Why Coiled Tubing Fails and How to Avoid Failures in Your Well
Transcript of Why Coiled Tubing Fails and How to Avoid Failures in Your Well
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Why Coiled Tubing Works and How to Make Sure it Works on Your Job
... in Your Well
Why Coiled Tubing Fails and How to Avoid Failure on Your Job
... in Your Wellor
Steven M. TiptonThe University of Tulsa
Society of Petroleum Engineers Distinguished Lecturer Programwww.spe.org/dl
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The Oilfield is a Rough Place
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Outline• Introduction / Mechanics Primer• CT Fatigue Research
– TU Coiled Tubing Mechanics Research Consortium
• Surface Defects• Analytical Modeling
– FlexorTU5• Inspection
– 3D laser imaging
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Background• 1989 - developed CoilLIFE – Schlumberger
• 1994 - JIP to revise CoilLIFE and develop CT test machine
• 1995 - developed coiled tubing fatigue module forNOV-CTES (Achilles)
• 1996 - 2000 University of Tulsa JIP’s to studyCT mechanical and fatigue behavior
• 2000 – present: University of Tulsa Coiled TubingMechanics Research Consortium
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What is “Coiled Tubing”• Exactly what it sounds like• Continuously Milled Tubing• Rolled from Flat Strip
– Resistance Welded Seam– HSLA (High Strength Low Alloy)
• 55-120 KSI– CRA
• Chrome Alloys, Titanium
• Diameters: 0.75” – 4.5”• Thicknesses: up to 0.25”• Lengths > 30k ft
– 10K – 20k ft most common
Courtesy Quality Tubing
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Typical CT Rigup
Coil Tubing
Reel ~ 10-20K ft
“Gooseneck”
Guide Arch
Injector
What is it used for?
• Anything “jointedtubulars” are used for!
• Drilling
• Completion
• Servicing
Courtesy ICoTA
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Coiled Tubing Units
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Coiled Tubing
2009 SAE Baja Vehicle
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Bending Strains
Bottom material in COMPRESSION
Top material in TENSION
“Strain” defined as % material stretchesin Tension or shortens in Compression
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repeats each tripspool
straight straighthole
arch archspool
Bending Cycles
time
stra
in
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Bending Strains Lead to• ULTRA-Low Cycle Fatigue
– e.g., lives ≈ 10 – 100 cycles to failure• Normal Low Cycle >103 Cycles to Fatigue• Engine Parts >106 Cycles to Fatigue
• Ballooning– >30% even when pressure is <50% yield
• Permanent Elongation– 6-10 ft per 10,000 ft trip, even for axial
loading well below yield
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Huge Cyclic Plastic Strains (>2%!)
Bending Strains
“Ratcheting”Incremental diameter growth >30%
Lives < 10 cycles!
Even with pressure below
50% of yield
Wall Thinning
bending axis
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Permanent Elongation
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Bending Strain Distribution
Strain
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Strain
εx,m = r/R
r
R
Bending Strain Distribution
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Bending Stress Distribution
StressSy
-Sy
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Bending Stress Distribution
StressSy
-Sy
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Causes Permanent Elongation
StressF
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CT Fatigue Research
CoilLIFEFatigueTestMachine
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CT Fatigue Research
CoilLIFEFatigueTestMachine
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CT Fatigue Research
CoilLIFEFatigueTestMachine
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TU CT Fatigue Test Facility
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Accelerated CT Fatigue Test Machine
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5 4 3 2 1 1 2 3 4 5 6 7 8NUMBER OF FAILURES
NUMBER OF FAILURES
Fatigue Crack Development
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Fatigue Crack Development
MOST fatiguecracks initiateat the inner surface
Fatigue behavior of CTdefies engineering logic!
Appears as a “pinhole” failure
cross section through tubing wall
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120 ksi, 1.5" x 0.134" x 48"R
0
50
100
150
200
250
300
350
400
450
500
0 2000 4000 6000 8000 10000 12000 14000 16000
Pressure (psi)
Cyc
les
to F
ailu
re
CT Life Prediction Routine
50% (mean) prediction
95% confidence prediction
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• Factors that influence life:– Diameter– Pressure– Wall Thickness– Bending Radius
• Spool• Gooseneck
– Material Grade
CT Life Prediction
70 – 120 ksi
Larger diameter – larger bending strains
Thinner walls – higher tangential stresses
Higher pressure – higher tangential stresses
Smaller radius – larger bending strains
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Effect of Tubing Diameter
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Gooseneck Radius of Curvature
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Material Selection
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Fatigue Life Estimation
σf’, b , εf’, S=f(σh, Sy’, Δεx)E’, K’, n’
LCFTesting
Constant PressureFixtureTesting
FittingRoutine
Plasticity / FittingRoutine
MATERIAL DATA SETLCF Data
CT Fatigue Model
εx
εx
time
ComplexField Loading
time
P
Coil Tubing Parameter
time
D
inputinput
output
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0
100
% L
ife E
xpen
ded
Distance along String
Fatigue Life Profile
• Implemented with String Management Software:CT Life Prediction
warning level
• Fatigue eliminated as a primary failure mode!
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What About Defects?
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Means of Imposition
• Milling (cut into surface)• Impressing• Corrosion Pits (chemical milling)• EDM – Different shapes possible
a b fd ec g h i
dw
x
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Means of Imposition
Milled Ball Impressed Ball
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Influence of Surface DefectsLow Pressure Impressed vs Milled 1/8" Balls
0
0.2
0.4
0.6
0.8
1
1.2
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45
d/t
%D
efec
t Life
to B
asel
ine
Life
Impressed (t=0.156)
Milled (t=0.156)
Impressed (t=0.204)
Milled (t=0.204)
scatter
Depth / Wall Thickness Ratio
Rat
io o
f D
efec
t Life
to B
asel
ine
Life
2.375” 80 ksi tubing, tested on 72” radius fixture
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Repairing Defects
• What happens after a defect is discovered?
– Scrap / Replace string (expensive)
– Cut and splice (welding –expensive)
– Grind repair
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Repairing DefectsOne Option: Butt Welding
Unfinished inside of the butt weld constitutes a severe structural discontinuity!
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Butt Welding Fatigue Data
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10
20
30
40
50
60
70
80
90
% o
f Unw
elde
d Sa
mpl
e Li
fe
Manual Welds:average = 38%std. dev = 18%
Orbital Welds:average = 41%std. dev = 11%
Manual Average Orbital Average
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Grind Repairing
replaced withsmooth, gradual thickness change
Abrupt, localized discontinuity
Technique is important and grind repair guidelines have been developed
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Fatigue Life Predictive Model
No defect:229
Cut defect:76
Repaired:129
Impressed:214
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Defects Must be FOUND
• Visual Inspection• Magnetic Flux Leakage
– Most popular approach for CT inspection– MFL can only DETECT defects– Can’t provide any information about defect
size or severity• 3D Laser Imaging for NDE
– Complete geometry of defect is captured digitally
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3D Laser Imaging System(Low-Cost)
Longitudinal Milled Defect
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Commercial Laser System
Sawcut
Impressed ballMilled ball
Milled Cylinder
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Impressed ball Machined ball
Color-coded Digital Data
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w
d
Ap
Automated Defect Measurements
rmax
rmin
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Color-coded Digital Data & Photographic Image
Useful to distinguish cut or impressed
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Sour and Corrosive Environments
•Similar effects: CT to Pipe•H2S
•HIC (Hydrogen Induced Cracking)•SSC (Sulfide Stress Cracking)
•SCC (Stress Corrosion Cracking)
•Residual Stresses•Accelerates Corrosion
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Manufacturing and Materials defects/flaws
•From 7 to 20 reels per 1,000 reels (~last 3 yrs)
–0.7 to 2% –Only Those Reported
•Failures occur that are not fully investigated
–Especially later in CT Life
Courtesy BJ Services
‘Cold’ Weld
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Conclusions• Fatigue is inherent to the use of CT• Large CT Bending Strains Cause
– Fatigue, diameter growth, elongation• Scatter is Associated with Fatigue Data • Fatigue Software can Predict Influence of
– Material, diameter, wall thickness, bending radius, internal pressure, statistical scatter, defects, grind repairs
• Field Monitoring Software used to Assess Every Section of Tubing Along the Entire String – Avoids failures in the field
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Conclusions
• Defects can have first order influence (cut)– Or negligible influence (impressed)
• Repairing by Grinding is a viable option• Software helps to quantify defect severity
(and effectiveness of a grind repair )• 3D Laser Scanning
– Holds the potential to detect defects– Capable of measuring every detail about their
geometry
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ConclusionsTo Avoid Failures on Your Job
.... or In Your well:• Control the Factors that Contribute to
Fatigue Damage Accumulation– Minimize tubing diameter– Maximize spool and
gooseneck sizes– Select material most
suited for application– Minimize tripping– Minimize tubing movement
with high pressure
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Conclusions• Avoid Defects
– Eliminate sources of defects - handling, injector debris…
– Monitor/Inspect– Environmentally
Induced Cracking• Storage corrosion
– Repair shallow defect using grind repair techniques
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Why Coiled Tubing Fails and How to Avoid Failure on Your Job
... in Your WellSteven M. Tipton
The University of Tulsa
918-850-0252
www.coiledtubingutulsa.org
Why Coiled Tubing Succeeds and How to Assure Success on Your Job
... in Your Well
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