Primary transfer sub for dual concentric drillpipe

摘要: A downhole transfer sub for use with dual concentric drillstring and rotary drilling apparatus a percussion motor. The sub, positoned immediately above the motor, transfers downstream fluid from annular space in double-wall drillpipe to inlet bore of also upstream exhausted entrained cuttings bottom borehole central return ground surface. Alternatively, drill may be operated as motor removed, which case will direct-connected at its lower end conventional bit.