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LINKpipe:A tool offering the answer as to

When will cracks become dangerous ?and

Which cracks can be accepted ?

Presentation at the Remote MonitoringConference, Britannia Hotel, Trondheim

5. November 2003

By

Prof. Christian ThaulowProf. Bjørn Skallerud

LINKftr AS

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The challenge:Metallic structures/steel structures,

like pipelines are subject to extreme loads.

Despite well founded design rules, representing the extraction of experience and theoretical knowledge collected over generations, structures still fail.

Last 6 years, a number of 228 serious incidents were recorded in the Norwegian sector of the North Sea.

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The challenge, cont’d:

Even though pipeline failures do occur from time to time, designs my still be conservative in other instances.

This represents a cost and a waste.

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The Tradition:Design engineers of structures and

pipelines have continually advanced their knowledge and improved their analysis- and design tools:

Advanced computer models are utilised to optimise the design – Safe designs, with minimised margins within the design rules.

However, this is a design analysis valid for a flawless material; based on the theoretical strength of a metal with no flaws or cracks (design for plastic collapse and buckling).

3D finite element mesh for a pipe segment

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The problem:The problem is that structural materials

never are found in a flawless state - there will always be cracks and other imperfections:

Through welding of structures new imperfections are introduced - girth welds in pipelines are critical sections with a higher than average risk for failure.

Erosion, corrosion and other degrading processes may introduce new defects through years of operation.

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The Solution:

LINKpipe - a tool for direct calculations

A Shell element modelPlus the novel Line Spring element=LINKPIPE

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Direct claculations:Fracture mechanics theory and practise has, through the LINKpipe SW been merged with thedesign engineers Finite Element models:

Effectively mergingthe expertise of thepipeline design engineer, with thatof the materials expert.

Line spring location

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LINKftr

THE LINK BETWEEN LOCAL FAILURE AND STRUCTURAL RESPONSETHE LINK BETWEEN LOCAL FAILURE AND STRUCTURAL RESPONSE

LINKfailure LINKresponsLINKtransfer

=

B

B

A

A

B

B

A

A

QQQQ

qqqq

DDDDDDDDDDDDDDDD

2

1

2

1

2

1

2

1

44434241

34333231

24232221

14131211

)()(

haD ep

ijLine Spring Tangent stiffness matrix

BAiq

, Generalized displacements atnodes A and B

iQGeneralized Force at nodes Aand B, in tension and inbending (N,M),

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LINKresponse

displacement

fracture ?

displacement

load

load

displacement

fracture ?

displacement

load

load

load

load

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LINKfailure

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LINKtransfer

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3D with crack

Line Springsconnecting theshell elements

Shell element with linespring

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LINKPIPE FEATURES

1.The calculations are performed directly on the structurewith realistic:-size and location of defect-geometry-tensile/bending loading-internal/external pressure

2.LINKPIPE combines structural analysis (plastic collapse, buckling) and local defect analysis (fracture)

3.LINKPIPE is merging the competence of materials- and structural engineering in a user-friendly way. No in-depth expert knowledge is needed

4.The speed of calculation is extremely high, close to real time.The defect size and dimensions of the structure can be changedwithin a few seconds

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LINKPIPE FEATURES....

5.LINKPIPE opens up for statistical evaluations

-no analytical equations are neede-scatter in material data, loads, dimensions etccan be examined-sensitivity analysis-Monte Carlo simulations-partial safety factors for a given system or project

OPTIMISATION OF COST AND SAFETY

6. Real time processing of data opens up for ”eField” applications

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One example of application: free spanning oil/gas pipelines

Large costs regarding inspection and monitoringLINK_PIPE: quantify risk, reduce costs during operation

Is this defect critical? LINK_PIPE

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3D FE

calculations

Shell elements FE

calculations

with line spring

Analytical

equations

Acc

urac

y

Costs

Accuracy

Cos

ts

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a

2c

Calculation time3D 60.000 sec (cpu)LINKpipe 100 sec (cpu)

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Cost Benefits

12min and 10sec200 NOK

25 hours25.000 NOK

TOTAL

10 minutes2 hoursPostprocessing

2 minutes17 hours calculationtime, gives3 hours inefficiency

Processing

10 seconds20 hoursPreprosessing

LINKpipeConventional FE Models

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LINKPIPE BENEFITS1.Dramatic reduction in man-hours

EXAMPLE: Surface defects in pipelines3D calculations 25.000 NOKLINKPIPE 200 NOK

2. Direct calculations are more realistic than analyticalapproachesThis gives better utilization of materials and design solutions

3. Extremely high speed of calculation and generation of new modelsScatter can be treated realisticlyCosts and Safety can be optimised

4.Data from a monitoring systems can directly be transformed toconcequencesReduced need to store and transmitt dataReduced need for monitoring, inspection and control

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FORMULATION AND NUMERICAL PROCEDURES

• Rect ANDES FE, co-rotated kinematics, consistent tangent• Stress resultants, linear/power law hardening for the shell element, consistent

tangent• Rect line spring FE, co-rotated kinematics, power law hardening, alternative

stress updates tried (expl, impl euler), yield surface with corners, calculatesfracture mechanics quantities such as J-integral, CTOD, T-stress(constraint)

• Increm-iterative solution of global eqs using Newton-Raphson and governingdisplacement/rotation control

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Co-rotated thin shell kinematics

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Formulation works well for large disp/rot, e.g. inelastic collapse ofpinched cylinder

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Shell finite element

• Rectangular ANDES element, 6 DOF at each node, higher order membraneand curvature interpolation according to assumed natural deviatoric strains

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Line spring finite element: simplify 3D crack problem to 2D, has a sound fracture mechanics basis from slip line analysis of the crack ligament

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Line spring relationships

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Line spring fe discretization, 8 DOF, elongation and rotation (opening of the crack)

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Visulatisation of J-integral/CTOD in crack

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Pipe in bending, D/t=80 and 60

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Local buckling versus fracture?

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The LINKteam would like to thank you for your attention

NTNU/LINKftr SINTEFJack Ødegårdresearch managerSINTEF materials technologyfracture mechanics and testing

Christian Thaulowprofessormaterial technology

Bjørn Skallerudprofessormechanics Bård Nyhus

researherfailure assessment

Kjell Holtheprofessormechanics

Espen Bergproduct developmentsiv ing Erling Østby

researcherfracture mechanics

Andreas SandvikPhD studentProbabilistic fracture mechanics

K R JayadevanPost docfracture mechanics

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LINKftrInnovasjonssenter GløshaugenRichard Birkelandsvei 1c7491 Trondheim

voice: 73 551861, 93059460mail: christian.thaulow@immtek.ntnu.no

bjoern.skallerud@mtf.ntnu.no

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LINKpipewhen will cracks become dangerous ?

• LINKpipe offers an immediate response on the consequence of cracks in a pipeline. This will strongly reduce the need for monitoring and inspection. Monitoring data on cracking activity in pipelines, are withinseconds transformed to meaningful information on the risk and consequence of growing cracks that may have been detected. The e-fieldmay become a reality.

• LINKpipe represents a transformation from the conservative calculationsand time consuming expert evaluations: This novel and innovative SW offers fast and direct calculations on the performance of induvidualstructures, segments of pipelines etc. under actual operating conditions. These calculations may be done by non experts in fracture mechanics.

• LINKpipe offers an optimal utilisation of materials under actualoperating conditions - such optimisation may be done, fully satisfyingyour needs for safety and a financially healthy and optimised operation.

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Applications of LINKftr

• In early stage concept studies and design analyses where welds (inhomogeneous material) and cracks (imperfections) needs to be considered

• In fitnes-for-purpose calculations with respect to defect tolerances

• In planning inspection programmes

• In optimisation of safety and costs through sensitivity analysis

• In calibration of safety factors

• In examination of the mutual effect of several potential “weak”zones on structural redundancy

• In materials selection

• In local geometry optimisation

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Two-parameter fracture mechanics: CTOD and constraint

FRACTURETOUGHNESS

[J, K, CTOD]

GEOMETRY / CONSTRAINT [T,Q,M]

SENB (a/W = 0.3)

CT (a/W = 0.5)

SENB (a/W = 0.5)

SENTPIPE

Black line (thick) FRACTURE RESISTANCE

Blue lines (thin) CRACK DRIVING FORCE

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