The burst loads on the casing must be evaluated to ensure the internal yield resistance of the pipe is not exceeded.
The burst load is the force applied by the fluid inside the casing which acts to rupture the pipe in the absence of external pressure.
The loads are normally caused by mud hydrostatic pressure inside the casing and perhaps some surface pressure. Fluids on the outside of the casing, called back-up fluids, supply a hydrostatic pressure that helps resist pipe burst.
The resulting effective burst pressure is the internal pipe pressure minus the external pressure. Burst conditions are established and the least expensive pipe that will satisfy the burst pressure is tentatively selected.
The API burst pressure rating of casing is given by:
Ym: is the minimum yield strength of the pipe
t : is the wall thickness in inches.
De: is the outer diameter in inches.
Example:Calculate the burst rating for 7” 23# J-55 casing.
From Table 2, Ym for J-55 casing is 55,000 psi. The ID of 7” 23# casing in 6.377”, therefore,