drill string or a retrievable plug thereafter the downhole deployment valve 50 is closed as illustrated in fig2 by applying hydraulic pressure from the surface fluid reservoir through the control line after the downhole deployment valve 50 is closed the wellbore 10 is separated into a first region 85 and a second region 90 the wellbore pressure in the first region is then reduced to substantially zero by manipulating the rotating control head 75 and the choke manifold system in one embodiment the downhole deployment valve 50 is equipped with downhole sensors that transmit an electrical signal to the surface allowing measurement and reading of real time downhole pressures when the wellbore pressure in the first region 85 is reduced to substantially zero the balance of the drill string 105 is tripped out of the wellbore 10 in a similar manner as the procedure for tripping pipe in a dead well during the trip into the wellbore 10 the drill string 105 is rerun to a depth directly above the downhole deployment valve 50 where a pipe heavy condition exists subsequently pressure is applied to the wellbore 10 to equalize the pressure in the first region 85 and the second region 90 when the pressures in the regions 85 90 are substantially equal hydraulic pressure from the surface fluid reservoir is applied through the control line to open the downhole deployment valve 50 thereby opening the pathway into region 90 of the wellbore 10 fig3 illustrates the drilling assembly 100 forming another section of the wellbore 10 during an underbalanced or a managed pressure drilling operation generally the wellbore 10 is formed by rotating the drill bit 110 while urging.