Waterjetting 34d - Drilling small holes through steel and concrete

In the earlier posts in this section I have concentrated more on cutting the profiles of a hole, and the different ways in which this can be done efficiently. However there are many cases where the hole has to travel into the work piece to a depth greater than can be easily achieved without the nozzle entering the hole, or where the hole is being drilled into something with no realistic “other side.”

In these cases, choice of the best strategies to remove the central core of material from the hole varies, depending in part on how large the hole is, and in what sort of material. For relatively small diameter holes, then the jet itself can drill a hole large enough for the head to enter the target. (In part this is because the diameter of the nozzle and supporting pipe can either be made quite small, or the jet can be made more diffuse.) In other cases the cutting head has to be designed to remove that central volume in a way that allows the fragments to pass the nozzle assembly as it feeds down the hole.

To illustrate the first approach consider some work that took place at the University of Alabama-Huntsville. The tool that was needed had to drill down through the reinforced concrete of bridge abutments and foundations to give access to tools that would evaluate their condition, and that of the surrounding ground. However putting large holes in such structures is, in itself, a potentially weakening process and can provide a path for subsequent corrosion. The tool developed therefore was reduced in size to around 1/4th inch in diameter, capable of drilling holes of around 3/8th of an inch in size through the reinforced concrete, using a Direct Injection of Abrasion or suspension jet, operated at 5,000 psi.

Figure 1. Portable abrasive suspension injection system. Operates at 5,000 psi (pump is on the left of the platform). This unit fits in the bed of a pickup truck and cost less than $10,000 for the MS&T HPWL lab to build. The device built in Alabama cost roughly $14,000). The tank at the front mixes the abrasive with polymer and water before feeding it into the two supply tanks that alternately supply the feed line to the nozzle. The polymer is used to keep the abrasive in suspension and not to settle back out of the suspension.

The University team coined the name Multi-Intrusive Testing, (MIT) for the method and showed that it was capable of driving holes more than 3-ft deep, and of the required small size. In this case the use of a suspension jet, where the abrasive is added to the high-pressure water upstream of the nozzle assembly means that there is only a single feed line going down-hole. This is one of the features that allows the operational size of the tool to be reduced.

As existing infrastructure (bridges, roads etc) exists through the changes of typical yearly weather cycles the concrete and surrounding materials will slowly deteriorate, and erosion over time can remove the support for a bridge, corrode the internal components and wear away critical parts of the overall assembly. The, fortunately relatively infrequent, collapse of major bridges shows the risks of leaving this damage without repair, yet the very massive nature of many of the structures makes it difficult to detect this damage before it reaches critical size. Hence the need for the tool.

Laboratory tests showed that the rig could drill holes up to 3-ft deep, with the drill, which was rotated at speeds between 60 and 120 rpm, being capable of drilling through not only the concrete, but also the rebar embedded within it.

Figure 2. Hole drilled through reinforced concrete showing the ability to penetrate the steel reinforcing within the concrete. (after Graettinger et al ibid).

Within the geometry of the very small holes of this type, which are required to minimize damage to the structure, it is difficult to offset the nozzle to a sufficient angle that the hole is large enough for the nozzle to pass. In most cases this needs to be at an angle greater than 12 degrees, since shallower angles tend to have the abrasive and jet rebound into the center of the core without fully cutting into the wall to a depth to ensure that the hole diameter is maintained.

One way of overcoming this problem is to force the jet to diverge as it leaves the nozzle, since the induced spreading (typically 15 degrees or more) not only ensures that all the material ahead of the jet is removed, but also allows the wall diameter to be maintained.

The problem with the design, as with many similar drilling applications for waterjet and abrasive waterjet systems, is that the jet cuts to the required diameter at some distance ahead of the nozzle body itself. This can be illustrated with a picture of a diffused waterjet nozzle, that had been used, at MS&T, to drill through a sheet of steel (representing a borehole cased section), a layer of concrete (which would act as the sealing element in wells drilled to recover oil and natural gas for example) and then out into the sandstone rock beyond.

Figure 3. Dispersed abrasive jet (not rotated) used to drill through the simulated borehole wall. Note the two black lines that define the edges of the hole as it cuts into the steel plate.

Although the hole diameter was acceptable for passage of the tool once the steel was penetrated, as with the rebar in the earlier example, the hole diameter through the steel was too small to allow the head to progress. The answer, as has been discussed in earlier posts, is to advance the shroud (the outer cover over the nozzle) forward until it touches the edge of the cutting jet diameter at the required size. At this point, should the jet not initially cut the hole to the required size, then the drill will stop advancing, but the jet will be in contact with the obstructing material – and in a short interval will penetrate through it allowing the jet to drill further forward.

Difffusing the jet and removing the material will work for hole diameters up to about four inches, and we have used such a technique to drill through gravel and similar beds of loose material (with care, since while the jets have little disturbing force on the material around the hole much vibration can cause that material to destabilize and continually collapse into the hole being drilled – which ends up defeating the object of the exercise). I’ll talk about approaches for larger holes next time.

Waterjetting 32d - Cutting with polymer in the water

In the recent past I have written about the use of polymers in high-pressure jets and that they can significantly improve jetting performance, with no additional changes in the power or pump and equipment used in the work. This is because of two different effects that the polymers have. Firstly they reduce turbulence in the flow from the pump to the nozzle, reducing pressure loss and increasing fluid flow, for the same pump power. As a practical consequence since the fluid flow will be greater for the same pump pressure, this will require that a larger orifice diameter be used to handle the greater, or – for the same flow rate and nozzle diameter, the pump can be operated at a lower pressure.

Figure 1. Comparison between a conventional jet and one containing the polymer additive marketed as SUPERWATER. (after Glenn Howells)

This is not immediately apparent, since the jet carrying the polymer appears smaller, but this is due to the second effect of the polymer, which is to tend to glue the water together, so that it is not dispersed as easily by the surrounding fluid – air in this case. In fact, for the same pump pressure and orifice diameter the lower jet will be operating at a higher pressure (since there is less pressure loss in the line) and there will be more water coming out of the polymer-supplied orifice at a higher velocity. But because it is not spreading into the air, it appears smaller.

This will improve the cohesion of the jet as it moves away from the jet, and as noted in an earlier post, this means that the jet will cut to a greater range from the jet, since it maintains the required critical pressure further.

The impact that the more coherent jet has on performance can be seen where the jet is used to cut into two different types of limestone, one oolitic and one crystalline.

Figure 3. Depths of cut of the polymer-containing jet (left) and plain waterjet, operating at the same pump pressure, nozzle diameter and standoff distance (Glenn Howells)

Note that at the distance where the normal waterjet has broken into droplets (as seen by the nature of the cut surface) and the jet has barely enough energy to remove the surface layer, where the jet contains polymer it retains the ability to cut.

Further, and this is more critical where cut quality is more important, the jet cut is much straighter and cleaner than the dispersed and wider normal cut. This can be seen where a different, more crystalline limestone has been cut closer to the nozzle.

Figure 4. Change in the cut shape to a narrower, deeper cut where polymer is added to the jet stream (rhs)in cutting limestone. (after Glenn Howells)

The benefit of the improved performance changes therefore with the distance of the target from the nozzle, with the more dramatic improvement being seen as the target gets further from the nozzle. Looking at the data from the original work that we did in Leeds, back in the early days of this study, this can perhaps be better realized through the use of a 3-D plot.

Figure 5. Improvement in cutting performance as a function of distance from the nozzle and jet pressure.

Note that, in these trials, the polymer improved the jet performance relatively more at lower pressures and greater standoff distance. Part of the reason for this (in hindsight close to 50-years after running the tests) is that when the jet was cutting at the lower pressures it was closer to the threshold pressure of the rock and this any drop in jet pressure had a more significant impact on cut depth than occurred at the higher pressures, where the gain was not relative to such a low benchmark.

For a number of years, until our research took us into fields where use of the polymer was precluded for several reasons, we routinely used a polymer (generally Superwater, marketed by Berkeley Chemical) rather than Polyox because it gave a relatively consistent and significant improvement in performance plus, being a liquid, it was relatively simple and inexpensive to buy a small metering unit (about the size of a small case) which would feed the polymer into the water supply line to the pump at the required concentration – typically 0.1 to 0.3%.

It tends to work better in improving abrasive jet cutting when it is used with a Direct Injection of Abrasive (DIAjet) or Abrasive Slurry Jet (ASJ) system than with conventional abrasive waterjet (AWJ) systems. The reason for this is that the AWJ system has the abrasive fed into the water stream at the mixing chamber just before the jet leaves the nozzle to strike the target. Within the mixing chamber the abrasive has to penetrate into the waterjet stream in order to acquire the jet velocity, and to distribute across the jet and give an even cut on the target.

Where the abrasive is feeding in from one side of the jet and the waterjet stream is more coherent, it becomes more difficult for the abrasive to penetrate the stream, and if the design is not adjusted accordingly, the cutting performance can be diminished, particularly relative to the gain that can be achieved where the combination is carried out effectively.

On the other hand with the ASJ systems the abrasive is mixed with the water far upstream of the nozzle and the are already thoroughly mixed together, so that the added cohesion of the jet will help to provide the acceleration that the particles need to reach close to the waterjet velocity, and achieve the improved cutting performance required.

Where an ultrahigh pressure jet does not contain abrasive, the polymer can be of benefit as a means of improving cut quality – as evidenced from this comparison in cutting a shoe sole pattern, both with and without the Superwater polymer.

Figure 6. Shoe sole cut comparison with and without Superwater. Note the smoother cut, with less fraying of the back side of the cut with the polymer. (after Glenn Howells)

Waterjetting 32b - early work with polymers

If we lower the viscosity of the fluid in a jet hitting a target, then the jet can penetrate into the fissures and flaws of that surface more easily and in this way it will see an improved cutting performance. On the other hand the thinner fluids have less resistance to the air or other fluids through which the jet has to pass on its way from the nozzle to the target, and so the amount of energy that is delivered to the target can be lower than with a less viscose jet. Some of the changes in fluid chemistry can, however, be reached using small amounts of different chemical additives that can have quite a profound effect on performance without totally changing the fluid being used.

Consider that before the jet gets to the nozzle it has already completed most of the journey from the pump to the target. And along the way, as I pointed out in one of the earlier posts I wrote, pressure losses along the line, due largely to friction, can overwhelm the power supplied by the pump. There was an occasion where a colleague at another research group was running a system where, because of the small diameter of the tubing he was using, the pressure drop from the pump was more than 75% before the water even left the nozzle. A slight increase in tubing diameter would have made the system much more powerful, with no other increase in cost.

But sometimes that option isn’t really available. I remember my first introduction to the use of long-chain polymers was after reading an article in The Sunday Times which described an investigation by the New York Fire Department. One of the problems in fighting high-rise fires is that the firemen have to haul their hoses up the long flights of stairs and 2-inch diameter hoses become quite heavy as they are moved up from level to level. In the late 1960’s (in the administration of Mayor John Lindsay) tests were carried out to see if this could be improved. Normally, when a smaller 1.5 inch diameter hose was used, the pressure loss in pumping 100 gpm through 1.5-inch diameter hose is around 25 psi per hundred feet, with 98% of that loss being due to the turbulence of the water.

Figure 1. Demonstration of the benefit of polymers before John Lindsey, the Mayor of New York City (seated far right). May 13, 1969. The two jets are of equal size and fed from pumps at the same pressure. The jet barely reaching the holding tank is the standard water, the other contains polyox.

What the demonstration showed was that more water could be sent through the smaller hose, (roughly 70%) so that it could be used, instead of the larger hose, and with better effect on putting out nearby fires.

One interesting historical comment is that the demonstration occurred in 1969, however back at the University of Leeds, I was aware of the research in 1967, and had included, as chapter 5 of my dissertation, a study of the benefits of using the polymer. Similarly Dr. Norman Franz, then at the University of Michigan, (and thereafter at UBC) had also become aware of the work, and – being smarter – had patented his findings, that were then licensed by the fore-runner (McCartney Manufacturing Company) of KMT.

The benefits were not just that the polymer reduced friction in the line from the pump, transitioning the flow from turbulent to laminar, with the consequent reduction in friction losses, but these also extended beyond the nozzle. I have always suggested that the easiest way to think of this is to compare the benefits of using the polymers to that of picking up spaghetti from a plate. Very short strands of the pasta need the eater to use a spoon, or simple fork to try and grab a small amount for a bite. On the other hand, where the pasta strands are longer, and well mixed together, when one pulls one out of the pile it is attached to another few that also rise, and before long the fork is full of a seemingly endless set of attached strands.

Given that the molecular weight of Polymerized Ethylene Oxide (Polyox) (which was first introduced by Union Carbide and is now a Dow product) can range up into the 8 million range, it is easy to make the analogy. As a result the intertwining of the long molecules facilitates passage down the feed line, but also tends to maintain the jet coherence once it leaves the nozzle.

The jet is thus not only allowed to pass through the feed line at a lower (often significantly so) pressure loss than with conventional water, but also, once the jet leaves the nozzle, the stream remains coherent to a much greater distance. This can be seen in Figure 1 at low pressure, however that result has been similarly validated at pressure of up to 50 ksi in the years since that original work was completed.

Figure 2. Effect of adding a small amount of polymer to the cutting performance of a jet at 10,000 psi.

I will spend the next couple of posts discussing the general concept of polymer additions but will close this with another anecdote. After I had carried out the first test series – which yielded the graph shown in Figure 2, I wrote to Union Carbide, who had been kind enough to send me about half-a-pound of dry product for that study. Given the favorable results we wanted to expand the study – but I received a note back from the European sales office of the company, together with an additional box of roughly three-quarters of a pound of product. The note let me know that I had now received their entire allocation of product for the continent of Europe, and that, in consequence, my research needed to be conservative from thereon out, since there was unlikely to be any more supply for some considerable time. (Which was why the research could not be pursued at any greater length until I had moved to UMR and acquired my own system – which took an additional four years).

It was not until years later, as spreadsheets became ubiquitous, and data analysis facilitated from the hand-calculations that we were reduced to before the days of computers, that I was finally able to fully comprehend the data that we had, but I will leave that discussion until the next post.

Waterjetting 31d - thickening a waterjet to improve downstream pressure

There is a trick that one can learn while a teenager, which comes with the introduction “I so strong that I can blow a brick over!” Upon finding a suitable victim to impress, the brick is placed over a deflated balloon, which is then inflated, raising the brick which then, if suitably placed, topples onto its side – proving the strength of your lungs.

The critical part of the activity is to have the air that you blow be confined within the balloon, and equally exert pressure over the surface of the brick, so that a low pressure translates into a much more significant and powerful force. It is the confinement of the pressure that allows the build-up that moves the brick.

In most cases the use of high-pressure water as a cutting tool does not see much confinement of the water over the cutting process, with the water flowing into the cut, removing some material, and then flowing on out. Yet the water still has considerable energy as it leaves, and this means that the process is usually quite inefficient. How then can the contained energy in the jet be used in a secondary way to improve the removal efficiency of the process?

One answer to the question comes with the use of long-chain polymeric additives. These have recently seen a fair amount of publicity because of their use as the “slick water” components of the “slick-water-fracking” tools that have helped improve the production of oil and natural gas from long horizontal wells drilled into the hydrocarbon deposits in places such as the Bakken fields of North Dakota and the Barnett and Eagle Ford Shales in Texas. The long horizontal wells are separated into short intervals, within which the pressure within the well is raised until the surrounding rock cracks (fracks) with s series of cracks that extend out into the surrounding rock. The crack makes it easier for the hydrocarbons in the rock to escape and reach the well, improving the recovery to the point that the well can be economic to operate. The reason that the “slick water” is used is that the crack would normally close back up after the borehole pressure was lowered back down. In order to stop this happening the fluid in the well during the frack is made up with long-chain polymers and also contains grains of a sand or similar proppant. When the crack is formed the fluid in the well flows into the crack, carrying the sand with it, and this then holds the crack open when the pressure falls.

The polymer thickens the water which makes up most of the fracking fluid, so that it can carry more of the sand, and at the same time, the polymer reduces the friction of the water against the rock, so that it is easier for the water and sand to penetrate deeper into the cracks. Once the proppant particles catch against the sides of the crack they become held in place, while the fluid moves on and eventually returns back out of the well.

Back in the days when I was carrying out the research for my doctorate, I had used the long-chain polymer Polymerized ethylene oxide (Polyox) to reduce the friction in the delivery line from the pump to the nozzle. The increased cohesion of the jet (which I will cover in posts that follow this) meant that it would cut to a greater distance from the nozzle, with less decline in cutting power. However the increased cohesion of the jet had an additional benefit, which was noted by Chapman Young, as part of his development of tools for removing loose rock from around tunnels.

In a typical tunnel excavation, the miners drill a pattern of holes in the face of the tunnel, and then partially fill these with explosive, which is then set off in a controlled pattern of blasting. The central core of rock on the face is broken out by the explosive force, but some portion of the rock at the edge of the blast is only loosened from the solid, and still hangs in place. One of the more dangerous mining jobs (which I have done) is to take a long pry bar and insert this behind the loose pieces of rock around the opening, hoping to be able to wedge these loose, so that they no longer pose a risk to miners who then pass underneath.

Seeking to automate this process Dr Young and his colleagues tried using high-pressure waterjets to blast these lumps free from the wall. Subsequently investigators at Colorado School of Mines have shown that the jets give an improved cleaning of the wall, over other methods – but as normally applied they do not have the confined power to be able to get behind the block with sufficient confined force to be able to pry larger blocks free.

And this is where the balloon analogy comes in, because Dr Young realized that if he could increase the viscocity of the water in the stream sufficiently so that, for a short instance, it would be confined behind the block and could acquire some of the pressure from the following impacting jet, then enough pressure over a large enough area would provide the force needed to dislodge the block. He tried it, and it worked.

It is not, however, a simple process to carry out, since the jet path must be carefully aimed to ensure that there is enough confinement of the water behind the target block for pressure to be built up, and this requires that the jet contain a relatively high concentration of polymer. That in itself brings another problem, which can be anticipated by the “slick water” nickname. Where the water gets onto the floor the friction reduction properties mean that it makes it quite difficult to walk on the wetted rock. Now while that, in turn, opens up a new avenue for business (the chemical is sometimes referred to as Banana water in riot control) it makes it unpopular with those that have to work with it in the confines of a mining tunnel and so the technology has not caught on. But it does provide an introduction to the topic of different cutting fluids, which will be the next topic of discussion.