Drill collar is connected to the drill pipe, which plays the role of overall weighting to prevent hollow slanting, the inner diameter of the drill collar is smaller than the drill pipe, and the weight is larger than the drill pipe.
In the drilling process, the drilling pressure is formed by the weight of the drill pipe, so the lower part of the drill pipe is subjected to compressive stress. When drilling, the value of the drilling pressure is generally greater than the critical value of the pressure bar stability of the drill pipe, because the lower part of the pressurised part of the drill pipe will be bent under the action of self-weight. The bent drill pipe will cause alternating stresses during rotation, leading to fatigue damage of the drill pipe. In order to reduce the degree of fatigue damage, the bending stress should be reduced. This requires minimising the length of the compression section and increasing its rigidity. Therefore, instead of using a drill pipe, a drill collar is used for the pressurised section at the lower end of the drill pipe.