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High vibrational loads occur due to self-excited high-frequency torsional oscillations (HFTO) that are excited by the bit-rock interaction while drilling in hard and dense formations. These critical vibration loads with an associated acceleration above 100 g can result in premature failure of downhole components, leading to an increase of non-productive time and reduced reliability. Beside the formation, drilling parameters influence the occurrence of HFTO. This paper presents a novel method to determine stable and unstable combinations of operational parameters and an associated strategy to mitigate HFTO. The inputs for the derived algorithm are the drill string model, high-frequency downhole data that is related to HFTO and downhole rotary speed and downhole weight on bit (WOB) measurements representing the instantaneous operational parameters. The data is measured by a downhole tool for vibration and load measurements that collects data during triggered events. The high-frequency data is used to determine the energy input into critical HFTO modes regarding changes of drilling parameters. Herein, events are used that naturally occur during the drilling process, e.g. stick/slip. The amplitude change of HFTO for the current combination of bit rotary speed and WOB is used to determine the stability of operational parameters regarding HFTO. The resulting stability maps display operational parameters that are susceptible to HFTO and stable zones without HFTO. The utilization of downhole fluctuations allows a derivation of the stability for large operational parameter ranges and not just for the current set point. The method is investigated in a post well analysis of two runs where a superposition of high and low frequency torsional vibrations occurs. In contrast to conventional methods that are based on large data sets, the derived method requires only a few seconds of high frequency data to determine an accurate stability map. The study shows different scenarios of HFTO with and without stick/slip or low-frequency torsional oscillations and discusses the optimal HFTO-mitigation strategy within the given operational parameter envelope. For both runs, a similar threshold rotary speed is determined that needs to be exceeded to mitigate HFTO. This observation is also confirmed during drilling where stable drilling above the derived bit rotary speed threshold can be observed. Hence, increasing RPM above this threshold reduces HFTO occurrence while increasing ROP in contrast to the conservative way to reduce HFTO amplitude by reducing the bit rotary speed. The method enables the analysis of the stability of HFTO and gives a quantitative measure to investigate bit, rock and other influencing factors. The resulting stability maps are used to propose stable regimes for operational parameters without HFTO to the driller in the upcoming runs and in similar environments, and thus enable an increase in drilling efficiency and reliability.
High vibrational loads occur due to self-excited high-frequency torsional oscillations (HFTO) that are excited by the bit-rock interaction while drilling in hard and dense formations. These critical vibration loads with an associated acceleration above 100 g can result in premature failure of downhole components, leading to an increase of non-productive time and reduced reliability. Beside the formation, drilling parameters influence the occurrence of HFTO. This paper presents a novel method to determine stable and unstable combinations of operational parameters and an associated strategy to mitigate HFTO. The inputs for the derived algorithm are the drill string model, high-frequency downhole data that is related to HFTO and downhole rotary speed and downhole weight on bit (WOB) measurements representing the instantaneous operational parameters. The data is measured by a downhole tool for vibration and load measurements that collects data during triggered events. The high-frequency data is used to determine the energy input into critical HFTO modes regarding changes of drilling parameters. Herein, events are used that naturally occur during the drilling process, e.g. stick/slip. The amplitude change of HFTO for the current combination of bit rotary speed and WOB is used to determine the stability of operational parameters regarding HFTO. The resulting stability maps display operational parameters that are susceptible to HFTO and stable zones without HFTO. The utilization of downhole fluctuations allows a derivation of the stability for large operational parameter ranges and not just for the current set point. The method is investigated in a post well analysis of two runs where a superposition of high and low frequency torsional vibrations occurs. In contrast to conventional methods that are based on large data sets, the derived method requires only a few seconds of high frequency data to determine an accurate stability map. The study shows different scenarios of HFTO with and without stick/slip or low-frequency torsional oscillations and discusses the optimal HFTO-mitigation strategy within the given operational parameter envelope. For both runs, a similar threshold rotary speed is determined that needs to be exceeded to mitigate HFTO. This observation is also confirmed during drilling where stable drilling above the derived bit rotary speed threshold can be observed. Hence, increasing RPM above this threshold reduces HFTO occurrence while increasing ROP in contrast to the conservative way to reduce HFTO amplitude by reducing the bit rotary speed. The method enables the analysis of the stability of HFTO and gives a quantitative measure to investigate bit, rock and other influencing factors. The resulting stability maps are used to propose stable regimes for operational parameters without HFTO to the driller in the upcoming runs and in similar environments, and thus enable an increase in drilling efficiency and reliability.
High-Frequency Torsional Oscillations (HFTO) are bit induced self-excited vibrations which can cause downhole tool failures and reduce the reliability of downhole tools. It is essential to understand the interaction of HFTO with other vibrational phenomena to develop effective HFTO mitigation strategies. While coupling with axial vibrations and stick-slip have already been studied extensively, the interaction of HFTO with lateral vibrations has received less attention. This paper analyzes this interaction based on a bit rock interaction model that accounts for the superimposed movement of whirl and HFTO at the bit. The excitation of HFTO can be attributed to a velocity-dependent characteristic of the cutting torque. A model that incorporates the superposition of lateral and torsional movement at the bit is used to calculate the velocity-dependent bit torque based on three components: the cutting velocity at each cutter, the normal force distribution along bit blades, and velocity-dependent forces between the bit and the rock. The velocity distribution is based on a kinematic model that superimposes the lateral motion of whirling and the rotational motion of HFTO. The normal force distribution is derived from the bit blade and cutter configuration, and the velocity-dependent force characteristics at each bit element is based on findings of laboratory tests with single cutting elements. A continuous multivariable function determines the nonlinear drilling torque characteristic depending on the amplitudes of HFTO and backward whirl. Evaluation of the simulated drilling torque shows that HFTO cannot be excited in presence of bit backward whirl. Specifically, it was found that an increasing rate of bit backward whirl leads to a bit torque characteristic generating less energy input or even energy output to HFTO. This is caused by backwards cutters with negative cutting torques corresponding to high energy dissipation. Forward whirl, on the other hand, cannot suppress HFTO. Comparison with laboratory data confirms these results. Cutter geometry and normal force distribution do not appear to have a significant effect on the results in either case. The new bit model provides a physics-based explanation of why bit backward whirl and torsional vibrations cannot be observed simultaneously. The influence of parameters, like the cutting-edge geometry, can be evaluated much faster than, for example, with particle or finite element models.
The push in the drilling industry for drilling longer wells, and faster and more difficult trajectories leads to increasing challenges with the dynamics of the drilling systems in particular in expensive high end offshore wells. Dynamic effects are more severe and occur more frequently increasing load and damage to tools so that mitigation by dedicated tools or drilling procedures is necessary. Generally, downhole drilling dynamics effects can be categorized by their direction of action. Vibration excited in the axial direction, along the tool axis, mainly originates from the bit rock interaction, e.g. bit bounce (Dykstra et. al. 1994). Vibration in the lateral direction, perpendicular to the tool axis, can be generated from interaction with the borehole wall. Here forward and backward whirl motions are possible. In case of backward whirl the origin can be, for example, the whirling bit (Brett et. al. 1990) or a stabilizer. Low clearance results in high whirl frequencies accompanied by high loads (Oueslati et. al. 2014). The earliest identified torsional vibration phenomenon in the drilling industry is stick-slip where the entire string oscillates with a frequency close to its first eigenfrequency. Due to the drill string length this frequency is low, at usually less than 1 Hz. Due to more advanced dynamics measurement tools (Makkar et. al. 2012, Akimov et. al. 2018, Townsend et. al. 2021) that record data at higher sampling rates, another torsional dynamics phenomenon was identified in recent years: high-frequency torsional oscillations (HFTO) (Jain et. al. 2014, Zhang et. al. 2017). The vibration frequency of HFTO could be identified to be in the range between ~50 Hz to above 400 Hz depending on the bottomhole assembly (BHA) and string configuration. The root cause for this type of vibration is a self-excitation mechanism in the bit-rock cutting process in hard rock, when drilling with PDC bits onshore and offshore (Oueslati et. al 2013, Oueslati et. al. 2014). HFTO can result in high dynamic torque and rotational and tangential acceleration amplitudes stressing the drilling tools. The combination of high frequencies and high amplitudes can quickly result in fatigue damage on tools and components. Examples are displayed in figure 1. The dynamic torque fatigues the material, and 45-degree cracks occur and grow (Zhang et. al. 2017). The acceleration leads to relative movement due to inertia forces which can vibrate cables off circuit boards or generate "vibration dust" where vibrating parts are in contact which each other.
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