Drill collars are thick-walled pipe that provide stiffness and concentration of weight at the bit.
Drill collars comprise heavy, thick-walled tube, usually steel, that are part of the bottom hole assembly (BHA) which can be placed in compression and and which are placed between the drill pipe and bit in the drill stem to provide:
1) Weight to the Bit,
2) An Rotational Inertia, and
3) Stiffness which predictably bends as a pendulum during rotation based on the number of stabilizers with the BHA.
Drill collars can have a "slick" outside surface or spirals can be placed in the surface to reduce sticking tendencies of the collar to the mud caked side wall of the bore when the rotation is stopped.
Drill collars may also have elevator and slip profiles built into the pipe bode to improve handling.
Bottom Hole Assembly (BHA)
The flexure of drill collars and hevi-wate drill pipe about a fulcrum know as a drill stabilizer allows bending of the drill string during rotation so that the direction of drilling can be controlled. The components and placement of drill collars, stabilizers and hevi-wate drill pipe are calculated to allow and/or prevent bending of the drill string during drilling. If the BHA is arranged for bending it is typically called a pendulum assembly because it forms the pattern of a pendulum when rotated as the same time that it is bent using the weight of the BHA.
Hevi-Wate Drill Pipe Bending
Drill Collar Mass Inertia
The thickness of steel walls and the mass or weight of the drill collars are important because they provide weight on the drilling bit so that it can cut into the rock and provide the inertia to keep the bottom hole assembly rotating when the bit digs into the rock and/or when significant friction exists.
Drill collars are relatively stiff but they can bend during rotation. For example, a drill string can bend by 1 to 3 degrees over 30 metres or 100 feet to bore a hole which is directionally orientated. The above diagram shows that by (1) placing a stabiliser to act as a fulcrum point close the the bit, the drill collars can bend and the BHA can be made to build or increase the angle of the hole it is boring; or alternatively by (2) placing a stabiliser acting as a fulcrum point away from the bit, the BHA and resulting borehole can drop or decrease in angle; or alternatively by (3) placing multiple stabilisers along the BHA to prevent bending and form what is known as a packed BHA, the BHA can drill a relatively straight hole.