Abstract Downhole separation of rotary shouldered connections in drill strings cause immense financial loss to oil and gas operators. Unfortunately, there seems to be an increasing trend in these downhole failures, particularly in deep water operations where the surface torque is limited by a top drive setting below the actual makeup torque of the drill pipe connections. This paper is intended to address the potential impact of the torque-limit setting at the surface on downhole torsional vibrations along the drill string when the operational torque approaches or exceeds that top-drive torque limit. The paper also discusses metallurgical testing methods that were used to identify the signatures of this failure type from multiple case studies. The study uses a simplified analytical vibration model to evaluate the downhole dynamic torque along the drill pipe string and its dependence on the operation parameters, particularly top drive rotary speed. By applying the initial conditions directly correlated to failure cases, time-dependence of downhole drill string torque is calculated and compared to the recorded drilling data. The vibration modeling process using simplified analytical models correlates well with the overtorque events in the failure cases modeled. The results also indicate that under certain circumstances downhole dynamic torque in the drill string could potentially be much higher than the top-drive torque limit setting at the surface. The metallurgical testing evidence provides support for the dynamic torque models in that damages were seen in the same patterns as predicted by the modeling. This modeling process, then, can be used to understand vibration problems in the drill string and correct for them by changing the top drive torque-limit settings and the rotary speeds.