The operation was moving all equipment off the main deck in preparation for a rig move. The crane was being used to move an oxygen & acetylene bottle rack from the starboard side of the main deck to the pipe rack. The Crane Operator was flagged by a Roustabout to pick up the bottle rack. When the load was high enough for the Crane Operator to have a visual, the Roustabout stopped flagging and began to move to the landing site. The Crane Operator began to swing the load but the load was not high enough to clear one of the pipe rack posts. The bottle rack struck the post in such a way that the door bar on the oxygen side of the rack was knocked open and several bottles fell out of the rack onto the main deck. Valves were broken off 2 of the bottles but fortunately the bottles were empty. No one was injured but clearly, had the bottles not been empty, this incident would have been catastrophic.