2nd Mode Shape Torsional Vibration - Observation, Identification, Mitigation

Abstract

This paper investigates the drill pipe twist-offs that occurred while drilling an S-shaped well in the 12 ¼" section. The presence of the 2nd mode shape of torsional vibration was observed and linked to the failure. Conditions that lead to a 2nd mode shape being triggered and sustained are discussed in addition to recommendations to prevent fatigue failures. 2nd mode shape torsional vibration is a standing wave with two nodes created along the drill string, with the first one being the top drive and 2nd located along the drill string. The drill pipe experiences the highest torque fluctuations in the rotational nodes during torsional vibration. A high-resolution drilling dynamics recorder was utilized to record a spectrum of torsional vibration frequencies, confirming the presence of the 2nd mode shape of torsional vibration. The multiple degrees of freedom spring-mass model was used to identify the node location, which was later compared to the locations of the actual twist-offs. The presence of the 2nd mode torsional vibration, its transition from the fundamental mode to the 2nd mode, and the conditions for the transition to happen were observed. Recommendations were developed on how to recognize the 2nd mode shape with the surface instrumentation, on the drill string design, and preventive inspection. Knowing the location of the node gives a possibility of selective inspection of the drill pipes once the 2nd mode shape was observed while drilling, minimizing the risk of fatigue failures. Also, the drill string design could be revised by increasing drill pipe strength within the nodes. The twist-off locations were found to match the calculated location of the node caused by the 2nd mode shape with a high degree of accuracy.