PETE 203
DRILLING ENGINEERING
CASING DESIGN
Objectives
Understand primary functions of casing
Recognize the various types of casing strings
used.
Understand the procedures used in the design
of casing strings.
Types of Strings of Casing
1. Drive pipe or structural pile
{Gulf Coast and offshore
only} 150-300 below mud-
Diameter
Example
16-60
30
16-48
20
8 5/8-20
13 3/8
line.
2. Conductor string. 100 1,600 (BML)
3. Surface pipe. 2,000 - 4,000
(BML)
Types of Strings of Casing
Diameter
4. Intermediate String
5. Production String (Csg.)
6. Liner(s)
7. Tubing String(s)
7 5/8-13 3/8
4 1/2-9 5/8
Example
9 5/8
7
Example Hole and String Sizes (in)
Hole Size
Pipe Size
36
Structural casing
30
26
Conductor string
20
17 1/2
Surface pipe
13 3/8
12 1/4
IntermediateString
9 5/8
8 3/4
Production Liner
Example Hole and String Sizes (in)
Hole Size
Pipe Size
36
Structural casing
30
26
Conductor string
20
17 1/2
Surface pipe
13 3/8
12 1/4
IntermediateString
9 5/8
8 3/4
Production Liner
Example Hole and String Sizes (in)
Structural casing
Mudline
Conductor string
250
1,000
Surface pipe
Intermediate String
Production Liner
4,000
Example Casing Programs
CONDUCTOR (1 or 2) (40 300)
Prevent eroding surface sediments and rig
foundation by circulating the drilling fluid to the
shale shaker.
Protects subsequent casing strings from corrosion.
Install director system on it.
SURFACE (300' - 5000')
Control caving and washing out of poorly
consolidated surface beds.
Protect fresh water sands from possible
contamination by drilling fluid mud, oil or gas and or
salt water from lower zone.
Install BOP on it.
INTERMEDIATE CSG (1 or 2)
It depends on well depth and geology in specific area.
Primary Purpose: To seal off troublesome zones which:
Contaminate drilling fluid.
Jeopardize drilling progress with possible pipe sticking,
excessive hole enlargement.
Contain abnormal pressure fluids, protect formation
below the surface casing from higher pressure
credited by mud.
PRODUCTION CSG
1.It is set through productive interval to;
2. Segregate pay zone.
3. Can be used to produce fluid instead of tubing.
DRILLING LINER
1. Lower in cost.
2. Functions like intermediate.
PRODUCTION LINER
1. Lower in cost
2. Functions like production.
6.2 Standardization of Casing
API:
American Petroleum Institute Standards.
Range of length
Outside diameter
Wt/ft.
Type of coupling
Steel grade
Classification of CSG.
1. Outside diameter of pipe
(e.g. 9 5/8)
2. Wall thickness
(e.g. 1/2)
3. Grade of material
(e.g. N-80)
4. Type to threads and couplings
(e.g. API LCSG)
5. Length of each joint (RANGE)
(e.g. Range 3)
6. Nominal weight
(Avg. wt/ft incl. Wt. Coupling) (e.g. 47lb/ft)
Length of Casing Joints
RANGE
16-25 ft
RANGE
25-34 ft
RANGE
> 34 ft.
OUTSIDE DIAMETER
Tolerance
(4.5 -20")
0.75%
Usually slightly oversized.
Minimum permissible
Wall thickness = 87.5% of nominal wall thickness.
Nominal = approximate average
Drift diameter : Check ID
Minimum mandrel diameter that must pass
Unobstructed through the pipe.
Insures a bit size less than drift diameter.
WEIGHT PER FOOT
Nominal wt/ft :
is not true wt/ft but is useful
for
identification purposes as an
approximate
average (wt/ft)
Plain-end wt/ft: is the wt/ft of the pipe body
excluding
the threaded portion and coupling wt.
Average wt/ft:
total wt. of avg. joint of
threaded pipe
with a coupling attached power
tight of one of avg. joint.
For design calculation nominal wt/ft. is often used.
6.3 Type of Coupling
A coupling is a casing connector which is made
of casing material.
Most common types of casing. Joints are
externally threaded from each end.
API specification the coupling should be of the
same grade as the pipe body.
Casing Threads and Couplings
API round threads - short
{ CSG }
API round thread - long
{ LCSG }
Buttress
{ BCSG }
Extreme line
{ XCSG }
Other
CSG & LCSG
These connectors have the same basic design:
Threads are round shaped and are spaced to give eight
threads/inch.
Sometimes they are called API 8-round threads.
Threads are cut with a taper of 3/4 in/ft.
These are commonly used connectors because of their
proven reliability, ease of manufacture and low cost.
Cut with a 60 angle, and has round peaks and roots.
Thread compound must be used to fill the voids and obtain a
seal.
Tensile strength of the joint
Joint Efficiency =
< 100%
Tensile strength of the pipe body
API BCSG CONNECTOR
Joint efficiency is 100% in most cases.
It is tapered but longer thread run out
inch/ft. for upto 75/8 inch.
Thread shape is square to reduce unzipping
tendency.
5 threads cut to the inch.
1 inch/ft for 16 inch csg.
API XCSG CONNECTOR
It is integral joint.
Pipe thicker near the wall.
OD is less than other API couplings
Sealing mechanism is metal to metal seal
between the metal and the box.
Much more expensive.
API Connectors
STRENGTH
Strength is designated by casing GRADE
Grade code : Letter and number H-40, J-55, C75, L-80
Letter is arbitrary.
Number designates the minimum yield strength of
steel in thousands of psi.
Yield strength :tensile stress required to produce a
total elongation per unit length of 0.005 on a
standard test specimen.
Minimum yield strength = 80% of average yield
strength observed.
Grades of Casing Recognized by the API
API Connectors
Tensile force balance on pipe body
Example 7.1:
Compute the body-yield
strength for 20-in., K55 casing with a nominal
wall thickness of 0.635
in. and a nominal weight
per foot of 133 lbf/ft.
Ften yield * A s
Tensile force balance on pipe body
K55
Solution:
This pipe has a minimum
yield strength of 55,000 psi
and an ID of:
Ften yield * A s
d 20.00 2(0.635) 18.730in.
Tensile force balance on pipe body
Thus, the cross-sectional area of steel is
2
2
A s ( 20 18.73 ) 38.63sq.in.
4
and a minimum pipe-body yield
is predicted by Eq. 7.1 at
an axial force of:
Ften yield * A s
Ften 55,000 (38.63 ) 2,125,000 lbf
Pipe Body Yield Strength
2 2
Py (D d )Yp
4
where
Py pipe body yield strength, lbf
Yp specified minimum yield strength, psi
D outside diameter of pipe, in
d inside diameter of pipe, in
Pipe Body Yield Strength
Example
What is yield strength of body of 7, 26 #/ft,
P-110 casing?
2
Py (D d2 )Yp
4
2
Py (7 6.276 2 )110,000 830,402 lbf
4
Py 830,000 lbf
(to the nearest 1,000 lbf).
agrees with Tables
Internal Yield Pressure for Pipe (Burst)
2Yp t
P 0.875
D
where
FT
FP
FP = DLP
P internal yield pressure, psi
Yp minimum yield strength, psi
DLP = 2tLYP
t nominal wall thickness, in
D O.D. of pipe, in
2Yp t
P
FT = 2tLYP
Example
For 7, 26 #/ft P-110 pipe
2Yp t
P 0.875
(7 - 6.276)
0.875 * 2 * 110,000 *
2*7
9,955
P 9,960 psi
(to the nearest 10 psi)
agrees with Tables.
TABLE 7.8 COMMONLY USED BIT SIZES THAT WILL PASS THROUGH API CASING
Casing
Size
(O.D., in)
Weight
Per Foot
(lbm/ft)
Internal
Diameter
(in.)
Drift
Diameter
(in.)
Commonly
Used
Bit Sizes
(in.)
9.5
10.5
11.6
13.5
4.09
4.052
4.000
3.920
3.965
3.927
3.875
3.795
3 7/8
3
11.5
13.0
15.0
18.0
4.560
4.494
4.408
4.276
4.435
4.369
4.283
4.151
4
3 7/8
Casing Design - Collapse
Casing Design - Tension
Casing Design - Burst
(from internal pressure)
Internal Yield Pressure for pipe
Internal Yield Pressure for couplings
Internal pressure leak resistance
Internal
Pressure
Casing Design - Burst
Example 1
Design a 7 Csg. String to 10,000 ft.
Pore pressure gradient = 0.5 psi/ft
Design factor, Ni=1.1
Design for burst only.
Burst Example
1. Calculate probable reservoir pressure.
pres
psi
0 .5
* 10,000 ft 5,000 psi
ft
2. Calculate required pipe internal yield
pressure rating
pi pres * Ni 5,000 * 1.1 5,500 psi
23 lb/ft
26 lb/ft
N-80
Example
3. Select the appropriate csg. grade and wt.
from the Casing tables:
Burst Pressure required = 5,500 psi
7, J-55, 26 lb/ft has BURST Rating of 4,980 psi
7, N-80, 23 lb/ft has BURST Rating of 6,340 psi
7, N-80, 26 lb/ft has BURST Rating of 7,249 psi
Use N-80 CSG, 23 lb/ft
API Design Factors
Required
(typical)
Design
10,000 psi
Collapse
1.125
100,000 lbf
Tension
1.8
10,000 psi
Burst
1.1
11,250 psi
180,000 lbf
11,000 psi
Table 3
Grade
D/t Ratio
H40
16.44 and less
J & K55
14.8 and less
C75
13.67 and
N80
13.38 and less
C95
12.83 and less
P105
12.56 and less
P110
12.42 and less
c 2Ym D / t 1 / D / t
(4)
