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Casing Design
University of Petroleum,China
By Jimmy Wang
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Content
section 1 functions of casing
section 2 casing types
section 3 strength properties
section 4 casing specification
section 5 casing design
section 6 other considerations
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The general picture of casing
After this topic, we should know the
following:
1 the functions of oil well casing
2 the various types of casing strings
3 the procedure used in the design of
casing strings
Purpose
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Casing
Casing string
Surface casing
Intermediate casing
Production casing
Liner
Drilling liner
Tube
Formation
Tensile force
Collapse strength
Collapse pressure
Collapse resistance
Burst strength
Burst pressure
Burst resistance
Compression load
Pressure coefficient
Technical words
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section 1 Functions of Casing
1 to keep the hole open and to provide a
support for weak, or fractured formations;
2 to isolate porous media with different
fluid/pressure regimes from contaminating the
pay zone;
3 to prevent contamination of near-surface
freshwater zones;
Can you give
some functions?
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4 to provide a passage for hydrocarbon
fluids;
5 to provide a suitable connection for
the wellhead equipment ;
6 to provide a hole of known diameter
and depth to facilitate the running of
testing and completion equipment.
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section 2 Casing Types
1 the necessity of classification
(1) the presence of high-pressured zones at
different depths;
(2) the presence of weak, unconsolidated
formations or sloughing shaly zones
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2 the classification of casing
(1) Stove pipeA. Functions
a. To prevent washout of near-surface
unconsolidated formations ;
b. To provide a circulation system for the
drilling mud ;
c. To ensure the stability of the ground surface
upon which the rig is sited.
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B. Sizefrom 26 in to 42 in
C. feature
a. cannot carry any wellhead equipment
b. can be driven into the ground with a pile
driver.
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(2) Conductor pipe
A. Functions
a to protect nearsurface unconsolidated
formations;
b to seal off shallow-water zones;c to provide a circuit for the drilling mud ;
d to protect the foundations of the
platform(offshore).B. Size
from 18 5/8 in to 30 in
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C. Setting depth
from the surface to some shallow depth
D.Features
a one or more BOPs may be mounted on this
casing;
b always cemented to the surface;
c either to support the subsequent casing
strings and wellhead , or simply cut at the
surface after setting the surface casing.
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(3) Surface casing
A. Functions
a to prevent caving of weak formations that
are encountered at shallow depths.
b to ensure that the formations at the casing
shoe will not fracture at high hydrostaticpressures which maybe used later.
c to prevent shallow blowouts as drilling
process.
B. Size
13 3/8 in(in the Middle East)
18 5/8 in or 20 in (in North Sea)
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C. Setting depth
chosen to protect troublesome formations, thief
zones, water sands, shallow hydrocarbon zones
and build-up sections of deviated wells.
D. Feature
BOPs are connected to the top of the string
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(4) Intermediate casing
A. Functions
a to seal off a severe-loss zones;
b to protect problem formations,such assalt sections or caving shales;
c to prevent communication behind the
casing between the lower hydrocarbon
zones and upper water formations.
B. Size
the most common size is 9 5/8 in
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C. Setting depth
usually set in the transition zone below or above an
over-pressured zone
D. Feature
a good cementation of the casing must be ensured
b multistage cementing may be used to cement long
strings of intermediate casing
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(4) Production casing
A. Functions
a to isolate producing zones;
b to provide reservoir fluid control
c to permit selective production in mutizone
production.
B. Size
the normal size is 7 in
C. Feature
a the last casing string;
b the well will be completed through the
string.
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(4) Liner casing
A. Introduction of liner casing
a not to reach the surface;
b hung on the intermediate casing
B. Setting depths
set at the bottom and hung from the intermediate casing
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C. Advantages:
a total costs of the production string is reduced;
b running and cementing times is reduced ;
c the length of reduced diameter is reduced.
D Disadvantages:
a possible leak across a liner hanger;
b difficulty in obtaining a good primary cementation due
to the annulus between the liner and the hole.
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section 3 Strength properties
Casing strength properties are normally
specified as:
(1) yield strength
(2) collapse strength
(3) burst strength
Strength
properties
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1 yield strength
Load-elongation graph
A
B
C
Yield strength
Ultimate strength
Fracture strength
Load
O Elongation
(1) O-A-B
This part is a straight line and can be called as the
elastic range.
Strength
properties
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Hookes law is only applicable to this portion:
=E (3-1)
Where =applied stress=load/cross-section area
E = Youngs modulus
=deformation=elongation/original length
A. to result in no damage to the internal structure;
removal of the load will resume its original shape and
length.B. Point B is defined as yield strength.
Strength
properties
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Note:
when quoting the strength of the casing, it iscustomary to use the yield strength of casing.
API define the yield strength as the tensile stressrequired to produce a total elongation of 0.5% of the
gauge length.
Strength
properties
(2) B-C
to result in a change in the internal structure and in a
loss of strength;
removal of the load will not resume its original shape
and length.
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(3) Axial tension
Ften= yieldAs (3-2)
Ften=(/4) yield(dn2-d2) (3-3)
Strength
properties
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Compute the body-yield strength for 20-in, K-55 casing with
a nominal wall thickness of 0.635 in and a nominal weightper foot of 133 lb/ft.
Solution. This pipe has a minimum yield strength of 55,000
psi and an ID of
d=20.00-2(0.635) = 18.730 in.
Thus, the cross-sectional area of steel is
As=(/4)(202-18.732)=38.63 sq in.
and minimum pipe-body yield is predicted by at an axial
load of
Ften =55,000(38.63)=2,125,000 lbf.
Example
Strength
properties
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2 collapse strength
(1) Concept
Collapse strength is defined as the
maximum external pressure to
collapse a specimen of casing.
(2) Types
A. Elastic collapse: specimen fails
before it deforms.
B. Plastic collapse: specimen
deforms before it fails.
Strength
properties
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Strength
properties
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(3) Elastic collapse
The elastic collapse pressure Pc
, can be determined
from the following formula:
Where E: Youngs modulus of steel;
: Poissons ratio;
t: casing thickness;
D: the outside diameter of casing
(3-4)Strength
properties
bar
t
D
t
D
EPc 22
1
1
1
2
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In Imperial units, E=30106psi and =0.3; hence theequation (3-4) simplifies to
psi
t
D
t
DPc 2
6
1
1095.46
In metric units, the equation (3-4) becomes
bar
t
D
t
DPc 2
6
1
10198.2
(3-5)
(3-6)
Strength
properties
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(4) Plastic collapse
The minimum collapse pressure Pp in the plastic range maybe determined from the following equation :
Where A,B and C are constants depending on the grade of
steel used and Y is yield strength.
(3-7)CBtD
AYPp
/
Strength
properties
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(5) Transition collapse pressure
The collapse behavior PT, in the transition zone between
elastic and plastic failure is described by the following
formula:
(3-8)
Where F and G are constants can be given by A,B and C.
psiGtD
FYPT
/
Strength
properties
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3 Burst strength
(1) Concept
defined as the maximum value of internal
pressure required to cause the steel to yield.
(2) The minimum burst pressure for a casing can begot by the following Barlows formula:
D
YtP
br
2875.0
The coefficient 0.875 can be deduced if the imperial
units are used in the above equation.
(3-9)
Strength
properties
Example
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ExampleCompute the burst-pressure rating for 20-in,K-55
casing with a normal wall thickness of 0.635 in
and a normal weight per foot of 133 lb/ft.
Solution
The burst-pressure rating is computed by use of the
above equation.
Pbr=0.8752550000.63520=3056 psi
Rounded to the nearest 10psi, this value becomes
3060psi. This burst-pressure rating corresponds to the
minimum expected internal pressure at which permanent
pipe deformation could take place, if the pipe is subjected
to no external pressure or axial loads.
Strength
properties
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E. The types of liner
a drilling liners: to isolate lost circulate or abnormally
pressured zones to permit deeper drilling
b production liners: to replace a full casing to provide
isolation across the producing or injection zones
c tie-back liner: a section of casing extending upwards
from the top of an existing liner to the surface or
wellhead
d scab liner: used to repair existing damaged casing
e scab tie-back liner:a section of casing extending from
the top o fan existing liner.
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section 4 Casing specification
Casing specification is referred to the
following parameters:
a. Outside diameter and wall thickness;
b. Weight per unit length;c. Type of coupling and thread;
d. Length of joints;
e. Grade of steel.
Casing
specification
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1 Outside diameter and wall thickness
Different depths
Different pressure
Different diameter
and wall thickness
Economy
Casing
specification
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2 Weight per unit length
API defines three types of casing weight :
(1) nominal weight;
(2) plain end weight;
(3) threaded and coupleded weight.
Casing
specification
(1) i l i ht
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(1) nominal weight
Used for the purpose of identification of
casing types during ordering.
Expressed in lb/ft or kg/m.
Not exact weights and normally based on
the calculated, theoretical weight per
foot for a 20ft length of threaded or
coupled joint.
Casing
specification
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Nominal weight,Wn , is calculated from
the following formula:Wn=10.68(D-t)t+0.0722D
2 lb/ft
whereD: outside diameter, in;
t: wall thickness, in.
(4-1)
Casing
specification
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(2) plain end weight
The plain end weight is the weight isthe weight without the inclusion of
threads and couplings.
The plain end weight can be calculatedby use of the following formula,taken
from API Standards:
Wpe=10.68(D-t)t lb/f twhereD: outside diameter, in;
t: wall thickness, in.
(4-2)
Casing
specification
(3) threaded and coupleded weight
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(3) threaded and coupleded weight
The threaded and coupleded weight is the
average weight of a joint including the threadsat both ends and a coupling at one end when
power-tight.
It can be calculated by use of the following formula:
W=(20-(NL+2J)/24)Wpe+weight of coupl ing-weight r emoved in threading two pipe end
20
Where W=threaded and coupled weight (lb/ft);
NL=coupling length (in);
J=distance from end of pipe to center of coupling in the
power-tight position (in);
Wpe=plain end weight.
(4-3)
Casing
specification
3 T f li d th d
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3 Types of coupling and thread
A coupling is a short section of casing and isused to connect two casing joints.
A casing joint is externally threaded at both
ends. The most common type of coupling is
internally threaded from each end.
API specifies that a coupling should be of the
same grade as the pipe body.
Casing
specification
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In general, the casing and coupling are specified by the type
of threads (or connection) cut in the pipe or coupling.
API defines three principal elements of thread:
(1) thread height or depth, defined as the distance between
the thread crest and the thread root measured normal to
the axis of the thread;
(2) lead, defined as the distance from one point on a threadto a corresponding point on the adjacent thread, as
measured parallel to the thread axis;
(3) taper, defined as the change in diameter of a thread
expressed in inches per foot of thread length;
(4) thread form-- most casing threads are squared or V-
shaped.
Casing
specification
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The following are the most widely used connections.
(a) API 8 round thread;
(b) Buttress thread;
(c) VAM thread;
(d) Extreme line threaded coupling;
(e) Buttress double seal (BDS) thread.
Casing
specification
Casing
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Casing
specification
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VAM thread configuration
Casing
specification
4 Length of joint
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4 Length of joint
API has the specified three ranges in which a
pipe length must lie.These are as follows:
Range Length (ft) Average
length (ft)1 16-25 22
2 25-34 31
3 Over 34 42
Casing
specification
5 Grade of steel
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5 Grade of steel
API lists eight different grades of casing,as
follows:
Casing
specification
6 The failure modes of casingCasing
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6 The failure modes of casing specification
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section 5 Casing design
Casing design is influenced by the following factors:
a. The loading conditions during drilling and
production;
b. The strength properties of the casing seat and ofavailable casing;
c. The degree of deterioration to which the pipe will be
subjected during the entire life of the well;
d. The requirements of completion and production;e. Safety;
f. Economy;
g. The availability of casing.
Casing
design
1 Design criteria
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1 Design criteria
A tensile force
(a) originate from casing-own-weight,bending
forces, and shock loading.
(b) the weakest point is located at the uppermost
joint of the string.
Casing
design
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B Collapse pressure
(a)Originate from the column of mud
(b) Collapse pressure
C=m gh
Where m : mud density
h: depth; g: acceleration due to gravity.
(c) the collapse is zero at the top;
the collapse is the highest at the bottom
(d) The collapse pressure never exceeds the
collapse resistance.
(5-1)
Casing
design
Casing
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C Burst pressure
(a) normally based on the maximum formation pressure
that can be encountered during the drilling of next holesection.
(b) At the top it is the highest.
At the bottom it is the least.
D Compression load
(a) Originate when casings carry inner strings.
(b) Since production casings don not carry inner strings,
they dont develop any compression load.
Casing
design
E Other loadings
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E Other loadings
(a) bending with tongs during make-up;
(b) pull-up of the joint and slip crushing
(c) corrosion and fatigue failure of the body and
threads;
(d) pipe wear due to running wire line tools andstring assembly;
(e) additional loadings strings treatment operations
such as squeeze-cementing ,acidising and hydraulicfractures.
Casing
design
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F Conclusions
(a) Only tensile force, collapse pressure,burst pressure
and compression load will be considered in the design.
(b) Other loadings,with the exception of (e) cannot bedetermined directly and be accounted for through the
use of safety factors.
Casing
design
2 S f t f tCasing
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2 Safety factors
Casing design is not an exact technique, because of the
uncertainties in the determining the actual loadings and
also because of the change in casing properties withtime,resulting from corrosion and wear.
A safety factor is used to allow for such uncertainties in
the casing design and to ensure that the rated
performance of the casing is always greater than anyexpected loading.
Usual safety factors are:
collapse: 0.85---1.125
burst: 1---1.1
tension: 1.61.8
design
3 combination stringsCasing
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3 combination strings
From the above table, the requirements for burst andtension criteria are different from the requirement for
collapse .
Hence a compromise must be reached when designing
for casing.
How to reach the compromise?
Maximum
tension
Maximum
Burst pressure
Maximum
collapse pressure
At the top
High grade or
heavy casing
At the top
High grade or
heavy casing
At the bottom
High grade or
heavy casing
design
Thi i i hi d i th f f bi ti
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This compromise is achieved in the form of a combination
string. In other words,casing of various grades or different
weights are used at different depths of a hole.
top
middle
bottom
Strong and heavy casing
Light yet heavy casing
Heavy casing
The combination string method represents the most
economical way of selecting casing consistent with safety.
Well hole
Casing
design
4 Biaxial effects C i
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4 Biaxial effects
(1) Concept
The combination of stress due to the weight of thecasing and external pressure are referred to the
Biaxial stresses.
(2) The ellipse of plasticity
Holmquist and Nadia in 1939 give the equationabout the relationship for the effect of axial stress on
collapse or burst .
yieldrztrrt 2)()()(
222
(5-2)
Where r ,t ,and z are the radial, tangential ,and verticalstresses, respectively.
Casing
design
h b i d h i d
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From the above equation and other equation and we can
deduce the following equation :
yield
iz
yield
iz
yield
it ppp
2
1
4
31
2
(5-3)
Casing
design
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Conclusion
Axial tension has a detrimental effect on
collapse-pressure rating and a beneficial
effect on burst-pressure rating.
Axial compression has a detrimental effect
on burst-pressure rating and and a beneficial
effect on collapse-pressure .
Casing
design
5 Graphic method for casing design
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5 Graphic method for casing design
The method is first described in 1965 in a series of articles by
Goins et al and has been adopted by many oil companies.
(1) Collapse line
A determined as follows:
(a) calculate the external load due to the mud column,H;
(b) calculate the internal load due to the mud inside the
casing, H1;
(c) calculate the collapse pressure C, as the difference
between H and H1,
C=H-H1
B Join the zero coordinates at the surface with the
value of C at the casing shoe to get thepressure---
depth graph.
(5-4)
Casing
design
(2) Burst line C i
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(2) Burst line
A determined as follows:
(a) calculate the external load due to an assumed mud
column of 0.465psi/ft.
(b) calculate the internal load to the formation pressure.
(c) calculate the burst pressure as the difference
between (a) and (b).
Casing
design
B
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a at the shoe
external pressure=CSDGm
internal pressure=Pf(TDCSD)G
burst = internal pressure external pressure
= Pf
(TDCSD)G CSDGm
b at the surface
external pressure=0
internal pressure=Pf
TD
Gburst = PfTD G
(5-5)
(5-6)
Casing
design
Where G=gradient of formation fluid;
Gm=the gradient of mud.
(3) Tensile forces
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(3) Tensile forces
A determined as follows:
(a) calculate the weight of casing in air;
(b) calculate the buoyancy force;
(b) to calculate total tensile loads and compare them
with the joint or pipe body yield values when the casing
is finally chosen.
(c) calculate the bending force in deviated wells;
(d) calculate shock loads due to arresting casing.
B considerations in selection of casing
(a) to check that the casing can carry its own weightin mud in the initial selection;
(b) to calculate total tensile loads and compare them
with the joint or pipe body yield values in the final
selection.
Casing
design
Example 1
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Example 1
Question:The following three grades of 133/8 in (340 mm)casings are available in a company store. It is required to run a
combination string based on collapse and tension only. The casing is
run in 67 pcf(1.0734 kg/l) mud to 6200 ft (1890 m). Safety factors
are 1.8 for tension and a minimum of 0.85 for collapse.
Grade Weight Collapse Yield strength
lbm/ft psi 1000 lb
body coupling
K55 54.5 1130 853 636
K55 68 1950 1069 1300L80 72 2670 1661 1693
Joint type: LTC for K55, 54.5 lb/ft and BTS for remaining grades.
Casing
design
SolutionC i
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(1) Collapse
Collapse pressure=676200/144=2884.7 psi
On a graph of depth against pressure draw a collapse
pressure line between zero at surface and 2885 psi at 6200
ft. Draw the collapse resistances of the three grades as
vertical lines, as shown in the next Figure .
From the Figure, selection based on collapse is as shown
in the next page table. (Note: Minimum safety factor in
collapse=collapse resistance of casing divided by collapse
pressure of mud column.)
Casing
design
Note that the last grade was only suitable down to a depth of 5400 ft for
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a safety factor of 1.However, since a minimum safety factor of 0.85 is to
be used, this grade is suitable down to 6200 ft,with the lowest safety
factor being 0.93 at TD. Above 6200 ft the safety factor value in
collapse increases and assumes a maximum value of
=1.7
Depth Grade and weight Length of section Minimum safety factor
0-2500 ft K55, 54.5 Ibm/ft 2500 ft 1
2500-4200 ft K55, 68 Ibm/ft 1700 ft 1
4200-6200 ft L-80, 72 lb/ft 2000 ft 184/(1840*0.1053)=0.93
1950
(2500*67/144)
Casing
design
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design
(2) TensionCasing
design
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( )
Casing-carrying capacity must be checked from the bottom joint to the
surface. Two values of yield strength are given in the table of strength
properties. One specifies the yield strength of pipe body and the other the
yield strength of the coupling. The lower of these two values is used for the
calculation of the safety factor in tension. Therefore, starting from the
bottom, see table below.
design
Si i i f f f 1 8 i b d i i
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Since a minimum safety factor of 1.8 is to be used in tension,
the K55, 54.5 lbm/ft (81.2 kg/m) may be used if it is designed
to carry a maximum weight, W, given by:
1.8=6.36*1000/W
W=353.33 lb
Hence, usable weight of section of 54.5# = (Total weight which
can be carried)(weight of lower casing grades)
weight of section of 54.5#=353 333259 600=93 733 lb
and
length of usable section of K55,
54.5#=93733lb/(54.5lbm/ft)=1720ft
Casing
design
Casing
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Remaining top length = 2500 - 1720 = 780 ft
A heavy casing must be used for the top 780 ft. Try K55,68#(next heavy casing).
Total weight that can be carried by the top joint of K55 is:
= 353 333 + 78068 = 406 373 lb
g
design
SF in tension for K55, 68# at top joint Casing
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SF in tension for K55, 68# at top joint
=1069*1000/406373=2.6
Hence, the final casing selection, based on collapse and tension,is as follows:
Casing
design
In exploration wells the designer often discards grades which give a
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marginal safety factor. In fact, the above selection could well be simplified
further to obtain added safety factors and to eliminate the risk of using the
wrong joint in a critical section of the well. In this example grade K55,
54.5# (81.2 kg/m), is the weakest grade and can therefore be eliminated
from our selection. Hence, final selection can be made as follows:
Casing
design
(4) Buoyancy
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( ) y y
Consider a cylinder of 1m (or 1ft) in length, of density
s ,which is fully immersed in a fluid of density ofm, of
outside diameterdo and inside diameter ofdi.
A
Air weight of cylinder=
or Wa=Ass g
B
Buoyancy force of cylinder=
or Wm=Asm g
gdd mio
14
22
gdd mio
14
22
(5-7)
(5-8)
Casing
design
C Casing
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the effective or buoyant weight of the casing
WB=Wa
Wm
=Wa(1m/ s)=WaBF
where BF = (1m/ s) and is called buoyancy factor.(5-9)
C s g
design
Example 2 Casingdesign
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pQuestion:7 in (177.8 mm) casing, 26#(38.7 kg/m), is
to be set at 17 000 ft (5182 m). If the internal diameter
is 6.276 in (159.4 mm), determine the buoyancy forceand buoyancy factor assuming that the mud density is
93.5 lbm/ft3(1.498 kg/l).
Solution
Weight of casing in air = 26 17 000 = 442 000 lb
Buoyancy factor =(1m/ s)=(193.5/489.5)=0.895
where density of steel = 489.5 lb/ft3 (7.85 kg/l)
Buoyant weight of casing= 0.809 442 000 = 357 578 lb
Buoyancy force = 442 000 - 357 578 = 84 422 lb
design
(5) Bending forceCasing
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(5) e d g o ce
Arise when casing is run in highly deviated wells or in
wells with severe dog-leg problems.
Assume:
(1) a beam subjected to pure bending;
(2) plane transverse sections will remain plane after bending;
(3) the radius is large in comparison with the transverse dimensions;
During the pure bending, the upper surface stretches and is in tension,
while the lower surface shortens and is in compression .
NA (neutral axis): a surface exists between the compressed and
stretched surfaces and has no longitudinal deformation.
design
HJ at a distance y from NA and has the same length as KL at the NA.
Aft b di th f HJ d f t f di R d i l d d
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After bending the surface HJ deforms to an arc of radius R and included
angle d.
Thus the longitudinal strain,e, in the H`J` is
R
y
Rd
RddyR
HJ
HJJHe
((5-10)
Casing
design
From Hooks law, we can get
(5 11)
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=Ee=Ey/R
If the original length of the beam is L and the total deformation angle is
, then
NA=R=L
(5-11)
(5-12)
From the above 2 equations, we can get
=Ey/(L/)=E y/L (5-13)
The maximum tensile stress occurs at the upper extreme
end of the beam at y=D/2, where D is the diameter of thebeam. Thus,
=E D/(2L) (5-14)
Casing
design
Also bending force (FB)= A Casing
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Also, bending force (FB)= A
where A is the cross-sectional area.
Hence, FB=EDA /(2L) (5-15)
When is expressed in degrees, while the above formula becomes
FB=EDA /(2L) *(/180) (5-16)
Casing
design
Equation (3-19) in field units Casing
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q ( )
A imperial units
E=modulus of elasticity of steel:30106
psi;
D=in; A=in2; L=ft; =degrees
Therefore, FB=218.17102DA /L (5-17)
In practice, the rate of change per 100ft is used to indicate the
degree of dog-leg severity. Hence , replacing L by 100in equation(3-22)
gives FB=218DA (5-18)
FB=63DWN lb (5-19)
Casing
design
metric unitsFB=63DWN lb (5-0)
(6) Shock loads Casing
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(6) Shock loads
Significant shocks loading can develop if a casing string
is suddenly stopped.Axial stresses result from sudden velocity changes
changes in a manner analogous to water-hammer in a
pipe caused by a sudden value closure.
Elastic theory leads to the following equation for axialshock loads resulting from instantaneously stopping the
casing:
sz Ev (5-19)
Wherezis the change in axial stress caused by the shock
load, vis the change in pipe velocity, E is Youngsmodulus, ands is the density of steel.
design
After average values for Youngs modulus and steel density
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are substituted ,this equation becomes:
vz
1780 (5-20)
Wherez : psi
v: ft/sec.
Casing
design
Casing Design Example Casingdesign
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An exploration well is to be drilled to a total depthof13 900 ft (4327 m). Relevant data are as follows.
Drilling program:0-350 ft (107 m), 26 in (660.4 mm) hole
350-6200 ft (1890 m), 171/2in (444.5 mm) hole
6200-10 400 ft (3170 m), 121/4in (3l 1.2 mm) hole
10 400-13 900 ft (4237 m), 8
1
/2in (215.9 mm) hole
Casing program:20 in(508 mm) casing to be set at 350 ft (107 m)
133/8in(339.7 mm) casing to be set at 6200 ft (1890 m)
91/8in(244.5 mm) casing to be set at 10 400 ft (3170 m)
7 in (177.8 mm) casing to be set at 13 900 ft (4237 m)
The casing head housing will be installed on the 20 incasing. The 7 in casing will be run to the surface.
Mud programme: Casingdesign
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Down to 350 ft (107 m),
mud weight is 65 pcf (1.041 kg/l)
Down to 6200 ft (1890 m),
mud weight is 67 pcf (1.073 kg/l)
Down to 10400 ft (3170 m),
mud weight is 73 pcf (1.169 kg/l)
Down to 13 900 ft (4237 m),mud weight is 87 pcf (1.394 kg/l)
Safety factors:Burst = 1.1
Collapse = 0.85
Tension = 1.8
Formation fluid gradient:
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0-6200 ft (1890 m), Pf= 0.465 psi/ft (0.105 bar/m)
6200-10 400 ft (3170 m),Pf= 0.48 psi/ft (0.1086 bar/m)
10 400-13 900 ft (4237 m), Pf= 0.57 psi/ft (0.1289 bar/m)
The 12 in hole experiences a maximum dog-legseverity of3o/100 ft. Other sections of the well experiencenegligible deviation. Shock loads are to be included inthe design of 9 5/8 in and 7 in casing strings.For collapse, burst and yield strength values referto sometables.Design suitable casing strings for the given hole sizes,taking into consideration the available casinggrades andthe maximum expected pressures.
Casing
design
solutionCasing
design
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1. Conductor pipe (20 in casing)This pipe is set at 350 ft (107 m) and will besubjected to formation pressure from the nexthole drilled to a depth of 6200 ft (1890 m). Itwill be assumed that no gas exists at thisshallow depth and kick calculations will bebased on a water kick situation in whichformation gradient is 0.465 psi/ft (0.105bar/m). Note that if gas is known to exist atshallow depths, it must be included in thecalculations.
Collapse Casingdesign
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Collapse pressure at surface = 0
Collapse pressure at 350 ft =
where mud weight is lbm/ft3.Therefore,collapse pressure at 350 ft
65350/144= 158 psi (11 bar)
mud weight depth144
This pressure acts on the outside of the casingand for the worst possible situation assume thatthe casing is 100% evacuated (as is the case ina complete-loss circulation situation).
Burst
Casing
design
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Burst pressure = internal pressure- external pressurea) Burst at shoeFrom Figure 10.11,
formation pressure at next TD = 62000.465or
Pf = 2883 psi (199 bar)Internal pressure = Pf - (TD - CSD)G
= 2883 - (6200 - 350)0.465= 163 psi (11 bar)
where G = gradient of invading fluid= 0.465 psi/ft.External pressure=casing settingdepthmud gradient
external pressure = (35065)/144= 158 psi (11 bar)
Figure 10.11
Burst at shoe = internal pressure- external pressure
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= 163 - 158= 5 psi (0.4 bar)
(b) Burst at surfaceBurst at surface = Pf TDG
= 2883 62000.465= 0
It should be noted that the zero values wereobtained as a result of the fact that a salt-water kickis considered. If instead a gas kick is considered, theburst pressure values at the shoe and surface will be2135 psi and 2140 psi, respectively.
Casing
design
Selection
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A graph is not normally required and selection is determined bycomparing the strength properties of available casing withexisting pressures.From Table 10.4 it can be seen that all the available gradeshave collapse and burst values above those calculated above.Hence, select grade K555, 94#,having collapse pressure= 520psi (36 bar), burst pressure=2110 psi (145 bar) and yieldstrength= 1 479 000 lb (6579 kN). It should be noted that gradeK55, 94# is the lightest and the cheapest of the three availablegrades.Since the casing head housing is installed on the 20 in casing,the latter will be subjected to compression forces resultingfrom the weights of subsequent casing strings.This casing will be checked later to determine whether it iscapable of carrying other casing strings.
Casing
design
2. 133/8 in casing Casingdesign
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This string is set at 6200 ft and will be subjected, in the event
of a kick, to formation pressures from the next hole drilled to a
TD of 10 400 ft.
CollapseCollapse pressure at surface = 0
Collapse pressure at 6200 ft(1890m)=676200/144
=285psi(199 bar)
The collapse line is drawn between 0 at the
surface and 2885 psi at 6200 ft, as shown in Figure
10.12.
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Casing
design
From Table 10.5 the collapse resistances of the
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available grades as adjusted for a safety factor of0.85 are as follows:The collapse resistance values are plotted asvertical lines, as shown in Figure 10.12
Casing
design
Burst Casingdesign
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Formation pressure from next TD= 10 4000.48
= 4992 psi (344 bar)
(see Figure 10.13).
Burst at shoe = internal pressure- external pressureInternal pressure = Pf - (TD - CSD)G
= 4992 - (10 400 - 6200)0.1
= 4572 psi (315 bar)
( where G = gradient of invadingfluid, assumed to be gas having a0.1 psi/ft gradient)
External pressure = CSD x 0.465
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where 0.465 psi/ft is the gradient of mud outside thecasing. Therefore,
external pressure = 6200 x 0.465= 2883 psi (199 bar)
Thus,Burst at shoe = 4572 - 2883
= 1689 psi (116 bar)Burst at surface = internal pressure
- external pressureExternal pressure = 0Internal pressure = Pf - (TD) G
Casing
design
Therefore,burst at surface = Pf - (TD)G
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= 4992 - 10 4000.1= 3952 psi (273 bar)
The burst line can now be drawn between 1689 psiatthe shoe and 3952 psi at the surface; see Figure10.12.
From Table 10.5, of casing properties, the burstresistances of the available grades are givenbelow,together with adjustment for SF = 1.1.
Casing
design
Selection
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Selection should consider the lightest weightsfirst, as these grades are the cheapest. On the
basis of collapse only, Figure 10.12 indicatesthat the given grades are suitable for thefollowing depths:
0-3050 ft K55, 54.5#
3050-4950 ft K55,68#
4950-6200 ft L80, 72#
On the basis of burst only, Figure 10.12 gives
the following selection:
0-2400 ft L80, 72#
2400-4200 ft K55, 68#
4200-6200 ft K55, 54.5#
Casing
design
When selection is based on both collapse and
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burst,Figure 10.12 indicates that grade K55,54.5#does not satisfy the burst requirement from 0
to 4200 ft. Also,grade K55, 68# does not satisfyburst from 0 to 2400 ft.Hence, selection from 0 to2400 ft is limited to grade L80, 72#.
Below 2400 ft, grade K55, 68# is suitable for
collapse from 0 to 4950 ft and for burst from 2400ft to 4200 ft. Hence, the middle section consistsof K55,68#from 2400 to 4200 ft.
The last section of hole can only be satisfied bygrade LB0, 72# in both collapse and burst; seeFigure10.12. Hence, selection based on collapseand burst is(see table below):
Casing
design
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Note that grade K55, 54.5# has been rejected,since it does not satisfy both collapse and
burst at once along any section of the hole.
Tension If bending and shock forces are ignored, thesuitability of selected grades in tension can be checked bycomparing the weight in air carried by each section with its
yield strength. For the 93 in and 7 in casing,effects ofbending and shock loading will be included and buoyantweight will be considered to reduce the possibility of over-designing. Hence, starting from the bottom, see table at thetop of next page.
Casing
design
Weight of section grade and weight cumulative weight safety factor
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(1000 lb) (1000 lb) =yieldstrengthcumul
ative weight
144.0 L80,72# 144.0 1650/144=11.5
122.4 K55,68# 266.4 835/266.4=3.13
172.8 L80,72# 439.2 1650/439.2=3.8
Note that yield strength values are obtained from the givenTable as the lowest value of either the body or couplingyield strength.
The safety factor must, at least, be equal to the requiredvalue of 1.8 if any of the selected grades is to satisfy thecriterion of tension. The table overleaf produces values ofSF of greater than 1.8, which indicates that the gradessatisfy collapse, burst and tension.
Casing
design
Pressure testing After the casing is landed and cemented, itis the practice to test the casing prior to drilling thecasing shoe. The testing pressure employed by some operating
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casing shoe. The testing pressure employed by some operatingcompanies is 60% of the burst rating of the weakest grade ofcasing in the string.Hence,testing pressure of 133 in
= 60% x burst pressure of K55, 68//
= 60% x 3450
= 2070 psi (143 bars)
During pressure testing an extra tensile force is exerted onthe casing and the SF should, again, be > 1.8 for the topjoint (or the joint of weakest grade). Hence,total tensileforce during pressure testing at top joint
= buoyant weight of casing
+ tensile force due to pressure testing
=weightin airBF + (/4) (ID)2testing pressure
Casing
design
BF=(1 / )=1 67/489 5=0 863
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BF=(1m/ s)=1-67/489.5=0.863
From the given table we can get the inside diameter of L80,72# as 12.347 in (313.6 mm).
Therefore,
total tensile force = (439.2 x 0.863) 1000
+ (/4) (12.347)2 2070
= 379 030 + 247 847
= 626 877 lb
SF in tension for top joint = 1 661 000/ 626 877
=2.65
Casing
design
Biaxial effects Check the weakest grade of selectedcasing for biaxial effects as follows.
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g
Tensile ratio = weight carried by weakest joint
yield strength of body (or coupling)Weakest grade selected is the K55, 68#, having a body yieldstrength of 1 069 000 lb and a coupling strength(LTC) of835000 lb.
Hence,
tensile ratio =266.41000/835000=0.319
For a tensile ratio of 0.319, Table 10.8 showsthat the collapse resistance of the casing is
reduced to approximately 80% of its original(under zero load) value.Hence, collapse resistanceof K55, 68# = 0.81950
under biaxial loading = 1560 psi(108 bars)
Casing
design
Collapse pressure due to mud at 2400 ft (i.e. topjoint of grade of the K55 68#)
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joint of grade of the K55, 68#)
=672400/144=1117 psi(77bars)
Therefore,
SF in collapse for top joint of K55, 68#
=collapse resistance collapse pressure
=1560/1117
=1.4
Final selection
Depth Grade and weight
0-2400 ft (732 m) L80, 72#(107 kg/m)
24004200 ft (1280 m) K55, 68#(101 kg/m)
4200-6200 ft (1890 m) L80, 72#(107 kg/m)
Casing
design
3. 9 5/8incasing
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The 95/8in casing is set at 10400 ft and willbe subjected, in the event of a kick, to
formation pressures from the next hole drilledto a TD of 13 900 ft.
Collapse
At surface
collapse pressure = 0
At shoe
collapse pressure =7310400/144
= 5272 psi (363.5 bars)
Draw a line between 0 and 5272 psi as shown inFigure 10.14.
Casing
design
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From Table 10.6 collapse properties ofavailablecasing are as follows:
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Grade Weight (lbm/ft) Collapse pressure
SF = 1 SF = 1.1C75 43.5 3750 3750/1.1=4412
L80 47.0 4750 4750/1.1=5888
C95 53.5 7330 7330/1.1=8624
The above collapse resistances can be drawn as vertical lines,
as shown in Figure 10.14.
Casing
design
Burst
The 95/8in casing will be subjected in the event of a kick
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The 9 /8in casing will be subjected in the event of a kick,to a formation pressure of:
0.57 psi/ft13 900 ft = 7923 psi (546 bar)
Burst at shoe = internal pressure
- external pressure
Burst at shoe = [Pf - (TD - CSD) x G]
- CSD 0.465
A gas kick is considered for this string; thus, G = 0.1psi/ft.
Therefore,
burst at shoe = 7923 - (13 900 - 10 400)
0.1 - 10 4000.465
= 2737 psi (189 bars)
(where TD = next hole depth = 13 900 ft).
Casing
design
Burst at surface = Pf TDG
Therefore,
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Therefore,
Burst at surface = 7923 - 13 9000.1
= 6533 psi (450.4 bar)
The burst line can now be plotted between 6533 psi at thesurface (i.e. at zero depth) and 2737 psi at 10 400 ft,asshown in Figure 10.14
From Table 10.6 burst pressures of available gradesof 9~ incasing as adjusted for an SF = 1.1 are:
Grade Weight (lbm/ft) Collapse pressure
SF = 1 SF = 1.1
C75 43.5 5930 5390/1.1=5391L80 47.0 6870 6870/1.1=6245
C95 53.5 7330 9410/1.1=8555
Burst resistance lines are plotted, as shown in Figure10.14.
Casing
design
Selection based on collapse and burstF Fi 10 14 l ti b d ll d
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From Figure10.14, selection based on collapse andburst is as shown at the table.
Buoyant weight of casing = 474.75BF
BF= 173/489.5 =0.851
Buoyant weight of casing = 474.75 0.851
= 404.012 1000 lbCasing
design
Tension The suitability of the selected gradesintension will be investigated by considering the
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g y gtotal tensile forces resulting from casing buoyantweight,bending force and shock load. Starting from
the bottom, the weight carried by each section canbe calculated, as follows:
Depth (ft) Weight of each section Weight in air carried( 1000 lb) by top joint of each
section
10 400--8700 79.90 79.90
8700-3200 239.25 79.90 + 239.25 = 319.153200-800 112.80 319.15 + 112.8 = 431.95
800-0 42.80 431.95 + 42.8 = 474.75
Casing
design
By use of the equations
bending force = 63 D W
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bending force = 63 D WN
drag force = 3200 WN
where WN is the weight per unit length, Table10.10 can be constructed. Table 10.10 shows thatall the selected grades satisfy the tensionrequirement.
Casing
design
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Pressure testingTesting pressure
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= 60% of burst pressure of lowest grade (C75, 43.5#)
= 0.6 5930
= 3558 psi (245 bar)
During pressure testing, an extra tensile force is generatedand selected grades with marginal SF should be checked. At800 ft grade L80, 47# has the
lowest SF of 1.8 (see Table
10.10); hence, this gradeshould be checked.
During pressure testing,
total tensile force
= buoyant load+ tensile force due to pressure testing
From table 10.10,
buoyant force at 800 ft = 361.21(1000 lb)
Casing
design
Total tensile load at 800 ft
= 361 21 100+ (/4)(8 681)2 3558
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= 361.21 100+ (/4)(8.681)2 3558
580.623 100
SF in tension = 1086/580.623=1.87
Biaxial effectsCheck the weakest grade selected.Grade C75, 43.5#is the weakest grade, carrying a total buoyantload of 248.41 1000 lb, as shown in Table 10.10.
Tensile ratio =weight carried / yield strength
=248.41/942
=0.264
Casing
design
From Table 10.8 it can be seen that, for a tensile ratio of0.264, the collapse resistance reduces to 84% of its
i i l l H ll i t f C75 43 5#
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original value. Hence,collapse resistance of C75, 43.5#under biaxial loading
= 0.84 x 3750
= 3150 psi (217.6 bar)
SF in collapse= collapse resistance under biaxialloadingcollapse pressure at 3200 ft
=3150 (733200/144)=1.94
Final selection
Depth Grade and weight
0-800 ft (244 m) C95, 53.5# (79.7 kg/m)
800-3200 ft (976 m) L80, 47# (70 kg/m)
3200-9700 ft (2957 m) C75, 43.5# (64.8 kg/m)
9700-10 400 ft (3170 m) L80, 47# (70 kg/m)
Casing
design
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Casing
design
4. 7in casing
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This string is set at 13 900 fi (4237 m).Collapse
Collapse pressure at surface = 0Collapse pressure at 13 900 ft=8713900/14
= 8398 psi (579 bar)
This pressure acts on the outside of the casing,and for the worst possible situation assume thatthere is zero pressure inside the casing. Draw thecollapse pressure line, as shown in Figure 10.15,between 0 psi at the surface and 8398 psi at 13900 ft.
Casing
design
Collapse resistances, from Table 10.7, are as follows:
Grade Weight (lbm/ft) Collapse resistance
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SF = 1 SF = 0.85
K55 26.0 4320 4320/0.85=5082
L80 29.0 7020 7020/0.85=8259
C95 29.0 7820 7820/0.85=9200
Collapse resistances can now be drawn as verticallines in Figure 10.15.
Casing
design
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Casing
design
BurstBurst pressure = internal pressure
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p p
- external pressure
(a) Burst at shoe
Internal pressure = Pf = 0.57 13 900
= 7923 psi (546.5 bar)
External pressure = Gmud CSD
For added safety, the external pressure resistinginternal pressure is assumed to be that of a mud columnoutside the casing, even though the casing is cemented.Also, the mud is assumed to deteriorate so that itsgradient decreases to that of salt water, largely becauseof settlement of solids. Hence,
G = 0.465 psi/ft (0.1052 bar/m)
Burst at shoe 7923 - 13 900 0.465
= 1460 psi (100 bars)
Casing
design
(b) Burst pressure at surface
= internal pressure - external pressure
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internal pressure external pressure
7923 - 13 900
gradient of invading fluid (assumed gas)
= 7923 - 13 900 0.1
6533 psi (450.4 bars)
Worst conditions In practice, hydrocarbonproduction is carried out through a tubing (single
or dual) sealed in a packer, as shown in Figure10.16. Thus, under ideal conditions only thecasing shoe will be subjected to burst effects.
Casing
design
However, a situation may arise inpractice when
the production tubing leaks gas to the7 in casing.
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In this case, the surface pressure (6533 psi) isnow acting on the column of packer fluid betweenthe casing and the tubing; see Figure 10.16.
Casing
design
Hence, burst calculations for production casing should bemodified as follows.
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(a) Burst pressure at shoe = surface pressure + hydrostaticpressure of packer fluid -external pressure.
Normally
packer fluid - drilling mud = 87 pcf (1.394 kg/l)
Burst at shoe = 6533 +8713900/144- 13 900 x 0.465
= 8467 psi (584 bar)
(b) Burst at surface = 6533 psi (450 bar)
Note: All these calculations assume that there is no cementoutside the casing.
Casing
design
The burst line is drawn between 6533 psi at thesurface and 8467 psi at 13 900 ft, as shown in
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Figure10.15. From Table 10.7 burst resistances asadjusted for an SF of 1.1 for the available gradesare:
Grade Weight (lbm/ft) Burst resistance
SF = 1 SF = 1.1
K55 26.0 4980 4980/1.1=4527
LB0 29.0 8160 8160/1.1=7418
C95 29.0 9690 9690/1.1=8809
Adjusted burst lines (for SF= 1.1) can now bedrawn as vertical lines in Figure 10.15.
Casing
design
Selection based on burst and collapseFrom Figure10 15 selection based on burst and
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From Figure10.15 selection based on burst andcollapse is as follows:
Depth (ft) Grade and weight
0-6100 LB0, 29#
6100-13 900 C95, 29#
TensionThe suitability of selected grades in tensioncan be checked by considering the cumulative
weight carried by each section. Hence, startingfrom the bottom, see the table in next page.
Casing
design
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The above table considered the weight of casingsections in air only and a marginal safetyfactor of 1.68was obtained. This value is belowthe required SF of1.8 and it is instructive tocheck the suitability of this grade by addingthe effects of shock loading (bending effectsare assumed to be negligible) as shown inTable10.11.
From Table 10.11 it is evident that grade L8029# is not suitable as a top joint. Furtherrefinement can be made when pressure testing isconsidered.
Casing
design
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Pressure testingNormally, casing is tested to 60% of its mill burst pressure.
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Taking the lowest grade (L80),therefore,
test pressure = 0.6 x 8160 = 4896 psi (338 bar)
The weakest joint is the top joint of the lowergrade.Therefore, load at top joint=buoyant weight of casing+ tensile force, resulting from extra pressure on inside ofcasing
= Wair BF + /4(di)2 test pressure
where di is the internal diameter of the casing = 6.184 inand BF = 1 -(87/489.5)= 0.822.
Therefore,
total load at surface
= 0.822 x 403.1 1000 /4(6.184)2 4896
= 478 400 lb
Casing
design
Therefore,
SF in tension during pressure test
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g p=676000/478400= 1.41
Thus, the top joint of L80 must be replaced by ahigher grade casing if the SF in tension of 1.8is to be maintained throughout the running andtesting of the casing. Hence, the maximum load, W,
that L80 can carry and still produce an SF = 1.8is given by
1.8=676 000/W
Therefore,
W= 375 556 lb
Casing
design
Hence,
weight of usable L80 section
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weight of usable L80 section
= total weight carried (W) - air weight of C95, 29#
= 375 556 - air weight of C95, 29#
= 375 556 - 226 200
= 149 356 lb
and
usable length of L80, 29# = 149 356 lb/ (29 lb/ft )
=55150ft
Casing
design
From Table 10.11 the total tensile load at topjoint is still 424.15 1000 lb, since the two
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different grades have the'same weight per foot of
29. If the weights were different, thenanother table should be constructed.
If grade C95 is used as the top section (950 ftlong),then
SF at top joint = 803000424150 1.89
(Note: Yield strength of C95, 29= 803 000 lb.)
During pressure tests,
SF803000/478400=1.68
Casing
design
Thus, even with the higher grade, the SF duringpressure testing is still below 1.8. To maintain
SF f1 8 d th t t b l 4896
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an SF of1.8, decrease the test pressure below 4896psi. Hence,
1.8=803000/(buoyant weight + tensile force due to pressure test )=803000/(331.348+(/4)(6.184)2P)
where P is the required pressure test.Therefore, P= 3821 psi (263 bar).Hence, the new selection is:
Depth (ft) Grade0-950 C95, 29#950-6100 L80, 29#
6100-13 400 C95, 29#
Casing
design
Biaxial effectsBi i l l di d ll i f
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Biaxial loading reduces collapse resistance ofcasing and is most critical at the joints of the
weakest grade. Two positions will be investigated.
(a) At 950 ft
tensile ratio
=buoyant weight carried by top joint of L80/ yield strength
tensile ratio (TR)
=(13 900-950)29BF/676000
= 0.46
Casing
design
Table 10.8 shows that for a TR =0.46, collapseresistance reduces to 69% of its original collapsealue(i e under ero load) Therefore actual
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value(i.e. under zero load). Therefore, actualcollapse resistance of LB0 = 0.69 x 7020 = 4844 psi
SF in collapseat 950 ft
=collapse resistance of casing collapse pressure of mud
=4844 (87950/144)
=8.4
(b) At 6100 ft
tensile ratio =154450/676000 = 0.23
Table 10.8 shows that for a TR=0.23, collapseresistance reduces to 86% of its original value.Therefore, adjusted collapse resistance of L80 =0.86 7020= 6037 psi at 6100 ft
Casing
design
SF in collapse at 6100 ft
6037/(876100/144)
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6037/(876100/144)
=1.6
Final selection
Depth (ft) Grade and weight
0-950 C95, 29#950-6100 LB0, 29#
6100-13 900 C95, 29#
Casing
design
section 6 other considerations
1 Effect of hydrogen sulfide on casing
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1 Effect of hydrogen sulfide on casingA Hydrogen embrittlement
When hydrogen sulfide is present, the rate at which the
hydrogen atoms (H) combine to form hydrogen gas (H2) is
reduced. As a result, atomic hydrogen (H) may enter the
metal at a significant rate before recombining. The presenceof this molecular hydrogen within the steel reduces its
ductility and causes it to break in brittle manner rather than
yield. This phenomenon known as hydrogen embrittlement.
The resulting failure is called sulfide cracking.
Water must be present for corrosion reaction to occur, whichgenerates hydrogen atoms. Dry hydrogen sulfide does not
cause embrittlement.
Other
considerations
B temperature effect on hydrogen embtittlement
Other
considerations
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Hydrogen embrittlement is especially significant high-strength steels at low temperature. Common carbon steels with
yield strengths below 90,000psi generally not fail by sulfide
cracking for temperature above 100F.
There is evidence that, as temperature increases, casing with a
higher minimum yield strength than 90,000 psi can be used
safely in wells that contain hydrogen sulfide in the produced
fluids. In deep, abnormally pressured walls, a practical casing
design is difficult to obtain without the use of some high-strength steel.
Kant and Greer have presented the results of experimental laboratory and
field tests of several steel grades that were exposed to hydrogen sulfide in
varying concentrations, at various temperatures, and at various stress levels.
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Shown in Fig. 7. i6 and 7.17 arc the maximum salt stress levels observed
(expressed as percent of minimum yield strength) for various steel grades,hydrogen sulfide concentrations, and exposure temperatures.
C incubation time
Failure resulting from hydrogen
b ittl t ft d t
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embrittlement often do not occur
immediately after exposure to
hydrogen sulfide. A time period duringwhich no damage is evident is
followed by a sudden failure. During
the time period before failure,called
incubation period, hydrogen is
diffusing to points of high stress.Fig.7.18 shows test results of the time
to failure for different RHNs
(Rockwell hardness number) and
different applied stresses. Fig. 7.19
shows the effect of hydrogen sulfide
concentration.
Other
considerations
2 Effect of field handling on casing
A Performance properties that a given joint of casing
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A Performance propertiesthat a given joint of casing
will exhibit in the field can be affected adversely by
several field operations. For example, burst strength is
affected significantly by the procedure and equipment
used to make up the pipe.
Tests have shown that burst strength can be reducedby as much as 70% by combinations of tong marks
that penetrate 17% of the wall thickness and 4% out-
ofroundness caused by excessive torque.
Other
considerations
B Mechanical deformationscan also occur while the
i i t t d t l ti hil it i i t th
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casing is transported to location or while it is run into the
hole. Any mechanical deformity in the pipe normally results
in considerable reduction in its collapse resistance. This is
especially true for casing with high dn/t ratios. A thinwall
tube that is deformed by 1% out-of-round will have its
collapse resistance lowered by 25%. Thus, the slightest
crushing by tongs, slips, or downhole conditions diminishesthe collapse resistance by a significant amount.Some of the
special hydrogen-sulfide-resistant casings,such as C90,
can be stress-hardened by careless handling.If this occurs,
the resistance to hydrogen embrittlement can be lost.
Other
considerations
3 compression in conductor pipe
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Since the conductor pipe carries the weight of
other strings, it must be checked for compressionloading.The procedure is to determine the total
buoyant weight of strings carried and then
compare this with the yield strength of the
conductor pipe. A minimum safety factor of 1.1
should be obtained.
In this analysis it is assumed that the tensile
strength of casing is equal to its compressivestrength.
Other
considerations
Example
One submerged weight of 133/8 in, 95/8 in and 7 in strings
Other
considerations
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One submerged weight of 13 /8 in, 9 /8 in and 7 in strings
in a mud weight of 0.465 psi/ft, so that the worst casing is
taken in account.
Hence, BF=(10.465/3.39)=0.863
where 3.39psi.ft is the pressure gradient of steel.
Casing Air weight 1000lb
13
3
/8 in 439.295/8 in 481.25
7in 403.1
Total air weight carried by conductor pipe
=1317050 lb
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3 7050 b
Total buoyant weight carried by conductor pipe
= 13170500.863
=1136614 lb
Yield strength of coupling of top joint of K55,94#=1479000
Hence ,
SF in compression= =1.3
1479000
1136614
Other
considerations
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section 6 other considerations
1 Effect of hydrogen sulfide on casing
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1 Effect of hydrogen sulfide on casingA Hydrogen embrittlement
When hydrogen sulfide is present, the rate at which the
hydrogen atoms (H) combine to form hydrogen gas (H2) is
reduced. As a result, atomic hydrogen (H) may enter the
metal at a significant rate before recombining. The presenceof this molecular hydrogen within the steel reduces its
ductility and causes it to break in brittle manner rather than
yield. This phenomenon known as hydrogen embrittlement.
The resulting failure is called sulfide cracking.
Water must be present for corrosion reaction to occur, whichgenerates hydrogen atoms. Dry hydrogen sulfide does not
cause embrittlement.
Other
considerations
B temperature effect on hydrogen embtittlement
Other
considerations
7/30/2019 Casing Design - Jimmy Wang
152/160
Hydrogen embrittlement is especially significant high-strength steels at low temperature. Common carbon steels with
yield strengths below 90,000psi generally not fail by sulfide
cracking for temperature above 100F.
There is evidence that, as temperature increases, casing with ahigher minimum yield strength than 90,000 psi can be used
safely in wells that contain hydrogen sulfide in the produced
fluids. In deep, abnormally pressured walls, a practical casing
design is difficult to obtain without the use of some high-strength steel.
Kant and Greer have presented the results of experimental laboratory and
field tests of several steel grades that were exposed to hydrogen sulfide in
varying concentrations, at various temperatures, and at various stress levels.
7/30/2019 Casing Design - Jimmy Wang
153/160
Shown in Fig. 7. i6 and 7.17 arc the maximum salt stress levels observed
(expressed as percent of minimum yield strength) for various steel grades,hydrogen sulfide concentrations, and exposure temperatures.
C incubation time
Failure resulting from hydrogen
embrittlement often do not occur
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154/160
embrittlement often do not occur
immediately after exposure to
hydrogen sulfide. A time period duringwhich no damage is evident is
followed by a sudden failure. During
the time period before failure,called
incubation period, hydrogen is
diffusing to points of high stress.Fig.7.18 shows test results of the time
to failure for different RHNs
(Rockwell hardness number) and
different applied stresses. Fig. 7.19
shows the effect of hydrogen sulfide
concentration.
Other
considerations
2 Effect of field handling on casing
A Performance properties that a given joint of casing
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A Performance propertiesthat a given joint of casing
will exhibit in the field can be affected adversely by
several field operations. For example, burst strength is
affected significantly by the procedure and equipment
used to make up the pipe.
Tests have shown that burst strength can be reducedby as much as 70% by combinations of tong marks
that penetrate 17% of the wall thickness and 4% out-
ofroundness caused by excessive torque.
Other
considerations
B Mechanical deformationscan also occur while the
casing is transported to location or while it is run into the
7/30/2019 Casing Design - Jimmy Wang
156/160
casing is transported to location or while it is run into the
hole. Any mechanical deformity in the pipe normally results
in considerable reduction in its collapse resistance. This is
especially true for casing with high dn/t ratios. A thinwall
tube that is deformed by 1% out-of-round will have its
collapse resistance lowered by 25%. Thus, the slightest
crushing by tongs, slips, or downhole conditions diminishesthe collapse resistance by a significant amount.Some of the
special hydrogen-sulfide-resistant casings,such as C90,
can be stress-hardened by careless handling.If this occurs,
the resistance to hydrogen embrittlement can be lost.
Other
considerations
3 compression in conductor pipe
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Since the conductor pipe carries the weight of
other strings, it must be checked for compressionloading.The procedure is to determine the total
buoyant weight of strings carried and then
compare this with the yield strength of the
conductor pipe. A minimum safety factor of 1.1
should be obtained.
In this analysis it is assumed that the tensile
strength of casing is equal to its compressivestrength.
Other
considerations
Example
One submerged weight of 133/8 in, 95/8 in and 7 in strings
Other
considerations
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8 8
in a mud weight of 0.465 psi/ft, so that the worst casing is
taken in account.
Hence, BF=(10.465/3.39)=0.863
where 3.39psi.ft is the pressure gradient of steel.
Casing Air weight 1000lb
133/8in
439.295/8 in 481.25
7in 403.1
Total air weight carried by conductor pipe
=1317050 lb
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Total buoyant weight carried by conductor pipe
= 13170500.863
=1136614 lb
Yield strength of coupling of top joint of K55,94#=1479000
Hence ,
SF in compression= =1.3
1479000
1136614
Other
considerations
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