Drillcollars are tubulars which have a much larger outer diameter and generally smaller inner diameter than drillpipe. A typical drillstring would consist of 9” O.D. x 2 13/16” I.D. drillcollars and 5” O.D. x 4.276” I.D. drillpipe. The drillcollars therefore have a significantly thicker wall than drillpipe. The function of drill collars are:
• To provide enough weight on bit for efficient drilling
• To keep the drillstring in tension, thereby reducing bending stresses and failures due to fatigue.
• To provide stiffness in the BHA for directional control.
Since the drillcollars have such a large wall thickness tooljoints are not necessary and the connection threads can be machined directly onto the body of the collar. The weakest point in the drill collars is the connection and therefore the correct make up torque must be applied to prevent failure. The external surface of a regular collar is round (slick), although other profiles are available.
Drill collars are normally supplied in Range 2 lengths (30-32 ft). The collars are manufactured from chrome-molybdenum alloy, which is fully heat treated over the entire length. The bore of the collar is accurately machined to ensure a smooth, balanced rotation. Drill collars are produced in a large range of sizes with various types of joint connection. The sizes and weight per foot of a range of drillcollar sizes are shown in Table 14. The weights that are quoted in Table 14 are the “weight in air” of the drillcollars.
It is very important that proper care is taken when handling drill collars. The shoulders and threads must be lubricated with the correct lubricant (containing 40-60% powdered metallic-zinc or lead).
Like drillpipe, collars are subjected to stresses due to:
• Buckling and bending forces
• Alternate compression and tension.
However, if properly made up, the shoulder/shoulder connection will be sufficient to resist these stresses. Figure 6 shows how numbered connections should be selected to provide an efficient seal, and adequate strength.
Institute of Petroleum Engineering, Heriot-Watt University