Mud can also be pumped from other
locations—choke and kill lines, for example—so debris and mud may flow back up
the drill pipe. However it gets there, the
debris is eventually sent back to a separation manifold on the drilling platform for
cleaning and reuse.
The BOP stack sits on the sea floor
directly above the wellhead. Its two halves
together are as tall as a six-story building and weigh over 400 tons. The top
half, called a Lower Marine Riser Package
(LMRP), contains two electronic control
pods with a pair of donut-shaped sealing devices, the Upper and Lower annular
preventers. The preventers seal around the
pipe and regulate upward fluid flow.
The lower half of the BOP stack contains
a series of shear and bore ‘rams,’ and the
valves and hydraulics that actuate them.
Depending on the circumstances, these
rams serve to ‘choke’ the well, or ‘kill’ it
if necessary. The BOP stack is a complex
device, and its job is critical: control pressure within the well, keep the drill pipe
centered, and sever and seal the pipe in
the event of an uncontrollable blowout. The
Deepwater Horizon’s BOP stack failed on all
On the night of the accident, shortly after
a routine “leak-off” test, manifold pressures
on the drilling platform increased five-fold.
Oil and gas began spilling onto the platform deck and the rig caught fire. Personnel
aboard the rig activated the Emergency
Disconnect Sequence (EDS) in an attempt to
shut down the well, but were unsuccessful.
How it happened
DNV’s analysis, supported by FEA data,
indicates that the primary cause of failure
was the blind shear ram’s inability to cut the
drill pipe and seal the wellbore. FEA clearly
showed that the shear ram should have been
able to do its job. The required force was in
the middle of the test and calculated data
range for a nominal, centered pipe model,
and there was nothing out of the ordinary
with the strength of the drill pipe itself. The
BSR should have worked, except the pipe
wasn’t where it was supposed to be.
When activation of the blind shear rams
occurred during ROV intervention, the
annular preventers were already sealed
around the drill pipe at the top of the BOP
stack. Physical evidence from the recovered portion of the drill pipe indicated that
the pipe was at the side of the wellbore
instead of the center.
The DNV engineers postulated that one
of the pipe’s tool joints was positioned just
below the upper annular preventer, which
was closed on the drill pipe. When upward
pressure became high enough, the pipe
had nowhere to go and the upward forces
caused the drill pipe to buckle—pushing
the pipe to the side of the wellbore. This
jammed a portion of the drill pipe between
the ram block faces, preventing the rams
from fully closing and sealing the wellbore.
The DNV team started with a buckling
analysis to determine what the necessary
load would have been to move the pipe to
the edge of the casing. The FEA models indicated that the applied pressures would have
been sufficient to force it out of position.
“First, using a combination of the shear
damage parameters, in conjunction with
the known elastic and strength values of the
material, our FEA data validated the estimated pressures needed to shear the pipe under
a center-load condition,” says Finneran.
The engineers simulated the situation
seen on the Deepwater. “Our shearing
analysis showed that, with the pipe off-
center, the rams would have been prevent-
ed from closing fully,” says Finneran. “This
allowed flow past the blind shear rams
and caused significant erosion of the blind
shear ram components and wellbore.”
“Under blowout conditions of this well,
it was possible for the drill pipe to buckle
between the UA (Upper Annular Preventer)
and U-VBR (Upper Variable Bore Ram),”
he adds. The DNV hypothesis was supported by their off-center BSR shearing FEA
model, which provided results consistent
with those damaged pipe segments, as well
as the erosion damage seen between the
rams, wellbore, and recovered drill pipe.
“I believe finite element analysis can be a
valuable tool in validating the capabilities
of both new designs and existing equipment,” Finneran says. “It allows you to
incorporate a number of scenarios and situations that are difficult, if not impossible,
to analyze through experimental testing."
He points out that incomplete raw material data is still a challenge because some
of the material damage parameters of the
actual materials used in Deepwater’s BOP
stack were unavailable, he and his team
had to substitute equivalent materials with
known values in their analyses.
“The industry as a whole would benefit
from developing material-specific data that
could help them to further fine-tune the
FEA of their designs,” he says.
“FEA can incorporate real world data
into highly accurate predictive models,”
Finneran says. “Given the right material
data, fluid flow characteristics, and down-hole pressures, we could simulate what
would happen under those conditions, giving you a reasonable amount of certainty
that your design will function under well
(Left) Photograph and (Right) laser scans of damaged blind
shear ram (BSR).