1

2

Gas Operations managerBy: Khaled Al Awadi

HOW A PART WALL DEFECT IN A PIPELINE FAILS

3

b. If the stress in the Pipeline is above a critical value, then the remaining ligament below the Part Wall Defect fails

and produces a Through-Wall Defecta. Pipeline contains a Part Wall Defect

l

dt

g. The Through Wall Defect ruptures, and Propagatesif the pressure is high, and/or if the pipe has a Low Toughness.

d. The through Wall Defect causes a Leakif the defect is Short, or if the pressure is Low.

c. A Through Wall Defect in a Pipeline.

e. The Through Wall Defect causes a Ruptureif the defect is Long, or if the pressure is High.

f. The Through Wall Defect ruptures, but

Arrestsif pressure is low, and/or pipe is High Toughness, or if the product is a liquid.

MORPHOLOGY OF CORROSION DEFECTS

4

Internal corrosionExternal corrosionCorrosion in the parent plateCorrosion approaching/in/crossing girth/seam welds

AxialCircumferentialSpiral

Single corrosion defectsColonies of interacting corrosion defects

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BACKGROUND TO METHODS FOR ASSESSING METAL LOSS DEFECTS

The methods described for assessing pipeline defects are based on research work undertaken at Battelle Memorial Institute (in the US) in the 1960s and early 1970s, on behalf of the American Gas Association (AGA).

Over a 12 year period, up to 1973, over 300 full scale tests were completed.92 tests on artificial through wall defects48 tests on artificial part wall defects (machined V-shaped notches)

DEFECT DIMENSIONS (THROUGH WALL DEFECT)

6

l

t

I (or 2c) = defect axial lengtht = pipe wall thickness

THROUGH WALL DEFECTS

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σσ

f M= −1

where

σf = failure stress= flow stress

M = Folias factor (bulging factor)σ

So, we need to understand what;Folias FactorFlow Stress

‘FOLIAS’ OR ‘BULGING’ FACTOR

8

2226.01 ⎟

⎠

⎞⎜⎝

⎛+=RtcM

2240.01 ⎟⎠

⎞⎜⎝

⎛+=RtcM

McRt

cRt

= +⎛⎝⎜

⎞⎠⎟−

⎛⎝⎜

⎞⎠⎟

1 03142

00008422 4

. .

where

2c = defect axial lengtht = pipe wall thicknessR = pipe radius

FOLIAS FACTOR

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0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0

2c/(Rt)^0.5 (normalised defect length)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

M^-

1

224.01 ⎟⎟

⎠

⎞⎜⎜⎝

⎛+=

RtcM

2226.01 ⎟⎟

⎠

⎞⎜⎜⎝

⎛+=

RtcM

422000843.023138.01 ⎟⎟

⎠

⎞⎜⎜⎝

⎛−⎟⎟

⎠

⎞⎜⎜⎝

⎛+=

Rtc

RtcM

MATERIAL PARAMETERS - ‘Flow Strength’

10

0 2 4 6 8 10 12STRAIN, %

0

100

200

300

400

500

600

700

800STRESS, (N/mm^2)

Ultimate tensile strength

Failure

Yield strength

Steel has a yield and UTS.Between these parameters we have work hardening - very difficult to modelWhen we have a defect in steel, it causes plasticity and hardeningTherefore, workers in the ‘60s proposed the concept ‘flow strength’ as a measure of the strength of steel in the presence of a defect.

Flow strength isbetween the yieldand uts.Most workers use(yield+UTS)/2

THE FLOW STRESS

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The flow stress is not a precisely defined term, it lies somewhere between the yield strength and the ultimate tensile strength of the material. A number of different definitions of the flow stress have been proposed (often depending on what form gives the best fit to the experimental data).

The following have been quoted in the published literature:σy + 10 ksi1.1 σy1.15σy(σy + σu)/20.9 σu

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SUMMARY - FAILURE OF THROUGH WALL DEFECTS

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7 8

2c/(Rt)^0.5

Failu

re S

tres

s/Yi

eld

Stre

ngth

RUPTURE

LEAK

This boundary is not sensitive to pressurising medium

2c or l

t

DEFECT DIMENSIONS (PART WALL DEFECT)

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ld

A

t

d = defect depthI (or 2c) = defect axial lengtht = pipe wall thickness

Corrosion Defect (Definition of Dimensions)[Longitudinal and Circumferential Orientation]

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l dA

t

lA

d

t

Pipe Axis

PART WALL DEFECTS

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σσ

f

dt

dt M

=−

−

1

11 or

σσ

f

AA

AA M

=−

−

1

11

0

0

where

σf = failure stress= flow stress

d = defect deptht = pipe wall thicknessA = cross sectional area of metal lossA0 = original cross sectional areaM = Folias factor (bulging factor)

2c

R

d

t

σ

FAILURE OF PART WALL DEFECTS - FAILURE

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0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7 8

2c/(Rt)^0.5

Failu

re S

tres

s/Yi

eld

Stre

ngth

1 - (d/t) = 0.6

0.5

0.3

0.2

0.10.05

0.4

1 - (d/t) = 0.6

FAIL

NO FAIL

2c (l)d

t

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A PIPELINE DEFECT ASSESSMENT ACCEPTANCE CHART

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

2c/(Rt)^0.5 (normalised defect length)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Failu

re S

tres

s/Yi

eld

Stre

ngth

DEFECT DEPTH = 60% OF WALL THICKNESS

FAILURE

NO FAILURE

A defect of depth 60%wt, and of this length, will not fail at this

stress level

A defect of depth 60%wt, and of this length, will fail at this

stress level

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FAILURE OF PART WALL DEFECTS - LEAK RUPTURE

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7 8

2c/(Rt)^0.5

Failu

re S

tres

s/Yi

eld

Stre

ngth

RUPTURE

LEAK

1 - (d/t) = 0.6

0.5

0.3

0.2

0.10.05

0.4

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SUMMARY - ASSESSING AN AXIAL METAL LOSS DEFECT

σσ

f

dt

dt M

=−

−

1

11

where

σf = hoop stress at failure= flow stress

d = defect depth2c = defect axial lengtht = pipe wall thicknessR = pipe radiusM = Folias factor (bulging factor)

2226.01 ⎟

⎠

⎞⎜⎝

⎛+=RtcM

σ0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7 8

2c/(Rt)^0.5

Failu

re S

tres

s/Yi

eld

Stre

ngth

20

1.3

1

0.72

Design Hydrotest Failure0

0.5

1

1.5D

esig

n Fa

ctor

Safety Factorbased on hydrotest

Safety Factorbased on failure

SAFETY FACTORS ON PIPELINE DESIGN PRESSURE

ESTIMATED REPAIR FACTOR

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An intelligent pig inspection report will often refer to the ERF (estimated repair factor) of a defect.The ERF calculation is another way of expressing an ASME B31G assessment.If the ERF is less than one the defect is acceptable to ASME B31G.If the ERF is greater than one the defect is not acceptable to ASME B31G.P’ =failure pressure

'PMAOPERF =

⎪⎪⎪⎪

⎩

⎪⎪⎪⎪

⎨

⎧

>⎥⎦⎤

⎢⎣⎡ −

≤

⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛

+−

⎟⎠⎞

⎜⎝⎛−

=

4.0Bfor11.1

04for

1321

321

1.1'

2

tdP

.B

Atdtd

PP

⎟⎟⎠

⎞⎜⎜⎝

⎛=

DtLA m893.0

P is the design pressure

1167.01.1

2

−⎟⎟⎠

⎞⎜⎜⎝

⎛−

=td

tdB

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PIPELINE DEFECT ASSESSMENT - USING THE HYDROTEST LEVEL AS A SAFETY MARGIN

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

2c/(Rt)^0.5 (normalised defect length)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0d/

t (no

rmal

ised

def

ect d

epth

)

DESIGN PRESSURE (72 percent SMYS)

HYDROTEST PRESSURE (100 percent SMYS)

Safety Margin