A little more on Jetting - surgery, art and diesel replacement

Being somewhat jet lagged today, after returning from Austria last night, I won't attempt anything that requires any great mental dexterity today, but rather continue on the theme about developments that are going on in my field that are likely not to be known to the more general public. One of the things that has been fascinating has been the way in which waterjets are being developed for use in the medical field. There was another illustration of this in a paper by Biskup et al, at Leibniz University in Hannover. When surgeons repair torn ligaments in the knee (an Anterior Cruciate Ligament Reconstruction) they use screws to hold the transplanted piece in place. Use of different materials for these screws has shown to have some long-term problems when they are made of metal. Recent work has shown that if these screws are themselves, however, made of bone, then, over time, the screw is integrated into the surrounding structure and becomes more stable. However it is hard to machine bone conventionally because if it gets heated, then the bone dies. If, however, it is shaped using an abrasive waterjet, to cut the thread, and the internal channel, which is also used to turn the screw, then temperatures can be controlled so that the bone is still viable. (Though it needs some chemical treatment to remove prions).

Back in 2006 the conference was held in Gdansk, and one of the interesting papers was on some work being done by Przemyslaw Borkowski at Koszalin University of Technology, in transferring photographs into inscribed pictures cut into metal. We took that idea and have moved it into a surface textural change that creates the picture as a 3-tone image in metal and rock (hence the "art") but there has been another development that has moved this into commercial availability. Nathan Webers and Carl Olsen of Omax Corporation, has modified the software on their cutting tables so that a controlled depth image can be inserted into a surface from an image. This, for example, is a lizard etched into aluminum.


The image was about 6 inches across and maybe half-an-inch deep.

It may make life a lot easier for those who have to carve images into stone, though removing a little of the artistic license with which sculptors apply their craft.

Incidentally Przemyslaw gave a talk at this conference on the use of waterjets in crushing coal. If you can take coal down to about 5 microns and mix it with water at 50% (roughly) GE have shown - by running a locomotive for more than 700 hours on the track - that it can replace diesel fuel. Looking at different available coals, it appears that brown coal is a little easier to break to the required size range than bituminous, which were the two varieties that he looked at. Using waterjets to do the crushing can make the process technically simpler and less energy intensive than other methods of getting the coal down to the required size.

The Waterjet Meeting in Graz

I was not planning on writing about the Conference that I have been at this week. It deals with the uses of high-pressure (typically about 55,000 psi) water jets. However there were a couple or three papers that had some relevance to the topic of energy, so I thought I would craft these into a small post.

The first one was a paper by Professor Soyama of Tohoko University in Japan, who has been looking at the events that occur during a cavitation cloud collapse. For those not familiar with what this is, when you adjust the flow pattern of a fluid so that forces try to stretch the fluid, the fluid ruptures into small bubbles. Instantaneously the bubbles have nothing in them, but they fill with vapor from the walls, over time. However there is rarely much time since, as the bubbles move into an area where there is a positive pressure in the water they collapse. But when they collapse it is not totally symmetrical. As a result tiny micro-jets (known in some circles as Munroe Jets) are formed, and these can generate impact pressures of up to a million psi (we proved this theoretical estimation, made originally by Al Ellis at UCSD). At the same time, as Professor Soyama notes, the temperatures that are locally generated can be very high, to the point where light can be generated in the 300 to 700 nm wavelength range. The combination of the two creates an condition where carbon dioxide, injected into the flow, can be (and in his laboratory was) converted into methane. This is a relatively new discovery, and at a scale that may likely be impractical to put to large-scale commercial development, but on the other hand . . . . . . .

The second paper was by Franz Trieb of BHDT GmbH who talked about the use of high pressure jets in helping the construction of prefabricated brick walls. At the rate of 400 sq. m. per day, for 3 workers. It uses the Redbloc system which glues well-formed brick courses together and gives the high quality wall. The water jets cut and trim shapes (such as door entries or windows) in the wall, which is then delivered to the construction site as a finished assembly, and can be rapidly assembled. The resulting house has a number of other benefits including lower cost.

The third item of interest was the description by attendees from KMT high-pressure systems who talked about the transition to gher pressures (up to 90,000 psi) where individual pieces can be cut about 50% faster rates than with conventional pressure. The overall result is a reduction in cutting costs, and lowering of the energy required to cut a part, and the water needed for the process.

Overall it has been a very interesting meeting, though likely my last. There was a mild incredulity when I brought up the subject of peak oil.

Drilling for Geothermal Energy

Just as the thread on my TOD post on Drilling was winding down, horizonstar posted a comment about the tools that Potter Drilling are developing for geothermal drilling, with $4 million worth of help from Google. The comment takes you to a Grist post, from which I am now going to pinch the top illustration.

Hole in granite drilled by Potter Drilling using hydrothermal spallation . Note the fine nature of the pile of material removed)

Now (if you will forgive the vanity) I will add a picture from my doctorate, and an experiment that I carried out in 1967.

Hole drilled through an 8-inch thick granite block using a 9,500 psi water jet. (Figure 7.13 in The Book )

Now if you look at the two holes you might think that the top hole is the better, and more efficiently drilled. This post is going to try and explain why in fact it is the bottom hole that is better and will become one of two posts that I plan on writing on the relative performance of different tools in drilling, including ideas such as lasers, electron beams and the infamous REAM. There is a video of the two stages of the development of the Potter drill in an article in Popular Science from a couple of months back that is worth watching, since it explains their idea.


So why isn’t their approach a good one – well there are a couple of reasons, let’s start with the basic idea of breaking rock. Way back when the world used a lot of coal men still mined it using a pick and a shovel. In using the pick the miner would attack cracks in the surface of the coal and grow the crack so that he could wedge out larger lumps of coal, rather than picking out the coal in small pieces. If he did it effectively he would use something on the order of 4 joules of energy to mine each cubic cm of coal he mined (4 j/cc).

If you are breaking out rock from the solid the amount of energy you need depends on the surface area of the rock that you have to form to break the rock out. Assume that it takes 1 unit of energy to hold the molecules across a sq. cm of rock together. If I want to split the rock through that cm of contact I am going to have to break all those connections and (if the process is 100% efficient) this will take just slightly more than that unit of energy to make the break. Now here is the important bit:
Granite split into two 50-cm x 1 m x 1 m pieces

If I take a cubic block of rock that is say 1 meter in size, and split it down the middle I will cut through 1 sq m of rock over the fracture that I create. So with that same amount of energy holding the rock together(which we call surface energy) it will take 100 x 100 = 10,000 units of energy to make that one crack.

Now if, instead of breaking that rock into just two bits I broke it into a sixty-four, by making three cuts vertically parallel to the front, three cuts vertically perpendicular to the front and three cuts horizontally , then I would have split the rock into 64 pieces each 25 cm on a side, but it would have taken 9 cuts of a sq m each, and required an input of 90,000 units of energy to break the rock into the smaller sizes.

Block broken into sixty-four pieces with nine meter-square cuts.

Thus the smaller the size the pieces are broken into then the more energy that you have to put into breaking the rock to make those smaller pieces. Consider that if you are breaking the rock into a fine powder (as the pieces are with the hole drilled at the top) then if those particles are 0.25 mm on a side then the energy input becomes that much greater (4,000 cuts along each direction – 12,000 cuts total at a total of 12,000 times the energy needed if we just broke the rock out in two big bits).

So breaking the rock out in bigger bits is better – but how can we do this with fluids. Well this is the difference between the thermal process and a water-jet based process. With the thermal process what you generally rely on for the fracture in granite is a phase change in the quartz element of the rock that occurs at about 1300 deg C. This causes the rock to spall and has been used in the granite industry for initial channeling around the blocks to be quarried. (It is very noisy and fairly slow – around 14 sq ft/hour of production). The particles produced are very fine, and the energy required is around 12,000 joules/cc.

Water cuts into the granite in a different way, first penetrating into the cracks between the different grains of the rock , and then as that water wedge is pressurized by the following jet, forced deeper into the crack, growing it and breaking the individual rock grains away from the surface. These grains are much bigger (so the hole wall that you can see in the second picture is rougher), so the energy required to drill can be down in the hundreds of joules/cc instead of thousands.

The other advantage that waterjets have (if configured with sand in the water) is that they can drill through any rock the drill comes up against. That is not the case with the thermal lance, since some rocks just melt into globby messes when heated, and that has to be pushed away (ask the folks at Los Alamos about the nuclear powered thermal drill that they invented one time – they showed how to do that). And the question is pushed to where when you are drilling holes thousands of meters deep.

And just to respond to the point about pipe-dreams that was mentioned about these ideas being impractical – Gulf actually drilled to about 5,000 m deep with an abrasive waterjet drill about 1970. So these ideas do have some practicality. But I’ll return to that aspect in my next post.

Oh, and just to show that you can break out meter sized blocks - this was from when we were excavating the rock under the Arch in St Louis to put in the OmniMax Theater.

Breaking out thousand-pound blocks of rock while excavating the Omnimax Theater under the Arch in St. Louis