Loss of primary well control - failure of riser
In floating drilling operations, an accidental disconnect or a failure of the marine riser close to the BOP stack will have a considerable effect on primary control.
The overbalance in the hole will suddenly be reduced by the difference in hydrostatic pressure between the original mud column in the marine riser above the BOP stack and sea water from sea level to the BOP stack.
In shallow waters, and when using a low density mud, sufficient overbalance may still be present. If other considerations allow, the mud . . .