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Tapered Strings
A tapered drill collar string is one in which the diameters of the collars change gradually over the length of the string. Its purpose is to reduce the potential for connection failures and fatigue damage by ensuring that changes in cross-sectional area from the larger to the smaller drill collar are not too abrupt.
The largest-diameter collars should be placed at the bottom of the string in a vertical hole interval, and should gradually be tapered upwards toward the smallest drill collars or heavy-wall drill pipe.
Smith International (1988) has published the following general rules-of-thumb for running tapered drill collar strings:
- The stiffness of the larger-diameter drill collar should not exceed 3.5 times that of the adjacent, smaller-diameter drill collar, where
Stiffness=EI
E= Young’s Modulus:
E for steel (Ni-200): 30.0⋅106 psi
E for Aluminum: 10.6⋅106 psi
I= Axial Moment of Inertia, in4
OD= pipe outside diameter, inches;
ID= pipe inside diameter, inches
- The section modulus (SM) of the larger drill collar should be no more than 5.5 times that of the smaller drill collar, where
- Do not reduce the drill collar outside diameter (OD) more than two inches at any crossover (for example, if the bottom collar OD is 10 inches,” the collar above the crossover should have an OD no smaller than 8 inches);
- Run at least three collars (i.e., one triple stand) of the next smaller size with each size change.
