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Exact values of loading are difficult to predict throughout 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 (check this paper about mud deterioration) 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.
Why we need a safety factor?
Casing Design is not an exact technique because of the uncertainties in determining the actual loadings ( which is related to Casing Design Loads Cases) as well as the deterioration of the casing itself due to corrosion and wear. We use API Casing Design Safety factors to allow for such uncertainties 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 downrated by a chosen safety factor value.
API Casing Design Safety Factors
The safety factor is determined by the ratio of the body resistance to the magnitude of the applied pressure. Below, you shall find the API SF for collapse, tension, burst, Triaxial, Compression calculations.
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- API SF for Casing Collapse Pressure design: 1.125
- API Safety factor for Casing Tension Strength design: 1.6
- API SF for Casing Burst: 1.1
- API Safety factor for Casing Triaxial Design: 1.25
- API SF for Casing Compression Design: 1.2
- Field Testing of Casing-string Design Factors , J.E. Saye; T.W.G. Richardson