An employee was inserting a 5" pipe into a large section of duct work. He needed a come-a-long that was at the other end of the pipe. He climbed onto the pipe and started walking on the pipe to reach the come-a-long. When he reach a flange, he stepped over it and then fell approximately 6' to the floor. The employee sustained bruised ribs, a bulging disc and a grade two kidney tear.