Dynamic characteristics in the form of two distinct resonant frequencies were measured by on board sensors during drilling operations being conducted with a steerable drilling assembly. Occasional main shaft failures were thought to be linked to these characteristics.
A highly detailed system model was generated and initial modal analyses used to identify system modes and calibrate the model. Following this, a transient dynamic simulation of the system was conducted. This enabled the effects of contact and friction along the assembly to be accounted for in the determination of the in-situ modes. FFT’s were extracted to determine the modal contributions during operation and to fully characterize the system dynamics
The model exhibited the same behaviour as the tool did in practice, and examination of the results enabled the conclusion to be drawn that a combination of mass distribution and and location of contact points between the rotating string and the hole always produced the same effective boundary conditions. This consistently resulted in a well defined, and now understood, dynamic response that could be better managed.