Dynamic Behaviour of a Bit-Motor-Thruster Assembly

Case studies show that the use of a thruster in the bottomhole assembly of a drillstring is beneficial for the operator because of a considerably improved rate of penetration and smooth drilling conditions. From a dynamics standpoint, the axial and torsional vibrations of the drillstring are improved by the use of a simple thruster device. The thruster acts like an 'anti-vibration' tool. Laboratory measurements of a full scale assembly containing a bit, a downhole mud motor and a thruster were carried out to quantify the forces, accelerations and dynamic pressures. Measurements were made at a 6kHz sampling rate and under various operating conditions. The signals were analysed by digital signal processing techniques to determine dynamic characteristics. The dynamics of the considered systems were then described by transfer functions. In these tests, the measured vibration response was used to calibrate the unknown parameters of a bit-motor-thruster dynamics time domain model that includes mud flow forcing effects. The computer application then predicts the dynamic behaviour of a complete drillstring while drilling.