Waterjetting 27e - Borehole Back Pressure Effects

In the earlier posts on this chapter of waterjet technology I have dealt with the changes in cutting performance when a waterjet stream cuts in to material that is either under pressure, or contains internal stresses that may not be obvious at first glance. In this post I will focus, instead, on the changes in performance when the borehole becomes filled with water under pressure.

Figure 1. 12-inch cores of sandstone that have been drilled by the same jet drill, at the same speed, but at borehole pressures of 0, 500 psi, 1,000 psi, 1,500 psi and 2,000 psi. (Jet pump pressure 10,000 psi; 970 rpm; 40 inches/min ROP)

The water used in the test also contained a small amount of polyethylene oxide (Polyox) that, at the time, was the only polymer readily available to enhance jet performance under water, although there are now liquids such as Superwater that similarly help.

It can be seen that even the change in pressure to 500 psi is sufficient to dramatically shorten the distance that the jet cuts through the material on a single pass, and the range then only shortens a little as pressure further increases. But the hole drilled at 2,000 psi is barely large enough to let the high pressure lance and nozzle assembly pass.

First an explanation of the equipment that we used to run the tests. A triaxial cell was used as the basic vessel to hold the core. This is so-called since it allows pressure to be applied around the rock core, and also since the cap can slide within seals, axial pressure given the third of the orthogonal directions for loading.

Figure 2. Triaxial cell used for the drilling experiments.

A valve was fitted on the flow line of water out of the chamber (just above the pressure dial) and this controlled the fluid pressure in the cell. The diameter of the outer (reaming) jet was 0.04 inches, and the rapid decay in range with the increase in pressure led to a second experiment, to see how changing the diameter changed the results. The equipment was modified for this test, the feed pipe to the nozzle was bent, so that, as it made a single circuit over the underlying rock, it would trace out a circular path rather cut a single hole. Then the top of the sample was cut at an angle so that, with the rotation the distance from the jet to the target would vary and the range of the jet could be seen. (Figure 4).

Figure 3. Modified equipment to find the effective jet range against back pressure.

A simplified factorial experiment was run with three nozzle diameters and five back pressures, measuring the depth of cut into the sandstone in each case.

Figure 4. The resulting cut when a 0.03 inch diameter jet was rotated over sandstone with a 1,000 psi back pressure in the cell. The 10,000 psi jet was brought up to pressure with the jet at the greatest standoff (hole at the bottom) and the back pressure was set before making a single pass over the sample. The depth of cut was averaged over several readings made along its length.

The data was then plotted (with the curve smoothed here for simplicity in discussion).

Figure 5. A plot of range of jet cutting ability as a function of hole back pressure for three different nozzle diameters.

The graph shows that, for this set of conditions, the larger the jet the better, and that the first 500 psi of back pressure has an immediate effect on jet cutting effectiveness. Jet size should be at least 0.064 inches when drilling against back pressure in the hole. There was a significant improvement in cutting ability when the polymer (at 300 ppm) was subsequently added to the water, in a later series of tests. The small number of tests carried out, however, were too small a sample to provide more than guidance as to concentration since all three levels tested (100, 200 and 300 ppm) all showed considerably improved depths of cut (increasing to a depth of almost 2 inches against a back pressure of 2,500 psi) when contrasted with the performance levels shown above. The polymer tests were carried out with a jet nozzle diameter of 0.064 inches.

There are two parts to the effect of the borehole pressure. The first is simply one of increasing the resistance of the water to jet penetration, and lowering the effective jet pressure (since that is effectively the jet pressure less the borehole pressure).

It is important to recognize that it is not just the drop in effective pressure that causes the effect. To check that this was the case a hole was drilled with the same conditions otherwise as the left-hand rock sample in Figure 1, except that the jet pressure was dropped to 5,000 psi. Thus the differential pressure of the jet across the nozzle was less than that in the case of the other four rock samples shown in Figure 1. Yet the hole was of the same approximate irregular geometry as that shown by the left-hand core of Figure 1 even with the lower differential pressure with the prominent cone cut ahead of the bit that is not evident in the other cases.

Mike Hood has shown the effect of loss in cutting range by using back-lit shadow images of a jet at different back pressures.

Figure 6. Illustration of the effect of fluid back pressure, the shadow image of the jet shows how back pressure reduces the range.

As mentioned above, the effects extend beyond reducing the jet range, and lowering the jet differential pressure. The increased confinement on the rock will compress the grains of the rock more tightly together, making it more difficult for the pressurized water to penetrate into the rock structure. This combines with the higher pressure required to grow the cracks to effectively reduce the ability of the jet to penetrate into the rock.

At the same time, if you listen as the back pressure is increased (we used a Lichtarowicz Cell the increasing pitch of the sound shows (as does the damage induced) that the collapse of the cavitation bubbles generated around the edges of the submerged jet is becoming more intense as the pressure increases. I have discussed how this can be used as a benefit in breaking up rock in an earlier post.

Waterjetting 27d: Drilling at a fixed diameter

In the last post I described how we initially came up with a simple design for drilling through material, using an axially aligned jet and a larger jet offset to one side at an optimal angle of around 20 degrees.

One of the problems with the use of this design is that the outer jet has to remove all the material in front of the nozzle during the time that it rotates around and advances the distance of the incremental feed rate. If it does not then there is a significant problem. Consider the case where the drill penetrates through a layer of limestone, while drilling otherwise in sandstone.

Figure 1. Sectioned waterjet drilled hole through a sandstone:limestone:sandstone sandwich of rock.

Note that although the hole does not deviate as it goes through the harder material since, unlike conventional drills, there is no mechanical contact between the high-angled rock and the nozzle assembly. But the hole reduces in size. If the hole reduces in size below the diameter of the nozzle holder, this will not contact the rock until it has passed behind the plane of the reaming jet. In other words the only way the blocking rock can be removed is to back the nozzle along the hole so that the reaming jet can hit the material blocking progress.

Figure 2. Drill passage blocked by protruding rock in the path of the nozzle body, but behind the cutting plane of the inclined jet.

One way to ensure that this is not a problem is to advance the drill at a slower rate, with the rate of penetration controlled by the ability to cut the hardest rock that the drill will pass through. The problem with that approach, and concurrently that of setting a fixed advance rate, is that, at the same advance rate and rotation speed, the drill will drill through different rocks at a different diameter. While this can be an advantage, in a limited number of cases that I will discuss in a later post, in most cases it is better if the hole is at a relatively constant diameter.

So how can we solve this problem?

One approach taken in Australia was to change the design and location of the cutting jets. Rather than have a single jet cutting out to the perimeter of the hole, two jets were used, but crossed over the axis and cut on the opposite side to their location. This had an additional advantage over the initial design in that, when drilling longer holes (and this went on to drill horizontal holes that ranged up to a kilometer in length IIRC) the head was balanced and so did not wobble and get out of alignment because of the force imbalance.

To overcome the problem of drilling at too small a diameter additional reaming jets were placed on the front of the nozzle assembly, so that he hole would be reamed to the diameter needed to allow the support hose access.

Figure 3. The addition of a pair of reaming jets. Note that offsetting the two front nozzles will also allow them to put a torque on the front part of the nozzle, which can therefore be self-rotating from the left hand of arrow A forward.

But the problem is not completely solved with these changes, since should any rock protrude into the hole in the distance A, so that it hits the larger diameter that follows, again it is not possible for the reaming jets to cut this rock without backing up the drill.

There is another problem, in drilling horizontal holes where the hole diameter can vary. Consider that if the drill goes into a softer material then, at constant advance (ROP), the hole diameter becomes larger. As the drill moves over this larger hole it will be riding on the floor of the hole, and thus the front of the drill will tip forward into the floor of the larger hole. This will incline the drill downwards, and so the hole will no longer be of constant alignment, but rather will gradually, over distance, tip increasingly downwards.

It is therefore critical that the hole be drilled at a relatively constant diameter (allowing for some hole roughness). How to achieve this? The answer is to put a gaging ring or collar of the required hole diameter, in the cutting plane of the rotating jets.

Figure 4. The use of a collar at the front of the nozzle to ensure the hole is cut to the right diameter.

It itself this isn’t sufficient to give the hole a constant diameter, since there is still the problem of drilling through materials of differing resistance. To overcome that problem we put a spring at the back of the drill, with a contact switch to a valve on the feed to the hydraulic motor powering the drill advance. Thus the drill would start to rotate, and the motor would increase the speed of advance until the collar bumped up against the rock. At that time the spring would compress, the contact switch would close, and the advance would momentarily stop. The drill would rotate around and remove the obstructing rock, the spring would expand opening the flow to the motor, and the drill would move forward. It may sound as though it would be a stuttering advance, but when we tried it in a mine you couldn’t tell that the mechanism was working, apart from the hole being of constant diameter, and by watching the spring. It drilled at between 7 and 12 ft a minute in an aggressive sandstone.

Figure 5. The drill assembly used underground. The hydraulic advance motor (it pulls the drill forward using the chain drive) can be seen under the drill sash (the red and grey bar – painted in 1 ft intervals).

In a normal drilling operation when a drill intersects a previously drilled hole at a shallow angle, then the second drill will follow the path of the first hole, and cannot drill through the opposing wall at that shallow angle. (We know this from experience having broken two drill steels trying while excavating the OmniMax Theater under the Arch in St. Louis). But with the waterjet drill we were able to make to second drill cross the intersection.

Figure 6. Photo down one drill hole, showing the point where the hole intersected a second, and crossed without deviation.

Hopefully there is now enough background so that next time I can talk a little more about the effects of borehole pressure on drilling performance.

Waterjetting 27c - Drilling nozzle design

In discussing how stress affects the ability of waterjets to drill rock, I have discussed the effect of the stress in the ground on drill performance, but before discussing the effect of the borehole pressure it is perhaps best to spend this post talking about the simplest drill bit design.

The diameter of the first jets we used to cut into rock were about 0.04 inches in size, with the nozzle holder used to hold the nozzle on the end of the supply pipe being at around an inch in diameter. As a result, if the jet was to cut a path into the rock, it would have to rotate around the face of the rock ahead of it, removing all the rock ahead of the assembly, and allowing the head to advance.

Figure 1. Original concept of a waterjet drill used to penetrate sandstone.

Of course, back when we first did this in the 1960’s the swivels weren’t available to allow us to rotate the high pressure line, and so we rotated the rock samples instead.

Figure 2. First holes drilled at the University of Leeds. Note the central cone.

Because the jet had to penetrate across the diameter of the hole, so as to remove the cone ahead of the tool, and since the jet would only cut around 2.5 times the jet diameter in width at any one time, the rate that the head could move forward was limited to a maximum of 0.1 x rotation speed (rpm) in inches/minute. And, because the rotation speed controlled the depth which the jet cut into the rock, the rpm had to be kept down to ensure that the jet cut to the full required diameter on each pass. The top speed we could achieve, even in relatively soft sandstone, was around 4 inches a minute.

One of those fortunate accidents that sometimes befalls research folk then occurred. I had asked Jim Blaine, our machinist, to make a new design, with one jet pointing forward and one off to the side, intending that the two be offset. However, due to a misunderstanding, he drilled the second, smaller hole along the jet axis, while offsetting the angled jet to cut further out from the diameter. Since the nozzle was built we proceeded to try it.

Figure 3. First dual jet nozzle design.

Because the axial jet removed the central core, we could offset the inclined jet so that it needed to cut a shorter distance in order to reach the required gage for the hole. That meant that we could rotate the nozzle faster, which in turn meant a faster drilling speed, much faster.

Figure 4. Hole diameter as a function of rate of penetration of the drill (in meters/minute), for two outer jet angles, and two rotation speeds.

Note that in the above figure, with a 30 degree outer jet, spinning at 970 rpm we were able to drill a hole at a speed of roughly 280 inches/minute instead of the previous 4 inches/min by adding only 25% more water to the bit with the second orifice.

As mentioned above, the limit on the advance rate was the depth which the jet cut into the wall, and the amount of rib between adjacent passes that the jet cut would leave.

Figure 5. An early hole drilled into Berea sandstone, at a slow advance rate, using a 10 ksi jet pressure.

Figure 6. Hole drilled into Berea sandstone at 970 rpm, 225 ipm advance rate, with a 15-degree inclined jet. Note that the hole perimeter has the equivalent of a thread cut into it.

It is pertinent to make a small observation over the advantage of that slightly roughened outer wall to the borehole. One of the ways in which miners hold up the roof while they are working underground, is to insert rods (known as roofbolts or rockbolts) into drilled holes placed in the surrounding rock. To improve the grip between these bolts and the wall, miners will also often insert packages of glue into the hole to fill the gap between the bolts and the rock wall.

Unfortunately when the hole is drilled with a conventional mechanical drilling bit, the walls of the hole are left relatively smooth. This means that the bolt has a poorer grip on the wall, and is more easily pulled out of the hole. The US Bureau of Mines ran anchorage tests for different rock wall finishes.

Figure 7. Effect of hole roughness on the anchor strength (US Bureau of Mines)

Conventionally a larger hole, with greater bearing surface, would give a stronger anchorage. This is shown by the greater load carried by the hole drilled with the 1-3/8th bit, over that drilled by the 1-1/4 inch bit. But both of these were smooth walled, and the bit drilled at 1-inch, with a roughened wall had almost three-times the pull strength even though of smaller size.

The roughness of the hole can be controlled by adjusting the feed rate, relative to the rotation speed, both as a function of the jet pressure, nozzle diameter and outer jet angle. It turned out, through experiment, that the optimal angle for the jet was at around 22.5-degrees, depending on the type of rock in which the drill was working.

The effect of rock properties plays a very significant role in the performance of the drill. And it was very easy, early in the program, to show that the important rock parameter was not the compressive strength of the material. To show this we drilled through prepared samples of an Indiana limestone and a sandstone, both of which had approximately the same (uniaxial) compressive strength. The advance rate was kept constant, as was the rotation speed, as the drill penetrated from one rock into the other, and then the hole was cut in half (as were the samples shown above).

Figure 8. Hole drilled from limestone into sandstone.

Although the hole maintained alignment, drilling straight forward through the steep interface between the two rocks (a problem with some conventional drills) the hole diameter changed dramatically.

How we changed the design to maintain hole diameter, and, at the same time, adjusted for changing borehole depth will be discussed next time.