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 density may be calculated to include a 'riser margin'. This will ensure that the formation pore pressure will remain under control if the hydrostatic pressure is suddenly reduced.
In deeper waters and when using heavier muds, it may not be practicable to include this margin and primary control may be lost. Secondary control must be activated immediately.
Depending on the marine riser diameter, strength, and water depth, collapse of the riser may occur if it should suddenly become partially evacuated.
Consideration should be given to employ a system to keep the riser full, such as the anti-collapse valve which is installed in the marine riser above the subsea BOP stack.
The drills on floating drilling rigs shall include sufficient practice of the special procedures for landing the string on the underwater BOP-stack.