Inadvertent unlocking of tubing hanger with the BOP annular open and pressure below the tubing hanger

During the abandonment of a well, while performing operations to remove hydrocarbons and excess pressure from the annulus, the tubing hanger was inadvertently unlocked from the horizontal xmas tree.

Following the unlocking of the hanger, well pressure lifted the hanger and upper completion, releasing gas from beneath. The gas quickly expanded in the marine riser, partially evacuating the sea water from the riser and expelling gas to the atmosphere.

Multiple H/C gas detectors activated, which triggered general alarm & call to rig muster. All personnel were safe via red zone management and no hydrocarbons were released to sea.