Drill collars are thick walled tubulars that are used at the bottom of the drill string (see Figure 3-1). Their principal purpose in the drill string is to provide the axial force needed to advance the drill bit (see Figure 1-2). When drilling a vertical borehole, the axial force is the weight of the drill collars. Drill collars are available in API range lengths given in Table 1-1. Figure 3-16 shows a BHA with Range 2 (~30 ft long) drill collars. Range 2 lengths are typical for double and triple land rotary drilling rigs. Also shown in Figure 3-16 are short drill collar lengths used to adjust positions of the stabilizers in the BHA. These shorter drill collar lengths are selected from Range 1 stock of drill collars.
Appendix B gives the dimensions and mechanical properties for API drill collars. These drill collars vary in outside diameter from 2 7/8 inches to 12 inches. Table B-1 shows that a drill collar having an outside diameter of 6 3/4 inches can be obtained with an inside diameter of 1 1/2 inches, 1 3/4 inches, 2 inches, 2 1/4 inches, 2 1/2 inches, 2 13/16 inches, 3 inches, 3 1/4 inches, and 3 1/2 inches. From this same table a 6 3/4 inch Column (1), by 2 13/16 inch Column (9), drill collar will have a weight per unit length of 100 lb/ft and a standard 30 ft length of this drill collar would weigh 3,000 lb. Table B-2 shows that the 6 3/4 inch Column (3), by 2 13/16 inch Column (11), drill collar is available with either, the 5 inch Double Streamline, or the 4 1/2 inch H-90 connections, Columns (1) and (2). The recommended make-up torque for each connection type is 22,426 ft-lb, 23,159 ft-lb, respectively, Column (11). These are the recommended make-up torques required to insure that the shoulder connections will not leak circulation fluid and will remain tight when used in a rotary drill operation.
Drill collars are usually fabricated from American Iron and Steel Institute (AISI) 4140 or 4145 heat treated steel. These are chrome-molybdenum steel alloys and have yield stresses in excess of 100,000 psi. In addition to drill collars fabricated of steel are special drill collars fabricated of nonmagnetic nickel alloys (e.g., usually Monel K-500). These nickel alloy drill collars (usually three) are used at the bottom of the drill string to allow magnetic compass like equipment to be used to survey the borehole as the well is drilled. These nickel alloy drill collars have material properties that are almost identical to that of the AISI 4140 heat treated steel of the standard drill collars.
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