Abstract
The amount of tensile pull that can be applied to free a stuck drill string is usually restricted by the drill string's minimum margin of overpull (MOP).1 To free a stuck string, the drill string is often rotated and pulled simultaneously. When this is done, the maximum permitted pull is reduced, depending on the amount of torque applied.
Application of pull, either based on MOP or the decreased tensile rating given the applied torque, has resulted in instances of overload failures of drill pipe. These failures have occurred at loads significantly lower than the drill strings rated capacity. In each of these failures, the formation had packed off around the string and circulation was lost.
This paper presents case studies and lessons learned from the analysis of such drill pipe failures. Metallographic examination of the failed drill pipe has revealed that the frictional forces produced while rotating the string induced sufficient heat to either locally temper the pipe or cause a phase transformation (change in grain structure) of the drill pipe material. As the material gets locally heated (often to temperatures above 1,500°F), the material hardness (and thus strength) decreases. The actual load capacity of the drill string depends on the temperatures reached downhole.
This paper discusses the factors that contribute to these failures and provides test data to support the findings. In addition, this paper presents the limitations of currently available methods used to determine the maximum allowable loads that may be applied when packed off with no circulation.
