Marine Riser Failure
While drilling in approximately
6,000 feet of water, a drillship recently experienced a catastrophic failure of the marine
riser. The drillship was equipped with a dual derrick, and dual activity was being
conducted at the time of the incident. On the forward rotary, where the marine riser was
installed, the rig crew was in the process of pulling out of the hole from total depth. On
the aft rotary, the rig crew was in the process of running 20-inch casing for an adjacent
well. The failure of the flanged marine riser occurred when drillpipe had been pulled a
couple hundred feet off-bottom. At that time, the rig experienced a heave motion followed
by a strong jarring action. The ROV, which had been launched to observe the running
of the 20-inch casing, was dispatched to examine the marine riser.
When the ROV reached
approximately 3,200 feet of water, it was determined that the riser had separated between
riser joint 39 and 40 and was unloading the synthetic-based mud that was in use at the
time. The drillpipe was observed to be intact at this depth. As the ROV traced the
drillpipe deeper, it was found penetrating the lower section of buoyant riser that was
free-standing from the seafloor to approximately 1000 feet from the mudline. The remainder
of the riser was found scattered on the seafloor surrounding the wellhead and BOP stack.
As the ROV scanned the BOP stack,
it was determined that the riser was cleanly parted about one foot above the lower marine
riser package. There was no flow observed from the well. When the riser parted, the “dead
man” system activated, and all fail safe valves, casing shear rams, and lower blind
shear rams were closed. The drillpipe was successfully sheared by this activation. At a
later point, the ROV used a hot stab to activate a second set of upper blind shear rams to
provide another barrier on the wellbore. Although the well control equipment functioned as
designed, the parting of the marine riser resulted in a release of an undetermined amount
of synthetic based mud.
The subject accident is currently
under investigation by MMS. Upon its completion, the investigation report, as well as a
possible follow-up Safety Alert, will be made available to the public. Your attention is
directed to our conditions of approval for Applications for Permit to Drill involving the
use of subsea BOP stacks. The approval outlines our requirements for the shut-in
capability of the well in the event of an unplanned disconnect of the lower marine riser
package or the parting of the marine riser. It should be noted again that, in this
incident, the “deadman” system functioned properly and prevented the release of
well bore fluids into the water column.
