Abstract
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.
