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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

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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

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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

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

Drill String

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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

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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

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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

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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

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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

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Full scale fatigue testsdescription of test rigs

Bending arms SpecimenRotating masses Straingauge

1 m

Test rig for steel construction

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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

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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

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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

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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

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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

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description of test rigs

Full scale fatigue tests

ResonantTestRig.avi

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description of test rigsThe role of resonance

Frequencyf , Hz

Bending stress amplitudeσ0 , 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

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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

σ0

[MPa]

Exp. nucleationFatigue lifeExp. nucleation fit lineFatigue life fit line

Full scale fatigue tests

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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)

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test resultsAluminum construction test results

105

106

107

108

0

20

40

60

80

100

120

140

cycles

σ0

[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

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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

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Finite Element simulationsFE model dedicated to threaded connectionSteel construction FE model

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

σr = 0σz > σθ > 0

τrθ = τrz = τθz = 0

εθ ~ 0

Stress state is similar to plain strain condition.

r

z

θ

Thread root

Thread axis direction

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

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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

σz

εz

Presetting

σzm

σzaσz

σθεθp ~ 0εz

p > 0εr

p ~ -εzp

ν1

εθe ~ 0

FE model dedicated to threaded connection

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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

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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]

εpl

σz

σrσθ

Equiv

alen

tpla

stic

stra

inεpl

Stress path

Stress path along thread root bisector, after presetting

FE model dedicated to threaded connection

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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]

Δσz/2Δσθ/2Δσr/2

FE model dedicated to threaded connection

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Finite Element simulations

Steel construction FE model

0 1 2 3 40

300

600

900

1200

1500

Stress path coordinate [mm]

Stre

sses

[MPa]

Δσz/2Δσθ/2Δσr/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]

εpl

σz

σrσθ

Equiv

alen

tpla

stic

stra

inεpl

Subsequent cycles

σz

εz

Make up plusfirst cycle

σzm

σza

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

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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

σa

classic approach (Gerber, kf , surface effect)

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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

σa

σm

classic approach (Gerber, kf , surface effect)

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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)

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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)

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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 σ0 ↑

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

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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 σ0 ↑

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

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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??

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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.