Gulf of Mexico oil drilling platforms.

platform (129K) The yellow dots mark the location of the approximately 4000 oil well drilling platforms in the Gulf of Mexico. They are interconnected by 25,000 miles of pipes. Some of the platforms stand on stilts, some float but are anchored in place and some float freely. Note that none are near Florida. This is because Florida has decided that oil drilling is too much of risk to the tourist industry.

The Deepwater Horizon floating oil rig

The Macondo Prospect reservoir contains billions of dollars worth of oil. All you need is a 3 mile straw to tap it. The Deepwater Horizon floating oilrig was designed to insert the straw. It is owned by Transocean and leased to BP. It was built in South Korea in 2001 and it is state of the art in every way. It dynamically positions itself by means of satellites. It is not anchored to the sea floor. It actively resists winds and currents by means of propellers. It can also compensate for tides and wave action.

Its replacement cost is about one billion dollars. It is now on the sea bottom not far from the oil well it drilled.

The Deepwater Horizon has propellers and it can move on its own at speeds up to 4 knots. However, long sea voyages are out of the question. To get from North Korea to the Gulf of Mexico, the Deepwater Horizon would have to go around either South American or Africa, which is a long and difficult sea voyage in either case. The Black Marlin was designed to transport large things. Here is the Black Marlin transporting the Deepwater Horizon;

blackmarlin (629K)

You get to play with very large toys in the oil industry.

Some calculations before drilling the Macondo Prospect

We have seen how Achimedes principle leads to an understanding of how an oil reservoir forms in nature. Now let us try to understand what happens when we put the straw in the oil and gas reservoir known as the Macondo Prospect. It is beneath 5000 feet of water plus 13,000 feet of rock. In metric units the numbers are 1524 meters and 3962 meters, respectively.

What is the oil reservoir pressure?

Before we calculate the pressure, we have to decide what unit of pressure to use. There are several choices. I propose the bar, which is atmospheric pressure. It is a familiar and intuitive. For example; automobile tires are inflated to about two bars. You will experience one bar of extra pressure if you dive 10 meters or 34 feet into water. It turns out to be true, within 1 percent, that the water pressure under 10 meters of water is one bar. Or, equivalently, the pressure due to the weight of the water is one kilogram per square centimeter. The computation of pressure in bars becomes easy. You divide the depth in meters by 10. This only works for water which has a specific gravity (SP) of one, by definition.

So, the pressure, at the well head, at the bottom of the sea, over the Macondo Prospect, is 152 bars. The oil reservoir is beneath 3962 meters of rock. The specific gravity of rock is typically 2.5 and this extra weight increases the pressure. The pressure within the reservoir, due to 3962 meters of rock, is;

3962 * 2.5 / 10 = 990 bars.

The total pressure in the reservoir is 152 + 990 = 1142 bars. To get psi (pounds per square inch) you multiply by 14.7. That would be 16,800 psi. The most serious power washer I can find works at 5,000 psi.

However, the straw leading to the surface would never be filled with air. At various times, the straw would be filled with drilling mud, water or crude oil. We would like to know how much pressure to expect at the surface. To do this, the weight of the fluid would be subtracted from the reservoir pressure. The light sweet oil found in the Macondo Prospect has a specific gravity of 0.85. So the pressure at the bottom of the well due to the weight of the oil is;

0.85 * ( 1524 + 3962 ) / 10 = 466 bars.

So, at the surface, with oil in the straw, the pressure would be 1142 - 466 = 676 bars. The methane always found dissolved in the oil causes complications which will be ignored for now.

A deep water oil well is complex. The production pipe, at the very bottom of the well is 7 inches 18 centimeters in diameter. The pipe at the ocean floor is 36 inches in diameter. In between, there are several different sizes, as this picture illustrates. To keep things simple, let's assume the pipe is 18 centimeters top to bottom. Let's calculate the force of the oil coming up the 18 centimeter pipe at the ocean's surface. That would be the cross section of the pipe times the pressure, or;

9 * 9 * pi * 676 = 172,000 kilograms or 378,000 pounds.

That is enough force to lift 6, fully loaded dump trucks straight up.

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Properties of methane.

Methane is a gas at ordinary temperatures and pressures. The gas that cooks your food and heats your house is mostly methane. But methane is a liquid at the pressures found within the Macondo reservoir. Methane as a liquid is miscible with crude oil. If you mix olive oil and canola oil you will see what miscible means. They don't separate the way a water and oil mixture would.

The methane in the Macondo reservoir becomes a gas if the pressure is reduced. At sea level, the volume of the gas is 600 times greater than the original liquid. Methane's conversion from liquid to gas happens fast when the pressure goes below 100 bars. This has implications when methane goes up the straw.

methanehydrate (68K) Methane and water does something unusual at high pressure. They form methane hydrate which is essentially little balls of ice enclosing methane molecules as the picture illustrates. Methane hydrate is a somewhat stable solid at atmospheric pressure, but it can suddenly decompose in pipes leading to pressure surges. Methane hydrate is the stuff that stopped BP's early attempt to cap the blowout preventer.

The science behind a blowout

Let's do some calculations. BP says they collected 15,000 barrels of oil and flared 30 million cubic feet of natural gas on 9 June. That works out to be 2000 cubic feet of methane per barrel. There must have been 3.33 cubic feet of liquid methane accompanying each barrel of oil before expansion by a factor of 600. Let's take that as representative of Macondo oil.

A barrel of oil is 5.61 cubic feet. Macondo prospect oil looses 37 percent of its volume when it reaches sea level because the methane that was dissolved in it escapes as a gas.

Let's presume 36,000 barrels per day are going up a 20 inch pipe. That flow rate is in the middle of the range of estimates in mid June. That is 1,500 barrels or 8,415 cubic feet per hour. A length of 20 inch pipe 0.46 feet long will hold a cubic foot of oil. Multiply that by the number of cubic feet the pipe must move in an hour. That is 3857 feet per hour or about 0.73 miles per hour at the sea floor. Now let's figure out how fast the methane is moving. The pipe must carry 1,500 times 2000 cubic feet of gas per hour. Multiply that by 0.46 and you get 1,375,000 feet per hour. Divide that by the number of feet in a mile (5280) to get 260 miles per hour. The oil is carried as a mist of droplets in the methane hurricane. "Blowout" is an apt description.

A blowout is what happens when you lose control of an oilwell.