Drillstring vibration is a complex and costly problem which when it occurs often results in damage to the drillstring, wear on tool joints and stabilizers and failures of MWD tools and BHA components, poor drilling efficiency and high NPT and adversely affect drilling efficiency (Fig. 1a & 1b). Drillstring vibrates as a result of load or displacement excitations applied at various locations and various frequencies. The primary causes of drillstring vibrations are bit/formation and drillstring/borehole interactions. Large vibration levels cause reduced rates of penetration and catastrophic failures while lower levels may lead to a reduced operating life over a sufficient time period. The dynamic nature of drillstring creates a unique problem in vibration analysis. Due to problems related to torsional and lateral vibration, some systems and methods were developed over the years to either eliminate or reduce the problem. For instance, in trying to reduce torsional vibration, methods such as reducing the weight on bit, or changing the speed of the top drive were adopted. It appears to reduce torsional vibration but also resulted in a poor drilling rate. Researchers over the years have developed systems to prevent torsional vibration while still maintaining good drilling rate. However, they have shortfalls which at the time were not visible. In this work, we considered that different drillstrings operate at varying rotary speeds and make different cutting impact with either the tricone bits or PDC bits when drilling directional wells. It depends on the natural frequency of the strings. The use of shocks subs does not provide a universal condition as shock subs are designed for one set of conditions. When the drilling environment changes, as it often does, shock subs become ineffective and often result in increased drilling vibrations, exacerbating the situation. Going forward, we analyzed previous models highlighting the shortfalls of the applicability. Also, we studied the effect of reduced WOB on Torsional vibration. The results though empirical were tested with field data. Key Words: drillstring vibration; vibration analysis; critical rotary speeds; PDC bits; directional wells
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