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Failure Assessment of an In-Service Pressure Vessel with Crack Flaw Using Failure Assessment Diagram (FAD) Method

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Failure assessment include fatigue assessment was performed for an in-service pressurize equipment utilized to support hydrocarbon proceesing activity as the response of crack-like flaw finding during phase array scanning inspection. The assessment required to ensure the integrity and the safety in the operation of deteriorated pressure vessel. The fitness-for-service assessment in this study are consist of failure assessment using Failure Assessment Diagram (FAD) and the fatigue assessment based on API 579-1/ASME FFS-1. The assessment has demonstrated that the current condition of the equipment was pass the assessment requirement and still has adequate strength and the fatigue damage due to actual operation pressure is an insignificant factor affecting the life of the equipment. This study also investigates the correlation between the geometry of the flaw and the stress increase ratio that is expressed in the exponential function as σC/σR = 4.18e0.82(LD/T^2)
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International Journal of Marine Engineering Innovation and Research, Vol. 9(2), June. 2024. 233-244
(pISSN: 2541-5972, eISSN: 2548-1479) 233
Failure Assessment of an In-service Pressure
Vessel with Crack Flaw Using Failure
Assessment Diagram
Wira Herucakra1, Christina Dewi R.I. Simanjuntak2
(Received: 18 May 2024 / Revised: 20 May 2024 /Accepted: 26 May 2024)
Abstract
Failure assessment include fatigue assessment was performed for an in-service pressurize equipment utilized to
support hydrocarbon proceesing activity as the response of crack-like flaw finding during phase array scanning inspection.
The assessment required to ensure the integrity and the safety in the operation of deteriorated pressure vessel. The fitness-
for-service assessment in this study are consist of failure assessment using Failure Assessment Diagram (FAD) and the
fatigue assessment based on API 579-1/ASME FFS-1. The assessment has demonstrated that the current condition of the
equipment was pass the assessment requirement and still has adequate strength and the fatigue damage due to actual
operation pressure is an insignificant factor affecting the life of the equipment. This study also investigates the correlation
between the geometry of the flaw and the stress increase ratio that is expressed in the exponential function as σC/σR =
4.18e0.82(LD/T^2)
Keywords
API RP 579-1/ASME FFS-1, Crack-liken flaw, Fitness-for-Service Assessment, Failure Assessment Diagram,
Pressurized Equipment.
I. INTRODUCTION
1
A pressure vessel is a pressurized equipment for
processing hydrocarbon from the initial separation,
processing, condition treating, and storage with the
common design and fabrication standard used is ASME
Boiler and Pressure Vessel Code, Section VIII [1]. The
pressurized vessels in the petroleum industry are such
hazardous equipment, that degradation may occur within
the service life of the equipment due to corrosion,
mechanical damage, and other flaw that require to be
maintained to avoid undesired catastrophic accidents.
Furthermore, besides robust design and construction, the
pressure vessel required to be inspected, monitored, and
assessed periodically to ensure the integrity and safety of
the equipment throughout the entire service life.
American Petroleum Institute also provides guidance for
the inspection, repair, and alteration of in-service
pressurized equipment [2] and can be used in
conjunction with the risk-based inspection recommended
practice for better focus and effective in the prioritizing
inspection planning [3].
During routine inspection using the ultrasonic scanning
screening method, some in-service pressurized
equipment was suspected to have a crack like flaw, and
by the detailed inspection utilizing phase array ultrasonic
thickness (Figure 1), the flaw was confirmed and the
geometry was identified. failure assessment is then
intended to be performed to ensure the integrity of
equipment under deteriorated conditions for safety in
Wira Herucakra, PT Dinamika Teknik Persada, South Tangerang,
15139, Indonesia. E-mail: wira.herucakra@dtp-eng.com
Christina Dewi R.I. Simanjuntak, PT Dinamika Teknik Persada, South
Tangerang, 15139, Indonesia. E-mail: christina@dtp-eng.com
operation and the fatigue assessment to estimate the
residual life of the equipment.
Failure assessment through the Fitness for service
(FFS) assessment Level 3 of crack-like flaw will be
performed based on API 579-1/ASME FFS-1 Part 9
using the Failure Assessment Diagram (FAD) method as
an early assessment to understand the behavior of the
flaw under actual operating conditions, whether the
deterioration condition is within acceptable criteria and
the flaw categorized as non-crack growth or the flaw
having potential to growth and required further advanced
evaluation. Fatigue Assessment was also performed to
estimate the life of equipment based on API 579-
1/ASME FFS-1 Part 14 [4]. A similar case was discussed
by Ghanbari as a discontinuity finding at a pressurize gas
separator vessel by Phase Array Ultrasonic Testing
(PAUT) device, the numerical 3D simulation, and the
fitness for service assessment using failure analysis
diagram (FAD) method based on API 579 was
performed to evaluate hydrogen-induced crack [5].
The first edition of API Recommended Practice 579
was developed and introduced in the 2000 by American
Petroleum Institute and The American Society of
Mechanical Engineers [6]. This standard provides
guidance on the assessment of deteriorated in-service
pressurized equipment with a wide range of damage
mechanisms that are not addressed in the design code,
including The Failure Assessment Diagram (FAD)
method for evaluating crack-like flaws that similar to
British Energy and the British Standard Institute method
[7]. The API 579 third edition and above provide
additional guidance related to fatigue assessment with a
multi-tiered approach and cycle counting method for
both welded joint and smooth bar fatigue methods [8].
Fitness for service assessment using FAD method has
been widely applied in for pressurized equipment, they
International Journal of Marine Engineering Innovation and Research, Vol. 9(2), June. 2024. 233-244
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Figure. 1. Phase array ultrasonic testing (PAUT) inspection activity (case no. 4.1 and 4.2, see table 4)
are, weld crack assessment for the hydrocarbon pipeline
with material grade of API X65 [9], structural integrity
assessment through the experimental and numerical
analysis for axially cracked pipelines [10], Experimental
and numerical analysis to estimate pressure failure of
cracked composed natural gas cylinder [11], Structural
Integrity analysis of steam generator turbine in nuclear
power plan with varied crack geometries and loading
condition [12], Failure assessment on the cracked
pressurize piping considering random and fuzzy
uncertainties [13], failure assessment of piping on
nuclear power plan contain defect at straight and elbows
[14], Assessment for 70 km gas pipeline with corrosion
defect based on magnetic flux leaked (MFL) intelligent
pig tools data [15], assessment of steel pipeline made of
API 5L X52 with corroded at elbow [16].
The Failure assessment diagram method is not limited
applicable for steel walled pressured equipment problem,
several non-pressurized equipment problem that was
assessed using FAD method, they are, FAD use to
estimate the initiating of brittle fracture at the end of
structural CJP groove welded joint with defect due to
post earthquake Kobe 1995 [17], failure assessment
structural square hollow section with crack at T-joint
[18], validation of BS7910:2005 assessment procedure
for structural square hollow section with crack at T-, Y-
and K-Joints [19], failure assessment of cracked X and K
joints of structural circular hollow section [20], study on
development of deformation limit using FAD method for
fatigue-cracked X-joint of structural hollow section
subjected in-plane flexure [21], Extensive assessment of
notched structural steel component [22], Assessment of
aero-engine turbine disk beyond normal operation
condition [23], an extensive failure and fatigue
assessment for component subjected with rolling contact
using FAD method with varies variable [24]. Non-steel
walled equipment that was assessed using FAD method,
they are, Failure assessment on Zr-2.5Nb alloy material
pressure tube used in the Canadian Deuterium Uranium
(CANDU) heavy water reactor due to delayed hydride
cracking [25], failure assessment for 316H stainless steel
containing creep crack [26], assessment of Ti-6Al-4V
titanium alloy laser welded plate containing undercut
defect [27], fracture assessment of notched short glass
fibre reinforced polyamide 6 (SGFR-PA6) [28], failure
assessment of nuclear steam generator tubes (SGTs)
made of Inconel 690 and incoloy 800 [29], extensive
assessment of additively manufactured (AM) specimen
containing noctes [30], strength analysis of lithium
hydride ceramic subjected thermal stresses during
sintering process [31]. II. METHOD
The API Recommended Practice 579-1/ASME FFS-1
Part 9 covers the fitness for service (FFS) assessment
procedure used to evaluate crack-like flaws in
components. The assessment procedure is based on the
Failure Assessment Diagram (FAD) Method, which is
summarized in Figure 2. The stress analysis concepts and
methods used in the API Recommended Practice 579-
1/ASME FFS-1 Part 9 are based on ASME B&PV Code,
Section VIII, Division 2 (VIII-2), part 5.
A. Flaw Characterization
The characterization of crack flaws is ruled in the API
579-1/ASME FFS-1 Section 9.3.6 for simplification of
the actual crack geometry model and to make more
amenable fracture mechanic analysis. The rule has
accounted for flaw shape, orientation, and interaction
that was tailored to characterize crack-like to lead
idealized models that are more severe than actual
geometry.
The mesh design technique adopted in this paper refers
to the guideline provided by Anderson based on the
crack plane model of a two-dimensional problem using
the quadrilateral element with focused “spider web”
mesh concentrated at the crack tip as illustrated in Figure
3 [32].
As idealized geometry was obtained, the finite element
model was then developed. Figure 4 presents of
developed crack plane two-dimensional finite element
model for embedded flaw taken form case no 5.1 to 5.3
(See Table 4). and the surface flaw model taken form
case no. 1.1 to 1.2 (See Table 4).
B. Fitness-for-Service Assessment Crack-Like Flaw
The level 3 assessment procedure provides the best
estimate of the structural integrity of a component crack-
like flaw. The level 3 assessment that will be used in this
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Figure. 2. Overview of failure assessment diagram (FAD) method [4]
Figure. 3. Boundary and focused mesh design model for crack plane of two-dimensional finite element problem [32]
Figure. 4. Two-dimensional finite element model. Left: embedded flaw (case no. 5.1 to 5.3, see Table 4), Right: surface flaw (case no. 1.1 and 1.2,
see Table 4)
International Journal of Marine Engineering Innovation and Research, Vol. 9(2), June. 2024. 233-244
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study is based on API Method A Assessment as
discussed in the 579-1/ASME FFS-1 Section 9.4.4.1
point a. As illustrated in Figure 2, the toughness ratio,
, is expressed as:
where is stress intensity attributed to the primary
loads using primary stress distribution, and is
material toughness. applied for the analysis is
129.37 MPa m0.5 refer to the experimental result
performed by Vishal that evaluate fracture parameter for
SA-516 Grade 70 material [33]. is defined as:
The load ratio, , is expressed in the following
equation:
where is the reference primary stress and is
yield strength of material. If the result of the assessment
falls inside the FAD curve, the result is acceptable and
the unstable crack growth will not occur. If the result of
the assessment fall outside the FAD and subsequent
point fall within the FAD, then the few amounts of crack
growth or stable ductile tearing will occur. Ductile
instability estimated when the result of the assessment
fall outside the FAD. The FAD curve is expressed as
follow:
C. Fatigue Assesment
The Assessment of Fatigue Damage Level 2 was used
in this paper, the procedure as mentioned in the API
Recommended Practice 579-1/ASME FFS-1 Part 14.
Method A: fatigue assessment using elastic stress
analysis and equivalent stress is selected in this case. In
this method, the fatigue damage is computed based on
effective total equivalent stress obtained from linear
elastic stress analysis, and a smooth bar fatigue curve.
The procedure fatigue assessment is referred to the steps
that are summarized in Figure 5.
The equivalent stress range, , within step no. 4.2
are experessed as:
where is Stress tensor at the location under
evaluation at the point for the cycle, and is
the stress tensor at the location under evaluation at the
point for the cycle
The effective alternating equivalent stress amplitude,
, within step no. 4.3 are expressed as:
where the fatigue penalty factor, evaluated based
on the following condition.
a. for ,
b. for , , refer to the
following equation
c. for ,
where is the primary plus secondary equivalent
stress range and is the Allowable limit on the
primary plus secondary stress range and the parameter m
and n are determined from the table 1.
Once the alternating equivalent stress amplitude, ,
computed, the permissible number of cycles, , can be
determined using fatigue curve for carbon for
temperature not exceeding 700°F and the ultimate tensile
strength not exceeding 80 ksi is selected to determine the
permissible number of cycles as presented in the figure
6.
Fatigue damage, , and accumulated fatigue
damage, , can be calculated based on equation (8) and
(9) respectively as follow:
Where is the actual number of the cycle.
III. RESULT
A. Equipment Description
Based on the previous inspection, the inspection result
found some flaw discontinuity on the pressure vessel
nozzle. A Fitness for Service (FFS) assessment is
required to be performed based on API 579. The general
data of pressured equipment to be considered in the
assessment are listed in Table 2. The actual crack flaw
geometry recorded from the phased array UT inspection
and then conservatively idealized based on the rule on
API 579-1/ASME FFS-1 Section 9.3.6 are presented in
Table 3.
Actual operating pressure records for each piece of
equipment are presented in Figure 7. Based on the
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Figure. 5. Fatigue Assessment Level 2 Method A (API 579, 2021)
TABLE. 1. FATIGUE PENALTY FACTOR (API 579, 2021)
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Figure. 6. Fatigue curve for carbon low alloy, series 4XX, high alloy steel and high tensile strength for temperature not exceeding 700° - σuts < 80
ksi (API 579, 2021)
TABLE. 2. GENERAL DATA OF EQUIPMENT
Tag ID
PV-01
PV-02
PV-03
PV-04
Name
Slug Catcher Inlet
Separator
Production Separator
Amine Contactor Inlet
KO Drum
Amine Contactor
Picture
Type
Horizontal
Vertical
Horizontal
Vertical
MAWP
1023.97 psig
1020.92 psig
1041.08 psig
1027.74 psig
Operating Pressure
507.63 psig
493.12 psig
493.12 psig
435.11 psig to 580.15
psig
Diam ID.
59.06 inch
48.03 inch
41.008 inch
105.12 inch
Length
15.75 feet
16.33 feet
8.67 feet
42.35 feet
Shell Nom. Thk.
1.89 inch
1.46 inch
1.5 inch
2.72
Corrosion allowance
0.24 inch
0.118 inch
0.118 inch
N/A
Shell Material
A-516 Gr 70N
A-516 Gr 70N
A-516 Gr 70N
A-516 Gr 60N
Yield strength
260 Mpa
260 Mpa
260 Mpa
250 Mpa
Tensile strength
485-620 Mpa
485-620 Mpa
485-620 Mpa
415-550 Mpa
Age
15 years
15 years
15 years
15 years
Figure. 7. Record of actual operating pressure
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TABLE. 3. CRACK GEOMETRY MODEL IDEALIZATION
Case
Actual geometry
Idealization
Flaw geometry idealization
PV-01
Slug Catcher
Inlet Separator
Nozzle N1
t = 47.5 mm
Embedded flaw 1
d1 = 28.87 mm
d2 = 33.88 mm
a1 = 25.05 mm
depth = 5.01 mm
Embedded flaw 2
d1 = 37.37 mm
d2 = 45.75 mm
a2 = 4.19 mm
depth = 8.38 mm
Length flaw 1 and 2 = 58 mm
Offset = 6 mm
S1 = 3.49 mm
S2 = 0.00 mm
S1 < max (a1, a2)
3.49 mm < 4.19 mm
→ Interaction of two
Embedded flaw
Combined flaw
d1 = 28.87 mm
d2 = 45.75 mm
depth = 16.88 mm
Length = 61.5 mm (18°: 6-9
o/c)
d2/t = 0.03 < 0.2
→ surface flaw
Length = 61.5 mm
Depth = 18.63 mm
PV-01
Slug Catcher
Inlet Separator
Nozzle N5
t = 47.5 mm
Embedded flaw
d1 = 28.89 mm
d2 = 40.81 mm
depth = 11.92 mm
Length = 148 mm (43°: 6-9 o/c)
Offset = 6 mm
d2/t = 0.14 < 0.2
→ surface flaw
Length = 131 mm
Depth = 11.9 mm
PV-02
Production Separator
Nozzle N4
t=37 mm
Embedded flaw
d1 = 11.83 mm
d2 = 15.60 mm
depth = 3.77 mm
length = 10.18 mm (18.2°: 12-5
o/c)
offset = 12.96 mm
d1/t = 0.3 > 0.2
→ Embedded flaw
PV-03
Amine Contactor Inlet
KO Drum
Nozzle K5B
t = 25 mm
Embedded flaw
d1 = 21.53 mm
d2 = 24.04 mm
depth = 2.5 mm
length = 29 mm
offset = 3 mm
d2/t = 0.03 < 0.2
→ surface flaw
Length = 30.92 mm (22.4°: 9-
12 o/c)
Depth = 3.46 mm
PV-04
Amine Contactor
Nozzle N4A
t = 69 mm
Embedded flaw
d1= 44 mm
d2 = 52 mm
depth = 8 mm
length = 218 mm (91.6°: 9-12
o/c)
offset = 11 mm
d2/t = 0.3 > 0.2
→ Embedded flaw
PV-04
Amine Contactor
Nozzle N4C
t = 69 mm
Embedded flaw
d1 = 52 mm
d2 = 57 mm
Depth = 5 mm
Length = 73 mm
Offset = 4 mm
d2/t = 0.17 < 0.2
→ surface flaw
Length = 103 mm (45.6°: 6-9
o/c)
Depth = 17 mm
Note:
t = shell thickness; d1= distance between edge of flaw to inner side of shell plate; d2 = distance between edge of flaw to outter side (surface)of
shell plate; a1 = half depth of first flaw; a2 = half depth of second flaw; s1=tranverse distance between two flaw; s2= longitudinal distance
between two flaw
recorded data, the actual operating pressure is far below
the Maximum Allowable Working Pressure (MAWP),
this fluctuates between a range of 49 to 56 percent of the
MAWP.
B. Failure Assessment Diagram (FAD) Result
The result of the Fitness for Service Assessment for
crack flaw through the FAD method is summarized in
Table 4 and plotted along the Fracture Assessment
Diagram as presented in Figure 8.
As discussed in Section 3.1 previously, the actual
operating pressure recorded is about 49 to 56 percent
below the Maximum Allowable Working Pressure,
hence, based on the FAD assessment result as
summarized in Table 4. Generally, the current
deteriorated condition of pressure vessel equipment has
adequate strength under operating pressure. Furthermore,
as the actual operating pressure value is below the FAD
curve, the existing flaw is not caused by operating
conditions but is suspected to have existed within the
construction phase.
C. Fatigue Assessment
Results of fatigue assessment for crack flaw condition
are summarized in Table 5. Generally, the alternating
stress, SALT, produced from fatigue load is below the
threshold line of the SN Curve (about 7 ksi) resulting in
the estimated allowable fatigue cycle for all equipment
being over 1.00 x 1010 cycle or can be considered as
infinite condition.
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TABLE. 4. FAD CALCULATION SUMMARY
#
Case
Condition
Pressure
σPref
LPr
KI
Kr
Remark
psig
Mpa
-
Mpa m0.5
-
1.1
PV-01
Slug Catcher Inlet
Separator
Nozzle N1
MAWP
1023.97
28.883
0.11
9.986
0.08
acceptable
1.2
OP
507.63
14.303
0.06
4.945
0.04
acceptable
2.1
PV-01
Slug Catcher
Nozzle N4
MAWP
1023.97
46.106
0.18
23.419
0.18
acceptable
2.2
OP
507.63
22.828
0.09
11.595
0.09
acceptable
3.1
PV-02
Prod. Separator
Nozzle N1
MAWP
1020.92
12.121
0.05
3.043
0.02
acceptable
3.2
OP
493.12
6.026
0.02
1.513
0.01
acceptable
4.1
PV-03
Amine Contactor Inlet
KO Drum
Nozzle K5B
MAWP
1041.08
27.099
0.10
6.609
0.05
acceptable
4.2
OP
493.12
13.211
0.05
3.222
0.02
acceptable
5.1
PV-04
Amine Contactor
Nozzle N4A
MAWP
1027.74
213.921
0.82
112.408
0.87
Unacceptable
5.2
OP
677.33
140.986
0.54
74.083
0.57
Acceptable
5.3
OP
580.15
120.758
0.46
63.454
0.49
Acceptable
5.3
OP
507.63
105.668
0.40
55.525
0.43
Acceptable
6.1
PV-04
Amine Contactor
Nozzle N4C
MAWP
1027.74
38.673
0.15
16.506
0.13
Acceptable
6.2
OP
677.33
25.488
0.10
10.879
0.08
Acceptable
6.3
OP
580.15
21.830
0.08
9.317
0.07
Acceptable
6.4
OP
507.63
19.100
0.07
8.152
0.06
Acceptable
Figure. 8. Plotted result of FAD calculation
TABLE. 5. FATIGUE ASSESSMENT RESULT
Case
Op. Pressure
Occurrence
SALT
Allowable Fatigue Cycle
bar
cycle
Ksi
Cycle
PV-01
34.1
192
1.502
Over 1.00 x 1011 Cycle
Slug Catcher
34.2
1344
1.507
Over 1.00 x 1011 Cycle
Inlet Separator
34.3
1728
1.511
Over 1.00 x 1011 Cycle
Nozzle N1
34.4
768
1.516
Over 1.00 x 1011 Cycle
34.5
960
1.520
Over 1.00 x 1011 Cycle
34.6
384
1.524
Over 1.00 x 1011 Cycle
34.7
192
1.529
Over 1.00 x 1011 Cycle
34.8
192
1.533
Over 1.00 x 1011 Cycle
34.9
192
1.538
Over 1.00 x 1011 Cycle
PV-01
34.1
192
2.641
Over 1.00 x 1011 Cycle
Slug Catcher
34.2
1344
2.648
Over 1.00 x 1011 Cycle
Inlet Separator
34.3
1728
2.656
Over 1.00 x 1011 Cycle
Nozzle N4
34.4
768
2.663
Over 1.00 x 1011 Cycle
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Case
Op. Pressure
Occurrence
SALT
Allowable Fatigue Cycle
bar
cycle
Ksi
Cycle
34.5
960
2.671
Over 1.00 x 1011 Cycle
34.6
384
2.679
Over 1.00 x 1011 Cycle
34.7
192
2.687
Over 1.00 x 1011 Cycle
34.8
192
2.695
Over 1.00 x 1011 Cycle
34.9
192
2.703
Over 1.00 x 1011 Cycle
PV-02
33.9
192
0.893
Over 1.00 x 1011 Cycle
Prod. Separator
34
1344
0.896
Over 1.00 x 1011 Cycle
Nozzle N4
34.2
1728
0.901
Over 1.00 x 1011 Cycle
34.3
768
0.904
Over 1.00 x 1011 Cycle
34.4
960
0.907
Over 1.00 x 1011 Cycle
34.5
384
0.909
Over 1.00 x 1011 Cycle
34.6
192
0.912
Over 1.00 x 1011 Cycle
34.7
192
0.915
Over 1.00 x 1011 Cycle
34.8
192
0.917
Over 1.00 x 1011 Cycle
PV-03
33.9
192
1.701
Over 1.00 x 1011 Cycle
Amine Contactor
34
1344
1.706
Over 1.00 x 1011 Cycle
Inlet KO Drum
34.2
1728
1.716
Over 1.00 x 1011 Cycle
Nozzle K5B
34.3
768
1.721
Over 1.00 x 1011 Cycle
34.4
960
1.726
Over 1.00 x 1011 Cycle
34.5
384
1.731
Over 1.00 x 1011 Cycle
34.6
192
1.736
Over 1.00 x 1011 Cycle
34.7
192
1.741
Over 1.00 x 1011 Cycle
34.8
192
1.746
Over 1.00 x 1011 Cycle
PV-04
33.3
192
7.626
1.50 x 1010 cycle
Amine Contactor
33.4
768
7.648
1.20 x 1010 cycle
Nozzle N4A
33.5
576
7.672
1.20 x 1010 cycle
33.6
1728
7.694
1.20 x 1010 cycle
33.7
768
7.718
1.20 x 1010 cycle
33.8
960
7.740
1.20 x 1010 cycle
33.9
384
7.763
1.20 x 1010 cycle
34
192
7.786
1.20 x 1010 cycle
34.1
192
7.810
1.20 x 1010 cycle
34.2
192
7.832
1.20 x 1010 cycle
PV-04
33.3
192
1.837
Over 1.00 x 1011 Cycle
Amine Contactor
33.4
768
1.840
Over 1.00 x 1011 Cycle
Nozzle N4C
33.5
576
1.769
Over 1.00 x 1011 Cycle
33.6
1728
1.774
Over 1.00 x 1011 Cycle
33.7
768
1.780
Over 1.00 x 1011 Cycle
33.8
960
1.782
Over 1.00 x 1011 Cycle
33.9
384
1.790
Over 1.00 x 1011 Cycle
34
192
1.795
Over 1.00 x 1011 Cycle
34.1
192
1.801
Over 1.00 x 1011 Cycle
34.2
192
1.806
Over 1.00 x 1011 Cycle
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Figure. 9. Stress increase ratio along thickness of plate for surface flaw
Figure. 10. Stress increase ratio along thickness of plate for embedded flaw
Figure. 11. Correlation between geometry of flaw with stress increase ratio
International Journal of Marine Engineering Innovation and Research, Vol. 9(2), June. 2024. 233-244
(pISSN: 2541-5972, eISSN: 2548-1479) 243
IV. DISCUSSION
Fitness for service (FFS) assessment for crack-like
flaws has been performed based on API Recommended
Practice 579-1/ASME FFS-1 Part 9 and has been
discussed in the previous section.
Figure 9 and Figure 10 present the stress resulted due
to a flaw condition, σC, with the reference stress (without
flaw), σR along the thickness of the shell plate. T
represents the thickness of the shell plate; H represents
the perpendicular distance between the inner side of the
plate to the surface plate at the reference location of the
crack tip; D represents the depth of the flaw; L represents
the length of the flaw.
For surface typed crack, the stress increase ratio, σCR
, are tend to significantly increase near crack tip H/T =
0.0 and gradually decrease and stable approximate the
reference stress value at the surface of shell plate, H/T =
0.1 and above. For surface crack, the stress ratio, σCR,
tends to significantly increase near crack tip H/T = 0.0
and gradually decrease and stabilize approximate the
reference stress value at the surface of the shell plate,
H/T = 0.1 and above. For the embedded type flaw, the
stress ratio also tends to increase significantly as the
reference depth approaches the location of the crack tip.
The magnitude of stress increase from surface and
embedded typed flaw are affected by crack geometry as
presented in Figure 11, the increase of length time to
depth of flaw relative to the shell plate thickness, LD/T2,
the higher stress increase will be produced. The
correlation between stress ratio with flaw geometry
through the exponential function can be used to predict
the increase of stress ratio expressed as σCR =
4.18e0.82(LD/T^2). V. CONCLUSION
Failure assessment include fatigue assessment have
been performed for pressurized equipment confirmed
with surface and embedded flaws based on the phased
array ultrasonic testing survey data. The assessment has
demonstrated that the current condition of the
deteriorated pressure vessel has adequate strength under
operating conditions, furthermore, the fatigue damage is
an insignificant factor affecting the life of the equipment.
The assessment also shows that the flaw geometry
affects the stress increase, the increase in depth and
length of the flaw will significantly increase the stress.
To maintain safety in the operation of deteriorated
pressurized equipment shall be:
a. Maintain their operation pressure in order to not
exceed about 56% of the maximum working
pressures (MAWP),
b. Perform inspection, monitoring, and testing, and
the assessment shall be performed periodically
based on API 510 and API 580 to ensure the
integrity of the equipment. In addition, and
c. Re-setting the pressure safety valve (PSV), if
required. ACKNOWLEDGEMENTS
This study was supported by PT. Dinamika Teknik
Persada for providing the data set, tools, and funding is
gratefully acknowledged.
CONFLICT OF INTEREST
No Potential conflict of interest was reported by the
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