Abstract: During setup procedures on an oil well drilling rig, coworkers were installing a flow ditch onto the pitcher nipple. Employee #1 was guiding the flow ditch as it was being pulled horizontally by a hoist line, and the hoist was being operated by a coworker on the drilling floor. The hoist operator could not observe the flow ditch installation because it was being done on the floor below him. Several coworkers on the floor below were calling to the operator, telling him when to pull and when to slack off. One coworker gave the signal to pull at the same time that Employee #1 wanted the hoist slacked off so he could reattach a sling. The pull on the hoist line caused the flow ditch to move suddenly. Employee #1's left arm became caught between a structural part of the sub-base and the rapidly moving flow ditch. His forearm was severely crushed.