A New Stick-Slip Prevention System

Abstract

Stick-slip is a dysfunction of rotary drilling, characterized by large cyclic variations of the drive torque and the rotational bit speed. It is recognized as a major source of problems, such as excessive bit wear, premature tool failures and poor drilling rate. This paper presents a new system for curing and preventing stick-slip motion. Like other competitive systems it fights the stick-slip oscillations by smart control of the drive. But in contrast to other active systems it does not use any kind of torque feed-back, not even the motor current. Fundamentally, the system is a PI-type speed controller that is tuned to effectively dampen torsional vibrations at the observed stick-slip frequency. In addition to automatic tuning of the speed controller the system includes a suite of support functions, such as automatic determination of the stick-slip frequency, estimation of the instantaneous bit rotation speed and calculation of the stick-slip severity, defined as the normalized downhole speed amplitude. All software is implemented in a standard programmable logic controller (PLC). The paper also includes results from a field test and from Hardware-In-the-Loop (HIL) simulation tests. In the latter tests utilizing the HIL, the software runs on a PLC communicating with an advanced real-time computer model for the drive and the drill string. HIL testing has proved to be cost efficient because the PLC software can easily be tested over a wide range of different downhole conditions that rarely occur in the field. Introduction The stick-slip phenomenon has been studied for more than two decades and it is recognized as a major source of problems, such as excessive bit wear, premature tool failures and poor drilling rate. The problems are closely related to the high peak speeds occurring during in the slip phase. The high rotation speeds in turn lead to extreme accelerations and forces, both in axial and lateral directions. A large number of papers and articles have addressed the stick-slip problem and a selection is included in the reference list. Many authors (Kyllingstad and Halsey 1988; Dufeyte and Henneuse 1991; Brett 1992, Pavove and Deplans 1994; Fear et al. 1997; Shuttleworth et al. 1998, Robnet 1999) focus on detecting stick-slip motion and on controlling the oscillations by operational means, such as adding friction reducers to the mud, increasing the rotation speed or reducing the weight on bit. Even though these remedies sometimes help, they are either insufficient or they represent a significant increase cost. Some papers also recommend applying smart control of the drive to dampen and prevent stick-slip oscillations. Halsey et al. (1988) demonstrated that torque feed-back from a dedicated string torque sensor could effectively cure stick-slip oscillations by adjusting the drive speed in response to the measured torque variations. As pointed out by Jansen and Steen (1995) the drawback of this approach is the need for a new and direct measurement of the string torque. They show that an alternative feedback based on available motor current and speed can be used. Their method has been patented (Warrall et al 1992) and the system, which is called Soft Torque Rotary System, has been commercially available for many years. Sananikone et al. (1992) and Pavone and Desplans (1994) also mention the use of a feed-back system for controlling torsional vibrations, but they give little or no details on how to do it.

Mitigating Stick-Slip in Deep Drilling Based on Optimization of PDC Bit Design

“…1, left) that can severely damage PDC cutters. Such vibrational issues have long been tackled in the field via efficient integrated approaches [24] and specific solutions have been developed to mitigate stick-slip, according to four main trends: definition of best drilling practices to avoid the unfavourable regime of stick-slip [1]; surface regulation systems designed to absorb upward torsional waves [39,25]; hardware solutions to reduce friction along the drillstring [14,2]; hardware technologies aiming at controlling the penetration of the drillbit into the rock [8,20]. However, stick-slip still occurs today in a wide range of drilling applications [27].…”

The Significance of Pro-active Online Monitoring with Stick-slip Mitigation

“…The efficacy of such a system has been extensively documented in recent years (Nessjoen 2011, Ertas 2013, Runia 2013, Attar 2014, Efteland 2014. The stick-slip software in this paper is based on Kyllingstad (2009). A mitigation example is seen in Fig.…”

“…This value may be updated as slow as every 30 -60 seconds displaying an average value. A more accurate and faster method is to calculate the downhole bit/BHA speed and derive a severity index metric (Kyllingstad 2009). Efteland (2014) validated these metrics by high-resolution downhole memory and wired pipe technology.…”

Observer-based output-feedback control to eliminate torsional drill-string vibrations

“…The rig is equipped with an AC top drive and fitted with a modern Soft Torque system [19]. However, for this depth and hole size stick-slip vibrations have been observed in the field for this rig.…”