Stick/slip is a severe type of torsional drillstring oscillation that affects the efficiency of the drilling process and can cause bit damage as well as drillstring failure. Different approaches to mitigate stick/slip oscillations are used in the field, ranging from procedures for optimizing drilling parameters, to passive downhole damping devices, and active surface controllers. To improve performance, some of these systems use mathematical models of the rotary system. Since the drilling process is subject to permanent changes, model-based control systems can only perform adequately when constantly updated with the actual conditions. To date, surface data has been used for this, but some key drilling process characteristics can only be identified with downhole data.This paper presents an innovative model-based stick/slip control system. It is based on a model of the drilling process that predicts the intensity of downhole vibration with respect to drilling parameters. Using downhole measurements, the model is adapted to the actual drilling process and employed to find optimal drilling parameters. To overcome the limited bandwidth of the typically used mud-pulse telemetry systems, models are identified downhole in the dynamics measurement tool.The paper presents field tests performed to evaluate the stick/slip control system and the investigation of how model parameters can be identified from measurements as well as how well the identified models represent the drilling process. Results show that this model can be used to predict the intensity of torsional oscillations and determine optimal drilling parameter values.The presented stick/slip control system can provide quantitative recommendations on changing drilling control parameters to mitigate stick/slip and can be used in an automated mode by directly connecting to the rig control system for monitoring drilling conditions and adjusting drilling parameters. Proper handling of downhole vibrations can significantly increase reliability.

Summary The investigation of oilwell-drillstring dynamics is essential for understanding the complex behavior of downhole-vibration phenomena. Experimental test rigs allow a reproduction of critical vibrations in a laboratory environment with defined boundary conditions. Furthermore, measurements of mechanical quantities become possible, allowing a closer insight into the complex mechanisms of drillstring dynamics. This article presents a new scaled rig for analyzing drillstring vibrations. A review of existing test rigs is given, and the new laboratory rig is presented. The experimental system consists of a rotating shaft representing a section of the bottomhole assembly between two stabilizers. It is capable of reproducing lateral drillstring vibrations with and without contact. Particularly, one can investigate the vibration phenomena of stick/slip, forward whirl, backward whirl, and snaking. The shaft is positioned horizontally and supported with a fixed and a floating bearing. An electric drive provides the required rotational speed. To induce an axial force into the shaft and to simulate the weight on bit, the test rig is composed of an axial-force module. The borehole/drillstring interaction is realized with a contact module that is positioned at the shaft's center. This contact module is equipped with two eddy-current sensors to measure the lateral deflection of the rotating shaft and three force sensors to determine the normal and tangential contact forces. The paper provides a comparison of the new scaled rig to existing test setups and points out differences. Unique features of the new test rig are the application of mechanical similarity laws for the test-rig design as well as the possibility to measure the contact forces. Measurements of vibration phenomena with and without contact are shown to outline the potential of the new scaled rig.

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