Waterjetting 37d - Underground Drilling with Waterjets

The last post showed some of the experiments that we carried out as we developed a tool for drilling "around corners", demonstrating the ability of a high-pressure water jetting system to turn from a vertical well bore and drill out a lateral well within a turning radius of about 9 inches. This was not our earliest work on drilling, and that earlier effort had been directed to the reaming of geothermal wells.

The earlier project was also funded initially by Sandia Labs, although in this case the objective was slightly different. Overall we set out to show that one could go down a vertical well bore, some 9-inches in diameter, and ream a cavity 6-feet across down in the "hot dry rock" of a geothermal deposit.

When we began we weren't sure that we could drill in stressed rock, or what those effects on drilling rate would be. So we were fortunate to get some help from Doe Run and work in one of their mines, as the following video shows.

Figure 1. Early video on Underground Waterjet Drilling

  It was this series of tests that showed how relatively quiet water jets could be in drilling, unlike the competition at the time. Below the fold I will add the second part of this program, where we demonstrated the technique we planned on using for the reaming program.

Figure 2. Reaming a Geothermal Well Demonstration

Although this program moved on to focus on other aspects of the problem (we also demonstrated that this was practical in cutting granite, which led on to the Stonehenge and Millennium Arch projects) it was not until we were tasked with reaming large scale rocket motors, some years later, that we returned to this program, and developed the tools further.

Waterjetting 27b - Drilling rock under stress

Last time I opened discussion on the topic of cutting a material that contained high levels of stress. This is a more common situation when working with rock, since – as a general rule of thumb – the vertical stress on a rock increases by 1 psi, for every foot deeper one goes into the earth. Thus, for example, if one goes down around 700 feet, the depth of a number of coal mines, then the background pressure on that rock is some 700 psi due to the weight of the rock that is pressing on it from above.

Now I should also mention that this is only a general rule, because, over the millennia, the rocks move, are split by earthquakes, overlain by volcanic eruptions and many other events that make that generalized statement less accurate for any given location. And one factor is that, if there weren’t such movements, then the natural horizontal stress on the undisturbed rock would be about a quarter of the vertical stress (the ratio is known as Poisson’s ratio, though usually derived for the resulting strain on the material, rather than the driving stress).

What one often finds, when these values are measured, is that the horizontal stress is higher than the above simple calculation would suggest. Which is a long way of saying that it is often difficult, without making a measurement, to know exactly what stress a rock is actually undergoing when found underground. But if some of the rock is removed (because it contains valuable ore) then the stress field redistributes, and some of the simpler assumptions come back into play. And we found that out when we drilled these holes:

Figure 1. Oval holes drilled into a lead-bearing sandstone;

You can see that we were drilling oval holes. The drill we were using used two high-pressure (10,000 psi) waterjets that were rotating at constant speed as we fed the drill into the rock. (And I’ll discuss the drill design and other stress effects in the next post). The small dark spots in the rock are galena, and as I will discuss in some future post, we were able to separate the galena from the sandstone at the drill, in part because of the way the waterjets penetrate, as I will discuss below.

Figure 2. Waterjet drill penetrating sandstone at up to 12 ft/min.

The region of the mine we were working in was around 700 ft. deep, and had been previously mined. Roughly half the rock volume had been removed, over a relatively large surface area, so that the pillars that were remaining were carrying roughly twice the load that they were before mining took place. On the other hand, since the rock on either side of the pillars had been removed, the vertical load was all that was acting on the rock within the body of the pillar, where we were drilling the holes. So very crudely the vertical stress, before we started drilling was around 1,500 psi in the rock.

Now, to explain why the holes are oval rather than round, consider that a waterjet works by getting into the cracks that exist in the rock, pressurizing the fluid and causing the crack to grow until it meets other cracks that together free a small piece of the rock mass. In this case the rock is made up of grains of sandstone and galena which have boundary cracks around each particle. By growing the cracks using this process, the rock is broken out into the individual grains of sand and galena.

But when the rock puts pressure on the rock, so the cracks are squeezed closed, and the water finds it harder to penetrate into them. This happens to the rock on the sides of the hole. As it is being formed, the load that was being carried by the rock being removed transfers to the rock on either side of the hole. Because the load is vertical this means that the jets find it harder to penetrate the rock on either side of the hole, and the horizontal diameter of the hole is therefore less than it would be otherwise.

Figure 3. Lines showing equal stress magnitude around a hole drilled into a rock loaded vertically. (This is purely representative and does not carry a scale, the lines are of diminishing intensity as they move away from the hole.)

On the other hand, as the load from the overlying rock moves out to either side of the hole, it comes off the rock at the top and bottom of the hole, and those cracks get larger, and were no longer being squeezed shut. As a result the jets found it easier to penetrate into the rock, and the vertical diameter of the hole is thus larger than it would be otherwise.

Put these two together and the result was that the jet drilled holes that were oval in shape, as shown in Figure 1.

As one way of making sure that this was really the cause of the change in hole shape, we used the waterjets to cut a slot around the perimeter of a part of the rock in the pillar. By making a horizontal cut above the slab that this outlined, we removed the vertical loading that the rock was seeing due to the overlying rock.

With no external loads on the rock, from either direction, it was as easy for the jets to cut into the rock in all directions, and, as a result, the holes that the jet drilled were round.

Figure 4. Round holes drilled in unstressed rock near the block of holes shown in Figure 1.

Again, while the effects are much larger when shown in cutting and drilling rock, the effects would be similar if we were cutting material that was under other internal stresses and which were then cut by a jet in a shop or other surface facility.

In the above case we were working in a mine where there was free access to the rock, the situation changes if we had been trying to drill down from the surface, and that will be the topic of the next post. In passing it should be noted that the waterjet drill was not only quieter, but also less powerful and smaller than the existing mechanical drill, and it could drill the rock faster.

Figure 5. Comparison of mechanical drill (upper) and the waterjet drilling equivalent (lower) on a drilling rig underground in a lead mine.(You need to look closely to see the drilling rod that is shown in Figure 2.)

Waterjetting 27a - Cutting materials with internal stress

Safety glass, or toughened glass is typically designed so that, when it fails it will break into small pieces with few of the relatively sharp, and thus dangerous, fragments formed by ordinary glass. It is used in making shower doors, and automobile windows. As such is differs from laminated glass (which I will discuss in a later post in this section). The toughened glass is formed by quickly cooling the glass, after it has been heated. And one way to check if a sheet has been treated this way is to look at it through polarized sunglasses. Tempered glass will show a pattern. The reason for that, and for the rapidity of the breakup of the glass, is that the lines show the internal stresses that the glass treatment deliberately leaves in the material. (There is an interesting variation on this way to tell the difference using an iPhone.)

Figure 1. Broken pieces of tempered glass, showing the small fragments that result. (Floydglass)

Because the treatment puts the outer parts of the glass into compression, while the inner part is in tension once cracks start to appear in the glass, then the glass is designed so that these stresses will cause the cracks to grow, bifurcate and join in patterns that cause the glass to shatter into less dangerous fragments. But this creates a considerable problem if there is a need to reshape the glass after it has been heat-treated.

Note that this treatment is the opposite of the result where glass is annealed, where – by cooling the glass at a slow rate – the internal stresses are much reduced, but as a result, when the glass breaks the fragments can be more damaging.

Figure 2. Sheets of annealed glass, showing how it may break from impact. (ADMglass)

Annealed glass is, as a general rule, relatively easy to cut with an abrasive waterjet system provided that certain simple precautions are taken. However, when it comes to cutting tempered glass, one of the suggestions is to anneal it first, so as to get rid of the internal stresses. Unfortunately, in the process this also removes the benefits of the tempered treatment.

When one tries, without other treatment, to cut tempered glass the results are not pretty. Edgar Hernandez has posted a video of what can be expected to happen.

The problem goes back to the basic way in which waterjets, and abrasive waterjets work in cutting through material. Simplistically waterjet impact will penetrate the cracks that exist in a target surface; the following slug of water then pressurizes the water within the crack, causing it to grow. As cracks get longer it takes less and less pressure, either internally within the crack, or in the surrounding material, for that crack to grow catastrophically to failure of the piece. Where there are relatively few natural cracks in the material – as happens with glass – then abrasive is introduced into the waterjet stream, so that the impact of the small particles will form small cracks when they hit the glass surface. Normally those cracks are relatively small, and when first cutting into or piercing the glass the pressure of the jet is often lowered so that the particle speed is also lower and the crack length that the particles create is also small and localized around the impact point, so that the integrity of the whole piece is not threatened.

Figure 3. Cracks around the impact of single particles of abrasive onto glass.

The problem, from a cutting aspect, with tempered glass is that the internal stresses that are deliberately placed into the glass are designed so that cracks do not have to be very long before the concentrated stress at the crack tip (which increases with crack length) reaches a point where it will continue to grow at an increasing rate to failure of the piece. The longest cut we have made in tempered glass before it shattered was about an inch-and-a-half.

Because the stress in the glass is an inherent part of the nature of that particular type of glass there is no really effective way of cutting the material, after it has been tempered. If a particular shape is required then the glass should be cut to final shape before it is tempered, and care should be taken to ensure that there aren’t any large cracks or chips along the edge of the glass before it is then tempered.

Stress problems aren’t restricted, however, to trying to cut tempered glass. When cutting larger pieces of metal one can also run into problems from stresses that were left in the material after it was initially formed. Perhaps the most common of these is found where a partial cut allows a stressed part to lift slightly above the plane of the rest of the material. If the part is being cut in steps, the raised piece can then move into the path of the cutting nozzle as it moves back over the piece. This can have some unfortunate consequences for the nozzle and focusing tube (there goes bitter experience speaking again).

Other problems that can crop up come from the shifting of the piece in the plane of the part, but where the stress relief moves the edges so that subsequent cuts into the part no longer comply with the blueprint for the cuts, since the material has shifted. This shift can be a relatively small movement – depending on the level of stress that was captured in the material, but it can be enough to take the final part out of tolerance, and thus it never hurts to be sure of the stress condition of the piece before starting to cut.

I’ll return to this theme, but with a different illustration of stress effects next time.