Deepwater Oil Spill - varying the junk mix and cutting the flow

The video from the ROVs monitoring the Deepwater oil leak from the well in the Gulf of Mexico show that the at around 10:45 am they may have started another junk shot injection, given that the flow from the riser has just increased, and various large particles have been coming past the camera. When they are injecting mud the contrast on the picture gets somewhat worse, and so it is probably better if you check this out for yourself. The riser is continuing to flow mud.

between about 10:45 am and noon 29/5/2010

In the earlier post I wrote about one way of tackling the leaks in the BOP, using spheres and triangles of rubber. But as you can see from the flows from the leak at the top of the riser, when the cracks get quite small the injected particles get swept past, and end up coming out of the end of the riser. So I am going to look at what the flow path size is, and another answer.


So let us run some numbers to see if we can estimate what the largest width that the particle has to bridge will be. That would be if the hole in the BOP is circular, since in any other geometry one of the dimensions will be smaller and catch the particle.

Assuming that, if for the sake of the discussion we accept that the well is leaking at 17,000 barrels a day, this translates into around 500 gallons of fluid a minute. (Divide by 24 to get flow per hour, divide by 60 to get flow per minute, and multiply by 42 to convert to gallons).

The next step uses an Excel table that I have generated over the years to calculate circuit flows. It has the orifice diameter on the left and across the top the pressure driving the flow. I have modified the table to show the diameter that would be required to allow 500 gpm to pass (roughly) and have highlighted where that flow is reached (roughly) for different combinations of flow and pressure. (i.e. the red numbers in the table, for a given driving pressure across the top read to the left to get the hole diameter that will give this flow).

For those interested in generating their own the equation I used for the value in space N13, for example, was

=$L$1*(3.1412*60*($A$13/2)^2*12*12.5*SQRT(N3))/231.

The values are very dependant on the discharge coefficient (L1) and this value can vary for flows through orifices from about 0.6 to 0.9. To get the largest diameter I made the value 0.6 (as the discharge coefficient increases the diameter of the hole needed to pass that flow reduces).



You can see that the largest dimension of the flow channel is just over 0.7 inches. (Which means that the BOP rams functioned over at least the majority of their stroke). The minimum is about half an inch, and if I change the discharge coefficient from 0.6 to 0.85 then the diameter range goes from 0.4 to 0.6 inches.

So from this we know that the maximum gap in the BOP is 0.7 inches in diameter. Now this is good news because it means that it is less than a third of the diameter of the feed line (which has an effective inner diameter of possibly 2.7 inches or so).

So we can continue, as they are, to send particles down through the riser to the BOP. But we also know that the flow path through the riser could be a long thin crack, rather than the round hole we used in the example above. So to address that problem a different particle shape and type is needed. Consider now what happens if we send some wire down the line with a thin rubber coating (so we don’t damage the fittings on the way down) and give it say a diameter of 0.4 inches. This is small enough to get through the pipes, but if the crack is narrower than this the wire, because of its shape, will be pulled across the crack, thus:

Wire fills a longer part of the crack if it is flexible enough to follow it.

Of course the cracks won’t run straight, and thicker wire is stiff, so after a while they will probably introduce wires of differing diameters. But this may be the next step in the process. There is, however a precaution that has to be taken. Wires tend to clump together and can build a blockage in the feed line if they are too long, so the pieces should be kept short, and fed in a little at a time, over a size range, to ensure that they help rather than hinder the bridging of the flow.

At the moment (noon) it looks as though they are still pumping mud, so they may be trying to use a slightly heavier mud in order to get balance, though again they are constrained on how heavy they can make this before they start losing it into the formations.