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1
SPE Distinguished Lecturer Program
The SPE Distinguished Lecturer Program is funded principally
through a grant from the SPE Foundation.
The society gratefully acknowledges the companies that support
this program by allowing their professionals to participate as
lecturers.
Special thanks to the American Institute of
Mining, Metallurgical, and Petroleum
Engineers (AIME) for its contribution tothe program.
Society of Petroleum Engineers
Distinguished Lecturer Programwww.spe.org/dl
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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 Well
or
Steven M. TiptonThe University of Tulsa
Society of Petroleum EngineersDistinguished 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 developCT test machine
1995 - developed coiled tubing fatigue module for
NOV-CTES (Achilles) 1996 - 2000 University of Tulsa JIPs to study
CT 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
http://../Tipton%20Stuff/For%20Dr.%20Tipton%20from%20Blount/Why%20CT%20Fails%20Global%20Symposium08/CTManufacturing.wmv7/23/2019 CT Fatigue Life
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Typical CT Rigup
Coil Tubing
Reel ~ 10-20K ft
Gooseneck
Guide Arch
Injector
What is it used for?
Anything jointed
tubulars 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 stretches
in Tension or shortens in Compression
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repeats each tripspool
straight straighthole
arch arch
spool
Bending Cycles
time
strain
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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
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Huge Cyclic Plastic Strains (>2%!)
Bending Strains
Ratcheting
Incrementaldiametergrowth >30%
Lives < 10 cycles!
Even withpressure below50% 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
x,m = r/R
r
R
Bending Strain Distribution
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Bending Stress Distribution
StressSy
-Sy
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Bending Stress Distribution
Stress
Sy
-Sy
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Causes Permanent Elongation
Stress
F
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CT Fatigue
Research
CoilLIFE
FatigueTest
Machine
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CT Fatigue
Research
CoilLIFE
FatigueTest
Machine
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CT Fatigue
Research
CoilLIFE
FatigueTest
Machine
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TU CT Fatigue Test Facility
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Accelerated CT Fatigue Test Machine
F i C k D l
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5 4 3 2 1 1 2 3 4 5 6 7 8
NUMBER OF
FAILURESNUMBER OF
FAILURES
Fatigue Crack Development
F i C k D l
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Fatigue Crack Development
MOST fatigue
cracks initiate
at the inner surface
Fatigue behavior of CT
defies engineering logic!
Appears as
a pinhole
failure
cross sectionthrough 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)
Cycle
sto
Failur
e
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
f , b , f, S=f(h, Sy, x)
E, K, n
LCFTesting
Constant PressureFixtureTesting
FittingRoutine
Plasticity / Fitt ingRoutine
MATERIAL DATA SETLCF Data
CT Fatigue Model
x
x
time
ComplexField Loading
time
P
Coil Tubing Parameter
time
D
inputinput
output
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0
100
%
LifeExpe
nded
Distance along String
Fatigue Life Profile
Implemented with String Management Software:
CT Life Prediction
warning level
Fatigue eliminated as a primary failure mode!
Wh t Ab t D f t ?
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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
d wx
M f I iti
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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
efectLifetoBaselineLife
Impressed (t=0.156)
Milled (t=0.156)
Impressed (t=0.204)
Milled (t=0.204)
scatter
Depth / Wall Thickness Ratio
Ratioof
DefectLifetoBaselineLife
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
R i i D f t
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Repairing Defects
One Option: Butt Welding
Unfinished inside of the butt
weld constitutes a severestructural discontinuity!
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Butt Welding Fatigue Data
0
10
20
30
40
50
60
70
80
90
%o
fUnweldedS
ampleLife
Manual Welds:
average = 38%
std. dev = 18%
Orbital Welds:
average = 41%
std. dev = 11%
Manual AverageOrbital Average
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Grind Repairing
replaced with
smooth, 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
Cant provide any information about defect
size or severity
3D Laser Imaging for NDE Complete geometry of defect is captured
digitally
3D Laser Imaging S stem
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3D Laser Imaging System
(Low-Cost)
Longitudinal Milled Defect
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Commercial Laser System
Sawcut
ImpressedballMilled ball
MilledCylinder
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Impressed ball Machined ball
Color-coded Digital Data
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w
d
Ap
Automated Defect Measurements
rmax
rmin
C l d d Di it l D t &
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Color-coded Digital Data &
Photographic Image
Useful todistinguish
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
Manufacturing and Materials
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Manufacturing and Materials
defects/flawsFrom 7 to 20 reels per
1,000 reels (~last 3 yrs)0.7 to 2%
Only Those Reported
Failures occur that are
not fully investigatedEspecially 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 EntireString 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
C l i
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ConclusionsTo Avoid Failures on Your Job
.... or In Your well:
Control the Factors that Contribute toFatigue Damage Accumulation
Minimize tubing diameter
Maximize spool andgooseneck sizes
Select material mostsuited for application
Minimize tripping
Minimize tubing movementwith high pressure
C l i
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Conclusions
Avoid Defects Eliminate sources of
defects - handling,injector debris
Monitor/Inspect
EnvironmentallyInduced 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 Well
Steven 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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Distinguished Lecturer Programwww spe org/dl 56
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