Waterjetting 34a - Drilling holes with water jets

Looking back over the Waterjet Index I realized that while I have addressed different aspects of drilling holes with waterjets in various ways, I haven’t really brought it all together as a focused topic. So, herewith, and in the next few posts, that coalescence. It begins with a bit of a recap.

It was clear, early in the work on waterjet applications, that one of the key problems to be addressed was that of the ingoing water having to fight its way past the spent water already exiting the hole that had been created. This is particularly true when making an initial pierce through a target material, where there is very little relative movement between the nozzle and the target. Given that the interaction between the two flows occurs within about a hundredth of a second, the effect on cutting efficiency is relatively immediate.

So how to overcome the problem? One way is to pulse the jet, and in some early work at Leeds we built such a pulsating unit and spun it in front of the nozzle, chopping the jet into segments and allowing one segment to leave the hole, before the next arrived.

Figure 1. Pulsating disc to rotate ahead of nozzle and “pulse” the jet.

This was inefficient, because the energy put into the segments diverted by the disc was lost, and it was also extremely noisy, to the point that tests had to be carried out after everyone else left the building.

The alternative was to rotate the sample (this was in the days before high-pressure swivels and couplings were available) and align the jet just off the axis of rotation, so that the jet cut a hole somewhat wider than itself, as it passed over the target and thus provided an open path for the rebounding spent water that did not interfere with the path of the fresh jet travelling to the impact point.

A comparison of relative penetration rates showed that while a fixed nozzle and steady jet had sensibly stopped penetrating after about a second, the pulsed jet continued to drill a hole, as did the rotating jet, but the latter was able to drill faster and remove more material.

Figure 2. Comparison of a steady jet, a pulsating jet and a rotating jet as they penetrate into rock over time. All jets are at the same diameter, pressure and standoff distance.

By moving the nozzle out to a greater radius the hole created could be enlarged. This allowed the nozzle to move forward into the cavity created and the process could be repeated. In this way, over several iterations, a waterjet at a pressure of about 9,500 psi drilled though a block of granite, uniaxial compressive strength around 30,000 psi, and a new drilling tool had been demonstrated.

Obviously it is impractical to keep enlarging the hole by reaming it wider from the surface to allow the nozzle body to enter the hole and advance to the bottom. The jet must, from the beginning, drill a hole large enough for the nozzle to advance. And the easy way to do this is to incline the nozzle (at an optimal angle of around 20 deg, depending on the pressure, the type of waterjet and the target material properties). And while we still did not have a rotating swivel, we could turn and raise the target, while directing the jet through a small inclined orifice.

Figure 3. Inclined jet drilling a hole through a rock

Skip forward a couple of years, and we were drilling rock, at a rate of about 4-inches a minute, with a single jet. Then Jim Blaine, the RMERC machinist at the time, misunderstood a drawing, and added a central axially aligned orifice to the nozzle geometry. And within days we had increased the speed of drilling by two orders of magnitude. (Though by this time we also had a working rotating coupling to help rotate the nozzle).

Figure 4. Modified drill nozzle geometry

Figure 5. Drilling rate of advance as a function of hole diameter and rotation speed.

When one is using a single jet to cut the required profile of the hole, which must exceed the diameter of the nozzle holder if the drill is to advance into the hole, the rotation speed of the drill must be fast enough, relative to the rotation that the ribs of material left between adjacent passes of the jet along the hole wall are either non-existent (where the adjacent passes overlap) or are small enough that the mechanical impact of the nozzle body can break them off with very little force. In the latter case, however, this can put a mechanical load onto the drill string. The drill is often made up of only a length of high-pressure tubing, with the nozzle threaded on the end, so that any significant mechanical force can distort it and cause the drill to misalign and no longer drill a straight hole, so that this contact is discouraged.

On the other hand the rock through which the drill passes will likely change in strength and composition quite frequently, and so the depth to which the jet will cut will also change. This means that the hole diameter may reduce, so that the hole is no longer large enough to allow the nozzle body to pass. To stop this becoming a problem a small ring is mounted ahead of the nozzle, in the plane that the jet reaches the hole diameter required in this particular case. Now when the jet fails to cut to that diameter then the ring will stop the drill advancing, while the jet cuts along the line of contact and enlarges the hole to the required size, then allowing the drill to move forward. This is helped where the drill is spring-loaded so that the compression of the spring stops the drill advance, and the relaxing of the spring, as the obstacle is removed, allows the drill to move forward again.

Figure 6. Gaging ring on the front of a drilling nozzle.

That drill development was relatively straightforward, and was demonstrated in the late 1970’s. Subsequently commercial drilling systems were developed that used high-pressure water to drill holes in mine rock. They had the advantage over conventional tools in that the hose feeding the nozzle could maneuver in a much smaller space than a conventional drill, and thus longer holes could be more easily drilled from narrow working areas.

Unfortunately it still proved difficult to drill all rock with a plain waterjet, despite the use of ultra-high pressure equipment, and two different approaches were then tried, which I will discuss in the next posts.

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.