Mud and the BOP
Mud and the christmas tree, aka a blowout preventer
(BOP) are the main means of control of a deepwater
oil well. People who drill for deepwater oil from
drill ships know the dangers. They know that an
unstoppable geyser of mud, methane and oil can erupt
if a mistake is made.
The patent for the rotary drill bit bit was assigned to Howard Hughs, Sr. in 1909. You could say the rotary drill bit launched the Age of Oil. The drill bit is screwed into the drill pipe which extends all the way from the drillship to the bottom of the well. The drillship rotates the drill pipe to drive the drill bit. Drill pipe comes in 33 foot sections that screw together. The drill pipe is made of strong steel and it is typically 7 inches in diameter with 2 inch thick walls.
Here are two videos explaining oil well drilling.
A highly engineered fluid,
called mud, is forced down the drill pipe and up the
outside of the drill pipe to the drillship. There,
the cuttings are removed and the mud is sent down for
another round trip. The weight of the mud serves
another purpose. It prevents oil from rising to the
It is possible to calculate the specific gravity (SP) of the mud required to exactly balance the pressure in the Macondo Prospect reservoir. It was explained previously how to calculate reservoir pressure which is 1142 bars. The pressure, in bars, due to the mud in the drill pipe at the depth (5486 meters) of the reservoir is;
pressure = SP * 5486 / 10
Set reservoir pressure equal to mud pressure and solve for the specific gravity;
SP = 1142 / ( 5486 / 10 )
SP = 2.08
Mud with a specific gravity of 2.08 will neither ooze down into the reservoir nor rise up. A near balanced state is desired when drilling a deepwater well.
It is common to encounter pockets of liquid methane while drilling. The methane is carried up with the mud. The methane becomes a gas and so occupies much greater space as it nears the surface. This rapidly displaces mud which shoots up. In industry parlance, the oil well has "kicked." The displaced mud also means the well is unbalanced. Small, infrequent kicks are only a small problem. They can be countered by using heavier mud and pumping it faster. Big and frequent kicks are a big problem. A really big kick is a blowout--there is no sharp line.
Almost all deepwater oil wells kick, but the Macondo Prospect well kicked more than most. This explains why BP was behind schedule and impatient to move to the next drilling site.
It is possible to make really heavy mud by adding barium sulfate--which is also used to make the barium enema used to visualize the colon in x-rays.
The blowout preventer (BOP)
suggests that the BOP does only one thing--prevent
blowouts. But in reality, the BOP does many things.
The BOP is the second as well as the last line of
defense against blowouts. Everything passes through
the BOP including drill bits, drill pipe, well
casings, well liners, oil, gas, mud, cement etc. To
the right is a schematic diagram of the Deepwater
Horizon BOP. Almost everything on the BOP is
duplicated to make it failsafe. I found the schematic
The original comes from the Department of Energy.
The schematic actually comes from a spreadsheet. I have added slightly to it. I will explain later.
The "kill" and "choke lines" carry mud to the lower part of the BOP. The lines originally ran alongside the riser to the Deepwater Horizon. While drilling, mud goes down the bore of the drill pipe.
One of the things the BOP can do is seal the gap between the drill pipe and the well casing. There are two of these and they are labeled "annular." They might be activated if a kick occurs. They prevent mud from coming up around the drill pipe.
If a hurricane is coming and you are in a hurry, you might use the casing shear ram to disconnect the drill ship from the well head. You have to remove the drill pipe first.
If a blowout has occurred and all counter measures have failed, you can activate the "blind shear ram". It is the last chance to avert a catastrophe. It has a blunt purpose. It shears the pipe casing and drill pipe. This takes considerable force as the drill pipe has thick walls and is made of strong steel.
The blind shear ram also completely and permanently blocks the BOP. It can be activated from several places on the drillship. It is also automatically activated when the BOP looses hydraulic or electrical connection with the drillship. The Deepwater Horizon disaster occurred as a direct result of the blind shear ram failing to do its job. The blind shear ram is the only major BOP component not duplicated.