API Drilling Collar Selections
Woods and Lubinski pointed out that using an unstabilized Drilling Bit and small OD drill collars can cause an undersized hole, making it difficult for Casing Running. They determined that the actual drift, or useful diameter, of the hole would be equal to the bit diameter plus the drill collar diameter, divided by two,
The above equation can be rewritten to determine the minimum drill collar OD which would insure passage of the larger Casing & Casing Coupling diameter. Substituting casing coupling diameter for drift diameter,
As a general rule, the largest drill collars that can be washed over and fished out should be selected for the best BHA Design.