Two roughnecks were working in BOP area to reposition the telescopic bell nipple. After working on the bell nipple, the Supervisor sent the winch line down through the rotary table opening. While the injured person (IP) was standing on the rotary beams he attached himself to the winch line hook and then unhooked his lanyard from the rotary table beams. As he disconnected his lanyard, the chest ring on his fall protection harness slipped out of the winch line hook causing the IP to be without fall protection which resulted in him falling 10-15 feet to the annular then another 15 feet to where he landed on his back over the BOP rams and eventually coming to rest on the ground near BOP hoses. The Rig’s Emergency Response Team stabilized the IP and he was transported to a local hospital for evaluation and treatment then transported to a hospital trauma facility. The medical assessment showed multiple fractures of the pelvis and back.