Drilling forces from the bit, drill string and mud motor rotor imbalance adversely affect the performance of steerable bottom hole assemblies (BHAs). Proper BHA design can significantly enhance tool reliability and drilling performance. Positioning of stabilizers and the use of drill collar flex sections to minimize the effects of rotor imbalance are of particular importance when designing steerable BHAs.

Evaluation of the performance of steerable BHAs run in a 17-1/2″ diameter hole indicates that the evolution from low torque/high speed mud motors to high torque/low speed mud motors has introduced connection fatigue problems associated with increased response of the latter to mud motor rotor imbalance.

High torque/low speed motors are characterized by the use of multilobe rotors. These rotors orbit eccentrically about the center of the motor housing at speeds several times greater than the output shaft speed. This creates large cyclic imbalance forces which impart large dynamic loads onto the BHA.

Proper positioning of stabilizers and the use of flex-sections in the BHA reduce the adverse cyclic bending moment levels below those of the low torque/high speed motors, thus eliminating their contribution to connection fatigue.

In addition to critical mud motor imbalance loads, natural lateral frequencies must be considered in the BHA design in order to avoid bit bending and whirling excitations. Finite element modal analysis of the proposed BHA design can determine these critical speeds, as well as speeds that may result in undergauge stabilizers bouncing from side to side in the hole.

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