Pipelines Defects Assements - Will Defect Fails ?

1

By: Khaled Al Awadi Gas Operations manager 2

HOW A PART WALL DEFECT IN A PIPELINE FAILS a. Pipeline contains a

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

Part Wall Defect

and produces a Through-Wall

l d

Defect

t

c. A Through Wall Defect in a Pipeline.

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

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

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

f. The Through Wall Defect ruptures, but

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

3

MORPHOLOGY OF CORROSION DEFECTS ˆ

Internal corrosion ˆ External corrosion ˆ Corrosion in the parent plate ˆ Corrosion approaching/in/crossing girth/seam welds ˆ

Axial ˆ Circumferential ˆ Spiral ˆ

Single corrosion defects ˆ Colonies of interacting corrosion defects

4

BACKGROUND TO METHODS FOR ASSESSING METAL LOSS DEFECTS ˆ 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). ˆOver a 12 year period, up to 1973, over 300 full scale tests were completed. ˆ92 tests on artificial through wall defects ˆ48 tests on artificial part wall defects (machined Vshaped notches)

5

DEFECT DIMENSIONS (THROUGH WALL DEFECT)

l

t

I (or 2c) t

= =

defect axial length pipe wall thickness

6

THROUGH WALL DEFECTS

σf −1 =M σ where

σf

σ

M

= = =

failure stress flow stress Folias factor (bulging factor)

So, we need to understand what; „Folias Factor „Flow Stress 7

‘FOLIAS’ OR ‘BULGING’ FACTOR ⎛ 2c ⎞ M = 1 + 0.26⎜ ⎟ ⎝ Rt ⎠

2

⎛ 2c ⎞ M = 1 + 0.40⎜ ⎟ ⎝ Rt ⎠

2

2

⎛ 2c ⎞ ⎛ 2c ⎞ M = 1+ 0314 . ⎜ . ⎟ − 000084 ⎜ ⎟ ⎝ Rt ⎠ ⎝ Rt ⎠

4

where

2c t R

= = =

defect axial length pipe wall thickness pipe radius 8

FOLIAS FACTOR 1.0

0.9

0.8

0.7

M^-1

0.6

⎛ 2c M = 1 + 0.26⎜⎜ ⎝ Rt

⎞ ⎟⎟ ⎠

2

0.5

0.4

0.3

⎛ 2c M = 1 + 0.4⎜⎜ ⎝ Rt

⎞ ⎟⎟ ⎠

2

2

⎛ 2c ⎞ ⎛ 2c ⎞ M = 1 + 0.3138⎜⎜ ⎟⎟ − 0.000843⎜⎜ ⎟⎟ ⎝ Rt ⎠ ⎝ Rt ⎠

4

0.2

0.1

0.0 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)

9

MATERIAL PARAMETERS - ‘Flow Strength’ STRESS, (N/mm^2) 800

Ultimate tensile strength

700

Flow strength is between the yield and uts. Most workers use (yield+UTS)/2

600 500

Yield strength

400 300 200

Failure

100 0 0

2

4

6

8

10

12

Steel has a yield and UTS. STRAIN, % Between these parameters we have work hardening - very difficult to model When we have a defect in steel, it causes plasticity and hardening Therefore, workers in the ‘60s proposed the concept ‘flow strength’ as a measure of the strength of steel in the presence of a defect. 10

THE FLOW STRESS ˆ

ˆ

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). ˆ The following have been quoted in the published literature:

!σy + 10 ksi !1.1 σy !1.15σy !(σy + σu)/2 !0.9 σu 11

SUMMARY - FAILURE OF THROUGH WALL DEFECTS This boundary is not sensitive to pressurising medium

1.2

Failure Stress/Yield Strength

2c or l 1

t 0.8

RUPTURE

0.6

LEAK

0.4

0.2

0 0

1

2

3

4

5

6

7

8

2c/(Rt)^0.5 12

DEFECT DIMENSIONS (PART WALL DEFECT)

A

l

d

t

d = I (or 2c) = t =

defect depth defect axial length pipe wall thickness

13

t

A

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

A

d

l

Pipe Axis

l

d t 14

PART WALL DEFECTS d 1− σf t = d 1 σ 1− t M

or

A 1− σf A0 = A 1 σ 1− A0 M

2c

d t

R

where

σf σ d t A A0 M

= = = = = = =

failure stress flow stress defect depth pipe wall thickness cross sectional area of metal loss original cross sectional area Folias factor (bulging factor)

15

FAILURE OF PART WALL DEFECTS - FAILURE 1.2

2c (l)

d

FAIL NO FAIL

Failure Stress/Yield Strength

t

1

0.8

1 - (d/t) = 0.6 0.6

0.5 0.4

0.4

0.3 0.2 0.2

0.1 0.05

0 0

1

2

3

4

5

6

7

8

2c/(Rt)^0.5 16

A PIPELINE DEFECT ASSESSMENT ACCEPTANCE CHART 1.2

FAILURE

Failure Stress/Yield Strength

1.0

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

0.8

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

0.6

DEFECT DEPTH = 60% OF WALL THICKNESS

0.4

NO FAILURE

0.2

0.0 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) 17

FAILURE OF PART WALL DEFECTS - LEAK RUPTURE

Failure Stress/Yield Strength

1.2

1

1 - (d/t) = 0.6

0.8

0.5

RUPTURE

0.6

0.4

LEAK

0.4

0.3 0.2

0.2

0.1 0.05 0 0

1

2

3

4

5

6

7

8

2c/(Rt)^0.5 18

SUMMARY - ASSESSING AN AXIAL METAL LOSS DEFECT d 1− σf t = d 1 σ 1− t M

1.2

⎛ 2c ⎞ M = 1 + 0.26⎜ ⎟ ⎝ Rt ⎠ where σσ

2

Failure Stress/Yield Strength

1

0.8

0.6

0.4

0.2

0 0

f

d 2c t R M

= = = = = = =

1

hoop stress at failure flow stress defect depth defect axial length pipe wall thickness pipe radius Folias factor (bulging factor)

2

3

4

5

6

7

8

2c/(Rt)^0.5

19

SAFETY FACTORS ON PIPELINE DESIGN PRESSURE 1.5

Design Factor

1.3

1

Safety Factor based on failure

1

0.72

0.5

Safety Factor based on hydrotest

0

Design

Hydrotest

Failure 20

ESTIMATED REPAIR FACTOR ˆ

An intelligent pig inspection report will often refer to the ERF (estimated repair factor) of a defect. ˆ The ERF calculation is another way of expressing an ASME B31G assessment. ˆ If the ERF is less than one the defect is acceptable to ASME B31G. ˆ If the ERF is greater than one the defect is not acceptable to ASME B31G. MAOP ˆ P’ =failure pressure 2 ERF = ⎛ ⎞ d t ⎧ ⎡ ⎤ ⎟⎟ −1 P' B = ⎜⎜ 2 d ⎛ ⎞ d t − 1 . 1 0 . 167 ⎪ ⎢ 1− ⎜ ⎟ ⎥ ⎝ ⎠ 3 t ⎪ ⎢ ⎥ ⎝ ⎠ for B ≤ 4.0 1 . 1 P ⎪⎪ ⎢ 2⎛ ⎥ ⎞ d P' = ⎨ ⎛ Lm ⎞ ⎢1 − ⎜ ⎟⎥ A = 0.893⎜⎜ ⎟⎟ ⎢⎣ 3 ⎜⎝ t A 2 + 1 ⎟⎠ ⎥⎦ ⎪ ⎝ Dt ⎠ ⎪ d ⎪1.1P ⎡1 − ⎤ for B > 4.0 ⎢⎣ t ⎥⎦ P is the design pressure ⎪⎩ 21

PIPELINE DEFECT ASSESSMENT - USING THE HYDROTEST LEVEL AS A SAFETY MARGIN 1.0

d/t (normalised defect depth)

0.9

0.8

0.7

0.6

DESIGN PRESSURE (72 percent SMYS)

0.5

Safety Margin

0.4

0.3

HYDROTEST PRESSURE (100 percent SMYS) 0.2

0.1

0.0 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) 22