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.
Slick-slip (SS) and lateral shocks are harmful to many expensive downhole tools such as rotary steerable systems, and downhole measurement and logging tools. Due to this concern, an asymmetric vibration damping device has been engineered to reduce these specified vibrational modes. This tool is uniquely specialized that, when placed and ran with the recommended drilling parameters, reduces stick-slip tendencies as well as dampen lateral shocks by placing the drill string near the tool in Forward Synchronous Whirl (FSW). As a result of placing the BHA (bottom hole assembly) into FSW, other harmful vibrational modes such as chaotic whirl and backward whirl are prevented from occurring. Stemming from the exclusive geometry of the tool, the tool will interrupt harmonic modes during rotation that can lead to harmful shocks and SS. Extensive research has been performed in laboratory and field testing using the vibration damping tool, and its success has been proven in multiple deep water applications (primarily with hole opening BHA's), but also on land with bi-center bits. Recently, a new approach was undertaken, utilizing two vibration damping tools in a single BHA. This was intended to control vibrations and reduce shocks that were causing extensive damage to Rotary Steerable System (RSS) horizontal applications in North America. The initial trial wells used the dual placement within the vertical and horizontal sections, drilling into the Cotton Valley formation in North Louisiana. These applications have historically seen very high vibration levels due to both the trajectory and challenging formations.
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