Teikoku Oil Company has experienced frequent and continuous drill collar failures on drill collar connections in deep wells, causing significant expense and inefficiency. Investigations about drill collar connection failure have included metallurgy, corrosion and stress analyses to elucidate the cause of failure happened on many of the drill collars.

Several metallurgical inspections reveled that all failed drill collars were produced under proper quality control. Sulfide stress corrosion cracking and hydrogen brittle wear were also considered, but any evidence was not detected.

Although stress analyses on connections both of pins and boxes were done, the values of triaxial stress showed that all failed drill collar have been used under the normal stress conditions. Both of bending stresses ratio and stiffness stresses was satisfied API RP7G respectively.

The remaining considerable cause of failure was fatigue failure. Teikoku Oil Company carried out a number of drill collar failures reviews on each size of drill collar, reviewing a significant number of wells.

The result of investigations showed that total drilling revolutions of drill collar till failure were very similar numbers. After these investigations, standards for the maximum total drilling revolutions of drill collar to prevent fatigue failure were decided for each size of drill collar. After the critical total revolutions, all drill collars used for drilling were sent to the maintenance shop for magnetic particle test.

Teikoku found that even though magnetic particle test was done, drill collar failure still occurred. Once establishing the critical total drilling revolutions for drill collars, drill collar failure significantly decreased. The company continues to perform magnetic particle tests by drilling crews at rig site for cost saving without any problem.


A lot of drill collar failures including the breakage of connection and the generation of the crack have happened in the wells Teikoku Oil Company has drilled in the past. Table1 shows the list of the drill collar failures in the wells until 1993. All drill collar failures have been happened in the PIN or the BOX, and occurred when drilling a large diameter holes which diameter was 12–1/4" or more. The inspection of a drill collar till then was the visual check connection and checking the inside and outside diameter of drill collar while break out the connection. And if some kind of failures was observed, the connection was re-cut or the drill collar was scraped. Moreover, after finish drilling the well, the Magnetic Particle Test was executed in the maintenance shop. It was thought that it was necessary to take some measures for the failures of drill collar at that time, and the examination were executed from some aspects. The checking of material of drill collar, corrosion analysis, and the stress analysis etc. were executed. However, it has been understood that all drill colors were produced under proper quality control, and the corrosion environment like the sulfide stress corrosion cracking etc. were not detected. Moreover, the all failed drill collars have been used under the normal stress conditions as a result of the stress analyses on connections both PIN and BOX. Bending Stress Ratio (BSR) and Stiffness Ratio (SR) of drill collar, which are showed in Table2, were also satisfied API RP7G respectively. However, the failures of drill collars happened frequently also in the well afterwards.

Attention of total revolutions of drill collars

In the also "Nishikubiki" well drilled from 1995 through 1996, the drill collar failures were observed frequently. At that time, it was thought that the main cause of such drill collar failures was fatigue. Then, the total revolution of drill collars until failure occurred ware examined about the failures that occurred in this well.

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