Development of Defect Assessment Methods for Pipelines

Development of Defect Assessment Methods for Pipelines – Part 1 Integrity Group Lunch and Learn Tom Bubenik July 19, 2007

Outline

Corrosion Defects – Types and Characteristics

Analysis Methods for Corrosion

Analysis Methods for Cracks (Part 1)

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01 August 2007

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Corrosion Defects – Types and Characteristics

External Metal Loss (Corrosion)

Groove or Narrow Axially Aligned Corrosion Preferential Corrosion

Complex areas of Interacting pits; general corrosion Isolated pitting Š Det Norske Veritas AS. All rights reserved

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Corrosion Morphology

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

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General Corrosion

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Covers large area

Edges generally smooth

Carbuncles or tubercles (knobby outward corrosion deposit)

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Pitting

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More localized than general corrosion but otherwise similar:

Edges generally smooth

Carbuncles or tubercles (knobby outward corrosion deposit)

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Pitting – DC Stray Current Corrosion

ECA Course - Section 3

Sharp-edged pitting attack

Sometimes has the appearance of chemical etching

January 21-22, 2004

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Pitting – AC Stray Current Corrosion

Pinhole

2-inch

Almost perfectly round

Smooth edges

Pimpled Pattern

Brown discoloration

1-inch January 21-22, 2004

ECA Course - Section 3

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Microbiologically Induced Corrosion (MIC)

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Deep localized pits

Woody appearance (sometimes)

Pits within pits

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Narrow Axial External Corrosion

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Follows path of coating tent (external)

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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

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Occurs in “colonies” with varying widths and depths

Cracking sometimes exists

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

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Other Metal Loss

Gouges and mechanically removed metal

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Other Metal Loss

Erosion

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Spiral Corrosion

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Carbon Dioxide Related

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Summary of Important Characteristics – Metal Loss Narrow Axially Aligned

Very Localized

Notch-like © Det Norske Veritas AS. All rights reserved

Complex Geometries 01 August 2007

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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 stress concentration due to bulging)

„

The same basic approach is accepted many places worldwide

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Circumferential Grooves: Girth weld corrosion

Crack-like anomalies

Corrosion Width

Applicability

B31G and RSTRENG Analyses General or Areal Corrosion

Wall Thinning

Pitting Includes MIC

Axial Grooves: Selective seam corrosion Holes

Axial Cracks: Crack-like anomalies

Corrosion Length 01 August 2007

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NG-18 Surface Flaw Equation

Failure Stress = where

Flow Stress

1-A/Ao 1-(A/Ao)/MT

A = Area removed by corrosion Ao = Original area, before corrosion MT = Folias or bulging factor = function of defect length, pipe diameter, and pipe thickness A = Area Removed Ao = Original Area = Metal-loss length*wall thickness

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Comments Failure Stress =

Flow Stress

1-A/Ao 1-(A/Ao)/MT

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?

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01 August 2007

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Terms Failure Stress =

Flow Stress

1-A/Ao 1-(A/Ao)/MT

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?

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Simplifications (B31G) Failure Stress = 1.1 x SMYS

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

d = defect depth t = thickness Mt = 'Folias' factor = f(L, Diameter, t)

where

A = Area Removed 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)

Failure/Predicted Failure Pressure

4 3.5 3 2.5 2 1.5 1 0.5 0 25

35 45 55 65 Specified Minimum Yield Strength (ksi)

75

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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

01 August 2007

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Slide 27

Modified B31G and RSTRENG σ Failure

A ⎤ ⎡ ⎢ 1− A ⎥ o ⎥ =σ ⎢ A −1 ⎥ ⎢1 − M ⎢⎣ Ao ⎥⎦ Flow Stress

Folias Factor

Area

1.1 SMYS

2-term L2/Dt < 20

2/3 Ld Parabola

SMYS + 68.95Mpa SMYS + 10,000 psi

3-term

0.85 Ld

RSTRENG SMYS + 68.95Mpa Effective Area SMYS + 10,000 psi

3-term

Profile, Iterative Calculation

Method B31G

RSTRENG 85% Area

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Modified B31G or “RSTRENG85”

Failure Stress =(SMYS+10 ksi)

where

1-0.85 x (d/t) 1-0.85 x (d/t)/Mt

d = defect depth t = thickness Mt = 'Folias' factor = f(L, Diameter, t) A = Area Removed

Quasi Flat-Bottom Approximation

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Modified B31G

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

Failure/Predicted Failure Pressure

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

4 - Hint: 3.5 3 2.5 2 1.5 1 0.5 0 25

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35 45 55 65 Specified Minimum Yield Strength (ksi)

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75

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Modified B31G

Failure/Predicted Failure Pressure

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

- Hint: 4 3.5 3 2.5 2 1.5 1 0.5 0 25

35 45 55 65 Specified Minimum Yield Strength (ksi)

75

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

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01 August 2007

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RSTRENG and Effective Areas

Using an “Effective Area” ensures the area used in the original surfaceflaw 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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Axial Length, inch 0

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Depth, mils

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250 01 August 2007

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Axial Length, inch 0

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Depth, mils

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Effective Length Effective Length is 7 inches (from 6” to 13”)

250 © Det Norske Veritas AS. All rights reserved

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24-inch x 0.375-inch, API 5L X52 2500

Predicted Failure Pressure, psig

2000

1500

Minimum Predicted Failure Pressure 1000

Results from 190 Iterative Calculations performed on 20-inch long flaw 500

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Axial Start Location 01 August 2007

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Effective Area Accuracy (before 1.39 safety factor) 3500

Predicted Failure Pressure (psi)

3000

Actual Corrosion Machined Defect Service/Hydro Failure

2500 2000

1500 1000 500

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3500

Actual Failure Pressure (psi)

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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 used where 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 Cracks Part 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 © Det Norske Veritas AS. All rights reserved

01 August 2007

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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.

F

F

F

F

We rarely use G or GIC in our analyses.

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Fracture Mechanics 101

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.

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01 August 2007

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Fracture Mechanics 101

The elastic stress field in an infinite plate is

Crack Tip Stresses 2

Elastic stresses

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.

S tress/Y ield S tress

σ = K ÷ √ (2 π r) 1.5

1

0.5

0 0

0.5

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1.5

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2.5

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Distance (1=crack tip)

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01 August 2007

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Fracture Mechanics 101

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:

da m = C × (ΔK ) dN where C and m are the “Paris Law” constant and exponent, respectively.

We often use KIC in estimating critical flaw depths.

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Fracture Mechanics 101

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.

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Stopping Point

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

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