Resonance is the tendency of a system to oscillate with greater amplitude at specific frequencies. When present in the downhole environment, this effect limits drilling performance. Often, this issue is resolved by employing Finite Element Analysis (FEA) to predict the critical or natural frequencies, which is validated by observing increased vibration levels when rotating at a critical speed. However, this approach is based solely on circumstantial evidence and does not confirm the vibration is occurring at the predicted frequency.
By using multiple Downhole Dynamics Data Recorders (DDDR) in a Bottom Hole Assembly (BHA), the actual vibration frequencies occurring downhole can be calculated and compared to predicted frequencies, thereby validating the FEA model.
This technique was recently used to identify the cause of recurring over-torqued connections in a deepwater application. Analysis of the DDDR data, alongside critical speed modeling, revealed that isolated vibrations within the drill collars were allowing connections to work themselves tighter during specific drilling intervals. These measured vibrations were shown to be resonance-induced by matching predicted natural frequencies with the calculated frequencies from the DDDR, where the observed vibrations increased and decreased in magnitude as the rotation corresponded to the modeled frequencies.
The innovative visualization of downhole vibration data and the validation of critical speed modeling techniques provide a step forward in drilling assembly optimization efforts. These findings will improve the industry’s understanding of critical speed modeling, which in turn will illuminate potential shortcomings of the current methods of vibration mitigation. Practitioners will now be able to design BHAs with more confidence in the results of specific modeling principles, which will ultimately improve performance, reliability and efficiency by eliminating harmful dynamic behaviors.