The derrickman had been pulling back a stand of drill pipe using the derrick tugger, unlatched the elevators, simultaneously the driller retracted the link tilt. As the elevators returned to well centre the chains connecting the link tilt to the bail arms took the weight of the bails and elevators, causing a shuddering action on the link tilt system. This caused the link tilt intermediate stop bar to pass the intermediate stops; resulting in the link tilt fully extending at an uncontrolled speed. The Intermediate stop was not in the correct position due to the fact a rope tied to the intermediate stop had been fastened to one of the bail arms. The continual action of the link tilt extending and retracting resulted in the rope attached to the bail arm moving downwards; which lead to the intermediate stop being gradually pulled downwards and out of position. This meant that when the link tilt was extended to the intermediate position the intermediate stop bar rested against the top of the intermediate stop resulting in hair trigger position. The shuddering action caused by the elevators returning to well centre was enough to allow the bar to pass the stops permitting full extension of the link tilt.
Failure of company Management System to capture and retain industry lessons learnt.Accepted practice in attaching the rope to the link tilt intermediate stop to the bail arm.Pneumatic operated link tilt system inherently fail-unsafe:When the link tilt is extended to the intermediate position, the link tilt bellow charges to full pressure against the intermediate stop. If the intermediate stops are rendered ineffective the link tilt will extend fully with force and speed.
Remove the rope attached to the link tilt intermediate stops when it is not required. Equipment connected to the link tilt system should not be connected to the bail arm, with the exception of the link tilt chains. Review the design of the link tilt system with the manufacture to seek a fail-safe system.
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