom type
Required Data
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q
zProbability:z Shell Int / Ext corrosion type / ratez Bottom Int / Ext corrosion type / rate
z Bottom stock-side protection
z Roof condition
z Last internal inspection..
Conclusions from Quantitative
Analysis
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y
z Had to create a judgment value for an
analytical taskz Adjusting the data will adjust the Risk 0.2.1
zThe bottom corrosion ratewas the singlebiggest driver of Risk
z Learned that the calculator is not a crystal
ball.
Next Two RBI Types
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yp
z Qualitative too subjective and open to
interpretationz Combo -
z Use the same basic analytical data
z Tank has a Secondary Containment
Lets Go Back In Time
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z Early 80s A refinery realized they needed to
know if they had a tank leakage problemzTried every technique known to man plus a
few more to determine if a tank might be
leaking
Old Tank Bottoms
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z 60s applied Fiberglass
z Up till 80s very few bottoms were replacedzTheir analysis showed that many of the FG
bottoms may be leaking even after a goodinspection
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Of 300 Tanks is One Leaking?z Researched all forms of pipeline and tank
testing / inspection.z Developed the secondary containment
bottom system.
How A Tank Leaks
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z API-581 Appendix O
z
99.72% chance the tank will weepbefore leak.(Table 2)
z 1983-1993 0.9% bottom leak (Table 3)
z
After 20 years Heavy oil some corrosionbeneath coating Finished none
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Conclusion
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z Develop a methodology for Consequence
z Develop a written processz Develop a process to verifyyour
methodology
z Some tanks are eligible for RBI!
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Similar Service
Similar Service Assessment:
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What is Similar Service Assessment?
The process by which corrosion rates and
inspection intervals are established for a
candidate tank using corrosion rates and otherrelevant service history from a control tank for
the purpose of establishing the next inspection
date.
Similar Service Assessment:
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Why Have a Similar Service
Appendix?To provide industry with performance-
based guidance on conducting a similar
service assessment.
Currently, there is no definition of similar
service and no clear guidance onconducting a similar service assessment.
Similar Service Assessment:
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If it looks like a duck, walks like a duck, and talks like a duck
then its probably a duck.
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Similar Service Assessment:
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How is a Similar Service AssessmentDone?
Collect data for control and candidate tanks.
Conduct assessment using Data Sheet from Appendix H.
Determine if Similar Service can be used for candidate tank.
Determine the corrosion rates to apply to the candidate tank. Establish the next internal inspection date for the candidate
tank.
Document internal inspection date for candidate tank.
As additional data becomes available, validate corrosion rateestablished for candidate tank.
Determine if additional inspection data changes theinspection date for the candidate tank.
Similar Service Assessment:Collect data for control and
candidate tanks
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candidate tanks
Conduct similar service
assessment using data sheet
Determine if similar service can
be applied to candidate tank
Determine the corrosion rate to
be applied to the candidate tank
Establish next internal inspection date
for the candidate tank
Document revised internal
inspection date for candidate tank
Validate revised inspection date for candidate
tank as additional information is available
Determine if additional inspection data
changes inspection date for candidate tank
Similar Service Assessment:
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Control Tank Tank(s) for which
corrosion rates and otherrelevant service historyare known anddocumented.
Candidate Tank Tank for which
corrosion rates are notknown
Similar Service Assessment:
CANDIDATE TANK(S)
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900 Miles
CONTROL TANK
CANDIDATE TANK(S)
Similar Service Assessment:
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Product Side
Side exposed tostored liquid or gasproduct
Soil Side Side exposed tosupporting soil,concrete, or othermaterial
Similar Service Assessment:
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Similar Service Assessment Criteria
Year tank erected Bottom material
Shell material
Corrosion allowance, bottom and shell
Bottom lining type, thickness and age
Cathodic protection
Similar Service Assessment:
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Similar Service Assessment Criteria (contd)
Double bottom?
Soil material in contact with bottom plate Soil or material type
pH, Alkalinity
Moisture Salinity
Resistivity
Oil type (If oiled sand foundation)
Soil /material cleanliness
Soil gradation
Chlorides
Sulfates
Similar Service Assessment:
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Similar Service Assessment Criteria (contd) Ambient conditions
Current service conditions Product classification
Specific gravity of liquid
Reid vapor pressure at 60F Normal operating temperature Inert gas blanket, if used
Water bottom, if used
Sulfur content
Length of t ime in service Product corrosivity
Previous service conditions
Similar Service Assessment:
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Similar Service Assessment Criteria (contd)
Product classification
Additional considerations
MFL or MFE data for tank bottom
Ultrasonic thickness measurement data
Fiber optic monitoring system data
Cathodic protection monitoring tube data
Tank bottom integrity testing data
Maintenance procedures, including frequency andmethod of tank cleaning
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Similar Service Assessment:
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Similar Service Assessment:
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What is the Objective of the Assessment?
Establish corrosion rates and inspectionintervals for a candidate tank using corrosion
rates and other relevant service history from a
control tank for the purpose of establishing thenext inspection date.
The concept is shown graphically on the next
slide.
Similar Service Assessment:
Tank was new in 1970
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Tank was new in 1970
Original bottom thickness = 1/4
At 20 years, thickness = 0.05
Metal loss = .25 - .05 = 0.20
Corrosion rate = .2*1000/20 =
10mpy
New 1/4 bottom installed in 1990
In same service*, the new bottom
can be expected to corrode at thesame rate, from which a
retirement date can be
calculated.
* All other factors being equal
Similar Service Assessment:
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Example of Use of Similar ServiceAssessment Data Sheet
Similar Service Assessment:
Section 1.0 - Tank Bottom Product Side Assessment
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Tank Characteristic Control
Tank
Candidate
Tank
Match
?
Year Tank Erected 1984 1986 No
Bottom Material A36 A36 Yes
Corrosion Allowance None 1/16 No
Bottom Lining Type None None Yes
Bottom Lining
Thickness
N/A N/A Yes
Bottom Lining Age N/A N/A Yes
Similar Service Assessment:
Current Service Control Tank Candidate Match?
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Current Service
Conditions
Control Tank Candidate
Tank
Match?
Current Product Name No. 2 Fuel Oil No. 2 Fuel Oil Yes
Product Classification 1 1 Yes
Specific Gravity of
Product
0.87 0.87 Yes
Normal Operating
Temperature
60F 60F Yes
Water Bottom? No No Yes
Sulfur Content < 1% < 1% Yes
Time in This Service 10 Years 15 Years No
Product Corrosivity Mild Mild Yes
Similar Service Assessment:
P i S i C l T k C did M h?
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Previous Service
Conditions
Control Tank Candidate
Tank
Match?
Previous Product Name Gasoline Diesel Fuel No
Product Classification 1 1 Yes
Specific Gravity of
Product
0.80 0.85 No
Normal Operating
Temperature
50F 60F No
Water Bottom? No No Yes
Sulfur Content < 1% < 1% Yes
Time in This Service 11 Years 6 Years No
Product Corrosivity Mild Mild Yes
Similar Service Assessment:
Section 2 0 - Tank Bottom Soil Side Assessment
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Tank Characteristic Control Tank Candidate
Tank
Match?
Year Tank Erected 1984 1986 No
Bottom Material A36 A36 Yes
Corrosion Allowance None 1/16 No
Double Bottom? None None Yes
Section 2.0 Tank Bottom Soil Side Assessment
Similar Service Assessment:
Soil / Material Control Tank Candidate Match?
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Soil / Material
Characteristics
Control Tank Candidate
Tank
Match?
Soil Type Crushed
stone & sand
Crushed
stone & sand
Yes
Soil pH 6.7 7.1 No
Soil Alkalinity No No Yes
Soil Moisture 15% 20% No
Soil Salinity Insignificant Insignificant Yes
Soil Resistivity 3500 ohm-cm 3000 ohm-cm No
Oil Type If Oiled Sand
Cushion
Diesel Oil None No
Soil Cleanliness Some sulfate
contaminants
No known
contaminants
No
Similar Service Assessment:
Current Operating Control Tank Candidate Match
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Current Operating
Conditions
Control Tank Candidate
Tank
Match
?
Normal Operating
Temperature
Ambient Ambient Yes
Cathodic Protection Yes Yes Yes
Ponding/Water Yes No No
Previous Operating
Conditions
Control Tank Candidate
Tank
Match
?
Normal Operating
Temperature
Ambient Ambient Yes
Cathodic Protection No No Yes
Ponding/Water Yes No No
Similar Service Assessment:
Si il S i A t C l i
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Similar Service Assessment Conclusions:
Does this assessment include additional
assessment documentation? Based on the criteria reviewed in this Similar
Service Evaluation Is Or Is Not recommended for this tank.
The corrosion rate to be applied to theproduct side of this tank is ______ mpy.
Comments
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TANKPAC - Condition
Monitoring for Storage
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Monitoring for Storage
Tank Bottoms
Acoustic Emissions
Information provided by:
Tank bottoms are the only structural part
of a tank with no access for inspection
during operation
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TANKPAC - In-Service Condition Assessment of TankBottoms Case Study for MHG Sales Growth.A $2-7K Traditional NDT test that just became a $40-45K complete inspection
package with zero competition
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Traditional NDT (Visual and MFL after opening
Acoust ic Emission TANKPAC
Complete API-653
Automated Ultrasonics (LSI)
Risk Based Inspection (RBI)
Failure of time-basedmaintenance
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Hot oil tank, 140 deg.C
Internally inspected 12months prior to failure,including UT+MFL. Thiscollapse was due to anarrow band of annularring corrosion. When thering split the very rapidloss of hot liquid pulled avacuum collapsing thetank shell.
Failure of time-basedmaintenance
50m CRUDE OIL TANK
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One of 40+ holes in thetank floor, although the tankwas not leaking duringservice, only the sludge and
debris were sealing thefloor. Attempts to re-suspend the sludge have
resulted in major leakageon many occasions.
Failure of time-basedmaintenance
Naptha Tank
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Naptha Tank
Leaking 100 cubic metres perday through a 1cm hole.Operations noticed lossesafter a week, but no visible
product, which wasdisappearing into ground.When the plates were cut the
cavity under the floor wasseveral cubic meters in size.
Failures of time-basedmaintenance
z 1mm diameter pinhole leak in a 25m diesel tank where
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z 1mm diameter pinhole leak in a 25m diesel tank, whereepoxy coating failed.
z Collapse of a 25m sulphur tank due to annular-to-shellcorrosion.
z 200 cubic meters per day leakage in a 75m crude tank, 100cu.m/hour in a 97m crude tank...
z
Many more examples.z Tanks removed from service, cleaned, sludge dumped, tank
inspected...and no repairs required..
z If time based internal inspection worked. this would not
be happening....
Summary and Requirements
z Summary:
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y
z Access to tank floors for inspection is difficult and costly.
z Leakage is no longer environmentally acceptable.
z Risk of catastrophic failure with severe annular ring damage.
z Cleaning costs can be >$200,000, + environmental waste problem.>> If no repairs are required these costs are wasted
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1989 PAL approached by customers, discussion on requirements fortank floor condition assessment.
z 1990 First AE trials on tank floors, Esso, BP, .
z 1992 User group formed, grew from 5 to 20+ Cos.
z 1996 User group became part of EEMUA, ~30 Cos (EngineeringEquipment Material Users Association) experience now >600 tank
floor tests, feedback on internal inspection of >150 tanks, procedureat rev.4.
z 1997 Procedure accepted by Saudi Aramco.
z 1998 Results of TANKPAC correlation studypresented at ECNDT
by Shell/Dow etc. >1000 tests now completed.z 1999 EEMUA recommendations to members.
Basis of Operation
Corrosion of steel causes
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Corrosion of steel causesAcoustic Emission (Yuyama,
Condello etc).This is detected by sensorson the outside of the tank.
Emission reaching threesensors is located.
Shown right: one hour of
emission from a tank with verysevere corrosion, characteristicsof emission change with scaling.
Signal amplitude >>
^Number of emissions
time
Corrosion of InternalZinc Anodes
I t l ifi i l d
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Internal sacrificial anodescorrode in place of tank floor.
Zinc blocks can be 1m in size.
Oilfield production tanks use
sacrificial anodes-very activeuntil used.
Special procedures used to
separate anode corrosion fromfloor corrosion.
Location of remaining activeanodes
TANKPAC: outline procedure
z Tank is isolated and allowed to settle.
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z Sensors are attached to the tank wall around the entirecircumference, ~1m above annular. One row, or two rows
where condensation or high noise is possible.
z The tank is monitored, duration is ~1-2 hours.
z The data is processed to eliminate unwanted noise.
z Note: effect of noise is conservative >increases grade.z The result is graded per procedure for the overallgrade on
an A(good) to E(bad) scale.
z
Location of 3+hit sources by triangulation.z Location and grading of 3+hit potential leaksources*.
z Discussion and recommendations.*Shell EWGAE paper
Sensor Mounting and Calibration
z Sensors are mounted ~1m
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z Sensors are mounted 1mabove bottom knuckle or
above sludge.z Paint smoothed or removed
if not adherent.
z A pencil lead fracture isused for calibration.z This is detectable at up to 70
metres on large product tanks.
z All sensors are checkedafter mounting.
Acoustic Sources detected
z Sources of interest:
S lli f i d
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z Spalling of corrosion products.
z Leak noise: flow interruption,or turbulence.
z Extraneous noise to be removed:
z Roof movement noise.
z Structural movement.
z External and pipe-borne noise.z Condensation.
z Particle impacts.
z Valve leakage.
Inputs to Grade and
Recommendations
z Overallactivity level, A-good condition, E-bad condition,
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Overall activity level, A good condition, E bad condition,B, C, D, intermediate conditions:
z Diameter, product, sludge height.z Normalise using: number of sensors, data filtered, threshold.
z Special procedures for sacrificial anodes, soft rubber lining.
z
Locate overall data:z The ~5-30% which hits>3 sensors, any concentrated
sources?
z Separate, locate, and grade potential leakdata:
z More severe local damage, Ato Escale.
z Retest recommendation based on above factors.
Recommendations Matrix
OverallGrade A B C D E
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PLDGrade 4 4 2 (2) (1) (n/a)
A 4 4 2 (2) (1)
B 4 4 2 2 (1)
C 2 2 1 1 1
D 2 1 1 1 1
E 1 1 1 1 1
Clearly leaking tanks often unable to grade, (should be opened anyway).
Limitations
z Detects and grades active corrosion only.
N t it bl f i th i t l diti f t k hi h
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z Not suitable for assessing the internal condition of tanks which arecleaned mechanically or chemically as this resetsthe condition,
(underside OK). Use history!!z Small leaks may be masked by active floor corrosion.
z Large leaks will mask overall floor condition.
z
Activity from active corrosion under insulation may mask floor condition.z Location may be unreliable on very active D/E tanks due to simultaneous
sources, (the tank needs opening anyway!).
z Not all tanks can be tested, due to noise/condensation etc.
z Complex procedure requires extensive training + control.
Quality Control and Trainingz Documentary quality control system under ISO 9002:
z Trained and certified engineers
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z Trained and certified engineers.
z Controlled TANKPAC procedures.
z Controlled TANKPAC Field worksheets .
z Quality plan for each test.
z Digital storage of data and full traceability.
z
Engineer training and certification:z ASNT II general AE
z TANKPAC procedure class and field training.
z TANKPAC written and practical examination.
z TANKPAC minimum experience requirement (~50 tanks).z PAC level III review of and approval of report.
Overall AE Grade vs. % of tanks for
crude and product tanks from major
sites%oftanks
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0
5
10
1520
25
30
3540
45
A B C D E
%CRUDE
%PROD.
%ALL
ALL SITES IN SURVEY% of tanks
TANKPACGRADE
sample size 598 tanks 6/96
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Naptha Tank-Before: E grade,
and after repair: A grade
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3-D view of E grade crude tank
and damage found
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110m GRP lined Crude Oil Tank:
TANKPAC and MFL
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Hot Fuel Oil Tank 50m
z Egrade overall.
z Annular ring very active
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z Annular ring very active.
z Dug underneath annular ring inmost active areas:
z Up to 8mm loss of metal on15mm annular plates
z Tank shut down immediately,avoiding failure (see next
slide).
Leaking Naptha Tank
z 100 cu.m/day loss
No visible indication of a
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z No visible indication of a
leakz Faint smell only
z TANKPAC test at 2%sensitivity due to noise-2mins. only:
z 1cm hole found at locationshown
Strategy for Using TANKPAC
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z TANKPAC is a maintenance planning tool.
z Use TANKPAC to help identify the tanks which requireinspection and repair, and leave others in-service until theircondition indicates action is required.
z
Use TANKPAC results to set the maintenance priority.
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