advanced integrity assessment of pipeline dents using ili data

Plant Reliability Specialists

Advanced Integrity Assessment of Pipeline Dents Using ILI Data ERRP Berlin; October 2010

20 October 2010 of 27

Advanced Integrity Assessment of Pipeline Dents Using ILI Data

Dr. Gregory W. Brown

US Structural Integrity Lead

Dr. Ted L. Anderson

Chief Technology Officer

Eric Scheibler

Consulting Engineer

Quest Integrity Group, LLC

2465 Central Avenue, Suite 110

Boulder, CO 80301 USA

[email protected]

Abstract

Improvements in inline inspection (ILI) and computing technology, coupled with the emergence of

fitness-for-service standards, have created an opportunity to advance the state of the art in pipeline

integrity assessment. This paper describes a novel approach for assessing dents in pipelines using data

from ILI tools.

Dents that are introduced during fabrication, installation, or by a third party are the most common source

of failure in pipelines. Traditional assessments are based on a simplistic characterization of the dent

(e.g. the ratio of the dent depth to the pipe diameter), combined with simple empirical equations. The

Quest Integrity Group has developed an advanced dent assessment methodology that combines a

detailed mapping of the dent from ILI data (either UT or a caliper pig) with 3D elastic-plastic finite

element analysis. Dent formation is simulated with three-dimensional (3D) finite element modeling.

Cyclic loading is then applied and remaining life is computed through a proprietary low-cycle fatigue

damage model. This advanced methodology can be applied to interacting anomalies such as dent/gouge

and dent/crack combinations.

This technology is demonstrated via correlation to experimental data from cyclic testing of dented pipes.

Application of this methodology to in-service pipelines under operating conditions is presented.

mailto:[email protected]




Table of Contents

ABSTRACT ......................................................................................................................................... 1

LIST OF FIGURES ................................................................................................................................ 3

1 OVERVIEW .................................................................................................................................. 4

2 DENT DETECTION WITH COMPRESSION WAVE ILI ........................................................................ 5

3 TRADITIONAL DENT ASSESSMENT ............................................................................................... 5

4 ADVANCED DENT ASSESSMENT ................................................................................................... 6

4.1 FINITE ELEMENT MODELING OF DENTS ................................................................................................... 6

4.2 LOW CYCLE FATIGUE DAMAGE MODEL ................................................................................................... 6

4.3 CRACK ANALYSES ................................................................................................................................ 7

4.4 API 1156 DENT RESULTS .................................................................................................................... 7

4.5 IN SERVICE PIPELINES ........................................................................................................................... 9

5 CONCLUSION ............................................................................................................................ 10

6 ACKNOWLEDGEMENTS ............................................................................................................. 11

7 REFERENCES ............................................................................................................................. 12

8 FIGURES AND TABLES ................................................................................................................ 13




List of Figures

Figure 1: Localized Corrosion Identified Using InvistaTM

and LifeQuestTM

. ...................................... 13

Figure 2: Dig Verification of Localized Corrosion. ........................................................................ 13

Figure 3: Hydrogen Blistering Identified Using InvistaTM

and LifeQuestTM

. ...................................... 14

Figure 4: X-ray Verification of Hydrogen Blistering. ..................................................................... 14

Figure 5: Pipe Denting Identified Using InvistaTM

and LifeQuestTM

. ................................................. 15

Figure 6: Verification of Pipe Denting. ......................................................................................... 15

Figure 7: 3D Elastic-Plastic FEA: Dent Formation. ........................................................................ 16

Figure 8: 3D Elastic-Plastic FEA: Dent Re-rounding (10 Cycles). ................................................... 16

Figure 9: Damage Parameter Validation Using Test Specimen. ....................................................... 17

Figure 10: ¼ Symmetric 3D FEA Model of Crack in ERW Seam. ................................................... 18

Figure 11: Pipe Experimental Examples from API 1156 Addendum [9]. .......................................... 19

Figure 12: 3D Pipe Denting Finite Element Model. ........................................................................ 20

Figure 13: FEA Dent Measurement After Elastic Rebound. ............................................................ 20

Figure 14: Effect of “Straight-Edge” Length on Dent Depth Measurement. ...................................... 21

Figure 15: Dent Re-Rounding During Pressure Cycles. .................................................................. 21

Figure 16: Dent Re-Rounding: Simulation vs. Experiment. ............................................................. 22

Figure 17: API 1156 Addendum [9] Observed Failure Locations. .................................................... 22

Figure 18: Damage Contours in 24% Dent. ................................................................................... 23

Figure 19: Hoop Stress (psi) for 6% and 24% Depth Dents. ............................................................ 23

Figure 20: Damage Accumulation in 12% Dent. ............................................................................ 24

Figure 21: 3D Finite Element Model of In-Service Pipeline. ........................................................... 24

Figure 22: Dent Re-Rounding of In-Service Pipelines. ................................................................... 25

Figure 23: Comparison of Re-Rounded Dent Shapes. ..................................................................... 25

Figure 24: Damage Contours for In-Service Pipelines. ................................................................... 26

Figure 25: Hoop Stress (psi) for 6% and 24% Depth Dents for In-Service Pipelines. ......................... 26

Figure 26: Damage Accumulation for Dents in In-Service Pipelines. ............................................... 27




1 Overview

Advances in inline inspection (ILI) technology have led to enhancements in both the quality and

quantity of pipeline inspection data. Corresponding improvements in fitness-for-service assessment

methods and technology are necessary to take full advantage of inspection data with higher resolution

and higher accuracy.

The fitness-for-service standard API 579-1/ASME FFS-1 [1] provides a comprehensive guideline for

assessing various flaw types and damage mechanisms in all pressure equipment including pipelines.

This standard incorporates three levels of assessment:

Level 1: This is a basic assessment that can be performed by properly trained inspectors or plant

engineers. A Level 1 assessment may involve simple hand calculations.

Level 2: This assessment level is more complex than Level 1, and should be performed only by

engineers trained in the API/ASME FFS standard. Most Level 2 calculations can be performed

with a spreadsheet.

Level 3: This is the most advanced assessment level, which should be performed only by

engineers with a high level of expertise and experience. A Level 3 assessment may include

computer simulation, such as finite element analysis (FEA) or computational fluid dynamics

(CFD).

These three assessment levels represent a trade-off between simplicity and accuracy. The simplified

assessment procedures are necessarily more conservative than more sophisticated engineering analyses.

With Level 1 assessments, the specified procedures must be followed exactly, and there is little or no

room for interpretation. Level 2 procedures provide some latitude to exercise sound engineering

judgment. For Level 3 assessments, the API/ASME standard provides a few overall guidelines, but the

details of the assessment are left to the user. The lack of specificity in Level 3 is by design. There is no

practical way to codify step-by-step procedures for advanced engineering analyses because every

situation is different, and there a wide range of approaches that may be suitable for a given situation.

The Quest Integrity Group has recently developed advanced assessment techniques for pipeline dents.

These level 3 assessments incorporate elastic-plastic finite element analysis. This involves modeling of

dent creation and re-rounding processes. The use of a damage model based on the Manson-Coffin

approach is used to model low cycle fatigue. This enables quantitative life prediction based on number

of pressure cycles to failure.




2 Dent Detection with Compression Wave ILI

The availability of high-fidelity ILI data has enabled advanced assessment of pipeline dents and gouges.

An ILI data set may cover several hundred kilometers of pipe, requiring the use intelligent software to

process and visualize the inspection data. Unlike MFL data, high resolution compression wave

ultrasonic (UT) may be displayed as digital maps of wall thickness and pipe geometry. This enables the

identification and dimensioning of discrete flaws, such as localized metal loss, denting, and blistering, as

well as continuous features, such as varying amounts of generalized metal loss.

The Quest Integrity Group has developed the LifeQuestTM

Pipeline software to process and visualize

data from high-resolution compression-wave UT ILI tools, including our InVistaTM

intelligent pigs [2].

LifeQuestTM

allows the visualization of large ILI datasets, enabling the identification and categorization

of discrete flaws such as dents. In addition, this software can perform a rapid Level 2 API/ASME wall

loss assessment over an entire ILI dataset, and compute the RSF and MAOP for each pipe section. The

areas of highest corrosion damage can be quickly identified by ranking the calculated RSF and MAOP

values.

The identification of localized corrosion is illustrated in figure 1, with the associated prove-up dig

provided in figure 2. This can be compared with the identification of hydrogen blistering shown in

figures 3 and 4. Figure 5 shows the identification and dimensioning of a pipeline dent. The depth and

shape of the dent can be determined due to the resolution and visualization of the ILI data. Figure 6

shows measurement of this dent. Though all these flaws can be categorized as “circular” indications,

different types of damage can be distinguished due to the resolution of the UT dataset. In addition to

providing information on general wall loss, advanced ILI data coupled with digital processing allows

identification and dimensioning of pipeline dents. Information regarding the re-rounded shape of dents

is required to determine failure location and remaining life.

3 Traditional Dent Assessment

Traditional dent assessments are often based on a simplistic characterization of the dent (e.g. the ratio of

the dent depth to the pipe diameter), possibly combined with simple empirical equations [3,4,5]. These

methods often apply a simple criterion for acceptance of dents, such as depth ratios of less than 2 to 6%.

However, information from pipeline operators has suggested low cycle fatigue failures of “shallow”

dents with depths of 2% or less. This raises concerns about the application of existing assessment

methodologies as a means to ensure pipeline integrity.

Pipe denting is a sufficiently complex phenomenon that Level 3 assessment technology is warranted.

Significant plastic strain (damage) occurs when the dent first forms. The pipe tends to re-round upon

pressure cycling, such that the observed deformation understates the true damage that has accumulated

in the pipe. The application of simple criterion to such “re-rounded” dents can provide non-conservative




estimates of remaining life. The size, shape, and location of the original dent affect the remaining life,

as do external factors such as the constraint provided by the surrounding soil.

4 Advanced Dent Assessment

In order to handle the complexities associated with dents, the Quest Integrity Group has developed a

Level 3 assessment methodology that relies on elastic-plastic finite element simulation. The formation

of the dent is simulated, along with the subsequent pressure cycling. The support of the surrounding soil

is incorporated as appropriate. The remaining life is computed through a proprietary low-cycle fatigue

damage model that has been incorporated into the elastic-plastic finite element simulation. Dimensional

data from ILI can also be used to build 3D finite element models of dented pipes. However the prior

damage created during the initial denting must be taken into account. Parametric studies modeling the

dent formation are used to infer the relationship between the current dimensions and the as-dented

configuration. Elastic-plastic finite element simulation can also be used to model interacting anomalies,

such as a crack in a dent.

4.1 Finite Element Modeling of Dents

To estimate the damage occurring in pipelines during dent formation and pressure cycling, 3D finite

element analysis (FEA) is used. This modeling uses elastic-plastic analysis to capture the strain

evolution during re-rounding and pressure cycling. Dimensional data from ILI can be used to capture

the shape of the dent, as well as the presence of nearby wall thinning.

This 3D finite element analysis is demonstrated with the typical modeling of dent formation in a pipe as

shown in figure 7. Note this model also includes the mapping of a slight out-of-roundness (bulging)

near the dent. Figure 8 shows the same model after 10 pressure cycles. Note the change in the shape of

the dent due to re-rounding. This illustrates the change in “observed” dent shape as compared to initial

dent shape as discussed in section 5.

The amount and shape of re-rounding, as well as damage incurred by the pipeline, depends on shape of

the initial dent, the support configuration, and the properties of the pipeline material. In particular, the

amount of strain hardening in the material can have a significant effect on the re-rounded shape, failure

location/mechanism, and remaining life. Calibration with material test data can be used to determine

appropriate material hardening parameters to be used in the FEA analysis.

4.2 Low Cycle Fatigue Damage Model

The damage occurring in the pipe during dent formation and pressure cycling is determined through a

proprietary low-cycle fatigue damage model. This allows the estimation of remaining life of the

pipeline. This damage model is based on the Manson-Coffin approach [6,7] for low cycle fatigue.




Damage accumulation due to reduction of deformability is defined by the Palmgren-Miner‟s rule. This

relates a scalar damage parameter to the plastic strain and number of cycles to failure:

where is the plastic strain amplitude, is the current number of cycles, and is the total number of

cycles to failure. For many pipeline (ductile) materials, the number of cycles to failure can be expressed

as a function of the plastic strain amplitude by the Manson-Coffin relationship [6,7]

where and are material constants. A discrete finite element implementation of this damage

methodology was used to enable quantitative life prediction based on the number of pressure cycles to

failure. Validation of the damage parameter was conducted using axisymmetric finite element analysis

of test specimens as shown in figure 9.

As discussed above, the loss of ductility places a crucial role in the accumulation of damage, and thus

remaining life of a dented pipe. Calibration with material testing, along with comparison of re-rounded

dent shapes can be used to determine appropriate material hardening parameters.

4.3 Crack Analyses

While low cycle fatigue life can be estimated from the proprietary damage model, other damage

mechanisms must be considered when assessing pipe integrity. Interacting anomalies, including cracks

in dents can be examined using advanced elastic-plastic finite element analysis. This can include the

insertion of cracks into dents, including mapping of the strain field resulting from dent formation.

Similarly, welding residual stresses can be included in the crack modeling as appropriate. Results from

pipeline dent-crack analyses can include remaining life for fatigue crack growth, critical crack sizing,

and pressure rerating. Crack insertion is illustrated with a typical 3D model of a crack in the seam of a

16 inch ERW pipe shown in figure 10.

4.4 API 1156 Dent Results

Validation of the Quest Integrity advanced dent assessment methodology was based on cyclic pressure

tests of dented pipes as presented in API publication 1156. [8,9] The experimental examples considered

dents in API 5L X52 (52,000 psi minimum yield) pipes with outer diameters of 12.75 inches and wall

thicknesses of 0.188 inch. A 4 inch diameter dome indenter was used to create initial dents to depths of

6, 12, 18, and 24 percent of the original diameter of the pipes.




During testing, the unpressurized pipes were supported with “saddle” type supports and indented to a

prescribed depth with a 4 inch dome indenter. The pipes were then subjected to pressure cycles of 0 to

72% of specified minimum yield (SMYS) to shape each dent around the indenter. The pressure was

then released and the indenter removed. The elastic spring-back of the dent could then be measured.

Pressure cycles of 36 to 72% SMYS and 6.5 to 78% SMYS were then applied to the pipe. The pressure

cycles were continued until failure (leak) was observed. Information taken from the API 1156

addendum [9] showing the samples considered for this validation are shown in figure 11. This figure

shows a table of the various dents considered, a photo of measurement of a dented pipe, and a plot of a

re-rounded dent profile.

A three-dimensional (3D) finite element model of the experimental set-up was constructed. This

included the pipe, the indenter, and the constraints due to the pipe supports. Owing to the symmetry in

the experimental configuration, a ¼ symmetric finite element model was used. The use of symmetry

allowed for increased mesh refinement in regions of interest while keeping a reasonable model size.

The FEA model used quadratic (20 node) brick elements and was run using the Abaqus [10] finite

element package. To accurately model the denting procedure, the simulation included nonlinear

geometry and contact interactions between indenter and pipe and between pipe and saddle supports.

Solving the simulations in parallel on 6 computer processors required typical wall clock times of 6+

hours. The finite element model for the denting simulations is shown in figure 12. As discussed in

section 4.2, a proprietary damage model was incorporated into the finite element analyses.

Following simulation of the dent formation and initial pressure cycling, the resulting dent profiles after

elastic rebound from FEA were compared to results from the API 1156 addendum [9]. Figure 13 shows

the dent profiles following elastic rebound. Measurement of dents in the addendum appeared to be

based on a “straight-edge” approach as suggested in figure 11. However, since there is significant

deformation of the pipes around the dents, the measured depth of each dent will depend on the length of

straight edge used. This is illustrated schematically in figure 14. The addendum did not provide details

on dent measurement. The small symbols on figure 13 show the assumed measurement locations for

comparison with the finite element results. These correspond to a straight-edge length of 24 inches.

The resulting errors between FEA and experimental dent depths varied from a few percent up to around

20%. However, these errors could be reduced by assuming a different straight-edge length.

The successive re-rounding of the dent with repeated pressure cycles is shown in figure 15 for an initial

dent depth of 18%. During pressure cycling, the depth of the dent decreased. However, formation of a

small peripheral bulge or “hump” at the edges of the dent was also observed. This was observed in the

experimental results as well, and is a characteristic re-rounded shape of deeper dents. Figure 16

compares the final dent shape of the FEA with the API 1156 results. Note that in the case of the FEA,

the final dent shape was after 9 pressure cycles. Note there was favorable agreement of the re-rounded

dent shapes. In particular, note the formation of “humps” at the edges of the deeper dents. The re-

rounded shape and formation of humps depends on the loss of ductility in the material during dent




formation and subsequent pressure cycling. The examination of re-rounded dent shapes can be used to

verify material hardening parameters. Depending on the initial dent depth, the material inside the dent

may harden and cycle elastically. This causes the creation of plastic “hinge” points at the humps on the

periphery of the dent. This can in turn lead to different failure locations as compared to a very shallow

dent. In the case of a shallow dent, failure due to low cycle fatigue may be expected at the center of the

dent, while for deeper dents, failure is expected at the periphery. This result is supported by

experimental observations in the API 1156 addendum as show in figure 17.

The damage occurring during dent formation and re-rounding was computed during the simulation using

the proprietary damage model discussed in section 4.2. Figure 18 shows damage for the 24% dent.

Note that the highest damage occurred directly under the indenter. However, significant damage is

observed occurring at the hump on the periphery of the dent. Figure 19 shows the hoop stress after 9

cycles for 6% and 24% depth dents. In the case of the shallow dent, tensile stresses are observed

directly under the indenter. These observed stresses and the damage observed suggested that failure for

the 6% dent would be expected at the center of the dent. However, for the 24% dent, compressive

stresses were observed directly under the indenter throughout the cross section, while tensile stresses

were observed on the periphery. In this case, failure would be expected to initiate on the periphery of

the dent. These results matched the observations from the API 1156 addendum as shown in figure 17.

Knowing the expected failure location, and using the incremental damaged computed for each pressure

cycle, the remaining life for low cycle fatigue failure can be estimated. The number of cycles to reach

critical damage (equal to 1) can be extrapolated to estimate remaining life. Figure 20 shows a plot of the

damage parameter versus load cycles for a 12% dent. The figure shows damage at the center of the

dent, and at the periphery. Extrapolation of damage at the hump suggests approximately 24,900 cycles

will be required to initiate a low cycle fatigue failure. As the damage curve tends to flatten with higher

number of cycles, this type extrapolation tends to give a conservative result. The API 1156 Addendum

observed failure after 24,886 pressure cycles of 36 to 72% SMYS followed by 14,949 cycles of 6.5 to

78% SMYS. Note that the estimated number of cycles is to initiate a fatigue failure. Depending, on the

through wall gradients, additional cycles may be required to propagate a defect through wall to cause an

observed leak.

4.5 In Service Pipelines

The Quest Integrity method for advanced dent assessment was applied to in-service pipelines.

Representative dents as provided by the pipeline operator were provided to Quest for assessment. Like

the API 1156 experimental validation, 3D simulation of the denting procedure was conducted. These

simulations were conducted on ¼ symmetric models using the Abaqus [10] finite element package. Soil

support was considered on the bottom half of the model, representing a half-buried pipeline. Various

observed dent depths were created using a 4 inch dome indenter. Following the initial dent creation,

pressure cycles of 0 to 78% SMYS were applied to the pipe. The proprietary damage model, as




discussed in section 4.2, was incorporated into the finite element analyses. The finite element model for

the in-service pipeline denting simulations is shown in figure 21.

Repeated pressure-cycling caused re-rounding and re-shaping of the dent. This is illustrated for dent

depths of 6, 12, and 24% as shown in figure 22. Like the API 1156 experimental validation, hump

formation at the periphery of deeper dents was observed. The amount of hump formation was more

significant than in the API 1156 results due to the different pipe support configuration. This is

illustrated in figure 23, showing comparative displacements (7x amplification) and equivalent plastic

strain contours. Soil constraint under the pipe prevented ovalization, leading to a more severe re-

rounded profile. Greater amounts of plastic strain are observed at the periphery of the dent due to the

increased “hinging” effects.

The damage occurring during dent formation and re-rounding was computed during the simulation using

the proprietary damage model discussed in section 4.2. Figure 24 shows damage contours for the 6%

and 12% dent depths. Note that the highest damage occurred directly under the indenter. However, as

the dent depth is increased, more damage is observed occurring at the hump on the periphery of the dent.

Figure 25 shows the hoop stress for 6% and 24% depth dents. In the case of the shallow dents, tensile

stresses are observed directly under the indenter. However, compressive stresses are observed on the

inner surface. However, for the 24% dent, compressive stresses were observed directly under the

indenter throughout the cross section, while tensile stresses were observed on the periphery. In this

case, failure would be expected to initiate on the periphery of the dent.

Figure 26 shows computed damage at the hump for various dent depths. Extrapolation of this damage to

a critical value of one can be used to estimate remaining life. For failure at the periphery of the dents,

failure initiation was suggested at 25,000, 4,950, and 1,970 cycles respectively for dent depths of 6%,

12%, and 24%. For dents identified using ILI inspection as discussed in section 2, these estimates

provided guidelines for future evaluation or remediation.

5 Conclusion

Improvements in inline inspection (ILI) and computing technology, coupled with the emergence of

fitness-for-service standards, have created an opportunity to advance the state of the art in pipeline

integrity assessment. This paper described a novel approach for assessing dents in pipelines using data

from ILI tools.

In order to handle the complexities associated with dents, the Quest Integrity Group developed a Level 3

assessment methodology that relies on elastic-plastic finite element simulation. The formation of the

dent was simulated, along with the subsequent pressure cycling. The support of the surrounding soil

was incorporated as appropriate. The remaining life was computed through a proprietary low-cycle




fatigue damage model that was incorporated into the elastic-plastic finite element simulation. This

allowed quantitative estimation of remaining life.

This technology was examined using dent validation studies presented in API publication 1156. [8,9]

and applied to dent assessment of in-service lines.

6 Acknowledgements

Much of the work described in this paper was funded by Koch Pipeline Company. The authors would

like to acknowledge the contributions of colleagues at the Quest Integrity Group who have participated

in the development of the advanced pipeline assessment technology described herein. These colleagues

include Devon Brendecke, Chris Tipple, Dan Revelle, Jim Rowe, and Greg Thorwald.




7 References

1. API 579-1/ASME FFS-1, Fitness-for-Service, jointly published by the American Petroleum

Institute and the American Society for Mechanical Engineers, June 2007

2. Papenfuss S., “Pigging the „UNPIGGABLE‟: New Technology Enables In-Line Inspection and

Analysis for Non-Traditional Pipelines”, 5th

MENDT Conference, Bahrain, November 2009.

3. ASME, “Gas Transmission and Distribution Systems”, B31.8, The American Society of

Mechanical Engineers, New York, NY, 2002.

4. Fowler JR., Alexander CR., Kovach PJ., Connelly LM., “Cyclic Pressure Fatigue Life of

Pipelines With Plain Dents, Dents With Gouges, and Dents With Welds”, Pipeline Research

Council International Inc., Falls Church, VA, 1994.

5. Baker M. (Kiefner and Associates), “Integrity Management Program: Dent Study”, Department

of Transportation, Office of Pipeline Safety, Delivery Number DTRS56-02-D70036, 2004.

6. Manson SS, “Behaviour of materials under conditions of thermal stress”, Technical Report

NACA-TR-1170, National Advisory Committee for Aeronautics, 1954.

7. Coffin LF. Jr, “A study of the effects of cyclic thermal stresses on a ductile metal”, Trans

American Society of Mechanical Engineers, 76:931-950, 1954.

8. Kiefner JR., Alexander CR., “Effects of Smooth and Rock Dents on Liquid Petroleum

Pipelines”, API Publication 1156, The American Petroleum Institute, November 1997.

9. Kiefner JR., Alexander CR., “Effects of Smooth and Rock Dents on Liquid Petroleum Pipelines

(Phase II)”, API Publication 1156 Addendum, The American Petroleum Institute, October 1999.

10. ABAQUS/Standard 6.9-3, Dassault Systèmes., 166 Valley St., Providence, RI,

www.abaqus.com.




8 Figures and Tables

Figure 1: Localized Corrosion Identified Using Invista

TM and LifeQuest

TM.

Figure 2: Dig Verification of Localized Corrosion.




Figure 3: Hydrogen Blistering Identified Using Invista

TM and LifeQuest

TM.

Figure 4: X-ray Verification of Hydrogen Blistering.




Figure 5: Pipe Denting Identified Using Invista

TM and LifeQuest

TM.

Figure 6: Verification of Pipe Denting.




Figure 7: 3D Elastic-Plastic FEA: Dent Formation.

Figure 8: 3D Elastic-Plastic FEA: Dent Re-rounding (10 Cycles).




Figure 9: Damage Parameter Validation Using Test Specimen.




Figure 10: ¼ Symmetric 3D FEA Model of Crack in ERW Seam.




Figure 11: Pipe Experimental Examples from API 1156 Addendum [9].

Effects of Smooth and Rock Dents on Liquid Petroleum Pipelines (Phase II), API 1156 Addendum, October 1999

Sample # Pipe geometry, Grade, D/t ratio initial depth (d/D, %) Re-round depth (d/D, %) 1st press cycle depth (d/D, %)

69 12.75 X 0.188 in, Grade X52, D/t=68 6 3.3 1.8

70 12.75 X 0.188 in, Grade X52, D/t=68 12 7.1 3.49

71 12.75 X 0.188 in, Grade X52, D/t=68 18 15.8 6.85

72 12.75 X 0.188 in, Grade X52, D/t=68 24 15.9 7.164in

dom

e indente

r




Figure 12: 3D Pipe Denting Finite Element Model.

Figure 13: FEA Dent Measurement After Elastic Rebound.

-3

-2.5

-2

-1.5

-1

-0.5

0

0 2 4 6 8 10 12 14 16 18

Rad

ial

de

pth

, in

Axial distance, in

Axial Dent Profile, Elastic Rebound

6% Modified σ-ε curve 12% Modified σ-ε curve

6% Modified σ-ε curve, relative error = -13% 12% Modified σ-ε curve, realtive error = -13%

18% Modified σ-ε curve 24% Modified σ-ε curve

18% Modified σ-ε curve, relative error = 22% 24% Modified σ-ε curve, relative error = 0.8%




Figure 14: Effect of “Straight-Edge” Length on Dent Depth Measurement.

Figure 15: Dent Re-Rounding During Pressure Cycles.

-2.5

-2

-1.5

-1

-0.5

0

0 2 4 6 8 10 12 14 16 18

Rad

ial

Dis

pla

cem

en

t (i

n)

Axial Length (in)

Decreasing Dent Depth with Succesive Pressure Cycling, Initial 18% d/OD

Elastic Rebound First pressure cycle Last analysis step

Increasing curvature




Figure 16: Dent Re-Rounding: Simulation vs. Experiment.

Figure 17: API 1156 Addendum [9] Observed Failure Locations.

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0 2 4 6 8 10 12 14 16 18

Rad

ial D

isp

lace

me

nt (

in)

Axial Length (in)

Dent Profile, Axial Cross Section (Final Step of Analysis)

6%, Modified σ-ε curve 12%, Modified σ-ε curve 18%, Modified σ-ε curve 24%, Modified σ-ε curve

Figure B-13, API 1156 Phase II




Figure 18: Damage Contours in 24% Dent.

Figure 19: Hoop Stress (psi) for 6% and 24% Depth Dents.

6% initial dent

24% initial dent

Compressive at OD

Tensile at OD

Tensile at OD




Figure 20: Damage Accumulation in 12% Dent.

Figure 21: 3D Finite Element Model of In-Service Pipeline.

0

0.005

0.01

0.015

0.02

0.025

0.03

0 2 4 6 8 10 12 14 16 18 20

Dam

age

(%)

Analysis Steps

Computed Damage, Modified σ-ε curve

12%, Node 4 'peak' 12%, Node 3809 'hump'




Figure 22: Dent Re-Rounding of In-Service Pipelines.

Figure 23: Comparison of Re-Rounded Dent Shapes.

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0 2 4 6 8 10 12 14 16 18

Rad

ial

dis

pla

cem

en

t, (

in)

Axial length, (in)

Dent Profile, Axial Cross Section (Modified σ-ε curve)

6%, Elastic rebound 12%, Elastic rebound 24%, Elastic rebound

6%, Last analysis step 12%, Last analysis step 24%, Last analysis step

Plot represents dent rerounding after the indenter is removed (solid line), and the final pressure cycle step (dashed line)

Note: increased curvature at the 24% initial dents periphery “hump”




Figure 24: Damage Contours for In-Service Pipelines.

Figure 25: Hoop Stress (psi) for 6% and 24% Depth Dents for In-Service Pipelines.

6% initial dent

12% initial dent

Increased damage at dent periphery “hump”

6% initial dent

24% initial dent

Compressive at OD

Tensile at OD

Tensile at OD




Figure 26: Damage Accumulation for Dents in In-Service Pipelines.

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

0.022

0 1 2 3 4 5 6 7 8

Dam

age

(%

)

Analysis Steps

Computed Damage

6%, Node 18633 "hump" 12%, Node 18633 "hump" 24%, Node 18633 "hump"

advanced integrity assessment of pipeline dents using ili data

Documents

pipeline integrity assessment

traditional dent assessment

advanced methodology

dent depth

dent detection

dent formation

ili data errp berlin

ili tools