High frequency of wash-outs and cracks in drillstring has been experienced in a specific formation in oil fields located in the Southern part of Iran. The rig time consumed on this challenge was high due to time spent on replacing damaged pipes. In a few cases fishing operations were necessary to recover twisted off drillstrings. The majority of drillstring failures happened during top hole drilling (17-1/2" hole section). So far several solutions have been tried out to overcome the drillstring failure, which to some extent has decreased the washout frequency.

In this work we tried to define the challenge as precisely as possible; where, how and why the problem occurred. The state-of-the-art with respect to cause/solution to pipe washouts is included in the paper. Possible reasons and practical solutions to this challenge are evaluated.

Analysis of relevant factors that affect drillstring failure revealed that drillstring vibration in torsional mode is the most probable factor to induce drillstring failure in the hazard formation. This factor is therefore discussed in detail. Inasmuch as the failure occurred just in one particular formation, the lithology that induced vibration is another factor that was investigated and analyzed in this study. The complete understanding of the cause of the problem has however still not been reached.

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