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Fatigue on drill string conicalthreaded connections,

test results and simulations

A. Baryshnikov

L. Bertini,

M. Beghini,

C. SantusENI S.p.A.

Milano.

Italy

University of Pisa,

mechanical dept.

Italy

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

- steel heavy construction vs. aluminum light construction

Full scale fatigue testsdescription of test rigs

- test results

Finite Element simulations- FE model dedicated to threaded connection

Fatigue modelsclassic 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 ofdrill pipes

connected through threaded

connections

Drill pipe length ~ 10m

Drill 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 technologyFatigue 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


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Fatigue locations along drill string

Fatigue accounts for 70 % of failures

Corrosion, 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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Steel construction Aluminum construction

Aluminum

body pipe

Steel thread

connection

(tool joint box)

Steel thread

connection

(tool joint pin)

Aluminum

body pipe


Steel pipe

Steel pipe

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Steel construction

fatigue locations

steel heavy construction vs. aluminum light construction

Aluminum construction

fatigue locations

Box fatigue

locationPin fatigue

location

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

Boxside

alluminum steel alluminum

Pinside

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Full scale fatigue testsdescription of test rigs

Bending arms SpecimenRotating masses Straingauge

1 m

Test rig forsteel construction

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description of test rigs

Test rig forsteel construction

F

t

F

t

-

Specimen

Rotating eccentric masses

--

Bending armBending arm

F2

Zt

de

H

The phase between the two couple of eccentric masses control the

stress amplitude

Full scale fatigue tests

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Test rig forsteel constructionDevice to change the phase

Bending

arms

Supporting springs

to allow oscillatingdisplacements Specimen


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Test rig foraluminum construction


Eccentric

rotating

mass

Rubber

wheels

Connection

to test

Electric

motor

Eccentric

rotating

mass

Stillmass Connection

to test

Rubberwheels

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Aluminum pipe

Steel tool joint

Fatiguesection

Fatigue

sectionAluminum pipe

0.5 m

Steel tool joint

Strain gauge


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X

Y

Z

Deformed shape

Undeformed shape

Fix

point 2

Fix

point 1

Eccentric rotatingmass

Specimen prop

at fix points


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ResonantTestRig.avi

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The role ofresonance

Frequency

f, Hz

Bending stress amplitude

0 , MPa

Resonance

conditionIdeal

behavior true behavior(damping)

For different

masses or phases

Working frequency window, near the resonance condition.

High slope, good for control.


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test results

Steel construction test results

Experimentalnucleation 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 set10

5

106

107

108

0

20

40

60

80

100

120

cycles

0

[M

Pa]

Exp. nucleationFatigue lifeExp. nucleation fit lineFatigue life fit line


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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


Crack fronts

2.5 cm

Detectable size

(exp. nucleation)

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test results

Aluminum construction test results

105

106

107

108

0

20

40

60

80

100

120

140

cycles

0

[MP

a]

TestsFit lineThe aluminum alloy here used

shows brittle behavior.

Then propagation phase can

not be distinguished from

dynamic behavior.


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Full scale fatigue teststest results

Aluminum 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 fracture

happens.

Until this condition, dynamic behavior

control is almost steady.

2 cm

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Finite Element simulationsFE model dedicated to threaded connection

Steel construction FE model

Under bending load the stress state is

biaxial at the thread root surface:

r= 0

z> > 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.

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Finite Element simulationsSteel construction FE model

Elastic shakedown at the last engaged thread root after

presetting:- linear kinematic hardeningcan be assumed

- limitedsubsequent stress amplitude

Subsequent

cycles

z

z

Presetting

z

m

za

z

p ~ 0

zp > 0

rp ~ -zp

1

1

e ~ 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

Element

discretization

at thread root

Bondedcontact

condition

Element

discretization

at thread root

Bondedcontact

condition

X

Y

Z

Axial

simmetry

Box

PinX

Y

Z

Axial

simmetry

Box

Pin

Elasto-

Plastic

material

model

Perfectelastic

material

model

Elasto-

Plastic

material

model

Perfectelastic

material

model

Elasto-plastic material

model, with linear kinematic

hardening behavior

Contact is modeled as

closed gap since no contact

loss is assumed.


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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]

S

tresses[MPa] pl

zr

Equivalentplastics

trainpl

Stress

path

Stress path along thread root bisector, afterpresetting


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Stress

path

Stress path along thread root bisector, afterelastic analysis.

0 1 2 3 40

300

600

900

1200

1500


Stresses[M

Pa]

z/2/2r/2


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0 1 2 3 40

300

600

900

1200

1500


Stresses[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


Stresses[M

Pa] pl

zr

Equivalentplasti

cstrainpl

Subsequent

cycles

z

z

Make up plus

first cycle

z

m

za


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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 modelsreference S-N curve

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

- heat treatment,

- grain orientation.

Nf

a

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 component

tested at positive mean stress ratios.

mean stress effect

a

m


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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 factorkf

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

orientation as thread root bisector

notch 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

-


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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.alternates

tress[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 100010

3

104

105

106

10710

3

104

105

106

107

Model prediction [cycles]

Exp.nucleation[cycles]

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.alternatestress

[MPa]

Failures

No failuresRun out

103 cycles

104

105

5 105

Fatigue limit 0

Pin stresses

Box stresses

0

0

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.nucleation

[cycles]

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 fatigueevaluation (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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ConclusionsWereexpensivefull 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 toavoidfull scale testing:

Specimens, as close as possible to real component conditions,are needed, to calibrate fatigue models.

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