Blowout Preventer or Bop on the Macondo well
The
Blowout Control system consists of mud, the blowout preventer, the kill line,
the choke line, and the choke manifold.
Blowout can occur when the hydrostatic pressure or the weight of the mud in the
hole falls below the pressure of the fluid coming from the reservoir. As soon
as formation fluid enters the well we have a problem because that fluid may be
under very high pressure and it will be lighter than the mud. The first
indication of pressure imbalance occurs when the mud begins to be pushed out of
the well – this is called a “kick”. If this is allowed to continue, all
the mud is pushed out and a blowout is said to have occurred.
The
mud column is the primary control against blowouts. It should always exert
sufficient pressure to prevent formation fluid from flowing into the well. If
primary control fails and a kick occurs, secondary control is provided by a
Blowout Preventer Stack (BOP).
Figure 1
RAM type BOP
The
BOP prevents blowouts by closing across the top of the well, whether the drill
string is in the well or not. The BOP stack is situated at the wellhead and is
connected to the wellhead by wellhead flanges.
There
are two types of blowout prevention mechanisms in the stack: the annular and
the ram. The annular preventer, a rubber ring mounted at the top of the stack,
which seals with the drill pipe or irregular shaped objects in the hole, is also
known as a bag preventer. The ram type preventers close over the well, one half
from each side of the BOP.
BOP open. This allows flow through the
annulus
(courtesy of ASME)
BOP closed. The rams close against the
drill pipe closing the annulus and shutting off flow.
(courtesy of ASME)
There
can be two, three or four ram type preventers in one stack, each performing a
different task.
Figure 2
Shear blades on the TOP RAM will cut through the pipe and seal the full face of
the well.
During drilling operations there is a possibility that formation fluids could
travel up the drill string rather than the annulus. To avoid that it is common
practice to install an in-line safety valve near the bottom of the string but
always, a Kelly cock is installed at the drill platform floor at the “kelly”. A
kelly cock may be installed at the top and/or the bottom of the Kelly. If a
blowout happens the Kelly must be shut manually.
 
Figure 3
Drill string safety valve
The Kelly is the heavy square or hexagonal steel member
suspended from the swivel and connected to the topmost joint of drill to turn
the drill stem as the rotary table turns.
Figure 4
Kelly cock
The
choke line takes the mud or invading fluid from the closed-in well to the choke
manifold which controls the rate of escape of the fluid and directs it back to
the mud tank or to a flare line. A hole full of mud that has the correct mud
weight and density should not be blown out. Unfortunately, human errors in
calculations can result in too little mud or too much water being added to the
circulating system, thus decreasing hydrostatic pressure.
If
the casing has been perforated or a plug drilled out, the wellbore may be
exposed to formation pressure greater than can be controlled by the column of
fluid in the wellbore. If a pipe is raised too fast, it can create a swabbing
effect, whereby the well fluid clings to the pipe as it is brought out, thereby
decreasing the volume of mud in the wellbore (followed by a drop in pressure).
Objects connected to the pipe or tubing, such as packers and testers, act as
pistons and increase pressure differential between the formation fluids and the
drilling mud.
The
choke and kill lines control kicks to prevent them from developing into
blowouts.
When
a potential blowout is detected, mud is pumped down the kill line to restore
pressure balance in the hole. When excess gases occur, the bag and ram type
BOPs are closed around the drill string. The gas pressure is released at the
choke manifold by slowly opening the choke, displacing mud as the bubble or
gas-entrapped mud approaches the surface. Without the choke, the gas would push
out the annulus mud between the drill string and the riser and control from the
weighted mud would be lost.
An
uncontrolled blowout would result if the gas pressure at the surface were to
exceed the pressure of the wellhead or the BOP.
In
the case of the BP incident in the Gulf of Mexico it appears that a gas kick in
the Macondo well resulted in a blowout after well control problems. Gas blew
back up the drill column with a subsequent explosion. At the time of the
incident drilling fluid was being displaced from the well with seawater in
preparation for setting the final cement plug. At 9:56pm the first of three
explosions rocked the Deepwater Horizon resulting in the death of eleven men and
massive oil contamination of the Gulf of Mexico.
We
now know that the explosion caused damage to the marine riser (designed to take
the oil from the well to the drill platform and also to house the drill
string). The shear ram BOP appears not to have closed completely and although
there are several theories as to why (including unauthorized modifications and
insufficient power to the rams) we are still not sure at this time as to the
true cause. We do know, however that BOP design has been around a long time
(since 1924) and has a reputation for reliability.
We
await the full closure of the well and a definitive Root Cause Investigation.
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