preventer different density fluid may be needed to offset choke line friction when the pump 24 in fig1 is operated it is preferable to use multiple riser auxiliary lines for fluid return to the platform if the riser system used makes this possible in order to reduce friction losses in the circulation system next at 88 the sea floor blowout preventer 14 in fig1 is closed to divert return flow through at least one of the auxiliary line s e g choke line 18 in fig1 such closure may include closing an annular seal not shown separately and or pipe rams not shown separately on the blowout preventer the choke line may be hydraulically connected to the wellbore for example by operating a valve 18 a in fig1 proximate the blowout preventer at 90 the drilling platform 39 s main drilling pump is stopped to cease pumping fluid through the drill string the control point pressure in the wellbore 25 in fig1 is then maintained by pumping fluid at a selected flow rate down the kill line 16 in fig1 during this time the upper end of the drill pipe may be disconnected from the drilling unit main pumps and a connection may be made or broken i e a segment of drill string may be added or removed from the drill string the fluid pressure during this time is maintained in the wellbore so that the ecd remains above the formation pore pressure thereby reducing the possibility of formation fluid entering the wellbore fig6 shows a flow chart of an example procedure used to resume drilling after maintain pressure as explained with reference to fig5.