During a rig move operation, a welder was conducting housekeeping activities on top of a reserve mud tank located outside the rig boundary. While attempting to discard an elbow fitting, a flange nut snagged on his coverall, causing him to lose balance. He attempted to grasp the handrails but was unable to maintain his grip and fell from the tank. The individual sustained non-life-threatening facial injuries and was promptly evacuated to the nearest hospital for medical evaluation and treatment