Why Kelly Bar Breaks During Construction?

The phenomenon of kelly bar breakage refers to the breakage or total disconnection of a section or oblique section of a kelly bar.

Analysis of broken kelly bar: No inspection, no repair, crack spreading, hard strata, brutal operation.

When drilling in the main working condition, the kelly bar is subjected to torque, shaft pressure, impact and other kelly bar stresses, and a section of the pipe body breaks instantly at the weak point. The kelly bar is so badly damaged that it cannot be used anymore, and it is necessary to stop work to repair or replace the kelly bar to resume construction.

Analysis of the causes of broken kelly bar


When the drive key, locking table and key strip are worn out, the slope chamfer appears, and when transferring torque and shaft pressure, they are squeezed to the inner side of the pipe, resulting in cracking of the pipe body, metal fatigue, and gradual diffusion of stress, which eventually leads to broken kelly bar.

Off set hole

The metal of the pipe body was fatigued due to the deviated hole, and the stress spread after the crack appeared, and the drill pipe was broken without timely repair.

Hard rock

Hard rock/rock with single-axis pressure of several tens of MPa, high rock strength, high drilling resistance, the need to output greater axial pressure shear crushing, drilling rod under huge axial pressure, prone to fracture.

Defects of drilling tools

The drilling tools are not selected according to the rock and soil conditions, the combination of drilling teeth is poor, resulting in heavy drilling load and resistance, as well as poor positioning and guidance, resulting in deviated holes, etc.

Brutal operation

High geological strength, large hole diameter, wear or defects in the drilling tools, resulting in a rise in drilling load, the operator does not look for the reason, but only high torque, high shaft pressure output, the front track has been supported, but also continue to pressurize, resulting in a heavy load on the kelly bar, such as the kelly bar has cracks, weak points or fatigue, it is likely to cause twisting or breaking.

Large hole diameter

Large hole diameter drilling has a high and heavy load, and the kelly bar is in high load condition for a long time, especially the core section, or a section of the kelly bar has cracks and is easy to twist off.

Long-term non-overhaul

Cracks in the kelly bar body are not found and repaired in time, resulting in stress diffusion at the cracks, which eventually leads to broken kelly bars.

No control of nodes

The kelly bar is not controlled at each node, and the blocking sleeve is impacted by the flat blocking sleeve, resulting in cracks at the blocking sleeve for a long time, and when the kelly bar is impacted again, the blocking sleeve is broken.

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