bottom of the well 108 in fig2 this may be achieved for example by raising a top drive or a kelly on the drilling platform or drilling vessel a bypass line such as isolation line 154 in fig2 may be opened prior to the closing of at least one bop such as upper annular bop 132 in fig2 opening the isolation line 154 in fig2 permits drilling mud to flow through the mlp 124 in fig2 after the upper annular bop 132 in fig2 sealingly engages the drill string 104 in fig2 the closing of the upper annular bop 132 in fig2 is a well control measure that may prevent a kick from circulating up from the bottom of the well 108 in fig2 to the srd 118 in fig2 and subsequently into the annulus 120 in fig2 of the marine riser 112 in fig2 the sac 116 in fig2 may typically be isolated from the well 108 in fig2 during normal drilling operations to prevent a gas influx from entering the marine riser 112 in fig2 however if the sac 116 in fig2 is not isolated from the well 108 in fig2 it may be isolated by closing an srd bypass line not shown or by closing sac isolation valves not shown the mlp 124 in fig2 inlet pressure and the.