Background

The Deepwater Horizon blowout story is not over and the whole story will take years to unfold. But for sure, an unstoppable geyser of mud, methane and oil did erupt 20 April. The blind shear ram did fail. The hurricane of methane did find a spark in the electrical equipment. The resulting explosions did kill 11 and the Deepwater Horizon did sink.

There seems to be no precipitating event. There was no storm, no tsunami and no collision. The Macondo Prospect was not especially difficult to drill. Hundreds of wells have been drilled at this depth successfully. The Deepwater Horizon was designed to drill much deeper and it had done so, repeatedly. The only technical problem was the numerous methane pockets which caused "kicks."

Here is a good account of what happened that day.

Deepwater Horizon Fire

It seem to me a series of short cuts and calculated risks that went awry caused the blowout rather then a blatant mistake. Personally, I don't see why there are not more blowouts. To prevent one, you have to do many things sequentially and correctly.

The Exxon Valdez spill is small in comparison. The Ixtoc blowout of 1979 was the largest ever but the Deepwater Horizon blowout is on track to exceed it. The Deepwater Horizon blowout occurs in deep water. That means methane dissolves in sea water before it reaches the surface. The oil, because of the high pressure, is broken into small droplets which rise very slowly. (Think of shaking a bottle of salad dressing.) The ocean is contaminated from top to bottom. Nothing like this has ever happened.

How could such a thing happen?

dragon (32K) The pressure of a deeply buried oil reservoir is like a dragon sleeping under the impermeable rock seal. When the seal is broken by the drillbit, the dragon is held in check by the weight of miles of heavy mud. But to produce the well, the mud must be replaced by much lighter water. The water is then pushed aside by the pressure of the oil and gas. If the pipes and valves work correctly, the oil and gas goes where intended. But, as can be seen, the dragon can sometimes escape.

Why is it so hard to stop an oil leak?

Understanding what the BOP can and cannot do is the key to answering this question. Let's do some more calculations. If the blind shear ram worked, and the oil flow was stopped, what would be the pressure just below the BOP? We previously calculated the pressure within the reservoir. It is 1143 bars. But there is a 3962 meter column of oil between the BOP and the reservoir. The pressure due to the oil column is;

0.85 * ( 3962 / 10 ) = 337 bars

The pressure at the bottom of BOP is;

1143 - 337 = 806 bars

The top of the BOP is in deep water which has a pressure of 152 bars. The blind shear ram within the BOP would have to withstand the difference between 806 bars and 152 bars which is 654 bars. This is a lot but the BOP was designed to do this.

bop schematic But the BOP has failed and the oil is flowing fast. In order to make the oil flow, there must be pressure drops. It is possible to figure out where the resistance to flow is by measuring the pressure drops.

(This BOP schematic also appears on the previous page. The right hand column has been added.) Pressure readings from within the BOP appear along the right hand side. The red numbers correspond to pressure differences. The red number "(4)" next to the blind shear ram proves it does not restrict flow much. A reading of 4 means the blind shear ram completely failed.

The "annulars" drop 29 bars, the casing shear rams and the pipe rams drop 42, another pipe ram drops 30, and, oddly, the test rams drop 49. Altogether, the pressure drop is 146 bars. The BOP is resisting the flow of oil and gas to some extent.

We are now in a position to understand the "top kill" attempt. The basic idea was to pump mud through both the "kill" and "choke" lines fast enough to drive mud down to the reservoir. The top kill was combined with a "junk shot." The idea was to use the kill and choke lines to introduce cunningly shaped pieces of rubber which would get caught in the upper parts of the BOP. BP ransacked the Gulf to get sufficient mud. BP had a 30,000 horsepower mud pump so they had that covered. The mud pumping rate was limited by the pressure the BOP could withstand. When that limit was reached, it was determined that the mud was going up, not down. Fearing damage to the BOP, BP shut down the mud pumps. Top kill failed because the BOP did little to slow mud flow out the top.

Destroying the BOP would have been disastrous for two reasons. Oil flow would increase and there would no chance of controlling the blowout.

A very nice description of the top kill attempt appears in the New York Times.

What would happen if the BOP failed?

If the blind shear ram had worked, the pressure at the base of the BOP would be 806 bars. The actual pressure is 299 bars. The difference, 507 bars, is due to resistance to flow.

What would happen if the BOP failed completely and no longer resisted flow? The engineering formulas needed to calculate flows are complex, but it is possible to extract an approximation. The approximation is this; The flow rate is proportional to the square root of the pressure drop. Plugging in the numbers;

flow rate = square root of ( 806 / 507 )

flow rate = 1.26

Oil flow rate would increase 26 percent and there would be little hope of stopping it.