2. Design Factors Collapse Corrected
Casing under axial load has an effective yield 15

3. Design Factors Collapse Corrected
Example
Compute the corrected collapse-pressure rating for 20”, K55, 133lb/ft for
In service conditions where the axial tension will be 1,000,000 lbf. Also compute
the minimum external pressure required for a failure if the internal pressure
will be 1000 psig.
Area of the steel is ”2

4. Design Factors Collapse Corrected
Sub into equation 15

5. Design Factors Collapse Corrected
D/t =
Collapse will be in transition mode

6. Design Factors Collapse Corrected
For Y = 36,385
F1 = 2.941
F2 = .0446
F3 = 645.1
F4 = 2.101
F5 = .0319
This is the corrected pressure differential, so for an internal pressure
1000 psig, the collapse pressure will be = 2267 psi

7. Design Factors Collapse Corrected
The boundaries for the failure regions also change with the change of
yield.
Upper limit for Yield – strength collapse

8. Design Factors Collapse Corrected
The Upper limit for Plastic Collapse range
The Lower limit for Elastic Collapse range
Transition range is between these limits

9. Design Factors Bending
In directional wells, the effect of wellbore curvature and vertical deviation angle on axial stress in the casing and couplings must be considered in the casing design.
When a casing is forced to bend, the axial tension on the convex side of the bend can increase greatly.
On the other hand, in relatively straight sections of hole with a significant vertical deviation angle, the axial stress caused by the weight of the pipe is reduced.

10. Design Factors Bending
The maximum increase in axial stress, sb, on the convex side of the pipe is given by Crandall and Dahl (1995)
sb = ± 0.5EdnK.
In oil field units, where dogleg severity, K, is expressed as the change in angle in degrees per 100 ft; E (lbf/in2) = stress/strain is elastic modulus.
sb = ± 218dnK.
In terms of an equivalent axial force
Fab = ± 218dnKAs.
In terms of weight per foot of pipe
Fab = ± 64dnKwp.

11. Design Factors Bending
Example: Determine the maximum axial stress for in, 39-lbf/ft, N-80 casing if the casing is subjected to a 400,000 lbf axial tension load in a portion of a directional wellbore having a dogleg severity of 40/100ft. Assuming uniform contact between the casing and the borehole wall.
Note that, the definition for the pipe body yield strength:

13. Design Factors Bending
Solution: Nominal API pipe body yield strength for this casing is 895,000 lbf, and the ID is The cross sectional area of steel in the pipe body is
A = p/4( – ) = in2.
The axial stress without bending: 400,000/ = 35,740 psi
The additional axial stress on the convex side of the pipe due to the bending
sb = ± 218dnK = 218(7.625)(4) = 6,649 psi
The total axial stress = 35, ,649 = 42,389 psi

14. Designing Casing Program
Design Selection
Casing strings most met all the future conditions in the well.
Drilling Completions -- Production
Designing Casing Program
Start with the smallest string, usually the production string
What size is required
Completion
Frac job
Production
Lift methods

15. Design Selection Intermediate casing strings
Number depends on hole conditions.
The last of the intermediate string must be large enough
so that the hole out from under it is large enough for
the production casing that is needed.
The next string up the hole, another intermediate or surface
must be large enough to drill the proper size hole.

17. Design Selection
Basically all wells will have a surface string and production string.
The number of intermediate strings depend on the well conditions
Formations that cause problems when drilling
Under of over pressured zones
Frac gradient
Lost circulation zones
Most of the conditions can be anticipated before drilling the well

18. Design Pipe Selection Combination Strings
For a more efficient design use multiple weights and grades.
Differing conditions in the wellbore require different specs.
Some cases a liner will be more economical

19. Design Pipe Selection
The string must be designed not to fail while running and
during operations.
The three failure modes, Tension, Burst and Collapse

20. Design Pipe Selection Example Intermediate String
Tension, the top joint must be able to support the
weight of the string. Joint strength
Burst, the burst rating of the job joint will control
the pressure during well control.
Collapse, there will be greater pressure outside the
bottom of the string while cementing.
The casing tables can be used for Joint Strength and Burst,
but collapse must be calculated for the load.