Failure occurred on a threaded joint casing at a shallow depth of about 80-100 feet near the drill bit about 30-40 feet from the bottom of the hole – on the male pin connection at the root of the last thread. We were asked to investigate.
The drilling includes two wings that expand once the bit outside the casing to create a 7 3/4 inch cutting area. Possibly while encountering an asymmetric hard formation, such as limestone, an eccentric loading occurred on the casing. This stress condition caused additional stresses on the threaded connection. We subsequently designed a new threaded profile to minimize this effect.
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