Sunday, November 23, 2014

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.

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