Paper No.

541

USE OF IN-LINE INSPECTION DATA FOR INTEGRITY MANAGEMENT

Patrick H. ViethKiefner & Associates, Inc.

P.O. BOX 268Worthington, Ohio 43085

Steven W. RustBattelle

505 King AvenueColumbus, Ohio 43201

Blaine P. AshworthTransCanada PipeLines

TransCanada PipeLines Tower111-5th Avenue

Calgary, Alberta, Canada T2P 3Y6

ABSTRACT

In-line inspection is a proven technology used by pipeline operators to monitor the integrity of their pipeline. Theinformation provided by the inspection can be used to identify immediate integrity concerns and can be used in thedevelopment of long term integrity plans. This paper provides a case history of methods developed and implemented for onepipeline operator to ensure the short-term and long-term integrity of their pipelines. The focus of this paper is the use of highresolution magnetic flux leakage (MFL) inspection tools in the detection, and sizing, and assessment of corrosion-causedmetal loss.

INTRODUCTION

TransCanada PipeLines (TCPL) operates 14,500 km of natural gas transmission pipelines. Their pipeline system iscomprised of up to seven (7) parallel pipelines which are routed through southern portions of Saskatchewan, Manitoba,Ontario, and Quebec. Most of these pipelines were constructed between 1956 and 1982, and they range in diameter from508 mm to 1066 mm (20-inch to 42-inch).

Two line breaks occurred in 1994 attributable to external corrosion-caused metal loss (galvanic corrosion). Theseline breaks were the first major service failures due to corrosion in their 40-year operating history. In response to thesefailures, a corrosion risk assessment model was developed. It was used to prioritize in-line inspections for locations that mayhave sustained corrosion-caused metal loss. Based upon the results of the risk assessment, a long range program wasdeveloped to inspect the entire pipeline system. Another corrosion failure occurred in 1996. In response to these failures,the in-line inspection program was accelerated such that the entire pipeline system

Copyright@1999 by NACE International. Requests for permission to publish this manueeript in any form, in part or in whole must be made in writing to NACEInternational, Conferences Division, P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in thispaper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.

(except sections coated with fusion bonded epoxy) would be inspected by the end of 1999. The number of inspections andkilometers of pipe inspected by year is summarized in Table 1.

The pipeline operator identified the need to develop a formalized in-line inspection (ILI) management program forconducting and responding to corrosion-caused metal loss inspections. The purpose of this formalized program was toensure accurate, consistent, and thorough handling of the data and to maximize the benefits of conducting an in-lineinspection. An overview of this program is described in Reference 1.

As part of this program, a two-phased excavation program was implemented. Phase 1 excavations are planned forlocations that have been identified by the tool to be potentially immediate integrity concerns. These locations are typicallyexcavated within 120 days of completing the inspection. Under specific circumstances, a location is excavated as soon aspossible and a pressure reduction is implemented until the excavation is completed.

Once the immediate integrity concerns have been addressed through the Phase 1 excavation program, the ILI dataare used to develop a long term program to address corrosion features that have not been excavated and to establish a re-inspection interval, Excavations that are identified through this analysis are referred to as the Phase 2 excavation program.The focus of the remainder of this paper is the process followed to identify potential Phase 2 excavations and to establish re-inspection intervals for the nearly 10,000 km of pipe inspected in 1997, 1998, and 1999.

APPROACH

The Phase 1 excavations are identified through a deterministic approach. That is, locations are identified forexcavation if the reported depth of corrosion is greater than or equal to 70°/0 wall loss or have a predicted failure stress lessthan or equal to 100% of the specified minimum yield strength (SMYS) of the pipe. While this approach is certainly valid toidenti& immediate integrity concerns, it does not provide a reasonable means for evaluating the status of corrosion along thepipeline nor does it provide the means for evaluating changes in other parameters such as operating stress level, variableestimated corrosion growth rates, or inspection tool performance (accuracy of reported defect dimensions). Therefore,another analysis tool has been developed to use the in-line inspection data for long term integrity management anddevelopment of the Phase 2 excavation program and to establish re-inspection intervals. This approach is referred to asProbability of Exceedance (POE).

The POE approach provides the means for systematically summarizing and illustrating the ILI results in a lessdeterministic fashion. The POE for each corrosion feature is calculated to evaluate the ‘probability’ of a leak or the‘probability’ of a rupture. It should be noted that the POE results presented herein are suitable for relative comparisons butthe absolute value of the POE results (e.g., 1 x 10-2) should be used with caution. In fact, the POE results are likelyconservative since one would have expected all of the Phase 1 excavations to result in repairs. However, less than 20’% ofthe excavations and corrosion assessments actually resulted in repairs once the corrosion features were excavated.

Two POE values are calculated for each corrosion feature reported by the tool; one to evaluate the ‘probability’ of aleak (depth of corrosion greater than 80% of the wall thickness) and the other to evaluate the ‘probability’ of a rupture(predicted failure stress less than the operating pressure of the pipeline).

POE for the Leak Criterion

The probability that the actual depth of corrosion exceeds 80% of the wall thickness (leak criterion) is presentedschematically by the three pig calls(l) presented in Figure 1. These pig calls have reported depths of 20%, 50°/0, and 80°Awall loss as shown along the x-axis in this figure. The horizontal line represents an actual depth of corrosion equal to 80’% ofthe wall thickness.

A distribution of expected actual depths of corrosion for each of these three pig calls is represented by the bell-shaped curve. The curves are established through a comparison between the depths of corrosion reported by the tool (pigcall depths) and the depths of corrosion measured in the field after completing an excavation.

For the 20’XOpig call, there is a relatively small probability that the actual depth of corrosion is greater than or equalto 80’% of the wall thickness. This is represented by the small portion of the distribution that extends above the horizontal

‘1) Corrosion features reported by ILI tools are commonly referred to as ‘pig calls’.

line representing an actual depth of 80% wall loss. The probability that a pig call with 209’. wall loss results in actualcorrosion greater than 800/0wall loss is relatively small but it is not zero.

For the 50% pig call, there is a greater probability that the actual depth of corrosion is greater than or equal to 80%of the wall thickness. This is represented by the larger portion of the distribution that extends above the horizontal linerepresenting an actual depth of 80°/0 wall loss.

For the 80’XOpig call, there is the greatest probability that the actual depth of corrosion is greater than or equal to80% of the wall thickness. This is represented by the larger portion of the distribution that extends above the horizontal linerepresenting an actual depth of 80°/0 wall loss.

POE for the Rupture Criterion

The probability that the predicted burst pressure is less than the maximum operating pressure is presentedschematically by the three pig calls in Figure 2. The results are presented in terms of Rupture Pressure Ratio (IU?R). AnRPR equal to 1.00 corresponds to a predicted failure stress equal to 100% of SMYS. Similarly, an RPR equal to 0.90corresponds to a predicted failure stress equal to 9094. of SMYS.

The pig calls in Figure 2 have calculated RPR values of 0.90, 1.00, and 1.10. These RPR values can be calculatedusing any corrosion assessment criterion (e.g., B31 G, RSTRENG 85’%0Area, or RSTRENG Effective Area based ondimensions reported by the tool). A distribution of expected actual RPR values for each of these three pig calls isrepresented by the bell-shaped curve. The horizontal line represents an actual RPR equal to 0.77 based upon an RSTRENGEffective Area analysis of the field measurement data.

For the 75% by 200 mm pig call with an RPR equal to 0.90, there is a relatively large probability that the RPR basedupon field measurements may be less than or equal to 0.77. This is represented by the portion of the distribution that extendsbelow the horizontal line representing an actual RPR equal to 0.77.

For the 50’7. by 200 mm pig call with an RPR equal to 1.00, there is a smaller probability that the RPR based uponfield measurements may be less than or equal to 0,77. This is represented by the portion of the distribution that extendsbelow the horizontal line representing an actual RPR equal to 0.77.

For the 20% by 200 mm pig call with an RPR equal to 1.10, there is a relatively small probability that the RPRbased upon field measurements may be less than or equal to 0.77. This is represented by the portion of the distribution thatextends below the horizontal line representing an actual RPR equal to 0.77. The probability that a pig call with 20% wallloss results in actual RPR less than 0.77 is relatively small but it is not zero.

Once the POE for each corrosion feature has been calculated based upon the Leak Criterion and the RuptureCriterion, the greater of the two POE results is maintained to characterize the corrosion feature.

APPLICATION OF THE POE RESULTS

One advantage of the POE approach is that it provides a reasonable method for comparing the results of multipleinspections, the status and distribution of corrosion along the pipeline, and variations in parameters such as operating stresslevel, variable estimated conosion growth rates, and/or inspection tool performance (accuracy of reported defectdimensions). Methods for incorporating these variables will be discussed later.

Once a POE has been calculated for every corrosion feature, the results can be analyzed and presented manydifferent ways. For example, the POE results can be presented on a feature-by-feature basis, a joint basis, or some otherdefined length interval such as 20 meters (60 feet) for a typical excavation or 1.0 km (0.62 miles) for a typical sectionconsidered for recoating or cathodic protection system changes. The POE results can be combined for any of these intervalsbased upon the following equation:

POE1n_, = l- fi (l-P,) (1)i=l

where Pi is the POE value calculated for each corrosion feature within the interval. For example, the POE for a pipe jointwith’n’ number of corrosion features (pig calls) is calculated as follows:

POEJOti, = 1 - (1 -P,)(I -P2)..(1 -Pn.l)(l -Pn) (2)

where Pi is the POE for the iti corrosion feature within the interval.

The tabulated results for one inspection are presented in Table 2. These results are presented in descending orderbased upon the POE result. The POE results are presented graphically in Figure 3 for the 100 corrosion features with thehighest POE. The columns of results in Table 2 are defined as follows:

column 1:column 2:column 3:column 4:column 5:Cohmm 6:column 7:Cohunn 8:column 9:column 10:column 11:column 12:column 13:column 14:column 15:

Identification of inspection runGirth weld number reported by ILI (sequentially numbered by 10’s)Absolute odometer distance (in meters) reported by ILIDiameter of pipe, mmNominal wall thickness of pipe, mmMaximum operating pressure of pipeline segment, psigSMYS of pipe, psiLength of corrosion feature reported by ILI, mmDepth of corrosion feature reported by ILI, % wall loss

Rupture Pressure Ratio (RPR) based upon the RSTRENG 85% Area CriterionPOE based upon ILI reported dimensionsPOE based upon 0.3 mm per year of corrosion growth after 2 years.POE based upon 0.3 mm per year of corrosion growth after 6 years.POE based upon 0.3 mm per year of corrosion growth after 10 years.POE based upon 0.3 mm per year of corrosion growth after 16 years.

A few observations from the results presented in Table 2 are worth pointing out. First, the corrosion features with the highestPOE results were most likely excavated during the Phase 1 Excavation program since either the RPR was calculated to beless than 1.00 or the depth was reported to be greater than 70’% of the wall thickness (see Columns 9 and 10). These resultsshow that the POE approach is consistent with the deterministic approach. Second, many of the corrosion features in Table 2are located in closed proximity. For example, the 2“dfeature and the 10* feature listed in Table 2 are located on the samepipe joint (Girth Weld Number 53700). Additionally, the 1“ and 2ndfeature listed in Table 2 are separated by only 2 pipejoints and could likely be examined through 1 excavation.

Once the POE results have been produced in a format similar to that presented in Table 2, these data can be assessedseveral ways. For example, the results in Figure 4 present the change in POE over time based upon a constant corrosiongrowth rate of 0.3 mm per year for an inspection completed in 1997.

As an example assume that the 10 features with the highest POE were remediated during the Phase 1 excavationprogram in 1997. The results presented in Figure 5 show the effects of corrosion growth and the examination of additionalcorrosion features over time, The upper, left curve in Figure 5 shows how the POE will increase over time as a result of thecorrosion growth of the features. These results show that the POE will become greater than 1.0 x 10-1in 2004 if noadditional features are excavated (see Point A). The next curve (squares) shows how the POE will increase over time as aresult of the corrosion growth of the features and an excavation program where 2 features per year are excavated.(2) Theseresults show that the POE will become greater than 1.0 x 10-1in2011 if 28 additional features are excavated (see Point B).

It should be noted that these 28 additional features maybe located in close proximity to one another and possiblyeven several are located on a single joint of pipe. These results can then be used to evaluate the expected cost of completingthese excavations in lieu of extending there-inspection from the year 2004 until the year 2011. These excavations can beplanned over time and additional cost savings may be experienced if a long term excavation program is initiated. Forexample, excavations for parallel pipelines can be planned over time such that mobilization costs can be reduced.

‘2) It is worth clarifying that the 2 features per year (resulting in 28 features through 2011) are the 2 features with

the highest POE each year.

Results similar to those presented in Figures 3,4, and 5 can be produced for each of the inspections completed andcan be compared to develop a consistent and defensible plan for conducting additional excavations and for establishing re-inspection intervals.

DISCUSSION OF THE RE.SULTS

The POE results presented within this are based on a pipeline that operates at 77% of SMYS and corrosion featuresthat will grow in time at a constant rate of 0.3 mm per year. However, assumptions other than these can be readilyincorporated into a POE analysis.

As an example of the effects operating stress level, assume that a pipeline actually operates at 53% of SMYS. ThePOE analysis can be modified to evaluate the probability that the actual failure stress is less 53’%.of SMYS based upon thecorrosion dimensions reported by the tool. Therefore, if the same corrosion feature is identified on this pipeline system and apipeline system operating at 7’77. of SMYS, the POE for the latter pipeline will be greater than that for the pipeline operatingat 53~o of SMYS.

Alternative corrosion growth rate models can also be incorporated into this analysis. If a more sophisticatedcorrosion growth rate model is available, the POE analysis can utilize these results for every corrosion feature. Therefore,the corrosion growth rate model will affect each corrosion feature over time based upon the more sophisticated corrosiongrowth rate model. A constant corrosion growth rate of 0.3 mm per year has been used in this paper.

The POE analysis methods can also be used in many ways, For example, the POE results can be used within a riskmodel. The POE results can model the probability of corrosion failure and the consequences can be modeled along thepipeline. In addition, the POE analysis methods can be developed and used in conjunction with other in-line inspection toolssuch as deformation tools and crack tools.

The development and validation of the POE analysis methods continue to evolve. The results presented in thispaper have been provided to describe a POE analysis method which can be used as a tool for developing long term integrityplans and options for several pipeline systems.

ACKNOWLEDGMENTS

The authors would like to acknowledge colleagues who have contributed significantly to the development of themethods presented in this paper. Elden R. Johnson of Alyeska Pipeline Service Company has been a key contributor to thedevelopment and application of the probability of exceedance approach. Fred R. Todt of Battelle has been a key contributorto the data management and statistical data analysis. The authors are gratefid for their contributions.

REFERENCES

1. Vieth, P.H., Sahney, R., and Ashworth, B. P., “TCPL In-Line Inspection Management Program”, American Societyof Mechanical Engineers, Proceedings of the International Pipeline Conference -1998, Calgmy, Alberta, June,1998,

Table 1. Summary of Inspections Planned through 1999

Number of Length of PipeYear of Inspection Inspection Runs inspected, km

1994 1 1391995 3 661996 8 780

1997 33 3,7001998(3) 19 3,6001999(4) 25 2,600

‘3) These inspections are in progress and planned

‘4) These inspections are planned for 1999.

through the end of 1998.

Table 2. Listing of POE Results for Inspection Number 1.

Section

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Inspection 1

Gh-thWeld

Number

53700

53730

60200

66210

54530

2610

53340

3840

2590

53730

3840

53350

56040

55040

54980

60990

57530

66210

60550

2590

30380

66200

5010

4010

59170

OdometerDktance

71573.75

71616.56

79559.06

86697.56

72560.00

4364.90

71143.56

6399.90

4321.30

71611.88

6400.30

71156.19

74604.25

73170.56

73103.69

80512.06

76381.69

86698.06

79990.50

4321.00

41123.19

86685.38

8288.30

6577.30

78340.88

Diameter,mm

864

864

864

864

864

864

864

864

864

864

864

864

864

864

864

864

864

864

864

864

864

864

864

864

864

NominalWall

Thicknessmm

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

9.53

MaximumOperatingPre3surq

psig

880

880

880

880

880

880

880

880

880

880

880

880

880

880

880

880

880

880

880

880

880

880

880

880

880

SMYS, lLIReportedpsi Length, mm

52000

52000

52000

52000

52000

52000

52000

52000

52000

52000

52000

52000

52000

52000

52000

52000

52000

52000

52000

52000

52000

52000

52000

52000

52000

604

570

455

327

399

351

282

387

575

379

556

222

324

285

281

25

267

33

220

250

231

222

214

276

156

ILIReportedDepth, %Wall Loss

48

41

35

40

35

33

35

28

23

26

20

36

26

27

27

64

28

63

32

27

29

30

31

24

42

(RSm&NG85% Area)

0.773

0.842

0.908

0.890

0.917

0.943

0.946

0.978

1.002

0.995

1.029

0.965

1.005

1.007

1.008

1.160

1.005

1.141

0.994

1.018

1.011

1.008

1.005

1.031

0.973

POEOyears

l.llE-01

3.56E-02

6.98E-03

6.82E-03

4.71E-03

1.78E-03

9.71E-04

7.35E-04

6.69E-04

3.86E-04

2.65E-04

2,25E-04

1.74E-04

1.05E-04

9.51E-05

9.35E-05

8.94E-05

6.83E-05

6.07E-05

3.83E-05

3.64E-05

3.46E-05

3.34E-05

3.18E-05

2.95E-05

POE2 years

2.34E-01

9.48E-02

2.47E-02

2.42E-02

1.77E-02

7.74E-03

4.58E-03

3.60E-03

3.32E-03

2.05E-03

1.48E-03

1.28E-03

1.02E-03

6.55E-04

6J30E-04

5.91E-04

5.68E-04

4.48E@l

4.03E-04

2.68E-04

2.56E-04

2.45E-04

2.37E-04

2.27E-04

2.13E-04

POE6 years

6.02E-01

3.72E-01

1.64E-01

1.62E-01

1.32E-01

7.56E-02

5.26E-02

4.44E-02

4. 19E-02

2.98E-02

2.35E-02

2.llE-02

1.80E-02

1.29E-02

1.21E-02

1.20E-02

1.16E-02

9.75E-03

9.02E-03

6.64E-03

6.42E-03

6.20E-03

6.05E-03

5.86E-03

5.57E-03

POE10 years

8.93E-01

7.45E-01

5.02E-01

4.99E-01

4.47E-01

3.26E-01

2.63E-01

2.37E-01

2.29E-01

1.85E-01

1.58E-01

1.48E-01

1.33E-01

1.07E-01

1.03E-01

1.02E-01

9.98E-02

8.85E-02

8.39E-02

6.80E-02

6.65E-02

6.49E-02

6.39E-02

6.25E-02

6.03E-02

POE16 years

9.97E-01

9.84E-01

9,31E-01

9.30E-01

9.llE-01

8.48E-01

8.OIE-01

7.77E-01

7.69E-01

7.19E-01

6.83E-01

6.68E-01

6.43E-01

5.93E-01

5.84E-01

5.82E-01

5.78E-01

5.51E-01

5.40E-01

4.95E-01

4.90E-01

4.86E41

4.82E-01

4.78E-01

4.71E-01

.—

80

..

... . .

roba

,.,

....”..—--

L

Iity o

f-.

)--.—. ...._.._.

&

danc(

\,.

/’

J

,.,,.-.

‘\

>,

.’

. . .

,, ,, ,,

~50%Pig all

.

A,--- ----

*’<

,‘. ,/‘.. -

80% Pig t+all

1 ‘“l‘--”-”--’---”.. \-.....

,+ I

o 10 20 30 40 50 60 70 80 90 100

Pig Call Depth, YO Wall Loss

Figure 1. POE for Evaluating the Likelihood of a Leak

1:””... . .

I

1.. .!

75%x 2001

,,, ,.,

,.‘.

/< ./

,

I

---’-’---’1----’-’---- ~

‘. /’2-= -— -.- . .. ...a.-———

----

, , , I t+

,1

.. . .. ... .. -.. . .. .+ 50%x 200my

I

/’/tI

I–—---.–--.-.4--------------

0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20

RPR Pig Call

Figure 2. POE for Evaluating the Likelihood of a Service Failure.

Number of Features

o 10 20 30 40 50 60 70 80 90 100

I.00E+OO

I.00E-01

1.00E-02

i “00E”03:% 1.00E-04

1.00E-06

1,00E-07

1,00E-08

1.00E+oo

I.00E-01

1.00E-02

1.00E-03

!A% 1.00E-04*a

: 1.00E-05&

1.00E-06

1,00E-07

1.00E-08

O years

Figure 3. POE Results for Each Feature from Inspection Number 1.

Number of Features

o 10 20 30 40 50 60 70 80 90 100

16 years

10 years

6 years

2 years

O years

Figure 4. POE Results for Each Feature from Inspection Number 1Which Shows the Effects of Corrosion Growth (0.3 mm per year).

Year

1.OE+OO

1.OE-01

1.OE-02

1.0E03

1.OE-07

1.OE-08

1.OE-09

1.OE-10

Figure5. POE Results for Inspection Number l to EvaluatePotential Excavation Sites versus Re-Inspection Interval.