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.