A dynamic evaluation method for safe operation of the bottom hole assembly with bit-push rotary steering drilling system

Key words:  rotary steering drilling  drill-string dynamic safety  natural frequency  dynamic stress  field test

Abstract:

The intermittent push-back of the bit pads against the wellbore wall during guided drilling (or bit-push steering drilling) can make the drill-string in a state with complicated nonlinear damping excitation. The violent vibration can cause a fatigue failure of the drill-string. In this paper, a dynamic stress calculation model of the drill-string was established, considering the contact collision and dynamic excitation between bit pads and borehole wall. The calculation results show that the dynamic stress of the bottom hole assembly (BHA) and the lateral force on the drill bit during the drilling process are in a volatile state. When the drill-string is in a critical rotation speed, it will result in severe vibration and great stress that can lead to a fatigue failure of the drill-string. The vibration acceleration of the BHA can be significantly reduced by adjusting the rotation speed of the drill-string to the intermediate value of two adjacent critical rotation speeds. The field test results have verified the feasibility to improve the BHA dynamic safety by adjusting its rotation speed, which can provide a basis for the optimization design of the drilling parameters for the safe operation of the bit-push rotary steering drilling system and similar tools.