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.