The Gulf Oil Spill
Whose to Blame as well as a
reflection of the science literacy
problem in the US
Deep Water Drilling:
The Dangers of High pressure
Geometry of the BOP
How an Oil Well Works
The oil entering at the bottom requires an internal pressure source to
push it to the top – this is usually supplied by associated gas deposit
Also Pressure by Direct Injection
Reservoir Pressure is key to driving the oil out
Simple Physics
► Without
any additional source of pressure
oil by itself will not be squeezed up the
tube. Thus crude oil without an internal
pressure sure is usually a Dry Well
► The deeper you drill the higher the internal
pressure you likely to tap. This is why you
need a pressure regulator a.k.a The Blow
Out Preventer
1. The top of the BOP is the bottom of
the seafloor about 5000 feet below
the surface. The confining pressure
from 5000 feet of water is 2250 PSI
(or 150 times higher than sea level
air pressure).
2. Deep Reservoir Pressure was
estimated by Coast Guard to be
8000 PSI
3. When this 8000 psi reservoir was tapped
there was obviously enough pressure to
allow oil to flow up vertically through
the 4000 meter long pipe – in turn this
reduces the reservoir pressure as its
now been tapped. One can calculate
the pressure in this 4000 meter column
of oil to be (in round numbers) 5000 psi.
4. This now means that the pressure at
the bottom of the BOP is 8000 -5000
= 3000 psi. Recall that the confining
pressure by the sea water is 2250
psi.
This means the differential pressure is
750 psi, which is less than the
confining pressure  or NO FLOW
5. Geological data suggested that average
reservoir pressures in these deep marine
pockets would be 20,000 – 40,000 psi. If you
use the 20,000 psi (which then creates a
differential pressure of about 18,000 psi) then
you calculate a flow rate of 65,000 barrels a
day for a 21 inch diameter pipe
6. The initial estimate of 1000 barrels a day
was therefore impossibly low. By the way,
if you leave your bathtub on for 24 hours
the volume flow rate would be equivalent to
500 barrels a day.
7. In addition, BOPs are rated at 15,000
PSI. The mere fact that it failed tells
you the pressure must be at least
that
Also, a typical deep marine rig will
process about 20,000 barrels a day.
The fact that BP got away for at least a
week by stating a figure of 1000
barrels a day is a sad testimony to
the science illiteracy of this country
and especially congress!
8. For pipe flow, the flow rate goes as the
square root of the pressure differential
so if you double the pressure differential
(40,000 PSI) then the flow rate increase
to 1.4 x 65,000 = 91,000 barrels a day.
Flow rate from Velocity Estimates
If you don’t know (or pretend not to know) the actual pressure
differential there is another method that can be used to estimate the
flow rate. The flow velocity comes from video camera measurements
only – no one every installed a velocity meter because it was too
difficult. Various techniques can be used to estimate the flow velocity
from video frame rate data. Those estimates come in at around 1
meter/sec (about 2 mph)
Velocity based flow estimate
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Diameter of pipe is 21 inches = 0.53m; pipe
cross section is pR2 = .2 square meters
Volume flow rate is then 1 m/sec * .8 = .2
cubic meters per second
.2 * 3600 seconds per hour = 720 cubic meters
per hour = 720,000 liters/hour
One bbl = 159 liters. Thus 720,000/159 = 4500
barrels/hour (in round numbers)
4500 x 24 = 108,000 barrels per day
Sources of Uncertainty
There is considerable confusion about which pipe
diameter to use. Some suggest that the 21 inch
pipe is merely the outlet pipe and that the interior
pipes are 14 inches in diameter. If you use 14
inches as the diameter this will reduce the flow
rate determined by the pressure differential
method by about a factor of 2. On the other hand,
it’s also possible that the internal reservoir
pressure was as high as 100,000 PSI there by
getting back your factor of 2.
Sources of Uncertainty
But the biggest uncertainty by far is the oil/gas mixture in the
flow. The previous calculations assume 100% crude oil. The
effect of mixing gas is two fold:
a) the effective density of the fluid lowers and could lower
greatly: pressure differential flow rates scale directly with fluid
density – if the effective density is down by a factor of 10, the
flow rate will be down as well.
b) The fluid velocity at exit is strongly determined by the gas oil
mixture. Pure crude will have an outflow velocity considerably
less than oil/gas mixture. The video always shows methane
bubbles present but the bubbling rate changes  this implies
variable oil to gas mixture with time.
Bottom Line
► Because
of the presence of gas in the
effluent it is scientifically impossible to
precisely know the volume flow rate.
► Worse case: 50,000 – 100,000 bbl/day
► Best case: 10,000 -25,000 bbl day
Note that the official Federal Task Force flow
committee arrives at a rate of 12,000 – 19,000 bbl/day
but that took an entire federal task force to do we just
did in twenty minutes.
Total Official Oil Budget
Whose to Blame – 3 Options
• BP as Corporate Greedy Assholes that
punctured a deep wound in Mother Earth and let
her bleed out
• Consumerism demand for the product drives the
industry to premature extraction techniques
where accidents happen and become the cost of
doing business
• The regulatory environment (MMS) that didn’t
understand simple physics.
The Price of Business