Fatigue on drill string conical threaded connections, test ... on drill string conical threaded connections, test results and ... connections Drill pipe length ... body pipe drill string and drill ...

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  • 1/35

    Fatigue on drill string conical threaded connections,

    test results and simulations

    A. Baryshnikov

    L. Bertini,M. Beghini,C. Santus

    ENI S.p.A.Milano.Italy

    University of Pisa,mechanical dept.Italy

  • 2/35

    Short introduction to drilling technology- drill string and drill pipes, fatigue failures on drill pipes- steel heavy construction vs. aluminum light construction

    Full scale fatigue tests- description of test rigs- test results

    Finite Element simulations- FE model dedicated to threaded connection

    Fatigue models- classic approach (Gerber, kf , surface effect)- test results correlation

    Conclusions

    Contents

  • 3/35

    Short introduction to drilling technologydrill string and drill pipes, fatigue failures on drill pipes

    Drill String

  • 4/35

    Short introduction to drilling technology

    Drill string:hundreds of drill pipesconnected through threaded connections

    Drill pipe length ~ 10mDrill string max. length ~ 5km

    Basic nomenclature

    Dog leg segment, for deviated drilling

    Drill bit

    drill string and drill pipes, fatigue failures on drill pipes

  • 5/35

    Short introduction to drilling technology

    Fatigue locations along drill string

    Rotating bending fatigue, due to dogleg on the upper part of the string

    Lateral bending fatigue, due to buckling on the lower part of the string

    drill string and drill pipes, fatigue failures on drill pipes

  • 6/35

    Fatigue locations along drill string

    Fatigue accounts for 70 % of failuresCorrosion, Stress-Corrosion, Wear, Static stresses are further detrimental effects in combination with fatigue

    Short introduction to drilling technologydrill string and drill pipes, fatigue failures on drill pipes

  • 7/35

    Short introduction to drilling technology

    Steel construction Aluminum construction

    Aluminumbody pipe

    Steel thread connection (tool joint box)

    Steel thread connection (tool joint pin)

    Aluminumbody pipe

    drill string and drill pipes, fatigue failures on drill pipes

    Steel pipe

    Steel pipe

  • 8/35

    Short introduction to drilling technology

    Steel constructionfatigue locations

    steel heavy construction vs. aluminum light construction

    Aluminum constructionfatigue locations

    Box fatiguelocation

    Pin fatiguelocation

    Last Engaged Thread- Notch effect- Mean stress effect

    (particularly for pin side)

    Conical shoulder Aluminum-Steel interface:- Fretting nucleation

    (different material stiffness)

    steel

    Fatigue location Fatigue location

    Box side

    alluminum steel alluminum

    Pin side

  • 9/35

    Full scale fatigue testsdescription of test rigs

    Bending arms SpecimenRotating masses Straingauge

    1 m

    Test rig for steel construction

  • 10/35

    description of test rigsTest rig for steel construction

    F

    tF

    t

    Specimen

    Rotating eccentric masses

    Bending armBending arm

    F2

    t

    de

    H

    The phase between the two couple of eccentric masses control the stress amplitude

    Full scale fatigue tests

  • 11/35

    description of test rigsTest rig for steel construction Device to change the phase

    Bending arms

    Supporting springs to allow oscillating displacements

    Specimen

    Full scale fatigue tests

  • 12/35

    description of test rigsTest rig for aluminum construction

    Full scale fatigue tests

    Eccentric rotating mass

    Rubber wheels

    Connection to test

    Electric motor

    Eccentric rotating mass

    Still mass Connection

    to test

    Rubber wheels

  • 13/35

    description of test rigsTest rig for aluminum construction

    Aluminum pipe

    Steel tool joint

    Fatiguesection

    FatiguesectionAluminum pipe

    0.5 m

    Steel tool joint

    Strain gauge

    Full scale fatigue tests

  • 14/35

    description of test rigsTest rig for aluminum construction

    X

    Y

    Z

    Deformed shape

    Undeformed shape

    Fixpoint 2

    Fixpoint 1

    Eccentric rotatingmass

    Specimen propat fix points

    Full scale fatigue tests

  • 15/35

    description of test rigs

    Full scale fatigue tests

    ResonantTestRig.avi

  • 16/35

    description of test rigsThe role of resonance

    Frequencyf , Hz

    Bending stress amplitude0 , MPa

    Resonance conditionIdeal

    behavior true behavior (damping)For different masses or phases

    Working frequency window, near the resonance condition.High slope, good for control.

    Full scale fatigue tests

  • 17/35

    test resultsSteel construction test results

    Experimental nucleation is fatigue life when the smaller crack can be detected through dynamic behavior control.

    The Exp. Nucleation life includes a large portion of propagation fatigue life.

    In other words nucleation/propagation can be resolved only when a large fatigue crack appears in the structure.

    Only pin side failure have obtained in this fatigue test set

    105

    106

    107

    108

    0

    20

    40

    60

    80

    100

    120

    cycles

    0

    [MPa]

    Exp. nucleationFatigue lifeExp. nucleation fit lineFatigue life fit line

    Full scale fatigue tests

  • 18/35

    test results

    Fatigue fracture section (pin)

    fatigue crack starting from last engaged thread root

    High toughness leads to a large wall-through crack, before brittle fracture

    (material: AISI 4145H)

    Steel construction test results

    Full scale fatigue tests

    Crack fronts

    2.5 cm

    Detectable size (exp. nucleation)

  • 19/35

    test resultsAluminum construction test results

    105

    106

    107

    108

    0

    20

    40

    60

    80

    100

    120

    140

    cycles

    0

    [MPa]

    TestsFit line

    The aluminum alloy here used shows brittle behavior.

    Then propagation phase can not be distinguished from dynamic behavior.

    Full scale fatigue tests

  • 20/35

    Full scale fatigue teststest resultsAluminum construction test results

    Crack surface, showing:- initiation point- brittle behavior

    Fracture toughness is not enough to allow wall-through crack.

    (material AA 7014-T6)

    After reaching this front, brittle fracturehappens.

    Until this condition, dynamic behavior control is almost steady.

    2 cm

  • 21/35

    Finite Element simulationsFE model dedicated to threaded connectionSteel construction FE model

    Under bending load the stress state is biaxial at the thread root surface:

    r = 0z > > 0

    r = rz = z = 0

    ~ 0

    Stress state is similar to plain strain condition.

    r

    z

    Thread root

    Thread axis direction

    The make up produces a strong presetting, and then a plastic zone around the thread root can be found.

  • 22/35

    Finite Element simulations

    Steel construction FE model

    Elastic shakedown at the last engaged thread root after presetting:- linear kinematic hardening can be assumed- limited subsequent stress amplitude

    Subsequent cycles

    z

    z

    Presetting

    zm

    zaz

    p ~ 0zp > 0rp ~ -zp

    1

    1

    e ~ 0

    FE model dedicated to threaded connection

  • 23/35

    Finite Element simulations

    2D axial symmetry, to avoid cumbersome 3D analysis

    Steel construction FE model

    Elementdiscretizationat thread root

    Bondedcontact condition

    Elementdiscretizationat thread root

    Bondedcontact condition

    X

    Y

    Z

    Axialsimmetry

    Box

    PinX

    Y

    Z

    Axialsimmetry

    Box

    Pin

    Elasto-Plasticmaterial model

    Perfectelasticmaterial model

    Elasto-Plasticmaterial model

    Perfectelasticmaterial model

    Elasto-plastic material model, with linear kinematichardening behavior

    Contact is modeled as closed gap since no contact loss is assumed.

    FE model dedicated to threaded connection

  • 24/35

    Finite Element simulations

    Steel construction FE model

    0 1 2 3 40

    300

    600

    900

    1200

    1500

    0 1 2 3 40

    0.002

    0.004

    0.006

    0.008

    0.01

    Stress path coordinate [mm]

    Str

    esse

    s[M

    Pa]

    pl

    z

    r

    Equiv

    alen

    tpla

    stic

    stra

    inpl

    Stress path

    Stress path along thread root bisector, after presetting

    FE model dedicated to threaded connection

  • 25/35

    Finite Element simulations

    Steel construction FE model

    Stress path

    Stress path along thread root bisector, after elastic analysis.

    0 1 2 3 40

    300

    600

    900

    1200

    1500

    Stress path coordinate [mm]

    Str

    esse

    s[M

    Pa]

    z/2/2r/2

    FE model dedicated to threaded connection

  • 26/35

    Finite Element simulations

    Steel construction FE model

    0 1 2 3 40

    300

    600

    900

    1200

    1500

    Stress path coordinate [mm]

    Stre

    sses

    [MPa]

    z/2/2r/2

    0 1 2 3 40

    300

    600

    900

    1200

    1500

    0 1 2 3 40

    0.002

    0.004

    0.006

    0.008

    0.01

    Stress path coordinate [mm]

    Str

    esse

    s[M

    Pa]

    pl

    z

    r

    Equiv

    alen

    tpla

    stic

    stra

    inpl

    Subsequent cycles

    z

    z

    Make up plusfirst cycle

    zm

    za

    FE model dedicated to threaded connection

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    Fatigue modelsclassic approach (Gerber, kf , surface effect)

    To propose a valid fatigue model the following issues need to be considered:

    - reference S-N curve, with plain specimens, to relate load to fatigue finite life

    - mean stress effect(the strong presetting of the connection induce high tensile stresses)

    - notch effect(high gradient at the thread root)

    - surface state effect(the machining to generate thread geometry can play a role in terms of fatigue nucleation)

    Steel construction fatigue life prediction model

  • 28/35

    Fatigue models

    reference S-N curve

    Several plain specimen were extracted from real component to test as close as possible in terms of:

    - heat treatment,

    - grain orientation.

    Nf

    a

    classic approach (Gerber, kf , surface effect)

  • 29/35

    Fatigue models

    To take into account mean stress, the Gerber (parabola) model is considered.

    Gerber parabola shows better fit with plain specimen extracted from real componenttested at positive mean stress ratios.

    mean stress effect

    a

    m

    classic approach (Gerber, kf , surface effect)

  • 30/35

    Fatigue models

    To take into account notch effect the following steps were considered:

    - Same notch radius to determine the fatigue notch factor kf

    - Also notched specimen are extracted from real component, and the notch bisector has same orientation as thread root bisector

    notch effect

    classic approach (Gerber, kf , surface effect)

  • 31/35

    Fatigue models

    Finally particular care is dedicated to the surface effect:

    - Small scale specimen extracted from thread geometry were tested to reproduce as close as possible surface conditions

    surface effect

    classic approach (Gerber, kf , surface effect)

  • 32/35

    Fatigue modelstest results correlationThe correlation is here presented as:- Equivalent stresses against material limit at different cycles (left)- logNpredicted logNExp.Nucl. diagram (right)

    Wide discrepancy in terms of cycles.

    Not so bad in terms of stresses.

    Eq. mean stress [MPa]

    Eq.

    alte

    rnat

    est

    ress

    [MPa]

    Failures

    No failuresRun out

    103 cycles

    104

    105

    5 105

    Fatigue limit 0

    Pin stresses

    Box stresses

    00

    100

    200

    200

    300

    400

    400

    500

    600 800 1000 103

    104

    105

    106

    107

    103

    104

    105

    106

    107

    Model prediction [cycles]

    Exp.

    nucl

    eati

    on

    [cycl

    es]

    Tests

  • 33/35

    Fatigue modelstest results correlationPossible sources of mismatch:- bad assessment of mean stress (uncertainty of make up presetting, possible material cyclic

    relaxation since it cycles at high mean stress)- big portion of propagation

    Eq. mean stress [MPa]

    Eq.

    alte

    rnat

    est

    ress

    [MPa]

    Failures

    No failuresRun out

    103 cycles

    104

    105

    5 105

    Fatigue limit 0

    Pin stresses

    Box stresses

    00

    100

    200

    200

    300

    400

    400

    500

    600 800 1000

    103

    104

    105

    106

    107

    103

    104

    105

    106

    107

    Model prediction [cycles]E

    xp.

    nucl

    eati

    on

    [cycl

    es]

    Tests

  • 34/35

    Conclusions Demanding full scale fatigue tests were proposed along with

    the description of resonance test rigs.

    Finite element dedicated to thread geometry was presented- elastic-plastic analysis was needed for the high presetting,- kinematic hardening was able to model elastic shakedown

    Fatigue model proposed deals with simple tools for fatigue evaluation (Gerber, kf , surface effect) and calibration of the model is based on small scale specimen as close as possible to real component conditions.

    To improve the correlation fatigue crack propagation should be included, but:

    how much is the nucleation/propagation crack length??

  • 35/35

    ConclusionsWere expensive full scale fatigue tests necessary ??YES, because:

    - Some fatigue issues are hard to be thought a-priori.- From small to full scale, propagation can play an important

    role. Though prediction is conservative, large mismatch can be found.

    If we have to avoid full scale testing:Specimens, as close as possible to real component conditions, are needed, to calibrate fatigue models.

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