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An Overview of API 579-1/ASME FFS-1 Fitness-For-Service
Assessment Standard with Applications to Case Studies
Conference Paper · December 2017
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An Overview of API 579-1/ASME FFS-1
Fitness-For-Service Assessment
Standard with Applications to Case
Studies
By
Mohammad M. Megahed
Professor of Solid Mechanics
Cairo University – Egypt
Keynote Lecture Presented at
Al-Azhar 14th International Conference on
Engineering, Architecture & Technology
12-14 December 2017
Cairo, Egypt
Layout of This Presentation
Concepts
 What is FFS assessment
 Objectives, Advantages, History, Contents
 The THREE Levels of Assessment
 Concept of Remaining Strength Factor (RSF)
 Concept of Failure Assessment Diagram (FAD)
Case Studies
 Case #1: Fire Damage in a Drilling Platform
 Case #2: Wall Cracking in Regeneration
Columns
 Case #3: Corroded Cantilever Pipe
 Case #4: Pipes Suffering from Pitting
Resource Documents of FFS Standard
1320 Pages (2016 Issue)
374 Pages (2007 Issue)
Objectives of FFS Assessment
 FFS assessment is a multi-disciplinary
approach to determine whether an equipment,
which is suffering from flaws or damage or
subjected to operating conditions higher than
design loads, is fit for continued service or
not.
 Final outcome of FFS assessment is a decision:
to run as is, repair, re-rate, alter, or retire the
equipment.
 FFS outcome may also include an estimate of
remaining life which is useful for planning
future inspection (in case of continued service)
and future budgeting (in case of equipment
retiring)
‫‪FFS Arabic Terminology‬‬
‫‪Arabic‬‬
‫تقييم لياقة المعدة لالستمرار فى الخدمة‬
‫من عدمه‬
‫استمرار المعدة فى الخدمة كما هى‬
‫اجراء اصالحات على المعدة‬
‫االستمرار فى الخدمة عند أحمال مخفضة‬
‫اجراء تغييرات فى التصميم‬
‫احالة المعدة الى االستيداع‬
‫‪English‬‬
‫‪Fitness for Service‬‬
‫‪Assessment-FFS‬‬
‫‪To run as is‬‬
‫‪To repair‬‬
‫‪To Re-Rate‬‬
‫‪To Alter‬‬
‫‪To Retire‬‬
Historical Background of FFS Assessment
 1990: Joint industry project was organized
by the Materials Properties Council (MPC) to
develop FFS guidelines for the refining
industry
 2000: Based on MPC final report, API issued
API-579 recommended practice (RP) for FFS
Assessment, which was welcomed by both
refinery and non-refinery industries
 2007: ASME joined forces with API and
issued API 579-1/ASME FFS-1 Standard
 2016: Latest edition of API 579-1/ASME FFS-1
Standard with a number of reorganizations,
Updates and addition ofr Part-14 on Fatigue
Contents of API 579 - Code Parts
Part 1 -
Introduction
Part 2 -
FFS Engineering Evaluation Procedure
Part 3 -
Brittle Fracture
Part 4 -
General Metal Loss
Part 5 -
Localized Metal Loss
Part 6 -
Pitting Corrosion
Part 7 -
Blisters, HIC, and SOHIC Damage
Part 8 -
Weld Misalignment and Shell Distortions
Part 9 -
Crack-Like Flaws
Part 10 -
Equipment Operating in the Creep Range
Part 11 -
Fire Damage
Part 12 -
Dents, Gouges, and Dent-Gouge Combinations
Part 13 -
Laminations
Part 14 -
Fatigue
List of Annexes of FFS Standards (2007 Issue)












Annex A -
Thickness, MAWP, and Stress Equations for a
FFS Assessment
Annex B1 - Stress Analysis Overview for a FFS Assessment
Annex B2 - Recommendations for Linearization of Stress
Results for Stress Classification
Annex B3 – Histogram Development and Cycle Counting for
Fatigue Analysis
Annex C - Compendium of Stress Intensity Factor
Solutions
Annex D - Compendium of Reference Stress Solutions
Annex E - Residual Stresses in a FFS Evaluation
Annex F - Material Properties for a FFS Assessment
Annex G – Damage Mechanisms
Annex H – Technical Basis and Validation
Annex I - Glossary of Terms and Definitions
Annex K – Crack Opening Areas
Multidisciplinary Nature of FFS
Assessment
Fitness of Service Assessment require
familiarity with the following fields:
 Stress Analysis
 Finite Element Analysis
 Metallurgy
 Materials Engineering
 Non-Destructive Examinations (NDE)
 Corrosion Science and Engineering
 Fracture Mechanics
API 579 Assessment Levels
 Level 1 assessment:
Most conservative




conservative screening criteria
Minimum amount of inspection and information
May be performed by an Inspector or Engineer
If result not acceptable, can resort to levels 2 or 3





more detailed evaluation
More detailed calculations needed
Would be done by an experienced Engineer
Produces more precise results
If result not acceptable, can resort to level 3





The most detailed evaluation
Detailed inspection and information required
Usually based on numerical techniques such as FEA
Most rigorous
Produces most precise results.
To be performed by experienced engineering specialist
 Level 2 assessment:
 Level 3 assessment:
Matching between Degradation
Mechanisms and FFS Parts- 2007 Version
Concept of Remaining Strength Factor (RSF)
RSF = LDC /LUC
RSF
LDC
LUC
= Remaining Strength Factor
= Limit Load of the Damaged Component
= Limit Load of the Un-Damaged Component
RSF is estimated by equations for Levels-1,2
RSF is computed by Non-linear FEA in Level-3
RSF is compared with an allowable value RSFa =0.9 say
If RSF<RSFa then the component can be operated at a
reduced pressure (Re-rated)
MAWPr/MAWP = RSF/RSFa
MAWP = Original maximum allowable pressure
MAWPr = Rerated pressure value
Deformed Shapes at limit loads of corroded
pipe with FCA = 0.4 mm, 1 mm and 1.6 mm
Calculation of RSF for
corroded cantilever
pipes with increasing
corrosion levels
Future
Operation
FCA P-Limit
RSF
Months after (mm) (MPa)
last inspection
0
0
37
100%
4
0.4
18
49%
10
1.0
15
41 %
16
1.6
12
32%
(a)
(b)
Part-9 Failure Assessment Diagram- FAD
Toughness Ratio Kr
1
0.8
Unacceptable Region
Cut-off for steels
with yield plateau
0.6
Cut-off for ASTM A508
Cut-off for
Cr-Mn steels
Cut-off for
stainless steels
0.4
Acceptable Region
0.2
0
0
0.5
1
1.5

Load Ratio L 
 ys
P
r
P
ref
2
2.5
14
Advantages of FFS Assessment
 Safe and reliable operation of aging
equipment
 Reduce downtime by eliminating
unnecessary repairs
 Extra time to plan shutdown and
replacement of equipment
 Improved yields, if rate of equipment
deterioration or life consumption can be
estimated
Case-1
Fitness-For-Service
Assessment of A Drilling
Platform Structure and
Piping following Fire
Damage
Introduction
 This investigation was called upon following a
major fire incident that took place on a Drilling
Platform 2004.
 The methodology used in the assessment of
fire damage (Part-11 of API-RP-579. This
recommended practice was the first issue of
FFS-579 (2000), see Fig.1
 In level-1, evidence is gathered and collected
to justify assigning a component to a certain
heat exposure zone. The fire damage was thus
categorized into six distinctive heat exposure
zones as illustrated in the platform computer
model shown in Fig. 2.
Fig.1. Consequences of Fire Damage
Fig.2. Identification of the 6 Heat Exposure Zones
Level-1 Assessment
The Three Levels of Assessment of Fire Damage
THREE levels of assessment of increasing complexity:
Level-1: Gather data and documents to justify assigning
a component to a certain heat exposure zone
Level-2: Conducted for Components that did not pass
level-1 and need to be assessed for continued
service. Requires hardness measurement in order to
determine the remaining strength of the affected
Material
Level-3: More complex assessment conducted for
components that did not pass level 2. Reliance
is made on grain size measurement and
metallurgical in-sito investigations
Specifications of Features of the 6 Fire Zones
 Categorization of fire damage into 6 distinctive zones of
fire exposure according to thermal effects on materials
 Zone-I: Ambient temperature during fire = No Exposure
 Zone-II: Ambient to 66o C = Smoke and water exposure
 Zone-III: 66oC – 204o C - Light heat exposure
 Zone-IV: 204oC – 427oC - Moderate heat exposure
 Zone-V : 427oC – 732oC - Heavy heat exposure
 Zone-VI: Greater than 732oC – Severe heat exposure
Level-2 Assessment
 Level-2 Assessment require measurement of
surface hardness in order to estimate the
remaining strength of component material
following exposure to fire.
 Many pipelines in addition to specific parts of
the platform steel structure were assessed
according to level-2.
 The investigation involved measurement of
hardness and remaining pipe wall thickness at
thousands of locations on the fire affected
zones.
 Estimated remaining ultimate tensile strengths
(UTS) were thus estimated
Hardness Survey on Fire-Affected Piping
Variation of BH Hardness .VS. Position of Test Header 6" and 8"
HB (MIN.)
200
HB (MAX.)
180
160
Av. for Gr-B
140
0
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28
Position
Along PIPE OP 6"
Along PIPE NO 6"
Along PIPE MN 6"
20
Along PIPE TW 8"
40
Along PIPE ST 6"
60
Along PIPE MS 6"
80
Min. for Gr-B
Along PIPE LK 6"
Along PIPE EF 6"
Along PIPE FG 6"
Along PIPE GH 6"
Along PIPE GH 6"
Along PIPE IJ 6"
Along PIPE JK 6"
100
Along PIPE AB 8"
HB
120
30 32 34 36 38 40
Results of FFS Assessment
 Affected pipes that need to be replaced
were identified
 Pipes that need to be de-rated were further
investigated. Allowable stress for affected
piping Safd is estimated using hardness
measurement and appropriate piping
design codes.
Safd =Min [0.25 Sutsht (SaT/SaA), SaT )
 Procedures for repair of the steel structure
were proposed.
Pressure De-Rating of Heat Affected Piping
Outcomes of FFS Assessment of Fire Damage
 FFS has identified fire-affected piping
 FFS provided piping de-rated capacities.
 Fire-affected zones of the platform structures
has been identified for possible repair actions.
 The root cause of the fire incident was
identified as an explosion in a riser due to local
thinning of riser wall thickness at the splash
zone.
Case-2
Fitness-For-Service
Assessment of Two
Regeneration Columns
Suffering from Wall
Cracking
Introduction
 A gas plant implements a sweetening process, in which
CO2 and small amounts of H2S are stripped-off the
produced gas through two Lean Benfield System Trains
# A, B. The process is conducted inside a vertical tower;
known as a “Regeneration Column”. Geometry of the
column: height = 28 m above the skirt, Di = 3 m, wall
thickness =16 mm
 The lower 18.6 m is fabricated from carbon steel (ASTMA516- Grade 70) with yield strength around 400 MPa,
while the upper 10 m is fabricated from stainless steel
316.
 A passivation technique of the inner wall was adopted
through circulating a Vanadium Pent-Oxide (V2O5)
through the lower section the regenerator column;
supposed to be effective for a normal duration of 5
years
Construction of the Regeneration Column
Design Conditions and Loads

Design Conditions
Design Pressure
Pd
= 4.1 barg,
Operating pressure Po
= 1.6 barg,
Po reduced to
= 0.4 barg.
Design temp. Td
= 140 oC ,
Operating temp. To
= 115 oC.
Column weight +Contents
= 1350 kN
Maximum eccentricity of weight
= 100 mm
Maximum wind speed
= 31 m/sec

Estimation of Stresses in the wall due to Operating Loads:
Hoop stress due to 0.4 barg
= +4 MPa
Axial stress due to 0.4 barg
= +2 MPa
Axial stress due to column weight = -9 MPa
Stress due to weight eccentricity
= + 1.2 MPa
Highest hoop stress due to P
= 4 MPa = 1% of Sy
Highest compressive stress due to all loads = -8 MPa = 2 %
of Sy
Problem History and Inspection Data
History
Year 1999: commissioning of the two columns: A, B
Year 2002: Uniform pitting observed at C.S./S.S. interface of
deepest pit =1.5 mm. Pits attributed to galvanic corrosion.
Year 2004: Leaks observed at small pinholes at weld locations in
the CS section, e.g. at shell girth welds, and piping connections.
Years 2004-2006: Increasing number of leaks + observed corrosion
in the vessel wall under the insulation
Damage was observed only in column A but not in column B
Most Important Inspection Results (April 2006)
Vessel Body: Branched long through-thickness crack at the girth
weld of Strakes 4 and 5.
Down-Comer: Non-penetrating internal circumferential cracks at the
welds connecting the down- comer piping to the vessel wall with
max. length of 220 mm.
N5 and N6 Nozzles: Non-penetrating internal circumferential cracks
at the HAZ of the welds connecting nozzles to vessel wall, lengths up
to 150-230 mm
Girth Weld Crack Defects
Through-Thickness
Branched crack ~80
mm long at shell
girth weld joining
strakes 4 and 5.
Scope: Use API-579 RP for FFS Assessment of
the most significant cracks in Column A
Cracks at toes of welds connecting
down-comer to shell
Crack
Cracks near welds of nozzles N5 , N6
Shell girth-weld
Methodology of Part-9 of API-579 RP for
Crack-Like Flaws: FAD
FAD combines the effects of stress field and stress
intensity factor into one assessment point
Level-3 Assessments
Assessment result for each cracking case is
shown on a simple plot known as the Failure
Assessment
Diagram
(FAD),
which
characterizes the border of safe operation in a
2-D plot.
Since the observed cracking pattern was
growing with time, it was compulsory to
implement Level-3 assessment, which is the
most stringent assessment procedure.
Assessment results of the most severe cracks
in the column wall identified both the present
condition and the remaining life of the column
based on Level-3 assessment.
Toughness Ratio Kr
FFS of Down-Comer Flaws Increasing
Crack Depth (a)
4
5
3.5
Case #
a [mm]
K iR MPa m
K iR / K mat
1
2
3
4
5
6.4
8
9.6
12.8
16
91.81
119.77
150.23
203.975
497.21
0.69
0.90
1.14
1.55
3.77
4
1.5
Not-Acceptable
3
1
2
1
0.5
Crack
Acceptable
0
0
0.5
1
1.5
Load Ratio LPr 

 ys
P
ref
2
2.5
36
FFS of the Branched Through-Thickness Girth
Weld
FFS Treatment of Branched
Crack to determine the
Equivalent Crack Length as 67
mm
Toughness Ratio Kr
1
0.8
0.6
0.4
0.2
0
0
0.5
1
1.5
Load Ratio LPr 

 ys
P
ref
2
2.5
Reboiler Vapour Return Nozzle N5 – Circumferential Crack
X58 with L= 150 mm is the most Serious
X58 =150 mm
X43 =120 mm
FFS of Crack N58 with L =150 mm in Nozzle N5 for
increasing Values of Crack Depth
Toughness Ratio Kr
3
Case #
a [mm]
1
2
3
4
8
11.2
12.8
16
2.5
4
2
m
96.6
114.8
128.3
299.7
K iR / K mat
0.733
0.871
0.973
2.274
Not-Acceptable
3
1
2
K iR MPa
1
0.5
Acceptable
0
0
0.5
1
1.5

Load Ratio L 
 ys
P
r
P
ref
2
2.5
Conclusions and Recommendations
Stresses in the tower wall due to Operating loads are
very small. Residual stresses are present at the girth
welds due to absence of PWHT.
Presence of residual stresses + lack of effectiveness of
the passivation technique are the main causes of
observed cracking located primarily at or near welds in
the CS section of the tower(s).
FAD showed that most of the material toughness is
exhausted at the through-thickness branched crack.
Immediate Repair is highly recommended.
Surface cracks at the down-comer weld could become
serious if the crack depth exceeds 50 % of shell wall
thickness. Inspection by ToFD should be conducted to
verify crack growth. Similar conclusions are made for
cracks at nozzles N5 and N6
Actions taken after First FFS Assessment
 ToFD Inspection was conducted in May 2007;
almost one year after April 2006 inspection
campaign
 ToFD
inspection
showed
that
cracks
are
propagating, and adjacent neighboring cracks are
combining together to form longer cracks.
 Further ToFD inspection on columns A,B showed
that cracks kept propagating at an alarming rate
with column A deteriorating faster than column B.
 A decision was thus made to replace the two
columns with new ones with improved design and
material of construction
 FFS Assessment has thus given the operator of the
columns enough lead time to re-design and
contract the new improved columns
Case-3
Fitness-For-Service
Assessment of a Corroded
Cantilever Pipe
Based on an Article entitled:
“Assessment of corrosion damage acceptance criteria in
API579-ASME/1 code” M. S. Attia · M. M. Megahed · M.
Ammar Darwish · S. Sundram, published in “The
International Journal of Mechanics and Materials in
Design” 01/2014; DOI:10.1007/s10999-014-9278-6 ·
1.20 Impact Factor
Problem Statement and Objectives
 A 4” sch. 80 API 5L Grade B steel pipe is suffering
from severe corrosion, and FFS assessment is
required.
 Wall thickness measurement by UT in 2006 and
2008 showed that the annual corrosion rate is
around 1.2 mm
 Pipe nominal thickness is 8.6 mm and metal loss is
localized in a straight section at 6 O’clock. The
minimum wall thickness recorded is around 3 mm
 The pipe carries internal pressure in addition to
mechanical loads due to attached valves
 This type of piping arrangements is of type-B and
therefore FFS assessment of level-1 may not be
made if supplemental mechanical loads can be
ignored; which is not the case. Thus Levels 2 and 3
only are conducted here
Schematic of the Corroded 4” Cantilever Pipe
Corroded
Region
Nominal wall thickness
=
Weight of Gate Valve
=
Weight of Globe Valve
=
MAWP = Design Pressure =
Current operating Pressure =
Material Yield Strength
=
Design Stress
=
8.6
mm
92
Newton
2220 Newton
9.3
MPa
1.6
MPa
241 MPa
138 MPa
UT Wall Thickness in the Corroded Region
Corrosion rate =1.2 mm/Y
From two consecutive
inspections
UT Grid size:
10 mm in axial direction
46 mm in circumferential
direction
Nominal Wall thickness
= 8.56 mm
Min. Wall thickness
At 180o = 3.0 mm
At 135o = 3.5 mm
At 225o = 4.8 mm
3D Wall Thickness Profile in Corroded Region
Results of Level-1 Assessment
Summary of Level-1 Acceptability Criteria
FCA (mm) from last inspection
Months of Future Operation
0.4
0.8
1.2
1.6
2.0
2.4
2.8
4
8
12
16
20
24
28
Acceptability of Average Measured Thickness
Condition-1:Tams-FCA > TminC
FALSE
FALSE
FALSE
FALSE
FALSE
FALSE
FALSE
Condition-2:Tamc-FCA >TminL
TRUE
TRUE
FALSE
FALSE
FALSE
FALSE
FALSE
Acceptability of MAWP
Condition-3/1: MAWPr>P-design
FALSE
FALSE
FALSE
FALSE
FALSE
FALSE
FALSE
Condition-3/2:MAWPr>P-Current
TRUE
TRUE
TRUE
TRUE
TRUE
TRUE
FALSE
FALSE
FALSE
Acceptability of Minimum Measured Thickness
Condition-4: tmm –FCA <= Tlim
TRUE
FALSE
FALSE
FALSE
FALSE
The pipe does not satisfy Level-1 acceptability criteria with respect
to measured thickness
The pipe should not be operated at the design pressure of 9.3 MPa
The pipe can be operated for future 24 months at the current
pressure of 1.6 MPa
Mechanical Loads acting on the 4” Pipe
Results of Level-2 FFS Assessment
Summary of Level-2 Acceptability Criteria
FCA (mm) from last inspection
Months of Future Operation
0.4
0.8
1.2
1.6
2.0
2.4
2.8
4
8
12
16
20
24
28
Acceptability of Average Measured Thickness
Condition-1:Tams-FCA > TminC
FALSE
FALSE
FALSE
FALSE
Condition-2:Tamc-FCA >TminL
TRUE
FALSE
FALSE
FALSE
Acceptability of MAWP
Condition-3/1: MAWPr>P-design
FALSE
FALSE
FALSE
FALSE
FALSE
FALSE
FALSE
Condition-3/2:MAWPr>P-Current
TRUE
TRUE
TRUE
TRUE
FALSE
FALSE
FALSE
FALSE
FALSE
Acceptability of Minimum Measured Thickness
Condition-4: tmm –FCA <= Tlim



TRUE
FALSE
FALSE
FALSE
FALSE
The pipe does not satisfy Level-2 acceptability criteria with
respect to measured thickness
The pipe should not be operated at the design pressure of 9.3
MPa
The pipe can be operated for future 16 months at the current
pressure of 1.6 MPa
Assessment of Remaining Life at Design and
Current Pressure Values using Level-2
Corroded Pipe FE Model used in Level-3 FFS
Material model = Elastic Perfectly-plastic
Material Yield Strength = 207 MPa
Limit load analysis is composed from 2 steps:
- Apply all mechanical loads and keep them in action
- Apply monotonically increasing pressure and observe
stresses and strains
- Limit pressure is reached when the pressure stabilizes while
strains keep increasing
3D Wall Thickness Profile in Corroded Region
Deformed Shapes at limit loads of corroded
pipe with FCA = 0.4 mm, 1 mm and 1.6 mm
Calculation of RSF for
corroded cantilever
pipes with increasing
corrosion levels
Months of
Future
Operation
0
4
10
16
FCA
(mm)
P-Limit
(MPa)
RSF
0
0.4
1.0
1.6
37
18
15
12
100%
49%
41 %
32%
(a)
(b)
1st Yield and Limit Pressures for corroded pipes after 4,10,16
months from date of last inspection compared to new pipe
Months of
Operation after
date of last
inspection
0
4
10
16
FCA (mm)
P-Limit
(MPa)
RSF
0
0.4
1.0
1.6
37
18
15
12
100%
49%
41 %
32%
240
200
Effective Stress [MPa]
FCA=0.4mm
Uncorroded
Py=13.5 MPa
160
120
PL=37 MPa
80
PL=18 MPa
Effective Stress [MPa]
240
40
0
200
160
FCA=0.4mm
FCA=1mm
120
FCA=1.6mm
80
40
0
0
5
10
15
20
25
Internal Pressure [MPa]
30
35
40
0
2
4
6
8
10
12
Internal Pressure [MPa]
14
16
18
20
Limit Pressures for the Corroded Pipe for Various
FCA values: (a) 0.4 mm, (b) 1 mm, (c) 1.6 mm
Variation of Effective Strain with Pressure for Corroded Pipes
for Various FCA values: (a) 0.4 mm, (b) 1 mm, (c) 1.6 mm
Conclusions and Recommendations
 The corroded pipe failed both Level-1 and
Level-2 FFS assessments for safe operation at
the design pressure of 9.3 MPa, but could be
operated at the current reduced pressure of
1.6 MPa for about 24 months
 Ignoring
Level-2
thickness
acceptability
criteria, a Remaining life of about 18 months is
estimated for operation at a reduced pressure
of 1.6 MPa
 Level-3 assessment shows safe operation at Pd
for up to 4 months only. Operation at a
reduced pressure 1.6 MPa can be tolerated for
about 16 months.
Case-4
Fitness-For-Service
Assessment for Pipes
Suffering from Pitting
Damage with Increasing
Severity
Introduction
 Pitting is an extremely localized corrosion in the form of
holes of metal loss in the metal.
 Usually corrosion exists in the form of pitting colonies.
Pits can be simplified as circular defects of metal loss.
 Each pit can be described by two geometrical factors; pit
diameter and pit depth.
 Part-6 of API 579-1/ASME FFS-1 presents assessment
procedures to determine remaining pressure carrying
capacities of pipes suffering from random pitting of
increasing severity.
 Three cases of random colonies of increasing pitting
severity are considered here for investigation by nonlinear FEA based on simplifying pits by shell elements
with reduced thickness.
 FEA predictions are compared with level-1 and level-2
assessment of part-6 of API 579-1/ASME FFS-1.
Different cross sectional shapes of pits
The 8 Standard Templates of Pitting Grades
 Part-6 of API 579-1/ASME FFS-1 contains 8 templates of
pitting charts representing 8 grades of increasing pitting
severity. Colonies corresponding to higher grades of
pitting simulate propagation with time of lower grades
and hence contain more pits within the same area.
Selection of pitting colonies
 Each pitting grade has an area of 150 mm x
150 mm. Only a region of an area of 57 mm x
57 mm was selected at the lower left corner of
each template, to satisfy the code requirement
of including at least 10 pits for the
assessment.
 Colonies of grades 1, 2 and 3 contain 12, 36
and 53 pits respectively of a widespread
scattered pitting. Higher grades are evolution
of lower grades with time. This evolution is
presented by defining pits of each grade by a
different color.
 Pipe wall thickness is 8 mm and with outer
diameter of 458.8 mm. The pipe material is
API-5L X80.
Evolution of pitting Grades with Time
Assessment of pitting colonies using part 6 of
API579-1/ASME-FFS-1
 Assessment procedures of part-6 are based on
determining the remaining strength factor
(RSF) for components suffering from pitting
corrosion.
 The code consists of three levels of
assessment; with levels 1 and 2 being of
analytical nature and level-3 assessment
relying on non-linear FEA of the pitted pipe.
 Level 1 assessment utilizes the 8 standard
pitting templates to carry out a preliminary
assessment.
 Level 2 assessment provides detailed
assessment which considers all parameters
defining the colony.
Level 1 Assessment
 Level 1 assessment is a preliminary analysis
utilizing standard pitting grades charts.
Further, the remaining strength factor RSF is
determined by the maximum pit depth within
the pitting colony.
 Check if RSF ≥ RSFa , where RSFa is the
allowable RSF is taken as 0.9. If the condition
is verified, then the pipe passes level 1
assessment and is safe for operating at the
calculated MAWP. If not, then the pipe is safe
for operating at a reduced pressure MAWPr
calculated according to
Determination of RSF using pitting
grades templates
The shown tables are
extracted from part 6
of
API579-1/ASMEFFS-1 2016 edition
provides RSF values
corresponding to five
levels of pit depths as
represented by the
ratio of remaining wall
thickness
at
the
deepest pit (Rwt ) for
use
in
level
1
assessment.
Results of Level 1 assessment
RSF
MAWP (MPa)
Colony Gr-1
0.95
7.3
Colony Gr-2
0.91
7.3
Colony Gr-3
0.83
6.7
Level 2 Assessment
 Level 2 assessment provides detailed analysis
which accounts for mutual interaction between
neighboring pits
 Pitting couples are defined based on the
nearest neighbor for each pit. Orientation of
each pitting couple with respect to pipe
longitudinal direction becomes important.
 For each couple, the spacing between the two
pits and the orientation of the line linking them
with respect to the pipe longitudinal direction
should be defined.
 Diameters and depths of the two pits forming a
couple are used in the assessment.
Determination of pitting couples
Results of Level 2 assessment
Colony Gr-1
Colony Gr-2
Colony Gr-3
RSF
0.94
0.85
0.83
MAWP (MPa)
7.3
6.9
6.7
Level 3 assessment (Nonlinear FEA)
 Level 3 assessment is based on nonlinear FEA. Shell
element is used to simulate the pipe wall. Pits are
modeled as circular defects with reduced thickness
equal to the remaining wall thickness of the pit.
 To estimate limit pressure for a pitted pipe, the “Twice
Elastic Slope (TES)” methodology was used. TES method
relies on constructing a line with a slope equal to twice
the elastic slope.
FE modelling for pitted pipe
Von-Mises Stress Distributions from FEA
Distributions of Radial deformation from FEA
Limit Pressures for Pitted Pipes by TES method
Perfect Pipe
Limit Pressure PL (MPa)
RSF
22.43
-
Colony
Gr-1
21.5
0.96
Colony
Gr-2
20.1
0.90
Colony
Gr- 3
19.9
0.89
Comparison between the results obtained
using FEA and part 6 of API-579/ASME-FFS-1
Level 1
Level 2
Level 3 (FEA)
Colony
Gr-1
0.95
0.94
0.96
RSF
Colony
Gr-2
0.91
0.85
0.90
Colony
Gr-3
0.83
0.83
0.89
Conclusions and Recommendations
 Level 1 assessment provides inaccurate
estimation for RSF due to its sole dependence
on the deepest pit depth and templates of
pitting grades.
 Level 2 assessment provides RSF values less
than that of Level-1 estimates due to its
consideration of the interaction between
neighboring pits
 Level-3 estimates of RSF provided RSF
estimates slightly higher than that of level 2
assessment. Level 3 provided detailed
indications on where burst will occur and how
neighboring pits interact.
 Research on Pitting still continuing…………
Acknowledgements
Acknowledgement is expressed here for
my friends, colleagues and students at
Cairo University (CU) who contributed
to the results reported in this
presentation:
1- Dr. Saad El-Raghy
2- Dr. Hesham Sobhi Sayed
3- Dr. Mohammad Salah Attia
4- Eng. Ahmad El-Taweel
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