Abstract

Drill string twist-offs are an expensive problem to the oil and gas drilling industry. With ever increasing rig and operating costs, there is an increased focus to avoid twist-off related expenses, such as: costs associated with time required to trip out of hole and replace the failed components, the cost of repair or replacement of failed equipment, fishing operations and possibly the cost of side-tracking. Oil and gas operators attempt to prevent drill string failures by optimizing the drill string and trajectory designs. Drill string manufacturers aim to engineer drill pipe and tools with ever increasing strength properties and maximum fatigue resistance. In addition to this, drilling contractors and service companies have extensive inspection programs to avoid failure. Despite these efforts, twist-offs remain an important problem for a large number of drilling applications. Recent research and analyses suggest that the majority of drill string failures are caused by fatigue. Since the root cause of a fatigue failure cannot always be identified from surface drilling parameters or laboratory analysis, the evaluation of drilling dynamics data is essential for understanding the cause of a twist-off.

This paper will present several case studies from the North Africa region to illustrate the important contribution of drill string vibration on fatigue-related failures. In all cases, high resolution dynamics recorders were incorporated into the BHA, or drill string, close to the location of the twist-off. Analysis of the dynamics data revealed that the drill string was exposed to severe lateral vibration prior to the twist-off events. Furthermore, the observed vibration was the major contributor to the sustained fatigue due to the resulting severe shock loading and increased bending stresses generated during vibration. The paper will also present mitigation practices or changes that were implemented based on the drilling dynamic analyses as well as potential solutions for the remaining case studies. Generally, these solutions are significantly more cost effective than the traditional approaches of reducing twist-offs such as: increased frequency of inspection programs or enhanced mechanical properties on connections and drill strings.

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