Two employees were attempting to install a mud bucket drain hose from the flow line to underneath the rig floor. Standing on the BOP scaffolding which had been installed prior to their shift, the two men found the hose connection to be just out of reach. Both men attached their lanyards onto scaffolding handrails. One man was attached to the handrail on the back piece of scaffolding and the other man to the handrail on the side piece of scaffolding. The men then climbed up onto the handrail on the side piece of scaffolding and reached out to couple the hose. Without warning, the side piece of scaffolding flipped, and fell to the ground. The employee whose lanyard was attached to the handrail on this piece of scaffolding was pulled approximately 15’ to the ground with the scaffolding (See pictures below). The employee suffered a severe laceration to his head, fractured vertebrae in his back and neck as well as three broken ribs. The other man grabbed nearby air lines to prevent himself falling. He then pulled himself back onto the remaining scaffolding avoiding injury.