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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
1ν
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.