External Corrosion Direct Assessment and Pipeline Integrity (ECDA)
Modeling of Corrosion NACE ECDA Seminar Jan...
Transcript of Modeling of Corrosion NACE ECDA Seminar Jan...
Pipeline Risk Management
Modeling of Corrosion
NACE ECDA Seminar
Jan 2009
What can a risk assessment do?
• where will the next failure
happen?
• when will a failure occur?
• how many failures next year?
where are the hot spots?
what is the best use of my resources?
what are the priorities?
Key Concepts
• Risk = (hazard likelihood) X (hazard consequence)
• Probability = Degree of Belief
• Risk Assessment -- Risk Management
• Management = choices in resource allocation
Historical (Informal) Risk Mgmt
ADVANTAGES:
• simple/intuitive
• consensus is often sought
• utilizes experience and engr judgment
• successful
Historical (Informal) Risk Mgmt
REASONS TO CHANGE:
• more at stake from mistakes
• inefficiencies/subjectivities
• lack of consistency
• need to consider complicated factors
• regulatory mandates
Gas IM Rule
Objectives• Prioritize pipeline segments
• Evaluate benefits of mitigation
• Determine most effective mitigation
• Evaluate effect of inspection intervals
• Assess the use of alternative assessment
• Allocate resources more effectively
ASME B31.8S, Section 5
Gas IM Rule
Points to consider• Account for relevant attributes
• Use conservative defaults for unknown data
• Identify significant risk-driving factors
• Sufficient segment discretization or resolution
• Predictive or “what-if” capability
• Updateable to reflect changes or new information
• Populating risk model is resource intensive
• Validate model, show to be plausible with respect to known
history and significance of threats
ASME B31.8S, Section 5
Threat Categories
• ASME B31.8 Supplement considers 3 categories
of threat:
– Time Dependent – May worsen over time;
require periodic reassessment
– Time Stable – Does not worsen over time; one-
time assessment is sufficient (unless conditions
of operation change)
– Time Independent – Occurs randomly; best
addressed by prevention
Bathtub Curve
Failures
Time
Threat Categories
Time Dependent Threats
• External corrosion
• Internal corrosion
• Stress-corrosion cracking (SCC)
• (Fatigue)
Threat Categories Time Independent
(Random) Threats
• Third-party/Mechanical damage
– Immediate failure
– Delayed failure (previously damaged)
– Vandalism
• Incorrect operations
• Weather related
– Cold weather
– Lightning
– Heavy rain, flood
– Earth movement
Threat CategoriesTime Stable Threats (resistance)
• Manufacturing-related flaws in
– Pipe body
– Pipe seam
• Welding / Fabrication-caused
flaws in
– Girth welds
– Fabrication welds
– Wrinkled / buckled bend
– Threads / couplings
• Defects present in equipment
– Gaskets, O-rings
– Control / relief devices
– Seals, packing
– Other equipment
ASME B31.8s
• Subject Matter Experts
• Relative Assessments
• Scenario Assessments
• Probabilistic Assessments
Better Way to Conceptualize
• Types of Models
– Absolute Results
– Relative Results
• Tools for All Models
– Probabilistic methods
• Scenarios
• Trees
– SME (input and validation)
Relative, Index, Scoring Models
• intuitive
• comprehensive
• ease of setup and use
• optimum for prioritization
• mainstream
• served us well in the past
Scoring Model Issues
• Difficult to anchor
• Potential for masking
• Technical compromises
– weightings
– scale direction
– interactions of variables (dep vs indep)
• Validation (reg reqmt)
• New Uses
Index Sum vs. Fail Probability
Scenario 1 Scenario 2
Index Score
Probability of Failure Score
Index Score
Probability of Failure Score
Third Party Damage 60 90
Corrosion 70 10
Design 80 90
Operations 70 90
280 280
Index Sum vs. Fail Prob
Scenario 1 Scenario 2
Index Score
Probability of Failure Score
Index Score
Probability of Failure Score
Third Party Damage 60 0.4 90 0.1
Corrosion 70 0.3 10 0.9
Design 80 0.2 90 0.1
Operations 70 0.3 90 0.1
280 76.5% 280 92.7%
Now
• plan for centerpiece of public scrutiny
• plan for legal challenges
• support integrity verification schedule
• determine appropriate reaction to risk
• anticipate desire for anchoring the numbers
• few computer limitations
Enhanced Modeling Approach
Probability of Failure
• Exposure
• Mitigation
• Resistance
Definitions
• Exposure: liklihood of an active failure mechanism reaching the pipe
when no mitigation applied
• Mitigation measure: prevents or reduces likelihood or intensity of the
exposure reaching the pipe
• Resistance: ability to resist failure given presence of exposure/threat
Information Use--Exposure, Mitigation, or
Resistance?
pipe wall thickness
air patrol frequency
soil resistivity
coating type
CP P-S voltage reading
date of pipe manufacture
operating procedures
nearby traffic type and volume
nearby AC power lines (2)
ILI date and type
pressure test psig
maintenance pigging
surge relief valve
casing pipe
flowrate
depth cover
training
SMYS
one-call system type
SCADA
geotech study
pipe wall lamination
wrinkle bend
Absolute Risk Values
Frequency of consequence
– Temporally
– Spatially
•Incidents per mile-year
•fatalities per mile-year
•dollars per km-decade
conseq prob
Dependent vs Independent
InteractionsAND Gates
– CP failure AND coating failure = failure of mitigation
– CP effectiveness = P/S reading AND P/S distance AND P/S age
OR gates
– PoF = PoF1 OR PoF2 OR PoF3
– Corr control = coating effectiveness OR CP effectiveness
Combination of Likely Events
0.8 AND 0.8 AND 0.8 AND 0.8 AND 0.8
= 0.8 x 0.8 x 0.8 x 0.8 x 0.8 = 0.3actually unlikely
0.8 OR 0.8 OR 0.8
= [1-(1-0.8) x (1-0.8) x (1-0.8)] = 0.992very likely
Failure Probabilities
Overall Pf is Prob Failure by [(Thd Pty) OR (Corr) OR (GeoHaz)…]
Ps = 1 - Pf
Overall Ps is Prob Surviving [(Thd Pty) AND (Corr) AND (GeoHaz)….]
So…
Pf overall = 1-[(1-Pfthdpty) x (1-Pfcorr) x (1-Pfgeohaz) x (1-Pfincops)]
Final PoF
PoF overall = PoFthdpty+ PoFTTF + PoFtheftsab+ PoFincops+ PoFgeohazard
PoF overall = 1-[(1-PoFthdpty) x (1-PoFTTF) x (1-PoFtheftsab) x (1-PoFincops) x
(1-PoFgeohazard)]
Guess PoF if 1%, 4%, 2%, 2%, 0%
Calc:
Probability of Failure
• Exposure
• Mitigation
• Resistance
Estimating Threat Exposure
• Events per mile-year for time independent /
random mechanism– third party
– incorrect operations
– weather & land movements
– equipment failures
• MPY for degradation mechanisms– ext corr
– int corr
– SCC / fatigue
Failure Rates
Failures/yr Years to Fail Approximate Rule Thumb
1,000,000 0.000001 Continuous failures
100,000 0.00001 fails ~10 times per hour
10,000 0.0001 fails ~1 times per hour
1,000 0.001 fails ~3 times per day
100 0.01 fails ~2 times per week
10 0.1 fails ~1 times per month
1 1 fails ~1 times per year
0.1 10 fails ~1 per 10 years
0.01 100 fails ~1 per 100 years
0.001 1,000 fails ~1 per 1000 years
0.0001 10,000 fails ~1 per 10,000 years
0.00001 100,000 fails ~1 per 100,000 years
0.000001 1,000,000 One in a million chance of failure
0.0000000001 1,000,000,000 Effectively, it never fails
Time Dependent Mech
PoF time-dep = f (TTF)
where
TTF = “time to failure”
TTF = (available pipe wall) / [(wall loss rate) x (1 – mitigation
effectiveness)]
Advantages of New Exposure
Estimates• Estimates can often be validated over time
• Estimate values from several causes are directly additive. E.g. falling
objects, landslide, subsidence, etc, each with their own frequency of
occurrence can be added together
• Estimates are in a form that consider segment-length effects and
supports PoF estimates in absolute terms
• Avoids need to standardize qualitative measures such as “high”,
“medium”, “low” avoids interpretation and erosion of definitions over
time and when different assessors become involved.
• Can directly incorporate pertinent company and industry historical
data.
• Forces SME to provide more considered values. It is more difficult to
present a number such as 1 hit every 2 years
Measuring Mitigation
Strong, single measure
or
Accumulation of lesser measures
Mitigation % = 1 - (remaining threat)
Remaining Threat = (remnant from mit1) AND (remnant from mit2) AND
(remnant from mit3) …
Measuring Mitigation
Mitigation % = 1-[(1-mit1) x (1-mit2) x (1-mit3)…]
In words: mitigation % = 1 - (remaining threat)
remaining threat = (remnant from mit1) AND (remnant from mit2) AND (remnant
from mit3) …
What is cumulative mitigation benefit from 3 measures that independently produce
effectiveness of 60%, 60%, and 50%?
Coating-CP Interaction
PoD of coating
OR gate
CP effectiveness
Calibrating Coating Fail Rate
calibrate coating fail rate using DOT stats
coat defect rate
dia
A, sq-
ft/linear ft
A, sq-
ft/mi fail/mil-yr
% corr
fail
coating
fail fctr
fails/per
sq ft/yr
24 6.3 33,175 0.001 0.3 100 9.0E-07 0.003 fails per yr per 33K sq ft
12 3.1 16,588 0.001 0.3 100 1.8E-06
10 2.6 13,823 0.001 0.3 100 2.2E-06
8 2.1 11,058 0.001 0.3 100 2.7E-06
6 1.6 8,294 0.001 0.3 100 3.6E-06
assume 100x as many coating failures as corr failures
% bare from CP current demand….
or
PoD for Coating
12” diameter
PoD = 1 - EXP[-[surface area, ft2)x(failure rate per ft2)]
L = 1 ft L = 10 ft L = 100 ft L = 1000 ft L = 5280 ft
excellent 5.0E-07 0.00% 0.00% 0.02% 0.16% 0.83%
good 2.5E-06 0.00% 0.02% 0.18% 1.75% 8.91%
fair 3.2E-04 2.46% 22.1% 91.8% 100% 100%
poor 4.0E-02 11.8% 71.4% 100% 100% 100%
absent 1.0E+07 100% 100% 100% 100% 100%
Probability of Defect in Segment, per yeardefect rate
per sq ft
Coating
Score
Corrosion Control Effectiveness
Coating Condition
Coating Defect Type
Prob of Defect Type per sq ft
Is CP fully eff?
Prob of CP
protecting pipe
Scenario Prob
Resultant MPY
Y 0.9 99.99% 0 none 99.9%
N 0.1 0.01% 0
Y 0.9 0.09% 0 hole 0.1%
N 0.1 0.01% 16
Y 0.1 0.0% 0
excellent
shielding 0.0% N 0.9 0.0% 16
Final probability of 16 mpy damage rate per sq ft 0.01% 16
Final probability of 0 mpy damage rate per sq ft 99.99% 0
Damage vs Failure
• Probability of Damage (PoD) = f (exposure, mitigation)
• Probability of Failure (PoF) = f (PoD, resistance)
Exposure PoD
Mitigation PoF
Resistance
Estimating Resistance
• pipe spec (original)
• historical issues
– low toughness
– hard spots
– seam type
– manufacturing
• pipe spec (current)
– ILI measurements
– calcs from pressure test
– visual inspections
– effect of estimated degradations
• required pipe strength
– normal internal pressure
– normal external loadings
Best Estimate of Pipe Wall Today
Press Test
1992
ILI
2005
Measurement error Degradation Since Meas
8 mpy x 15 yrs = 120 mils
8 mpy x 2 yrs = 16 mils+/- 15%
+/- 5%(inferred)
2007 Estimate
Best Estimate of Pipe Wall Today
Press Test 1
Press Test 2
Bell Hole 1
Bell Hole 2
ILI 1
ILI 2
NOP
Best Est Today
ILI Capability Matrix
Dent/ Ovality/
Gouge Buckling
Aggressive 5 5 100 10 20 50 50 5 100
Routine 10 10 100 50 50 50 50 10 100
Min 15 15 100 100 50 50 50 15 100
Aggressive 10 10 100 50 50 50 50 10 100
Routine 15 15 100 100 50 50 50 15 100
Min 20 20 100 100 50 50 50 20 100
5 5 100 100 20 20 5 5 100
TFI 20 20 5 10 50 50 50 20 5
EMAT 50 50 10 10 50 50 10 50 10
50 50 10 10 50 50 10 50 10
100 100 100 100 5 5 100 100 100
Press test 5 5 5 5 2 2 2 5 5
Defect Type
External
Corrosio
Internal
Corrosio
Axial
Crack
Circum
Crack Lamin
Ultrasound
Ultrasound shear wave crack tool
Caliper, sizing, gauging, inertial
Max Surviving Defect
Metal Loss
Inspection
Type
Validation (Pig-
Digs) Protocol Crack
MFL high
resolution
MFL std
resolution
NOP & Integrity
+ Integrity info -Rupture potential
-Higher stress
-Fatigue
Time
PoF
TTF to PoF
Time
PoF
TTF to PoF
PoF: TTF & TTF99
time
PoF PoF=100%
PoF=1%
TTF99
Examples
• TTF = 0.160” / [(16 mpy) x (1 - 0.9)] = 100 years
• TTF99 = 0.160” / (16 mpy) = 10 years
• PoF => lognormal or other =>0.001%
• TTF = 0.016” / [(16 mpy) x (1 - 0.9)] = 10 years
• TTF99 = 0.016” / (16 mpy) = 1 year
• PoF = 1/TTF = 1%
Approach Advantages
• more intuitive
• better models reality
• distinguishes between unmitigated exposure to a threat,
mitigation effectiveness, and system resistance--better risk
management decisions
• eliminates need for unrealistic and expensive re-weighting of
variables for new technologies or other changes
• flexibility to present results in either absolute terms or relative
terms
• more audit friendly