Exact values of loading are difficult to predict through out the life of the well.
For example, if mud of 75 pcf is on the outside of the casing during the running of the casing, this value cannot be expected to remain constant for the entire life of the well. The mud will become deteriorated with time and will reduce this value to perhaps a saltwater value of 64 pcf.
Therefore, calculations of burst values assuming a column of mud at 75 pcf are not realistic throughout the life of the well. If the initial casing design is marginal, then over a period of time in the event of a gas leak the casing may burst.
Since casing design is not an exact technique and because of the uncertainties in determining the actual loadings as well as the deterioration of the casing itself due to corrosion and wear, a safety factor is used to allow for such uncertainties in the casing design and to ensure that the rated performance of the casing is always greater than any expected loading.
In other words the casing strength is always down rated by a chosen safety factor value.
Usual safety factors are:
For Casing Collapse Pressure Calculations: 1.125
For Casing Tension Strength Calculation : 1.6
For Casing Burst Calculations: 1.1
The safety factor is determined by the ratio of the body resistance to the magnitude of the applied pressure.