A crewman was changing the rubber stops on the forward Pipe Racking System (PRS) Track at the 144′ (43.9m) level of the derrick. He was wearing a safety harness and had his safety lanyard tied off to the PRS track and was working from the PRS. During this task, the driller moved the PRS along the track at the request of the PRS programmer. This moved the unit out from underneath the man working aloft. The man aloft felt the movement and arrested his fall by grabbing onto his lanyard. He then lowered himself the full length of the lanyard and hung suspended from the PRS track. The driller was alerted to the situation, and stopped the movement of the PRS. A personnel workbasket was sent up and the man lowered to the floor with no injuries sustained.