Abstract: At approximately 2:00 a.m. on April 6, 2006, Employee #1 and a crew responded to a well site to investigate why the well was not holding a steady pressure. They prepared to shut down the well and check the condition of the choke valves. The well was shut down by closing the high pressure well, flowing at approximately at 4,000 psi, isolation valve with a remotely controlled-pneumatically operated valve and the secondary well isolation valve. These valves were located about 10-in. apart and when they both were closed a portion of pressurized gas was trapped between them. Next, they closed the line, flowing approximately at 1,000 psi, isolation valve to ensure that gas would not back-up into the open choke valves. The lines connecting the pneumatic valve and the line isolation valve were bled to atmospheric pressure. During this process two pressure sensors initiated a beeper-horn-siren to signal that the high pressure and the low pressure lines had lost pressure. Once the line was at atmospheric pressure, they popped the head from the high pressure choke valve. As Employee #1 and a roustabout were removing the seat from this valve, a coworker was trying to communicate to them but the beeper-horn-siren was too loud. The coworker went to the control panel and turned off the switch controlling the noise. The switch opened the remotely controlled-pneumatically operated valve and released the compressed gas trapped between the high pressure well isolation valve and the pneumatically operated valve. The 4,000 psi gas escaped directly to the high pressure choke valve that had been opened and the seat removed. The wrench being used to remove the valve seat was propelled, striking Employee #1 in the face, head and shoulder. Employee #1 was killed.