Casing Design 1.ppt

Casing and Casing Design
: Introduction
 Casing seat selection determines the total no.
Of casings required in a well
 Casing seat selection also determines the
depth of each casing.
 Bore-hole geometry determines the hole size
and their corresponding diameters of casings

: Objective
 Depth and diameter of a casing is known from
previous exercises
 The selected casing must withstand various
loads which might impose on casings during
operations/ entire life
 The designed casing must be as economical
as possible
 Casing design determines the grade(s),
nominal weight(s), and types of thread required
for a particular well considering safety & cost-
effectiveness

: Influencing Factors
 Loading conditions during drilling and
production
 Formation strength
 Thermal effects
 Corrosive environment
 Hole irregularities
 Availability of casings

Casing And Casing Design
: Input Data
 Formation & Fracture pressure profile
 Location of lost circulation and permeable
zones
 Location of salt zone
 Type of well (vertical/ directional/ horizontal)
 Temperature profile
 Presence of H2S, CO2 & Nacl

 Minimum hole size required
 Type of completion
 Sp. Gravity of packer fluid
 Worst case loads that may occur during
completion/production/ work over
operations
 Availability of casings / inventory
 Regulatory requirements

: Design Criteria
 Burst
 Collapse
 Axial tension / compression
 Biaxial
 Bending
 Buckling
 Corrosion

: Loading Conditions – Burst
BURST CONDITION
WHEN pi > pe
Pi
Pe
INTERNAL PRESSURE(LOAD) = pi
EXTERMAL PRESSURE(BACK-UP) =
pe
Net stress imposed on
casing or ‘resultant’
=load – back up = pi - pe

Conditions–collapse
Pi
Pe
EXTERNALPRESSURE(LOAD) = pe
INTERNAL PRESSURE(BACK-UP) = pi
COLLAPSE CONDITION
WHEN pe > pi
NET STRESS IMPOSED ON
CASING OR ‘RESULTANT’
=LOAD – BACK UP = Pe - Pi

: Loading Condition-tension
 Most axial tension arises from weight of
casing itself.
 Other tension loadings can arise due to
bending, drag, shock loading and during
pressure testing.
 Increase in temperature and pressure
can impose tension loadings in casing

Casing and casing design
casing design: safety factors
 Because of uncertainties in determining actual
loadings and as well as casing properties a
factor is used to allow for such uncertainties and
to ensure that casing properties always remain
greater than loadings.
 This factor is called ‘design factor’ or ‘safety
factor’
 Safety factor is defined as the ratio of rating of
casing and resultant loadings

: Safety factors (Contd.)
 For example, safety factor in burst
=
Burst resistance of casing
Resultant burst loading

: Safety Factors(contd.)
 Oil industry has no uniform policy on safety
factors of casing design
 Safety factors are normally decided by the
individual company in accordance of their
company policy.
 Following safety factors are used in ongc
(I) BURST – 1.1 to 1.125
(Ii) COLLAPSE – 0.85 (cemented portion)
-1.125 (uncemented portion)
(Iii) TENSION – 1.8 (without buoancy)
- 1.6 ( with buoyancy)

: Design approaches
 In oil industry, various approaches to design
casing are followed.
 However, two most widely used approaches
are ;
(i) Conventional
(Ii) Maximum load concept
 Approaches are different from one another
due to different assumptions in loads and
back ups

: Load Determination
(CONV-BURST)
(SURFACE-INTER-PROD)
CEMENT
CSD
SURFACE
NEXT SHOE
OPEN HOLE
Assumptions:
-A kick generates at next shoe
depth
-Mud inside casing & open
hole is thrown out by gas
and casing is full of gas
inside

: Assumptions
Back-up:
- Barytes in mud behind
casing would be settled
at bottom in course of
time and thereby saline
water column would
remain in annulus
CSD
SURFACE
NEXT SHOE
OPEN HOLE
SALINE
WATER

: Computation
CSD
SURFACE
NEXT SHOE
OPEN HOLE
SALINE
WATER
Load at surface -1 =
Formation pressure at next shoe
depth
e .0001138 x 0.65 x depth (metres))
Load at surface - 2 =
Formation pressure at next shoe
depth
- Hydrostatic pressure of gas
column

: Computation
SALINE
WATER
CSD
SURFACE
NEXT SHOE
OPEN
HOLE
Load at surface – 3 =
Fracture pressure at casing shoe depth
e .0001138 x 0.65 x depth (metres)
Use greater of the three above
in case of exploratory well for
safety and minimum one for
development well

: Computation
LOAD AT CASING SHOE
USING EQUILATERAL
TRIANGLE
BC
DE
Next shoe
depth
CSD
0
Pressure
Load at surface
Load at CSD
0
A B
C
D E
F
=
FB
FD
LOAD AT CASING SHOE
= GD + DE
G

: Computation
BACK UP AT SURFACE = 0
BACK UP AT CSD
=HYDROSTATIC PRESSURE OF SALT WATER
= 0.052x CSD X SP. GRAVITY OF SALT WATER
(PSI)
= CSD X SP. GRAVITY OF SALT WATER ÷ 10
(KG/ CM2 )

: Computation
RESULTANT
RESULTANT AT SURFACE = SURFACE PRESSURE – 0
RESULTANT
AT CSD = LOAD AT CSD– HYDROSTATIC PR. OF
SALT WATER

: Load Lines
GRAPHICAL REPRESENTATION
Load at surface
Load at CSD
Load line
Back up
line
Resultant
Pressure

COLLAPSE

(CONV- COLLAPSE) (SURFACE-INTER-PROD)
CSD
Active mud
in annulus
Casing
Load at surface = 0
Load at CSD ( in kg/cm2) =
hydrostatic Pr. Of mud used
during casing lowering
Load at CSD ( in kg/cm2) =
depth(m) X mudweight(gm/cc)/10

BACK-UP (CONV- COLLAPSE) (SURFACE-
INTER-PROD)
CSD
Active mud
in annulus
Casing
Assumption:
Casing is totally empty
Inside due to mud loss
During drilling next phase
In case of surface &
Intermediate casing.
In case of production
Casing, assumption is
Same but due to artificial
Lift & plugged formation

: Load Lines
GRAPHICAL REPRESENTATION (COLLAPSE)
Load at surface = 0
Back up at surface = 0
Resultant at surface = 0
Load at CSD = Hyd Pr. Active mud
Back up at CSD = 0
Resultant at CSD = Hyd. Pr of
Active mud
Load line = resultant
CSD
Pressure
Load line
Back up line
0

TENSION

: Determination
 Tension load is primarily due to the casing’s
own weight
 Tension load increases during pressure
testing of casing.
 Tension load also increases due to increase in
temperature
 Increase of sp. Gravity of mud both outside
and inside of casing increases tension in
casing.

: Computation
Tension load = weight of casing in air/ unit
length x depth
= Kg/ m x depth in metre (kgs)
= PPF(Lbs /ft) X Depth (metre) x1.489 (Kgs)

: Computation
Other axial loads – shock load
 Shock loading is often expressed as
F shock = 3200 wn lbs where, wn = ppf
= 1450 wn kgs where, wn = kg/m
 Considering average running speed of
185 ft/min or 56 metre/min

: Computation
Other axial loads – bending force
 Bending force fb = 63 dwn lbf
Where, D = OD in inch
Wn = nominal weight, ppf
 = rate of angle change/ 100ft.

: Computation
Other axial loads – temperature
 CHANGE IN AXIAL FORCE DUE TO
TEMPERATURE CHANGE = - E t
Where, E = young’ modulus of steel
= 30 X 106 psi for steel
 = Thermal coefficient of expansion
= 6.9 x 10-6 0F-1
T = average change in temperature( 0F)

: Computation
 Normally, shock load & bending loads are not
considered unless specific conditions are
expected in well.
 Also, in general, temperature will typically
have a secondary effect on tubular design
 These loads are not generally considered in
casing design

BIAXIAL

: Bi-axial
 All pipe strengths are based on uniaxial stress
state.
 Pipe in the well bore, however, is always
subjected to combined loading conditions.
 Fundamental basis of casing design is that if
stress in pipe wall exceed yield strength of
material, a failure condition exists
 Hence, yield strength is a measure of
maximum allowable stress.

: Bi-axial
 Published collapse resistances of casings are
under zero axial load.
 Axial tension reduces the yield strength of
material.
 In three modes out of four modes of collapse
resistances equations, except elastic collapse,
collapse strength is directly proportional to the
yield strength of material.
 It follows that tension decreases both yield
strength and collapse resistance of casing.

: Computation
 Graphical representation of hoop stress-axial
stress on % of yield biaxial ellipse is available.
 For easy application, a table comprising
factors ‘x’ and ‘y’ is calculated from the above
ellipse and readily available
 Reduced collapse resistance of casing under
axial loading can be determined from this
table

: Computation
Determination of reduced collapse resistance
of casing under axial loading using ‘x’&‘y’
factor
 X = axial load / pipe body yield strength
 Obtain value of ‘y’ from table corresponding to
‘x’
 Reduced collapse strength
= published collapse strength x ‘y’

: Comments On Biaxial Stress
 Neither approach is rigorous treatment of the
topic
 Depending on the type of load, burst & collapse
rating of zero axial stress increases or
decreases
 Tensile loads increases burst rating but
decreases collapse rating
 Compressive loads increases collapse rating
but decreases burst rating

: example
Design the casing using conventional
approach with the following input data:
(a) Casing size : 9-5/8”
(b) Casing shoe depth : 3000 m
(c) Next casing shoe depth : 4200 m
(d) Formation pressure at 3000m : 1.32 mwe
(e) Formation pressure at 4200 m : 1.6 mwe
(f) Sp. Gravity of mud during lowering : 1.36
(g) Sp. Gravity of mud in next phase : 1.65

: Example
(h) Fracture pressure at 3000m : 1.8 mwe
(i) Type of well : vertical/ exploratory
(j) Following casing are available:
N-80, 53.5 ppf, BTC– 2000 m
N-80, 47 ppf, BTC – 1500m
N-80, 43.5 ppf,BTC – 2000m
CONSIDER FOLLOWING SAFETY
FACTORS :
BURST – 1.1,collapse – 1.125
Tension – 1.8 (neglecting buoyancy)
biaxial effects are to be considered

: Burst
SOLUTION Inside Pressure
FORMATION PR. AT 4200M = 1.6 X 4200
10
= 672 Kg/ Cm2
SURFACE PR. =
672
e .0001138 X .65 X 4200
= 492 Kg/ Cm2
LOAD AT 3000 M
X=
180 X 3000
4200
= 128
= 492 + 128
= 620
3000
x
492
492
4200
672
180
1200
Kg/ Cm2

: Burst
0
492 - =
10
BACK UP AT SURFACE =
BACK UPAT CSD =
RESULTANT AT SURFACE = 0 492 Kg/cm2
RESULTANT AT CSD = 620 - 321 = 299 Kg/ cm2
1.07 X 3000 = 321Kg/ cm2
Outside Pressure

: Collapse
COLLAPSE LOAD AT SURFACE = 0
COLLAPSE LOAD AT CSD =
1.36 X 3000
10
= 408 Kg/ cm2
COLLAPSE BACK UPAT SURFACE = 0
COLLAPSE BACK UPAT CSD = 0
RESULTANT AT SURFACE = 0
RESULTANT AT CSD = 408 - 0 = 408 Kg/ cm2
Outside Pressure
Inside Pressure

: Graphical Representation
PRESSURE
3000
0 492
299
Resultant
burst
408
Collapse load
line
Collapse - backup
Collapse load line
= Collapse resultant
620
321
Burst load
line
Burst back up

: Graphical Representation
PRESSURE
3000
0 492
299
Resultant
burst
408
Collapse load
line
Collapse - backup
Burst back up
Equation of
resultant line is
y = 15.54x - 7645

CASING AND CASING DESIGN
: Selection Of Casing
Bottoms Up Casing Selection is Preferable. As such
minimum collapse pressure required for casing
= 408 x 1.125 = 459 Kg/ cm2
From Data Table, available casing with this collapse
resistance is N-80, 53.5 #
Next lower grade available casing is N-80, 47 # and
collapse rating of this casing is 334 Kg/ cm2. From
graph or calculation shown below, this casing can be
lowered up to 334 x 10
1.125x1.36
= 2183  2180 M
So, 2180 – 3000 : N-80. 53,5 #

: Biaxial Effects
Depth of N-80, 47# needs correction for Bi-axial effect
Maximum collapse effect is at 2180 M.
P.B.Y.S OF 47# CASING = 492 X 103 Kgs
TENSILE LOAD AT 2180 M= (3000-2180) x 53.5 x 1.488 = 65.28 x 103
Kgs
FACTOR ‘X’ =
65.28x 103
Kgs
492 x 103
Kgs
= 0.132
CORRESPONDING ‘Y’ VALUE = 0.958
COLLASE RATING AT ZERO AXIAL STRESS = 334 Kgs/ cm2
COLLAPSE RATING UNDER TENSILE LOAD
= 0.958 x 334 = 320 Kgs / cm2
REVISED COLLAPSE DESIGN FACTOR UNDER TENSILE LOAD
= 320 / 296 = 1.08 NOT SAFE

: Biaxial Effects
FACTOR ‘X’ =
72.44x 103 Kgs
492 x 103 Kgs = 0.147; ‘Y’ VALUE = 0.951
REDUCED COLLAPSE RATING = 0.951 x 334 = 317 Kgs
/ cm2
Casing could be lowered to:
From graph or calculation shown below, this casing can
be lowered up to
320 x 10
1.125x1.36
= 2091  2090 M
317 x 10
1.125x1.36 = 2071  2070 M
Taking L = 2050 M
Net Collapse Pressure at 2050 M = 2050 x 1.36 = 279 Kgs/ cm2
10
Again, length and hence weight has increased. It is
an iterative process. It needs to be done once or
twice.

: Biaxial Effects
Reduced Collapse Resistance due To Biaxial Load at
2050 M
X =
75.62x 103 Kgs
492 x 103
Kgs
= 0.153;‘Y’ VALUE = 0.950
Reduced collapse rating = 0.950 x 334 = 317 Kgs / cm2
Revised collapse design
factor = 317 / 279 = 1.136 Hence safe
Burst and tensile S.F. are much higher than desired
So, 2050 – 3000 : N-80. 53,5 #

: Burst
Next depth to which N-80, 47 # could be used for Burst and
Tension need to be checked. Burst rating = 483kg/cm2
Considering S.F.burst 1.1 the resultant burst load to which
the casing can be subjected to 483/ 1.1 = 439kg/cm2
From the similar triangle
ADE and ABC
AC/AE= BC/DE
AC =
3000 (492-439)/ (492-299)
Depth at which resultant
burst press 439kg/cm2
exists
=823M or 820M
O
D
O
A492
B C
321
299
439
E
3000
4200
Press kg/cm2
Depth M
672

So, N-80, 47 # can be used below 820M, i.e 2050-
820M
Thereafter N-80, 53.5# having Burst resistance
= 558 kg/cm2 can be used up to surface as it is
> the required pressure of 492x 1.1 =541
kg/cm2
So, 0 –820M: N-80, 53.5#
: Burst

: Selection
Casings which are selected are as follows :
0 to 820 M N-80. 53.5#
820 to 2050 M N-80 47 #
2050 to 3000M N-80 53.5#

: Burst & collapse
DEPTH Safety factors
(burst)
Safety factors
(collapse)
53.5# 47# 53.5# 47#
0 558/ 492
= 1.13
- - -
820 558/439
=1.27
483/439
=1.1
412/111
=3.71
288/111
=2.59
2050 558/360
= 1.55
483/360
=1.34
445/279
=1.59
317/279
= 1.136
3000 558/299
=1.86
- 465/408
=1.139
-

: Tension
TOTAL WEIGHT OF CASING IN AIR
= WEIGHT OF 53.5# (820M) + WEIGHT OF 47# (1230M)
+
WEIGHT OF 53.5# (950M)
= (53.5 x 820 + 47 x 1230 + 950 x 53.5) x 1.488 Kgs
= 226 927 Kgs
= 227 Tonne

: Tension
PIPE BODY YIELD, JOINT STRENGTH AND
TENSION SAFETY FACTORS
DEPTH TENSION
LOAD
( x103)
RATINGS SAFETY
FACTORS
Nom. Wt.
(# ppf)
P.B.Y.S
( x103)
Jt. Strength
(x103) BTC
0 227 53.5 563 601 2.48
820 162 53.5 563 601 3.47
820 162 47 492 526 3.03
2050 76 47 492 526 6.47

: Summary
Depth
(Mts)
Casing
(9 5/8” )
Collapse
S.F. (Min)
Burst S.F.
(Min)
Tensile
S.F. (Min)
0 - 820 N-80, 53.5 # 3.71 1.13 2.48
820 -
2050
N-80, 47 # 1.136 1.1 3.03
2050 -
3000
N-80, 53.5 # 1.139 1.55 High

Reduced collapse resistance of N-80, 53.5# at 2050 M
X =
: Annexure I
75.62x 103 Kgs
563 x 103
Kgs
= 0.134; ‘Y’ VALUE = 0.957
REDUCED COLLAPSE RATING = 0.957 x 465 = 445 Kgs / cm2
563 x 103
Kgs
Reduced collapse resistance of N-80, 53.5# at 820 M
X =
(950 x 53.5 + 1230 x 47) x 1.488
= 0.287; ‘Y’ VALUE = 0.886
REDUCED COLLAPSE RATING = 0.886 x 465 = 412 Kgs / cm2
Reduced collapse resistance of N-80, 47# at 820 M
X =
(950 x 53.5 + 1230 x 47) x 1.488
= 0.328; ‘Y’ VALUE = 0.863
REDUCED COLLAPSE RATING
Collapse Pressure at 820 M = 820 x 1.36 / 10 = 111Kgs / cm2
= 0.863 x 334 = 288 Kgs / cm2
492 x 103
Kgs

Casing Design 1.ppt

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Casing Design 1.ppt