advanced assessment of pipline inegrity using ili data.pdf

8/11/2019 Advanced Assessment of Pipline Inegrity Using ILI Data.pdf

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ADVANCED ASSESSMENT OF PIPELINE INTEGRITY USING ILI

DATA

Ted L Anderson

Quest Integrity Group, LLC

2465 Central Avenue

Boulder, Colorado 80301

USA

ABSTRACT

Improvements in inline inspection (ILI) and computing technology, coupled with theemergence of fitness-for-service standards, have created an opportunity to advance the state

of the art in pipeline integrity assessment. This paper describes novel approaches for

assessing cracks, wall loss, and dents in pipelines using data from ILI tools.

Crack detection ILI tools that rely on shear wave UT have improved significantly in

both detection probability and sizing accuracy. The Quest Integrity Group (QIG) employs

realistic fracture mechanics models that utilize 3D elastic-plastic finite element analysis. The

combination of advanced modeling and reliable inline inspection provides a superior

alternative to hydrostatic testing for ensuring pipeline integrity.

Inline inspection tools that measure wall loss with compression wave UT provide

superior results compared to MFL tools. The former outputs a digital map of individualthickness readings, which is ideally suited to effective area assessment methods such as

RSTRENG and the API 579 Level 2 Remaining Strength Factor (RSF) calculation. QIG has

developed software that can rapidly process large quantities of ILI wall loss data and evaluate

the maximum allowable operation pressure (MAOP) at discrete locations. The ranking of

these MAOP values serves as a rational and rapid means for prioritizing the severity of

corrosion throughout the line.

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 a simple empirical equation. QIG has developed an advanced dent

assessment that combines a detailed mapping of the dent from ILI data (either UT or a caliper

pig) with 3D elastic-plastic finite element analysis. A dimensionally accurate 3D model of

the dented pipe is subjected to cyclic loading, 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.


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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 2calculations 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, theAPI/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 combination of Level 3 fitness-for-service technology and high-fidelity ILI data

makes accurate predictions of burst pressure and remaining life feasible. In certain instances,

simplified assessments are not sufficient. In the case of crack assessments, for example,

supposedly conservative analyses have led to unconservative predictions in some cases.

Quest Integrity Group (QIG) has recently developed advanced assessment techniques

for cracks, wall loss, and dents. Level 3 assessments that incorporate elastic-plastic finite

element analysis are used for cracks and dents. We have adapted the API/ASME Level 2

assessment for wall loss in order to process large quantities of ILI compression wave UT

data. Each of these advanced assessments is described below.


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LEVEL 3 CRACK ASSESSMENT AS AN ALTERNATIVE TO

HYDROSTATIC TESTING

Traditional models for crack assessment are considered conservative because they tend to

underestimate burst pressure and critical crack size. One such approach is the NG-18 method

[2], which dates back to the early 1970s and is still widely used today. So-calledconservative methods such as NG-18 can actually be unconservative in some instances, as

described below.

Hydrostatic testing has traditionally been used to protect pipelines against unexpected

failures from cracks or other planar flaws. The hydrostatic test is designed to detect critical

flaws by causing leaks and ruptures under controlled conditions. In many cases, the NG-18

equation has been used to estimate the critical flaw dimensions at the test pressure. If the

pipe passes the hydrostatic test, it is assumed that no flaws larger than the calculated critical

dimensions are present. However, this assumption is not justified because the NG-18

equation and other simplified models typically underestimate the critical flaw size.

Figure 1 shows a bell curve that represents the population of crack-like flaws in apipeline. If a hydrostatic test is performed on this line, cracks on the upper tail of the bell

curve will be identified, as indicated by the area shaded in red. The NG-18 equation

significantly under-predicts the critical crack size. The yellow shaded area in Fig. 1

represents the population of flaws that were predicted to fail the test but did not. In other

words, larger-than-predicted cracks are left in the pipe following a hydrostatic test.

The scenario that is schematically illustrated in Fig. 1 is demonstrated with actual data

in Fig. 2. A 16-inch Schedule 10 pipeline, which was installed in 1955, has experienced hook

crack in ERW seams. These cracks have grown over time by fatigue due to pressure cycling.

As a result of several in-service failures, the operator instituted a hydrostatic testing program

in 1991. The NG-18 equation was used to predict the critical flaw dimensions at the test

pressure. A fatigue crack propagation analysis was then performed on the calculated critical

flaw sizes in order to infer an appropriate retest interval. The most recent full-line hydrostatic

test on this pipeline was performed in 1999. The corresponding critical flaw calculation from

the NG-18 equation is represented by the blue curve in Fig. 2. This pipe was inspected by a

shear wave UT ILI tool in 2008. A total of 139 cracks were reported, 62 of which were sized

by manual UT. The measured crack dimensions for these 62 flaws are plotted in Fig. 2. The

red line represents the predicted growth of the calculated critical flaws during the 9-year

period between the full-line hydro and the ILI. Had the NG-18 equation correctly predicted

the critical crack dimensions for the 1999 hydrostatic test, none of the flaws detected in 2008

would have fallen above the red curve. In reality, however, 9 of the 62 flaws sized by manual

UT fall above the curve. According to the NG-18 method, these 9 flaws should not have

survived the 1999 hydrostatic test.

The 1970svintage NG-18 equation is incapable of accurate predictions of critical

flaw size or burst pressure. A state-of-the-art Level 3 crack analysis provides a much more

accurate reflection of reality. Quest has applied a Level 3 assessment to the 16-inch pipeline

described above. Our assessment procedure contains the following features:


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Three-dimensional elastic-plastic finite element models of cracks in ERW seams.

Fracture toughness inferred from laboratory tests on samples extracted from the pipe

of interest.

Weld residual stress computed from a finite element simulation of the ERW process.

FIGURE 1. Schematic comparison of predicted and actual critical flaw size for a hydrostatic test. The

conservative analysis under-predicts the maximum flaw sizes that survive the hydrostatic test.

FIGURE 2. Comparison of predicted maximum flaw sizes that survived the 1999 hydrostatic test with actual

measured flaws following a 2008 ILI tool run. The NG-18 equation was used for critical flaw predictions.

0

0.05

0.1

0.15

0.2

0.25

0 2 4 6 8 10 12 14 16

CrackDepth,in

Crack Length, in

Comparison of Predicted and Actual Maximum Flaw Sizes in 2008

Based on the NG-18 Methodology

Computed Critical Flaw Size

(1999 Full Line Hydro)

Predicted Maximum Flaws

After 9 Years of Service

Actual Detected Flaws (2008)


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Figure 3 shows a typical 3D model of a crack in an ERW seam. A total of 35 such

analyses were run for the 16-inch ERW pipe, which encompassed a wide range of crack

dimensions. Figure 4 is a repeat of the comparison between predicted and measured flaws in

Fig. 2, but with predictions based on the Level 3 assessment. The blue curve represents the

calculated critical crack dimensions for the 1999 hydrostatic test, and the red curve

corresponds to the predicted growth of critical cracks after 9 years of service. All of theactual measured cracks fall below the red curve, which is the expected result. Note that the

Level 3 analysis in Fig. 4 predicts more crack growth in 9 years than the analysis based on

the NG-18 equation (Fig. 2). This is because large cracks grow faster than small cracks.

Since the NG-18 equation underestimated the critical crack dimensions for the 1999

hydrostatic test, the subsequent fatigue analysis underestimated the maximum possible

growth of cracks that survived the test.

FIGURE 3. Finite element model of a crack in an ERW seam. The model is symmetric.


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FIGURE 4. Repeat of Fig. 2, but with flaw size predictions based on the QIG Level 3 assessment.

The 16-inch line discussed above was due for a full-line hydrostatic test in September

2009, but the operator received a temporary deferment from the US Department of

Transportation (DOT). Quest is working with the operator to validate an alternative tohydrostatic testing that is based on a combination of ILI and Level 3 crack assessment.

Pending the results of this study, the DOT may permit the operator to permanently replace the

existing hydrostatic testing program with the alternative strategy.

Hydrostatic testing is a very expensive but ineffective means for identifying cracks

and other planar flaws in pipelines. Figure 5 schematically compares the relative

effectiveness of hydrostatic testing versus ILI. The former identifies only the largest flaws,

while the current generation of shear wave ILI tools can detect very small flaws. For

example, of the 139 reported cracks from the 2008 ILI of the 16-inch pipe, only 4 or 5 of

these cracks would have failed a full-line hydrostatic test. Given the ILI data, a Level 3

analysis can be used to establish repair criteria and re-inspection intervals. This alternativestrategy provides a greater degree of reliability at a significantly lower cost compared to the

traditional hydrostatic testing approach.

The shear wave UT ILI tool used to inspect the 16-inch ERW pipe has a 90%

probability of detection for cracks greater than 40 mils (1 mm) in depth. Thus this tool is far

more sensitive at detecting flaws compared to hydrostatic testing. However, there is still

room for improvement on flaw sizing accuracy with shear wave ILI data. In the case of the

inspection on the aforementioned 16-inch pipe, flaw depths were reported in ranges: 40-80

0

0.05

0.1

0.15

0.2

0.25

0 2 4 6 8 10 12 14 16

CrackDepth,in

Crack Length, in

Comparison of Predicted and Actual Maximum Flaw Sizes in 2008

Based on the Level 3 Analysis

Computed Critical Flaw Size

(1999 Full Line Hydro)

Predicted Maximum FlawsAfter 9 Years of Service

Actual Detected Flaws (2008)


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mils (1-2 mm), 80-160 mills (2-4 mm), and > 160 mils. While flaws shallower than 40 mils

(1 mm) can be detected, such indications were not reported because it is difficult to

distinguish cracks from extraneous reflections from the ERW seam.

FIGURE 5. Comparison of ILI crack detection capabilities with the ability of hydrostatic testing to identify

cracks.

FIGURE 6. Measured depths (with manual UT) of flaws reported to be within the 40-80 mil (1-2 mm) range

based on ILI UT data.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 20 40 60 80 100 120 140

CumulativeProbability

Measured Flaw Depth, mils

Actual versus Reported Flaw Depth

40-80 mil Reporting Range

Inspection Data

Weibull Fit

Reported Depth Range

20-mil deep cracks

detected by ILI


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FIGURE 7. Measured depths (with manual UT) of flaws reported to be within the 80-160 mil (2-4 mm) range

based on the ILI UT data.

Figures 6 and 7 are plots of the measured flaw depths for cracks reported in the 40-80

and 80-160 mil ranges, respectively. For flaws reported in the 40-80 mil range, the manual

UT measurements exhibit a significantly wider range of crack depths compared to the

reported range. Note that two 20-mil (0.5 mm) deep cracks were reported, which is an

indication of the high sensitivity of the ILI tool. For the 80-160 mil depth range, the

measured flaw depths generally fall within the reported range. This indicates that sizing

accuracy with ILI shear wave UT data is better for deeper cracks. Both populations of flaws

(40-80 and 80-160-mil reported ranges) follow Weibull statistical distributions. Given the

uncertainty between the actual depth of a given flaw and the reported range from the ILI data,

a probabilistic analysis is recommended.

RAPID ASSESSMENT OF METAL LOSS WITH COMPRESSION

WAVE UT ILI DATA

Metal loss in pipelines has traditionally been assessed with the ASME B31.G and RSTRENG

[3] methods. Given an ILI dataset covering several hundred kilometers of pipe, a manual

data analysis taking up to 3 months is typically performed prior to assessing the wall loss and

applying acceptance criteria. A primary purpose of this initial analysis is to identify and size

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 20 40 60 80 100 120 140 160 180 200

CumulativeProbability

Measured Flaw Depth, mils

Actual versus Reported Flaw Depth

80-160 mil Reporting Range

Inspection Data

Weibull Fit

Reported Depth Range


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discrete corrosion flaws. In addition to the time and cost associated with this painstaking

process, a major problem with this approach is that reality seldom conforms to the ideal of

discrete areas of wall thinning surrounded by uncorroded metal. Instead, wall thickness in a

corroded pipe varies continuously over the surface; obvious discrete flaws are the exception

rather than the rule.

This reality is evident in high-resolution compression wave UT data, which, unlikeMFL data, can be displayed as a digital map of wall thickness. Figure 8 compares the ideal

of discrete flaws with a color map of actual UT wall thickness data. Part of the UT data

analysts job is to take the non-ideal wall thickness data and force-fit it to the discrete flaw

ideal. The process is often referred to as flaw boxing, as the analyst defines the length and

width of the flaw with a rectangle that contains the corresponding wall loss data. When

applying the B31.G acceptance criteria, the only measurements that are used in the

assessment are the length and width of the boxed flaw, along with the minimum measured

wall within the box. In such cases, over 99% of the wall thickness data is discarded, and a

key advantage of high-resolution UT data relative to MFL is lost.

(a) Idealized case with discrete flaws.

(b) Actual UT data. This is a 2D unwrapped plot of wall thickness.

FIGURE 8. Comparison of actual UT wall loss data with the idealized case where discrete flaws are surrounded

by uncorroded material.


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The Level 2 assessment of metal loss in API 579-1/ASME FFS-1 2007 [1] is an

effective-area method that is similar to RSTRENG [3]. Flaw boxing is notrequired with the

API/ASME method, however. A river-bottom profile is constructed from the thickness

data, and a remaining strength factor (RSF) is computed, which can be used to compute a

maximum allowable operating pressure (MAOP). These calculations can be performed over

a short segment of pipe, or a single MAOP can be computed for an entire pipe sectionbetween girth welds. All valid wall thickness readings are considered with this assessment

method. This approach is not only less labor intensive than flaw boxing, it is much less

subjective, and results in a more technically sound MAOP.

Quest has developed the LifeQuestTM

Pipeline software to process and visualize data

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

intelligent

pigs [3]. LifeQuestTM

performs a Level 2 API/ASME wall loss assessment over an entire ILI

dataset, and computes 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.

Figure 9 is a screen shot from LifeQuestTM

Pipeline.

FIGURE 9. The LifeQuestTM

Pipeline software.


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LEVEL 3 DENT ASSESSMENT

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

warranted. Significant plastic strain 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 size, shape, and location of the original dent affect theremaining life, as does external factors such as the constraint provided by the surrounding

soil.

In order to handle the complexities associated with dents, Quest 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 be used to build 3D

finite element models of dented pipes. However the prior damage created during the initialdenting must be taken into account. We have performed parametric studies 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.

Figure 10(a) shows a typical 3D finite element model of a pipe after the formation of a dent.

Figure 10(b) shows the same model after 10 pressure cycles. Note that the pipe has re-

rounded.

ACKNOWLEDGEMENTS

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

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

participated in the development of the advanced pipeline assessment technology described

herein. These colleagues include Greg Brown, Devon Brendecke, Eric Scheibler, Dan

Revelle, Jim Rowe, and Greg Thorwald.


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(a) Immediately after formation of the dent.

(b) Re-rounding after 10 pressure cycles.

FIGURE 10. Elastic-plastic finite element simulation of dent formation and pressure cycling.


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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. Kiefner, J. F., Maxey, W. A., Eiber, R. J., and Duffy, A. R., Failure Stress Levels of

Flaws in Pressurized Cylinders. ASTM STP 536, American Society for Testing and

Materials, 1973.

3. "A Modified Criterion for Evaluating the Remaining Strength of Corroded Pipeline",

Pipeline Resarch Council International (PRCI)/AGA, Contract Number: PR-3-805,

Catalog Number: L51688.

4. Papenfuss, S., Pigging the UNPIGGABLE: New Technology Enables In-Line

Inspection and Analysis for Non-Traditional Pipelines 5th MENDT Conference,

Bahrain, November 2009.

.

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