CASING LOADS DESIGN AND GRAPHICAL METHOD
IntroductionOnce Casing Seating Depths and Casing Size of each casing string are established, the weight, grade and couplings used in each casing string can be determined.
Loads Affecting Casing StringsIn general, each casing string is designed to withstand the most severe loading conditions anticipated during casing placement and the life of the well. The loading conditions that are always considered are Casing Burst, Casing Collapse, and Casing Tension.
When appropriate, other loading conditions such as bending or buckling must also be considered. Because the loading conditions in a well tend to vary with depth, it is often possible to obtain a less expensive casing design with several different weights, grades, and couplings in a single casing string.
It is often impossible to predict the various loading conditions that a casing string will be subjected to during the life of the drilled well. The assumed design load must be severe enough that there is a very low probability of a more severe situation actually occurring and causing casing failure. When appropriate, the effects of casing wear and corrosion should be included in the design criteria. These effects tend to reduce the casing thickness and greatly increase the stresses where they occur.
Casing Design Criteria Means Most Economical CasingIn general, minimum cost is achieved when casing with the minimum weight per foot in the minimum grade that will meet the casing design criteria is selected.
For this illustration, only API casing and couplings will be considered in the example applications. It will be assumed that:
- The cost per foot increases with the burst strength.
- The cost per connector increases with increasing joint strength.