INVESTIGATION OF THE INFLUENCE OF SCREW CONNECTIONS IN DRILL STRINGS ON THE PROPAGATION OF TORSIONAL WAVES WITH RESPECT TO ADVANCED STICK-SLIP CONTROLLERS

Drill strings are used in the oil and gas industry to search for oil, gas, and geothermal resources and form extremely slender structures which makes them very sensitive to torsional and other vibrations. In order to immensely reduce torsional vibrations along the whole string, a wave based control method was developed at our institute. Numerical simulations and tests at an experimental setup showed very good results, but the implementation in a real drilling rig has not yet been taken place. One apparent difference in a real drill string will be the assembly of many rather short drill pipes, which is unregarded in conventional models and our small test rig. This might lead to improper behavior of our wave based control mechanism and shall be investigated is this paper. We present a model that accounts for a discontinuously built drill string and show the consequences for our advanced control method via numerical simulations.