A rig had finished driving a threaded connection 42 inch conductor pipe to 282 ft. below the mud line (BML) utilizing a third party hydraulic hammer. Prior to rigging down the hammer, a pre-job meeting was held. After which, the crew commenced rigging down the hammer. Slings were attached to the 12,000 lb. x 14 ft. drive down adapter and it was lifted out of the conductor pipe using the rig’s traveling block. Soon after clearing the conductor pipe, the adapter separated from its lifting plate and fell 9 ft. to the rig floor. It contacted the edge of the 42 inch box before landing on the rig floor toward the starboard forward corner. One floorman was struck on his right side by the adapter while trying to escape, sustaining injuries that subsequently proved to be fatal. Another floorman injured his right foot while escaping from the path of the adapter.