Sample Report - T 204 VTTI by Quest
Sample Report - T 204 VTTI by Quest
Sample Report - T 204 VTTI by Quest
Client: VTTI
Attn: Sony Manikandan
Written permission is required if this report is to be reproduced for promotional purposes or in part. The name of Quest Integrity Group may not
be used in advertising without consent. Any samples received by Quest Integrity in connection with this investigation that are not reclaimed by the
client within 8 weeks of the date of issue of this report will be disposed of.
Asset Longevity | Plant Performance
Executive Summary
The purpose of this analysis was to establish that repaired tank TK204 owned and operated by VTTI at the
Fujairah refinery in the United Arab Emirates is fit for continued service without the need for a hydro test
by application of fracture mechanics analysis per API 653 [1]. The tank is to undergo modifications
including the installation of a new annular ring.
In accordance with API 653 [1], a hydro test exemption, including a detailed stress analysis and fracture
mechanics assessment, was conducted. The stress analysis identified the locations and magnitudes of the
peak stresses to be used in the fracture mechanics assessment.
The finite element analysis and fracture mechanics calculations showed that the tank is considered fit for
service so long as no defects of critical size exist in the vicinity of new welds. The critical defect sizes
exceeded the defect sizes that would be cause for rejection during inspection. Therefore, the analysis
showed that repairs made to the tank do not require a hydro test based on the guidelines in API 653 [1].
This assessment considered the new stresses resulting from the modifications of the tank as well as the weld
inspections, both of which were necessary to exempt tank TK204 from a hydrostatic test. A weld joint
efficiency of 0.85 was assumed for all calculations as a factor of safety on the stress results for the fracture
mechanics assessment.
This assessment was based on the provided geometry, loading, and material information. Variations in
loading, method of fabrication, inspection accuracy, and material properties will increase uncertainty in the
results. Loadings not described in the provided information were not included in this assessment. Failure
mechanisms not explicitly listed were not covered by this assessment. Direct assessment of the welds (e.g.
residual stress modeling due to welding), or other failure mechanisms, were outside the scope of this report.
It is up to the operator to determine an appropriate margin of safety based on operational controls, inspection
methodology, and their overall integrity management strategy.
Contents
1. Introduction .................................................................................................................................4
6. Conclusions .................................................................................................................................8
7. References ...................................................................................................................................9
8. Figures .......................................................................................................................................10
1. Introduction
The purpose of this analysis was to establish that repaired tank TK204 owned and operated by VTTI at the
Fujairah refinery in the United Arab Emirates is fit for continued service without the need for a hydro test
by application of fracture mechanics analysis per API 653 [1]. The tank is to undergo modifications
including the installation of a new annular ring.
In order to determine if the repaired tank can be returned to service without the need for a hydrostatic test,
finite element analysis (FEA) was performed in order to identify the peak stresses in the new repair welds
when the tank was subject to hydrostatic test conditions. Using these stress results, a fracture mechanics
analysis was performed to establish the critical flaw sizes associated with the new repair welds. The fracture
mechanics analysis was performed in accordance with Part 9 of API 579-1/ASME FFS-1 2016 (API 579)
[2].
These critical flaw sizes were compared against a typical flaw detectability reference. This detectability
reference represents a typical flaw size which may still be present in the new repair welds after a thorough
inspection of the welds. If the critical flaw sizes are observed to exceed the detectability reference, unstable
crack propagation in the new repair welds is not expected and the tank can be considered exempt from the
need for a hydrostatic test.
FEA was performed in order to identify the peak stresses in the new repair welds for the tank under
hydrostatic testing conditions. A finite element model was created to examine the stresses in the bottom
fillet weld. The model simulated the storage tank under hydrostatic test conditions with the tank being filled
to a maximum fill height of 65.6 feet (20,000 mm) with 1.0 specific gravity fluid.
The mesh used for the bottom fillet weld model is shown in Figure 1. Ultrasonic thickness (UT)
measurements for course 1 were provided in a UT examination report from VTTI. The thicknesses used in
the models represented the minimum measured thickness for course 1, and the nominal thicknesses for
courses 2 through 8. These values and the other general tank dimensions are summarized in Table 1.
Detailed information for the finite element model is included in Appendix A.
3. Material Properties
Linear-elastic material properties were used for the FEA. Destructive testing was performed in order to
obtain values for yield strength, ultimate tensile strength and fracture toughness for the the new bottom fillet
weld using tensile testing and CTOD testing. This destructive testing was completed by Anderson and
Associates. Table 2 summarizes the material values. The test report from Anderson and Associates is
provided in Appendix B.
A contour plot of the bending stress in the floor plates at the bottom fillet weld is shown in Figure 2. A
contour plot of the bending stress in the shell plates at the bottom fillet weld is shown in Figure 3.
The peak stresses from the FEA are summarized in Table 3. The stresses presented in Table 3 were adjusted
by a factor of safety associated with a weld joint efficiency of 0.85.
The maximum bending stress in the floor plates occurred on the top side of the plate near the toe of the
inside chime weld.
Using the stress results presented in the previous section, a fracture mechanics analysis was performed to
establish the critical flaw sizes associated with the new repair welds. The fracture mechanics analysis was
completed in accordance with Part 9 of API 579 [2]. Detailed methodology for this analysis is provided in
Appendix B.
The critical flaw sizes for a surface flaw in the new bottom fillet weld are shown in Figure 4. The critical
flaw size calculations were completed using SignalTM FFS [3], a commercial software package developed
and sold by Quest Integrity. Reports detailing the input data and output plots and results from SignalTM FFS
are included in Appendix C.
According to ASME standard procedures, flaw depths greater than 1/16 inch (1.59 mm) are considered
readily identifiable with common inspection methods. Relevant rounded indications greater than 3/16 inch
(4.76 mm) are cause for rejection. A rounded indication is one of circular or elliptical shape with a length
equal to or less than three times the width. These criteria were used to establish a detectability reference as
being a 3/16” long x 1/16” deep surface flaw.
The critical flaw sizes calculated for the new repair welds exceeded the detectability reference. Therefore,
unstable crack propagation in the new repair welds is not expected and the tank can be considered exempt
from the need for a hydrostatic test. This statement is made assuming that ultrasonic, dye penetrant,
radiography and/or magnetic particle inspection will find existing flaws in the new repair welds as small as
4.76 mm long by 1.59 mm deep, and these flaws are eliminated before the tank is returned to service. The
inspection of the new repair welds shall be performed accordance with API 653 [1].
6. Conclusions
The purpose of this analysis was to establish that repaired tank TK204 owned and operated by VTTI at the
Fujairah refinery in the United Arab Emirates is fit for continued service without the need for a hydro test
by application of fracture mechanics analysis per API 653 [1]. The tank is to undergo modifications
including the installation of a new annular ring.
In accordance with API 653 [1], a hydro test exemption, including a detailed stress analysis and fracture
mechanics assessment, was conducted. The stress analysis identified the locations and magnitudes of the
peak hoop and bending stresses to be used in the fracture mechanics assessment.
The finite element analysis and fracture mechanics calculations showed that the tank is considered fit for
service so long as no defects of critical size exist in the vicinity of new welds. The critical defect sizes
exceeded the defect sizes that would be cause for rejection during inspection. Therefore, the analysis
showed that repairs made to the tank do not require a hydro test based on the guidelines in API 653 [1].
This assessment considered the new stresses resulting from the modifications of the tank as well as the weld
inspections, both of which were necessary to exempt tank TK204 from a hydrostatic test. A weld joint
efficiency of 0.85 was assumed for all calculations as a factor of safety on the stress results for the fracture
mechanics assessment.
This assessment was based on the provided geometry, loading, and material information. Variations in
loading, method of fabrication, inspection accuracy, and material properties will increase uncertainty in the
results. Loadings not described in the provided information were not included in this assessment. Failure
mechanisms not explicitly listed were not covered by this assessment. Direct assessment of the welds (e.g.
residual stress modeling due to welding), or other failure mechanisms, were outside the scope of this report.
It is up to the operator to determine an appropriate margin of safety based on operational controls, inspection
methodology, and their overall integrity management strategy.
7. References
1. The American Petroleum Institute Standard 653, Tank Inspection, Repair, Alteration, and
Reconstruction. Fourth Edition © 2012 American Petroleum Institute.
2. Fitness-for-Service, API 579/ASME FFS-1, June , 2016, API 579 Second Edition, The American
Society of Mechanical Engineers, 1220 L Street, NW, Washington, DC 29996-4070, USA.
3. Signal Fitness-for-Service commercial software, Quest Integrity Group LLC. 1965 57th Court
North, Suite 100, Boulder, CO, www.questintegrity.com.
4. ABAQUS/Standard 6.16, Dassault Systèmes Simulia Corp., 1301 Atwood Ave, Suite 101W,
Johnston, Rhode Island 02919, USA. www.abaqus.com.
8. Figures
Figure 2. Bending stress in floor plates at the floor to shell weld. Stress scale in psi, deformation scale 30x.
Figure 3. Bending stress in shell plates at the floor to shell weld. Stress scale in psi, deformation scale 30x.
6
Crack Depth a (mm)
2
detectability
reference
1
0
0 50 100 150 200 250 300
Crack Length 2c (mm)
An axisymmetric shell element model was used to evaluate the stresses associated with the repairs made to
this weld. This model was composed of linear axisymmetric volume elements (CAX4R). The model was
generated using the ABAQUS [4] finite element modeling program.
Gravity loads were applied along with a hydrostatic pressure load associated with the tank being filled with
water (SG=1.0).
Contact interaction was assigned between the bottom of the tank and the foundation. The tank foundation
was represented by an analytic rigid surface with an assigned contact stiffness. This allowed for the slight
upward bending of the bottom fillet weld due to the hydrostatic fluid load to be accurately captured.
Pursuant to your request number 110965 we have completed the testing on the
submitted welded samples from VTTI Fujairah Tank 204. This report details our
findings.
SPECIMENS
A. One old shell corner-welded to a new bottom plate was submitted for testing
TENSILE TESTING
1. One cross-HAZ reduced section tensile (RST) was removed from Sample A. The
results follow.
Yield Tensile
Strength Strength Elongation
Specimen [psi] [psi] [%]
Old Shell Corner-Welded to New
54,700 78,500 28.5
Bottom Plate
CTOD TESTING
2. One set of three front face SE(B) CTOD specimens was removed from Sample A
and notched in the bottom plate HAZ.
ANDERSON & ASSOCIATES, INC. – 919 F.M. ROAD 1959 - HOUSTON, TX 77034-5425 - (281) 481-5840 - FAX (281) 481-9115
ANDERSON & ASSOCIATES, INC. 170173 Quest Integrity VTTI Fujairah Tank 204 110965
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3. The SE(B) CTOD specimens were compression stress relieved per Annex C.2 of
ISO 15653. All specimens were then pre-cracked at room temperature and tested
per ISO 15653 at 40°F. The results are summarized as follows:
CTOD
Location Specimen [inches] Validity
Old Shell to New Bottom Plate 173-1 δm=0.030 Valid
Old Shell to New Bottom Plate 173-2 δm=0.030 Valid
Old Shell to New Bottom Plate 173-3 δm=0.028 Valid
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ANDERSON & ASSOCIATES, INC. 170173 Quest Integrity VTTI Fujairah Tank 204 110965
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ANDERSON & ASSOCIATES, INC. 170173 Quest Integrity VTTI Fujairah Tank 204 110965
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Asset Longevity | Plant Performance
A fracture mechanics assessment was performed to identify the critical flaw sizes in the new repair welds.
Critical flaw sizes were calculated per Part 9 of API 579 [2].
Crack stability (i.e. the potential for rupture) was assessed using the Failure Assessment Diagram (FAD)
which is described in API 579 [2] Part 9. The FAD enhances linear elastic fracture mechanics (LEFM)
assessments by incorporating ductility. This calculation started with a peak stress profile calculated from
the FEA of the un-cracked structure. The stress intensity factor (KI) was calculated along the crack front
for the through thickness stress profile. The stress intensity factor depended on the loading conditions, the
component geometry and the crack configuration. For a brittle material, the crack becomes unstable when
the stress intensity factor (KI) exceeds the fracture toughness (KIC).
The FAD extends the crack stability assessment to structures experiencing both brittle and ductile fracture.
The FAD is a plot with a limiting curve and points representing the structure of interest. Figure B1 shows
a sample FAD. The x-axis of the plot is the load ratio (Lr) which is the ratio between the reference stress
and the material yield strength. The reference stress is proportional to the far-field stress and is computed
based on the loading condition, the component geometry, and the crack configuration. The y-axis of the
plot is the toughness ratio (Kr) which is the ratio of the stress intensity factor (KI) computed for the primary
and secondary loads and the fracture toughness of the material (KIC). Through-thickness stress profiles from
the FEA were incorporated in the computation of Lr and Kr.
For a particular crack size of length 2c and depth a, an Lr - Kr point is computed and plotted on the FAD. A
point falling under the limiting curve is considered acceptable or safe. A point falling on the curve is
considered critical. A point falling outside the curve is considered unacceptable or unsafe. A point lying
towards the right end of the diagram fails due to plastic collapse. A point lying towards the upper left corner
of the diagram fails due to brittle fracture.
These methods for computing the stress intensity factor, KI, and the FAD are outlined in API 579 [2]. These
methods are widely accepted and are implemented in the commercial software package Signal™ FFS [3],
developed and sold by Quest Integrity.
With the applied stresses and material toughness values, critical defect sizes are calculated. The critical
flaw sizes are defined as the set of crack sizes which are computed as points on the FAD curve as described
above. The data is plotted as a graph of surface crack length versus crack depth. A semi-elliptical surface
crack was assumed for the analysis. The assumed flaw shape had a surface length, “2c” and depth, “a”. The
weld residual stresses were calculated using guidelines in API 579 Annex 9D. Since the R/t ratio for storage
tanks is very large, the flat plate solutions are applied.
Results Summary
Part 9 Level 2
(Refer To: June 2016 API 579-1/ASME FFS-1 )
Results Summary
Part 9 Level 2
(Refer To: June 2016 API 579-1/ASME FFS-1 )