A fatality occurred when two employees working on a floating MODU, were in the process of disconnecting the “U” tube of the number three tensioner from the isolation valve. This was being done in an effort to initiate repairs to the damaged marine riser tensioner (MRT). While removing the last bolt of the isolation valve flange, the internal parts of the valve blew out and struck the injured employee in the head and face. It was later discovered that they were actually dismantling the valve body because they had misidentified the flange as a separation point for the line. The employees had inadvertently released the 400 psi of air on the 3 inch line and the internal parts of the valve.