FILI High Air Pressure DTH Drill Bit

Function and Design Considerations of Drill Collars

Drill collars are components connected to the drill string, primarily designed to add weight and prevent hole deviation. The inner diameter of the drill collar is typically smaller than that of the drill pipe, but it is heavier. During drilling, the weight of the drill string (composed of the drill pipe) generates the necessary weight on the drill bit, resulting in compressive stress at the lower part of the drill string.

When drilling, the applied weight on the drill bit is usually greater than the critical buckling load of the drill string, causing the lower section to bend under its own weight. This bending can induce alternating stress as the drill string rotates, leading to fatigue failure. To mitigate the risk of fatigue damage, it is essential to reduce bending stress. This requires shortening the length of the compressed section and increasing its rigidity. Therefore, the lower section of the drill string that experiences compressive stress typically uses drill collars instead of drill pipes to enhance overall stability and durability.