Drill CollarsDrill collars are large diameter-small bore steel pipes which posses a great weight and great stiffness. Drill collars are run above the Drilling Bit and make up the predominant component of the Drilling BHA.
What is the functions of Drill Collars:
- Provide weight on the bit (WOB)
- Minimize bit stability problems from vibrations, wobbling and jumping
- Minimize directional control problems by providing stiffness to the BHA.
Drill Collar ThreadSince the drill collars have such a large wall thickness Tool Joints 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 Collar LengthsDrill 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 Collar SizesDrill 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 drill collar sizes are shown in Table 14. The weights that are quoted in Table 14 are the “weight in air” of the drill collars. 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).
Stress subjected to Drill Collars:
- Buckling and bending forces
- Alternate compression and tension.