Down The Hole Drilling Trouble Shooting – Treaded

March 11, 2018

General

This chapter contains some common types of failures occurring on DTH drilling tools
For most failures we can identify it’s probable cause and typical supporting evidence
Corrective actions to avoid recurrence of the failures are also described

Number 1

Problem
- Lower end thread failure due to fatigue cracking at the root of the first engaged thread near the base of the pin
Probable cause
- No evidence of contact between the shoulder of the box thread and the pin indicates that the thread has not been made up correctly
Corrective action
- Check for proper make up of threads before drilling starts

Number 2

Problem
- Upper end thread failure due to bending stresses
Probable cause
- Evidence of partial contact between the shoulder of box thread and the top sub indicates that the box has been bent during drilling
Corrective action
- Monitor outside wear on drill pipe and chamfer on box thread

Number 3

Problem
- Upper end thread failure due to fatigue cracks
Probable cause
- It is evident that this failure started concurrently on two sides of the pin and that is an indication that the pin has been wobbling during drilling
Corrective action
- Monitor that sufficient make-up torque is applied when connecting drill pipes

Number 4

Problem
- Thread failure near the pin box shoulder
Probable cause
- An examination of the resultant thread shows that the pin thread has been stretched in tension before failure due to too much make-up torque
Corrective action
- Limit hydraulic pressure to rotary head

More Articles