Development of Defect Assessment Methods for Pipelines

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Development of Defect Assessment Methods for Pipelines – Part 1

Integrity Group Lunch and Learn

Tom BubenikJuly 19, 2007

© Det Norske Veritas AS. All rights reserved Slide 201 August 2007

Outline

Corrosion Defects – Types and Characteristics

Analysis Methods for Corrosion

Analysis Methods for Cracks (Part 1)

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Corrosion Defects – Types and Characteristics


Groove or Narrow Axially Aligned Corrosion

Preferential Corrosion

Complex areas of Interacting pits;

general corrosion

Isolated pitting

External Metal Loss (Corrosion)

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Corrosion Morphology

Observations in the field and the corrosion morphology often help characterize root cause of the corrosion.


General Corrosion

Covers large area

Edges generally smooth

Carbuncles or tubercles (knobby outward corrosion deposit)

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Pitting

More localized than general corrosion but otherwise similar:

Edges generally smooth

Carbuncles or tubercles (knobby outward corrosion deposit)

ECA Course - Section 3 Slide 8January 21-22, 2004

Pitting –DC Stray Current Corrosion

Sharp-edged pitting attack

Sometimes has the appearance of chemical etching

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ECA Course - Section 3 Slide 9January 21-22, 2004

Pitting –AC Stray Current Corrosion

1-inch

2-inch

Pinhole

Almost perfectly round

Smooth edges

Pimpled Pattern

Brown discoloration


Microbiologically Induced Corrosion (MIC)

Deep localized pits

Woody appearance (sometimes)

Pits within pits

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Narrow Axial External Corrosion

Follows path of coating tent (external)


Preferential or Selective Corrosion

Very localized

Follows weld bond line or heat affected zone

May or may not have adjacent corrosion in base metal

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“Tree Bark” Corrosion

Occurs in “colonies” with varying widths and depths

Cracking sometimes exists

Cause and consequence not well understood (relative of SCC??)


Other Metal Loss

Gouges and mechanically removed metal

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Other Metal Loss

Erosion


Spiral Corrosion

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Carbon Dioxide Related


Summary of Important Characteristics –Metal Loss

Narrow Axially Aligned

Notch-like Complex Geometries

Very Localized

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Analysis Methods for Corrosion

Most Metal Loss is Analyzed Using ASME B31G or RSTRENG

Both are based on an analysis equation developed in the late 1960s and early 1970s

- B31G was originally referenced in Appendix G of the B31 Code.- RSTRENG is an acronym for the Remaining Strength of Corroded Pipe- Prior to development, the approach was to the degrade pressure carrying

capacity by the percent wall loss- The equation accounts for load shedding around shorter defects and it is

semi-empirical (includes analytic expression that accounts for stressconcentration due to bulging)

The same basic approach is accepted many places worldwide

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Applicability

Axial Grooves: Selective seam corrosion

Axial Cracks: Crack-like anomalies

Circu

mfer

entia

l Gro

oves

: Girth

we

ld co

rrosio

n

Crac

k-like

anom

alies

Holes

Corrosion Length

Corro

sion W

idth

PittingIncludes MIC

Wall Thinning

General or Areal Corrosion

B31G and RSTRENG Analyses

Ao = Original Area = Metal-loss length*wall thickness

NG-18 Surface Flaw Equation

A = Area Removed

1-A/Ao

1-(A/Ao)/MTFlow StressFailure Stress =

A = Area removed by corrosionAo = Original area, before corrosionMT = Folias or bulging factor

where

= function of defect length, pipe diameter,and pipe thickness

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Comments

The equation is empirical (curve fit) and based (in part) on intuition- It’s not a derivation, and there is nothing sacred about the form

Limiting Cases:- For very short defects, MT approaches one. What happens?- For very long defects, MT approaches infinity. What happens?

1-A/Ao



Terms

Flow Stress- This is an artificial concept that is meant to reflect the stress level at which

pipe without a defect will fail.- Why is this not equal to the tensile strength?

Folias or Buckling Factor- Equals the ratio of the stress intensity factor for a crack in a flat plate to that of

a crack in a cylinder.- Expressed as an infinite series – the number of terms used affects the

accuracy. Originally, a 2-term expressions was used; later a 3-term expression was introduced.

- What is the rationale for going from a 2-term expansion to a 3-term expansion?

1-A/Ao


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A = Area Removed

Simplifications (B31G)

d = defect deptht = thicknessMt = 'Folias' factor = f(L, Diameter, t)

where

1-(2/3)(d/t)

1-(2/3)(d/t)/MtFailure Stress =

1-(2/3)(d/t)/Mt1.1 x SMYSFailure Stress =

Parabolic Approximation

Same depth and length as before:Area removed = 2/3 * metal-loss length * maximum depth

A/Ao = (2/3 * length * depth) / (length * thickness)= 2/3 (depth/thickness)

Original B31G Accuracy(Before 1.39 Safety Factor)

25 35 45 55 65 75

2.5

2

1.5

0.5

1

0

3

3.5

4

Specified Minimum Yield Strength (ksi)

Failu

re/P

redi

cted

Fai

lure

Pre

ssur

e

25 35 45 55 65 7525 35 45 55 65 75

2.5

2

1.5

0.5

1

0

3

3.5

4

2.5

2

1.5

0.5

1

0

3

3.5

4

2.5

2

1.5

0.5

1

0

3

3.5

4


Failu

re/P

redi

cted

Fai

lure

Pre

ssur

e

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Modifications

Modifications were made because pipeline companies didn’t like excess conservatism. (This is the same reason the surface flaw equation and B31G were developed in the first place!)

Three types of modifications:- Flow stress- Bulging factor- Area approximations


Modified B31G and RSTRENG

Method Flow Stress

Folias Factor Area

B31G 1.1 SMYS 2-term L2/Dt < 20

2/3 Ld Parabola

RSTRENG 85% Area

SMYS + 68.95MpaSMYS + 10,000 psi

3-term

0.85 Ld

RSTRENG Effective Area

SMYS + 68.95MpaSMYS + 10,000 psi

3-term

Profile, Iterative

Calculation

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

−

−=

−11

1

MAA

AA

o

oFailure σσ

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Modified B31G or “RSTRENG85”

A = Area Removed

t = thicknessMt = 'Folias' factor = f(L, Diameter, t)

d = defect depthwhere

1-0.85 x (d/t)Failure Stress =

1-0.85 x (d/t)/Mt(SMYS+10 ksi)Failure Stress =

Quasi Flat-Bottom Approximation


Modified B31G

The only modification that significantly affects the failure pressure predictions is flow stress:

- Where does the change from [1.1 x SMYS] to [SMYS + 10 ksi] have the biggest effect?

- Hint:

25 35 45 55 65 75

2.5

2

1.5

0.5

1

0

3

3.5

4


Failu

re/P

redi

cted

Fai

lure

Pre

ssur

e

25 35 45 55 65 7525 35 45 55 65 75

2.5

2

1.5

0.5

1

0

3

3.5

4

2.5

2

1.5

0.5

1

0

3

3.5

4

2.5

2

1.5

0.5

1

0

3

3.5

4


Failu

re/P

redi

cted

Fai

lure

Pre

ssur

e

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Modified B31G

- Why was the area changed from [2/3 x L x d] to [0.85 x L x d]? - Hint:

- Changing to a 3-term expansion does nothing to the accuracy.

25 35 45 55 65 75

2.5

2

1.5

0.5

1

0

3

3.5

4


Failu

re/P

redi

cted

Fai

lure

Pre

ssur

e

25 35 45 55 65 7525 35 45 55 65 75

2.5

2

1.5

0.5

1

0

3

3.5

4

2.5

2

1.5

0.5

1

0

3

3.5

4

2.5

2

1.5

0.5

1

0

3

3.5

4


Failu

re/P

redi

cted

Fai

lure

Pre

ssur

e

RSTRENG and Effective Areas

Using an “Effective Area” ensures the area used in the original surface-flaw equation better reflects the true geometry.

RSTRENG, a frequently modified software package, was developed to simplify the effective area calculations

The predictions work well but require information on the profile of a defect

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0

50

100

150

200

250

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20Axial Length, inch

Dep

th, m

ils


0

50

100

150

200

250

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20Axial Length, inch

Dep

th, m

ils

Effective Length

Effective Length is 7 inches

(from 6” to 13”)

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24-inch x 0.375-inch, API 5L X52

0

500

1000

1500

2000

2500

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20Axial Start Location

Pred

icte

d Fa

ilure

Pre

ssur

e, p

sig

Results from 190 Iterative Calculations performed on 20-inch long flaw

Minimum PredictedFailure Pressure

Effective Area Accuracy(before 1.39 safety factor)

1000500 1500 2000 2500 35003000

Actual Failure Pressure (psi)

1000

500

1500

2000

2500

3500

3000

Pre

dict

ed F

ailu

re P

ress

ure

(psi

)

Actual CorrosionMachined DefectService/Hydro Failure

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Comments on B31G And RSTRENG

Each assumes failure by ductile deformation and cannot be used in low toughness regions (e.g., welds) or for cracks or crack-like

Each assumes failure due to pressure overload and cannot be usedwhere axial loads are high

They are also not appropriate for defects with large circumferential extents

They are sometimes unconservative for short deep defects that fail by leaking (and B31G cannot be used for defects greater than 60 to 80 percent deep)

They do not consider multiple or spiral defects

Analysis Methods for CracksPart 1

(and low toughness materials)

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Fundamentals of Fracture

Brittle fracture- No plastic deformation

Moderately ductile fracture with necking- Sometimes called a cup-and -cone fracture- Most common form of ductile fracture- Moderate plastic deformation

Highly ductile fracture- Large amounts of plastic deformation

All three can occur in pipelines


Fracture Mechanics 101

(Infinitesimal) crack growth releases some of the strain energy stored in a sample (i.e., it allows the body to relax).

For linearly elastic materials where no yielding occurs, the strain energy release rate is defined as G. You can calculate G.

The strain energy release rate at which fracture occurs is defined as GIC. This is the energy required to create new fracture surfaces. GIC is measured, not calculated.

- For brittle materials, GIC is invariant and does not vary with temperature.

We rarely use G or GIC in our analyses.

F

F

F

F

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For steels, the situation is more complicated because the material permanently deforms (plastically strains) while new fracture surfaces are created.

Linearly elastic fracture mechanics is based on “K”, which is a measure of the stress intensity at the crack tip.


Crack Tip Stresses

0

0.5

1

1.5

2

0 0.5 1 1.5 2 2.5 3Distance (1=crack tip)

Stre

ss/Y

ield

Stre

ss

Elastic stresses


The elastic stress field in an infinite plate is

σ = K ÷ √ (2 π r)

where “r” is the distance from the crack tip and “K”, the stress intensity factor, is defined as

K ≡ σapplied √ (π a)

where σapplied is the applied stress and “a” is half the crack length.

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The stress intensity factor at which fracture occurs is defined as KIC . - KIC accounts for limited yielding and the creation of new crack surfaces. It

depends on temperature and specimen geometry. - KIC is measured, not calculated.

K and KIC is used heavily in our analyses. In particular, we use K in fatigue analyses where crack growth is a function of the change in K:

where C and m are the “Paris Law” constant and exponent, respectively.

We often use KIC in estimating critical flaw depths.

( )mK C dNda

Δ×=



Steel, especially modern steels, have high toughness and fail in a true elastic-plastic mode with larger scale yielding.

For these materials, we use what’s called a J-integral to evaluate toughness. This integral is the strain energy release rate for a nonlinearly elastic material, and it reflects conditions that occur around the crack tip as a crack grows. While it is based on elastic behavior, it works well for most materials up to and including those that fail in a fully plastic mode.

The J-integral when crack growth begins is defined as JIC.

We use J and JIC for determining critical flaw sizes in CorLas.

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Summary Point

Fracture mechanics generally deals with materials that fail before reaching fully plastic (limit load) conditions. It is based on the energy used up in the vicinity of a crack tip as the crack grows.

We primarily use K, KIC, J, and JIC in our analyses. - K and KIC are for fatigue analyses for materials that have limited yielding

before failure.- J and JIC are used for determining critical flaw sizes. We sometimes use KIC

to estimate critical flaw sizes when there is limited yielding.


Stopping Point

The next lecture will cover how analysis methods take into account different forms of K, J, KIC and JIC for pipeline materials.

Documents

Development of Defect Assessment Methods for Pipelines