A drilling crew had finished bull heading 3-1/2 barrels of mud down the annulus of the well and the pressure was bled through the choke and was reading zero. The normal procedure would have been to close the annular during bull heading operation, but this time the top rams were closed. A tour change occurred and the driller, who just came on tour, opened the annular preventer and attempted to hoist the 2-7/8” drill pipe to pull out of the hole. A tool joint came up against the closed top pipe rams and the pipe parted at the rig floor. There were no injuries and a fishing job was not necessary given that the severed tool joint was still above the rig floor.