Present day, oil and gas industries continue to drill more complex, extended reach, deeper and higher dogleg severity wells. Due to this, drillstring elements are subjected to higher loads and several kinds of cyclic stresses. The success of drilling operations rely on keeping the integrity of BHA and the drillstring. One of the most common and catastrophic events related with BHA integrity is twist-off where the BHA component parted downhole. Twist off can be caused by many reasons, such as the load exceeding collar strength, high shock and vibration, corrosion, etc., but the primary cause of twist-offs is due to fatigue failure. Despite the stress is below the material strength specification, the drillstring would fail downhole. This is caused by fatigue accumulated over time. According to some reports fatigue failure contributes to over 60% of total dirllstring failures. Fatigue failure can occur under a wide range of conditions and environments, however there is always a higher risk of fatigue failure when rotating drill collars and tools through high doglegs interval during the drilling process.
With high rig operating costs, the increasing BHA complexity, and wellbore complexity, there is an increased focus to avoid twist-off related expenses such as:
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