CRACK REPAIR WITH COMPOSITE MATERIALS

1

OUR COMPANY

ClockSpring|NRI is shaping the future of

critical infrastructure through innovative

composites, valves, and concrete solutions.

With global headquarters in Houston, TX, the

company manufactures high-performance

critical infrastructure construction and repair

products and provides associated

engineering support and training services.

The High-Performance Critical

Infrastructure Company

The Global Leader of

Innovative Composite

Solutions

Award-Winning Inline

Insertion Valve Technology

Leading provider of

geopolymer, geosynthetic

and structural strengthening

solutions

CSNRI

• Solutions for safe and sustainable construction,

repair, and maintenance of critical infrastructure.

• Industry leading products such as Atlas™,

Clock Spring, DiamondWrap®, SynthoGlass® XT,

and ThermoWrap™ Inspectable.

• We support our products with best-in-class design,

engineering, testing, and training services.

• Our products are easy to install, cost-effective to

deploy, and durable for decades.

The Global Leader of Innovative Composite Solutions

• Overview of pipeline cracks

• Welds & SCC

• Seam weld cracks

• Testing history

• Repair theory

• Crack modeling

• Design life discussion

PRESENTATION TOPICS

• Defining a crack

• A defect splitting an item without breaking into separate parts.

• Has a “sharp tip” that may continue to grow.

• Can lead to structural failure if not addressed.

• Short-term burst considerations

• Will it fail at operating pressure?

• What pressure would it fail at?

• Long-term cyclic failure

• How quickly will the crack grow?

• How long until it fails due to burst?

WHAT IS A CRACK – THE BASICS

• Stress is a measurement of force spread over area

• Stress = Force / Area

• σ = F / A

• Strain is a measurement of displacement due to some load

• Strain = Change in length / initial length

• ε = (L-Lo)/Lo

• Crack oversimplification:

• If area is reduced due to cracking, stress increases

• If the crack grows large enough, material will fail

STRESS & STRAIN F

A σ

Lo

L

• As you apply force (pressure)

• Stress increases

• Strain increases

• Modulus of elasticity

• Ratio of stress to strain in the elastic region

• E = σ / ε

• Linear relationship

• Deformation is recoverable

• Up to yield strength

• Cycling can cause general fatigue failure

ELASTIC DEFORMATION

0

20

40

60

80

100

120

140

0 5 10 15 20 25 30 35 40 45

Stre

ss

Strain

Elastic Deformation

Elastic – recoverable deformation

Elastic Region

• Applying force beyond yield point:

• Stress and strain continue to increase

• Relationship is no longer linear

• Deformation is unrecoverable

• Any strain beyond yield point does not return

• Cycling in the plastic region typically isn’t good

• Elastic shakedown

• Cyclic ratcheting behavior

PLASTIC DEFORMATION

0

20

40

60

80

100

120

140

0 5 10 15 20 25 30 35 40 45

Stre

ss

Strain

Plastic Deformation

Plastic – unrecoverable deformation

Plastic Region

• Any cracks or sharp changes in

geometry affect stress flow

• Visualized with stress lines

• Build up in these stress loads are

known as stress concentrations

• Impact is captured as a factor (SCF)

• Essentially causes a higher local stress

than that of the overall system

• Maximum buildup is at crack tip

STRESS CONCENTRATION

• Crack tip plastic zone

• Region with high relative stress

• For each cyclic load:

• Stress increases at crack tip

• Leads to local plastic deformation

• Damage “causes” crack growth

• Eventually leads to failure

SIMPLIFIED APPLICATION TO CRACKS

CRACK-LIKE OR CRACK?

Crack-like

• No sharp tip

• Lower stress at tip

• Can be stable

• May turn into a crack with high

enough stresses / cycling

Crack

• Sharp tip

• High stress at tip

• Will generally grow

CRACKS FOR CONSIDERATION

Girth weld, seam weld, and Stress Corrosion Cracking (SCC)

• Cause of weld cracks

• Initial weld issues

• (such as debris, improper heating, or an undercut)

• During operation

• (due to general fatigue, thermal expansion or high stresses)

• Key features include

• Location of crack in relation to weld

• Location of crack in relation to wall thickness

• Depth and length of crack

CRACKS IN WELDING

Heat Affected Zone (HAZ) Weld material

Fusion line

• Many types of cracks

• Toe crack, root crack, fusion-line, crater crack, underbead crack, etc.

• Focus here is not on the type of crack or their differences

• Key point of “crack repair” is to limit actively growing crack

• Not analyze the type of crack

• In this regard, there are only two primary considerations:

• Cracks in the axial direction (axial cracks)

• Cracks in the hoop direction (circumferential cracks)

TYPES OF CRACKS

• Circumferential cracks grow primarily due to:

• Axial loads

• Bending

• Thermal expansion

• Axial cracks grow primarily due to:

• Hoop stress

• Pressure cycling

• Thermal expansion

CRACK GROWTH (OR PROPAGATION)

ENVIRONMENTALLY ASSISTED CRACKING

Stress

Material

Environment

Environmentally Assisted Cracking

Sustained Bending Torsion Tension

Cyclic Fatigue

Geometric Discontinuities Fracture Toughness

Chemistry Grade

Manufacturing

Coating Temperature

Corrosive Elements Cathodic Protection

Corrosion Kinetics

• Subset of environmentally assisted cracking

• Sustained stress

• Susceptible material

• Corrosive environment

• Grows by cracking trans- or inter-granularly

• Many different types exist and need to be treated differently

• Chloride induced SCC

• H2S induced SCC

• Circumferential SCC

STRESS CORROSION CRACKING

STRESS CORROSION CRACKING

• SCC – Remove one of the key factors

• Reduce the stress (a composite can help here…)

• Change the pipe

• Change the environment (…and here)

• Weld cracks

• Reduce perpendicular forces (such as putting the crack in compression)

• Reduce cyclic stress range in pipe (through stress sharing)

• Remove the crack

• Limit cycles

FIGHTING CRACK GROWTH

COMPOSITE REPAIRS AND CRACKS

Will composite repairs even work at all?

• Remediates crack-like features with no disturbance to the pipe

• Mostly invisible to inspection tools for future monitoring

• No change to standard installation process

• No shut down in operations

• No hot work required

BENEFIT OF COMPOSITE REPAIR ON CRACKS

• Currently in the standards, cracks must be

ground out before a composite can be applied.

• In 2015, a test program was initiated to look at

the use of the Atlas™ composite repair system

on longitudinal seam weld crack-like defects.

• Due to this testing, dozens of repairs were

approved and installed.

• This success spurred on more test programs.

BACKGROUND

• First program involved simulated weld flaws.

• Created using an EDM notch and pre-cycled for crack initiation.

• Study performed to determine feasibility of crack repair.

INITIAL SEAM WELD TESTING

• Two defects – 33% deep and 75% deep notches

• Various layers installed – 12-48 layers

• Samples were cycled 1,500 times then burst

• ~10%-72% SMYS

• Atlas™ repair system used

TEST PROGRAM #1

• End-user was happy with results for lower sized cracks

• Repaired dozens of crack-like features for cracks ~ 33% deep

• Project highlighted that these repairs are possible but additional testing is needed to

determine maximum crack depth and explore cyclic effects.

• Standard seam weld crack repair:

• 24 layers of Atlas™

• 3ft long on a 3” crack

• Not to exceed ~ 33% crack depth

PROJECT RESULTS

• A JIP was launched looking at two more crack depth’s:

• 15% deep

• 50% deep

• JIP focused more on cyclic crack propagation

• Run out condition of 250,000 cycles established

• Atlas design methodology was to shoot for 200,000 cycles

TEST PROGRAM #2 (JIP)

• Strong scatter, but good results

15% DEEP CRACK RESULTS

Atlas™

• Runout conditions met – no significant data obtained other than “yes – it lasts”.

• Clearly, the right system, installed correctly, can make a huge difference.

50% DEEP CRACK RESULTS

Atlas™

General Hypothesis:

1) A well installed composite repair will reduce localized stress at the crack.

2) Burst pressure predictions can be made using reduced stress

• Requires fracture toughness

3) Remaining pipe life can be estimated

• Requires material constants

DEVELOPING A MODEL

When flowing material through a pipe + composite :

1. Internal pressure pushes outward on the pipe wall

2. For the pipe to expand, the composite must also expand

3. To resist internal pressure, both materials must be stressed

4. Both rings will grow until the material’s combined stress

response equals the pressure load

5. Can determine equivalent pressure on composite and pipe

STEP 1 – DETERMINE STRESS DISTRIBUTION

Internal Pressure

• Stress distribution is known

• For crack growth, strain in pipe is

needed

• For use in fracture mechanics,

equivalent pressure in pipe is needed

STEP 1 – DETERMINING EQUIVALENT PRESSURE

𝑃𝐷

2= 𝑠𝑝trem + 𝑠𝑐𝑡𝑐

𝑃𝐷

2= 𝐸𝑝𝜀𝑝trem + 𝐸𝑐𝑡𝑐(𝜀𝑝 − 𝜀𝑙𝑖𝑣𝑒)

𝑃𝐷2

+ 𝐸𝑐𝑡𝑐𝜀𝑙𝑖𝑣𝑒

𝐸𝑝𝑡𝑟𝑒𝑚 + 𝐸𝑐𝑡𝑐= 𝜀𝑝 =

𝑃𝑒𝑞𝐷

2𝐸𝑝𝑡𝑟𝑒𝑚

𝑃𝑒𝑞 =𝑃𝐸𝑝𝑡𝑟𝑒𝑚 + 𝑃𝑙𝑖𝑣𝑒𝐸𝑐𝑡𝑐

𝐸𝑝𝑡𝑟𝑒𝑚 + 𝐸𝑐𝑡𝑐

• Determine stress distribution between multi-layered system

• Converting that stress into equivalent pressures

• Primary design factors come from:

• Install pressure

• Pipe and composite thickness

• Pipe and composite modulus

STEP 1 – SUMMARY

𝑃𝑒𝑞 =𝑃𝐸𝑝𝑡𝑟𝑒𝑚 + 𝑃𝑙𝑖𝑣𝑒𝐸𝑐𝑡𝑐

𝐸𝑝𝑡𝑟𝑒𝑚 + 𝐸𝑐𝑡𝑐

Fracture toughness terminology

• Stress intensity factor, K

• Stress state near the crack tip

• Critical stress intensity factor, K1c

• Also called fracture toughness (FT)

• Point at which crack becomes critical / unstable growth

STEP 2 – FRACTURE MECHANICS MODELING

• Actual failure mode is a mix between fracture and stress failure

• Brittle (Low FT) material likely to fail from stress intensity factors

• Strong (high FT) material likely to fail from standard stress failures

• Utilize Newman-Raju and API 579 Reference Stress to estimate burst

• Replace pressure inputs with calculated equivalent pressure

*Several other equations and estimates are available

STEP 2 – FAILURE MODE

𝐾𝐼 = 𝐹𝑃𝑅

2𝑡𝜋𝑎 𝜎𝑟𝑒𝑓 =

𝑔𝑃𝑏 + 𝑔𝑃𝑏2 + 9 𝑀𝑠𝑃𝑚 1 − 𝛼 2 2

3 1 − 𝛼 2

• Failure mode visualized by using Failure Assessment Diagram (FAD)

• Plots stress intensity factor / fracture toughness vs stress / failure stress

• Compares values to mixed mode failure assessment curve

• Inside the curve = safe to operate

• Outside the curve = likely to fail

STEP 2 – FAILURE ASSESSMENT DIAGRAM

33,439 psi-in^.5

25,824 psi-in^.5

34,695 psi-in^.5

0.6456

Unrepaired Pipe at Design Pressure

Estimated Fracture

Toughness Loading

Estimated FT Loading

/ Material FT Ratio

Repaired Pipe at Design Pressure

Failure Pressure

0.8360

0.8732

32,948 psi

25,444 psi

34,413 psi

Yield Stress used : 48000 psi

Flow Stress used : 56500 psi

Est. Stress Loading on

Remaining Pipe

Est. Stress Loading

/ Yield Stress Ratio

0.7169

0.5301Repaired Pipe at Design Pressure

Failure Pressure

Unrepaired Pipe at Design Pressure 0.6864

STEP 2 – FAD EXAMPLE

0.000

0.200

0.400

0.600

0.800

1.000

1.200

0.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400

Frac

ture

To

ugh

nes

s R

atio

Stress Load RatioUnrepaired Pipe at Design Pressure Repaired Pipe at Design Pressure Failure Pressure

Frac

ture

Rat

io -

KI /

KIC

Load Ratio - σ / σmax

STEP 2 – BURST VALIDATION WITH ATLAS™

R² = 0.9327

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

5,000

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000

Esti

mat

ed B

urst

Pre

ssur

e (p

si)

Actual Burst Pressure (psi)

(1) Failed outside repair close to estimated pipe yield conditions

(2) Failed outside repair Low FT values in pipe – model underpredicts performance

1

2

• The next step is to determine the cyclic fatigue life.

• The simplest equation is Paris-Erdogan equation.

• Cycle from effective pressure max – min

• “C” and “n” are material properties for the pipe

“a” is the crack depth “ΔK” is the change in fracture loading per cycle

STEP 3 – CYCLIC ANALYSIS

𝑑𝑎

𝑑𝑁= 𝐶Δ𝐾𝑛

I II III

𝐿𝑜𝑔𝑑𝑎

𝑑𝑁

𝐿𝑜𝑔 (Δ𝐾)

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

0 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000

Est

ima

ted

Cyc

les

to F

ailu

re

Cycles to Failure

STEP 3 – CYCLIC VALIDATION WITH ATLAS™

(1) All repairs performed much better than estimated

• Model showed to be underestimating failure points

• Aka – “conservative”

• Further work being done to be more accurate

• Since it uses standard fracture mechanics approach, capable of modeling many

different scenarios not exclusively tested

• Does require known material properties of pipe or conservative estimates

• Too many conservative estimates may make a repair look unattractive though

MODEL CONCLUSIONS

• Stress distribution and fracture mechanics works on other cracks as well!

• Different equations / approaches are needed

• Primary drivers are no longer internal pressure

• Testing on cracks in girth welds has not been performed yet

• Testing on SCC is scheduled for 2021

• Other consideration is removal of environment may prevent all crack growth

• Turns SCC into a “standard” growth type crack

GIRTH WELDS AND SCC

• Temporary

• Intended to be replaced at earliest convenience

• Repair either

• prevents failure from occurring until repair permanently or

• changes failure risk to acceptable.

• Permanent

• Intended to last a reasonable time without additional planned replacements

• Can be monitored or observed with standard inspection plans.

PERMANENT VS TEMPORARY

• Extreme crack depth > 50%

• May prevent burst

• Changes failure mode to leak instead of violent burst

• To be replaced asap

OR

• Cyclic frequency that is too aggressive

• Will slow crack growth

• Can’t accurately depict near term failures

• Again, Changes failure mode

• To be replaced asap

TEMPORARY SCENARIOS

• Low depth < 50%

• May prevent crack-like from turning into crack

• Crack size can be monitored with EMAT

• Restores serviceability

AND

• Cyclic frequency that is not too aggressive

• Will significantly slow crack growth

• May predict hundreds of years (~ permanent)

• Again, can be monitored

PERMANENT SCENARIOS

• Additional inputs needed :

• Fracture toughness estimate

• Crack size

• Operating conditions

• Estimated material properties

• Additional outputs :

• FAD burst analysis

• Estimated cyclic life remaining

WHAT’S THE PROCESS?

46

Check out other upcoming TEC talks at cs-nri.com/TECtalks

@clockspringNRI @cs-nri @csnri @clockspringnri