2. Design Calculations Combination Strings
Calculate the lower section, collapse
Example TD 4000 ft, MW 10 #, 8 5/8” casing
Assume empty pipe, the maximum condition
Using a SF of BHP 2496 psi
K55 24# PC psi
K55 32# PC psi
Lower section is 32# next section 24#
Depth of Bottom 24# section = 1370/.054 x MW x 1.2 = 2196 ft
Length of 32# section 1804 ft, weight of section 57,744 #
Stress 8327# se psi
F1 = F2 = F3 = F4 = F5 = .0348
Gives Elastic Lower Plastic Upper d/t 32.7
so transition mode

3. Design Calculations PcT = 1335 psi
Sub this pressure back in, new Depth 2139 ft, Length 1861 ft
weight 59,538 # stress psi se 50,202
New Pc = 1333 psi close enough
Check joint strength 24# 263,000# SF ,421#

4. Design Calculations Surface Casing
Cement is circulated, so design for running.
Mainly joint strength
Intermediate String
If cement is circulated, design for running.
Check collapse for conditions while cementing.
If not circulated, design must also take into account
drilling conditions, at the TOC. Mud in the annulus
and the possibility of empty casing.
Burst pressure below the BOP.

5. Design Calculations So take the 8 5/8” casing example,
The total weight of the string of 32 and 24# would be 113,120#
JS of the 24# with SF 1.9 is 138,000# so that would hold.
For circulated cement, assume the cement will weight 14.5 #/gal
the plug displaced with water, 8.33 #/gal or a difference of 6.7 #/gal
in the annulus or a gradient of .33 psi/ft.
For a TOP, example 200’ of cement, start the calculations at that point.
The burst for the top section is 2950 psi, suitable for 2500 psi wellhead

6. Design Calculations Production String Rarely circulated
Design for drilling, completion and production.
Collapse is designed using empty casing, possible
during production if pumped with a rod pump,
gas production without a packer
Burst for completions work, frac job down casing
top section is the weakest for burst
Joint strength for running the casing