coiled tubing loads and forces
DESCRIPTION
N/ATRANSCRIPT
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Coiled Tubing Loads and Forces
• Tensile
• Burst
• Collapse
• Torsion• Torsion
• Cyclic Fatigue
• Modeling
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Loads
• Tensile (last section and in deep well section)
• Burst (last section and in high pressure
section)
• Collapse• Collapse
• Buckling (defered to deviated well section)
• Torsional (nope, not a typo)
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Tension
• Weight produces stretch
• Increased by BHA weights
• Increased by friction on POOH
• Offset to some degree by well fluids• Offset to some degree by well fluids
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Uniaxial Tension
A
B D
σy
σAPI
σ=F/AF
A
E E
1 1
C 0.005 ε=δ/L
L
δ
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Tension failure mode for CT in the laboratory.
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Collapse more
common than neck
down
The collapse failure is more
common in the field because of
CT ovality and annular pressure
reducing collapse resistance.
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Axial Load Capacity
• The one-dimensional axial load capacity of the
tubing is considered to be the tension load
that will produce a stress in the tubing equal
to the minimum yield.to the minimum yield.
Ly = SyA
where: Ly = CT load cap. at yield, lbs
Sy = yield strength of the CT, psi
A = x-sect. area of CT, in2
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Load Capacity Example
• For a 1.5”, 0.109 wall CT of 70,000 psi yield
strength steel, the one-dimensional load
capacity at yield is:
Ly = 70,000 psi x 0.476 in2= 33,320 lb
an 80% operating factor is common..…
• Ly = (0.8)*33,320 = 26,656 lb
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Operating Safety Factor Suggestions
• 0.8 under best conditions - new strings, especially high strength strings
• 0.5 to 0.7 for field welds
– 0.7 for welds in lower section – 0.7 for welds in lower section
– 0.5 for welds in upper section
– 0.5 for questionable welds
• 0.4 to 0.5 for corroded strings
– consider refusing the string if corrosion severe
– refuse string if any evidence of pin holes
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Welds
The heating that occurs during the welding process
will cause the weld metal and the heat affected
zone around the weld to be physically different
from the surrounding, original metal.
An anode is created by this difference.
An anode can start here or here.
Heat
affected
zone
Weld metal (added
and different from
original base
metal)
Base metal
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Simplistic Depth Limits
Le = Ly(80%)/W
where:
Le = equivalent string length
Ly (80%) = 80% of CT load capacity
W = tubing weight (effective), lbs/ft
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Examples of Depth (length) Limits of 1.5" CT (no buoyancy)
CT OD wall weight yield 80% yield max string
(in) (in) (lb/ft) strength load length in air
(psi) (lbs) (ft)
1.5 0.095 1.426 70,000 23,482 16,466
Depth Limits, without buoyancy
1.5 0.095 1.426 70,000 23,482 16,466
1.5 0.109 1.619 70,000 26,672 16,474
1.5 0.134 1.955 70,000 32,200 16,470
1.5 0.087 1.313 100,000 30,896 23,531
1.5 0.109 1.619 100,000 38,104 23,536
1.5 0.134 1.955 100,000 46,000 23,529
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Other factors that figure in….
• POOH loads are increased by:
– frictional drag forces along walls
– frictional drag in fluids
– bending loads through deviated sections– bending loads through deviated sections
– BHA weights
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Weight Indicator Load -Verification
4500
9000 Measured
RIH (model)
POOH (model)
RE
AD
ING
(lb
f)
4,000 8,000
500
4500
MEASURED DEPTH OF STRING (ft)
WE
IGH
T IN
DIC
AT
OR
R
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Internal Yield Pressure (Burst)
PB = 2 (t wall-min)Sy/OD
Where:Where:
PB = internal yield or burst pressure, psi t wall-
min = thinnest wall, in
Sy = yield strength of the CT, psi
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Burst Pressure:
This one is really tricky!
• Depends on:
– CT size
– CT wall thickness
– CT strength– CT strength
– damage (dents, corrosion, ovality, fatigue)
– offsetting pressure (it’s a differential)
– mechanical loads? - (compression? - usually not a
factor)
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Theoritical Burst Calc. with Round TubeCT OD wall Yield Burst (theory)
(in) (in) (psi) (psi)
1.25 0.095 80,000 12160
1.25 0.095 70,000 106401.25 0.095 70,000 10640
1.25 0.075 70,000 8400
1.25 0.125 70,000 14000
1.25 0.156 70,000 17472
1.25 0.151 80,000 19328
The problem is that the tube isn’t round.
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Theoritical Burst Calc. with Round TubeCT OD wall Yield Burst (theory)
(in) (in) (psi) (psi)
1.25 0.151 70,000 16912
1.5 0.151 70,000 14093
1.75 0.151 70,000 12080
2 0.151 70,000 10570
2.375 0.151 70,000 8901
2.875 0.151 70,000 7353
3.5 0.151 70,000 6040
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Burst Press as function of YS
20,000
25,000
The Variation of Theoretical Burst in New, Round Pipe and
Yield Strength with Tension Load
0
5,000
10,000
15,000
0 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000
Tubing Load, psi
Bu
rst
Pre
ss, p
si
110 ksi
90 ksi
70 ksi
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Collapse Pressures
• Derated by tension
• charts are not accurate - tube not round
– One of the biggest misrepresentations in the CT
data is that of collapse pressure data. data is that of collapse pressure data.
– Personal Opinion - use these charts as the best
possible case and derate the prediction at least
30%.
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g22.tif
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Ovality
• Diameter increases most along sides and walls
thin proportionally.
• Ovality creates unequal stress on CT.
• Some total diameter swell• Some total diameter swell
Ovality = (OD max - OD min)/OD spec.
Solution? Measurement, Testing, Life models
and, oh yeah, Experience.8/25/2015 22
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COIL OVALITY
Ovality Vs Collapse Pressure
8000
10000
12000
Co
llap
se P
ressu
re (
psi)
120,000 Psi Yield
69000 lb String Weight
0
2000
4000
6000
0 1 2 3 4 5
Percent Ovality (%)
Co
llap
se P
ressu
re (
psi)
2"- .204 Wall
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Collapse Press as Function of YS
-4,000
-2,000
0
0 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000
-20,000
-18,000
-16,000
-14,000
-12,000
-10,000
-8,000
-6,000
Tubing Load, psi
Co
llap
se P
ress, p
si
70 ksi
90 ksi
110 ksi
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CT Collapses
• CT collapses from a few feet to over 1100 ft
have been reported. The problem is that CT is
often operated right on the edge of material
strength so any disturbance spike (sudden strength so any disturbance spike (sudden
application of load) that can push it to
collapse may trigger a collapse in several
hundred feet of tube - like a run in hose.
• Remember, tensile force changes as well
unloads?
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Worst (?) Cases
1. High annular
surface pressure
2. Long CT string
3. Heavy BHA
4. Large diameter
Ps
Most severe problem jobs
for CT collapse:
1. POOH with any BHA
2. POOH through severe
dogleg
3. Fishing (and jar action)
4. Trying to free stuck
Wt
Friction
BHA
5. Viscous annular
fluids
6. Highly ovaled or
damaged CT
strings or sections
7. Corrosion
4. Trying to free stuck
tubing
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Accuracy Problems
• For any constant shape and size piece of pipe,
an expression or method of prediction for
tension, collapse, or burst can be generated.
BUT, CT is a reel of variences handled by a BUT, CT is a reel of variences handled by a
system of extremes. The best we can do are
estimations.
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Torsion Yield Strength
Ty = Sy(OD4 - (OD - 2 t wall-min)4)/105.86 OD
Where:
T = Torsion Yield Strength, lb-ftTy = Torsion Yield Strength, lb-ft
t wall-min = thinnest wall, in
Sy = yield strength of the CT, psi
OD = CT OD
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Theoritical Torsion Strength vs CT OD for 0.151" Wall Thickness
8000
9000
10000
Th
eo
rit
ica
l T
ors
ion
Str
en
gth
, p
si
Torsion Strength for CT
Why bother with
torsion for CT?
0
1000
2000
3000
4000
5000
6000
7000
8000
1 1.5 2 2.5 3 3.5 4
CT OD, inch
Th
eo
rit
ica
l T
ors
ion
Str
en
gth
, p
si
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Torque
• Usually we don’t push the torque limit in
workovers
– need to rotate is very limited
– smaller motors are very limited in torque output– smaller motors are very limited in torque output
• This changes in CT Drilling, especially with big
motors
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Fillup
• Volumes vary with OD and wall thickness
• Remember, the volume of CT is not just what’s
in the well - it includes what’s on the reel.
• Friction can be a killer when rates are needed • Friction can be a killer when rates are needed
- remember: reel + well.
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1.2
1.4
1.6
1.8
Barr
els
Fill U
p p
er
1000 f
t.Barrels of Fill Up Volume Per 1000 ft of Coiled Tubing
(Remember to use entire reel length)
1-3/4”
1-1/2”
0
0.2
0.4
0.6
0.8
1
0 0.05 0.1 0.15 0.2
Barr
els
Fill U
p p
er
1000 f
t.
Coiled Tubing Wall Thickness, in.
1-1/2”
1-1/4”
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Force Application on CT
• Force to push CT through stuffing box/stripper
(opposite running)
• Force on CT from Well Head Pressures -
(upward)(upward)
• Force to overcome friction (opposite running)
• Force from weight of CT & BHA (downward)
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Other Forces and Loads
• Pressure Effects on Length/Force
• Temperature Effects on Length/Force
• Stretch
• Buckling loads• Buckling loads
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Swab/Surge Forces
• “Plunger force” - tremendous force exerted
event in small movements because of large
area affected.
• Close clearances and high tool movement • Close clearances and high tool movement
speeds increase the swab/surge force
• Circulation while pulling lessens swab/surge
loads
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Swab Forces
• Problems
– small hole volumes
• small gas influx causes large underbalance - get in trouble quickly
– large BHAs - swab force increased sharply– large BHAs - swab force increased sharply
– continuous, fast movement of CT
– horizontal holes
• gas storage area - isn’t apparent on surface gauge quickly - must monitor trip tanks.
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Swab Effect From Pipe Speed
Hole Size, in.
360 245 180 120
Pipe pulling Speed, fpm
360 245 180 120
8.5 276 167 124 98
6.5 589 344 256 192
5.75 921 524 394 289
14 lb/gal mud, 4.5” BHA
Adams8/25/2015 37
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CT Stretch - W/Buoyancy Effect
Selastic = 12 L Fbouyancy / A E
Where:
Selastic= elastic stretch of CT per 1000’, in.
F = corrected pull on tubing, lbFbouyancy = corrected pull on tubing, lb
L = tube length (where load applied), ft
A = cross sectional area of tubing
E = modulus = 30 x 106 psi
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Stretch Example for 5000 ft CT With and Without Load
Fluid Added CT CT
Weight Length Density Air Wt. Bouyed Load Stretch Stretch
CT OD Wall, in Area, in2
lb/ft (ft) (lb/gal (lbs) Wt, (lbs) (lbs) inches ft
1.25 0.109 0.391 1.33 5000 1.9 6640 6447 0 33.0 2.75
1.25 0.109 0.391 1.33 5000 1.9 6640 6447 500 35.5 2.96
1.25 0.109 0.391 1.33 5000 8.33 6640 5794 0 29.6 2.471.25 0.109 0.391 1.33 5000 8.33 6640 5794 0 29.6 2.47
1.25 0.109 0.391 1.33 5000 8.33 6640 5794 500 32.2 2.68
1.25 0.109 0.391 1.33 5000 10 6640 5625 0 28.8 2.40
1.25 0.109 0.391 1.33 5000 10 6640 5625 500 31.3 2.61
1.25 0.109 0.391 1.33 5000 12 6640 5422 0 27.7 2.31
1.25 0.109 0.391 1.33 5000 12 6640 5422 500 30.3 2.52
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Stretch Example for 5000 ft CT With and Without Load
Fluid Added CT CT
Weight Length Density Air Wt. Bouyed Load Stretch Stretch
CT OD Wall, in Area, in2 lb/ft (ft) (lb/gal (lbs) Wt, (lbs) (lbs) inches ft
1.5 0.109 0.476 1.62 5000 1.9 8095 7860 0 33.0 2.75
1.5 0.109 0.476 1.62 5000 1.9 8095 7860 500 35.1 2.93
1.5 0.109 0.476 1.62 5000 8.33 8095 7064 0 29.7 2.471.5 0.109 0.476 1.62 5000 8.33 8095 7064 0 29.7 2.47
1.5 0.109 0.476 1.62 5000 8.33 8095 7064 500 31.8 2.65
1.5 0.109 0.476 1.62 5000 10 8095 6857 0 28.8 2.40
1.5 0.109 0.476 1.62 5000 10 8095 6857 500 30.9 2.58
1.5 0.109 0.476 1.62 5000 12 8095 6610 0 27.8 2.31
1.5 0.109 0.476 1.62 5000 12 8095 6610 500 29.9 2.49
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Stretch Example for 5000 ft CT With and Without Load
Fluid Added CT CT
Weight Length Density Air Wt. Bouyed Load Stretch Stretch
CT OD Wall, in Area, in2
lb/ft (ft) (lb/gal (lbs) Wt, (lbs) (lbs) inches ft
2 0.109 0.648 2.20 5000 1.9 11005 10685 0 33.0 2.75
2 0.109 0.648 2.20 5000 1.9 11005 10685 500 34.5 2.88
2 0.109 0.648 2.20 5000 8.33 11005 9603 0 29.6 2.472 0.109 0.648 2.20 5000 8.33 11005 9603 0 29.6 2.47
2 0.109 0.648 2.20 5000 8.33 11005 9603 500 31.2 2.60
2 0.109 0.648 2.20 5000 10 11005 9322 0 28.8 2.40
2 0.109 0.648 2.20 5000 10 11005 9322 500 30.3 2.53
2 0.109 0.648 2.20 5000 12 11005 8986 0 27.7 2.31
2 0.109 0.648 2.20 5000 12 11005 8986 500 29.3 2.44
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CT in Horizontal Wells
1. Excellent method for spotting fluids
2. Reasonable method for setting equipment
and tools
3. Fair method for unloading
Sticking Points
1. Bend area
2. Lateral8/25/2015 42
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Max tool length through the bend
area….
Max length of stiff pipe or tool…
L=1/6[R2 - (R - ^d)2]1/2
where:
L = tool length, ftL = tool length, ft
R = curve radius, inches
^d = ID casing - OD tool (inches)
8/25/2015 44George E. King Engineering
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Prejob• Modelling
• Friction reducer - damage to reservoir?
– lubri beads
– pipe straighteners
• Obtain up-to-date completion drawing
• Check recent interventions (write-ups and talk to engineers)
– problems
– debris
– restrictions etc.
• Coil
– condition (derations) and footage
– previous history e.g. if cement then not suitable for acid
– drift
• Obtain well flowing data (rates, pressures, temperature etc.)
– slugging or dying tendency?8/25/2015 45
George E. King Engineering
GEKEngineering.com