Sunday, August 17, 2014

Tech Talk

This series will be on hiatus for a couple of weeks as I travel and fulfill a couple of other commitments. Expect to start up again at the beginning of September.

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Iceland may soon have a new volcano

Most of my focus as I watch the earthquake patterns in Iceland has been on the Mýrdalsjökull glacier, but it appears that the number of quakes up at Bárðarbunga has reached the point of concern.


Earthquake pattern in Iceland on Aug 17, 2014 (Iceland Met Office)

A closer look at the site seems to show that the activity is due to magma movement, and there is an ongoing ground swelling which supports this.


Closer look at the eruption pattern (Iceland Met Office)

Jón Frímann has been watching the site and has a more detailed description of what the instruments are showing. Note that the activity is under the ice cap, so if it breaks through this may well contribute to the size of the particles generated, and the resulting size and path of the debris cloud.

I will update below the fold if this situation gets worse.
UPDATED Aug 18th, twice since the Icelandic Met Office have now raised the warning to orange. (h/t Jón Frímann)

One day later and, depending on your point of view, the quakes are getting more focussed, or the activity is beginning to settle down. The stars denote quakes that are larger than 3.0, though these two barely make that distinction, and there is one around each of the foci of activity.


Figure 3. Earthquake activity August 18, 2014 (Iceland Met Office)

UPDATE 2 at 4:30 pm

As noted above the fold, the Icelandic Met Office has noted the increasing levels of activity and has upped the warning for the region to orange, which means:
ORANGE: Volcano shows heightened or escalating unrest with increased potential of eruption.
This is only superseded by RED, which occurs when either the eruption is imminent or already happening.

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Saturday, August 16, 2014

Waterjetting 24c - angled jets in cutting concrete

In this short section of the series I have been discussing some of the issues that relate to cutting through concrete. In today’s piece the discussion will continue, focusing on the angles that the jets are set at, when making repeated passes over an area to deepen the cut. The basic premise of the discussion holds true regardless of jet pressure, provided that the concrete is being removed by a moving continuous jet stream, rather than a pulsed jet system, which I will discuss next time.

As was mentioned at the beginning, the main way in waterjets remove concrete most efficiently is by washing out the cement around the individual particles of the aggregate, which, in turn, causes the particles to fall out of the slot, since they are no longer supported.

If a cutting head is built with the jets pointing vertically downwards, so that, as the head moves, so the jets spin over the surface and wash out the cement over a wider path, any cement that underlies a particle is not removed, and the particle remains held in place by the underlying cement column.

Regardless of how the nozzle is moved over the surface, with only vertical jets the path of the assembly very rapidly becomes blocked, and the nozzles can no longer move into the slot to deepen it.

The obvious solution to this is to incline the nozzles so that as they rotate over the surface, they can reach under individual particles and wash out the cement beneath them, removing their support. This also has the advantage of cutting a path into the concrete that is wider than the cutting head itself, so that on later passes the head can be lowered into the cut, shortening the standoff distance to the fresh surface and improving cutting efficiency.

Moving the head down a little on each pass also has the advantage that it exposes fresh layers of cement to the jet action and makes it more likely that all the cement within the desired slot is removed (and the aggregate with it) leaving a clear path for the assembly to move deeper into the slot.

So the question then arises as to what the most efficient angle is to tilt the nozzles to, relative to the perpendicular axis of the target surface. (I use that awkward phrase because not all targets are going to be flat horizontal bridge or garage decks).

Very shallow angles don’t work very well. The best demonstration of this was when we started cutting slots in granite, with an initial divergent jet angle of around 8 degrees. After the first few passes we noted that the slot was developing walls that sloped into the cut. As a result the slot was getting narrower with depth, and the nozzle assembly would no longer be able to move into the cut.


Figure 1. Tapering cut into granite. The nozzle had been advanced about a third of an inch after each pass of a dual-nozzle rotating head. Nevertheless the cut tapered as the cutting continued.

We had chosen that initial angle because it worked well when cutting slots in coal, but clearly in harder, less jointed material that was not the case. And so we, and others, have carried out tests to find out what the best angle would be for the cutting tests.

And, before I show the results, let me emphasize that these only hold true for a certain concrete mix. Where aggregate particle sizes are larger, the jet angle may need to change to make it easier to get around. The pattern of the jets on the surface, (affected by the ratio of the rotation speed of the head relative to the movement of the entire assembly over the surface) and the jet parameters themselves (jet pressure, nozzle diameter and standoff distance) also play a part. In this latter regard remember that the effective range for many waterjet streams is not that much more than a hundred diameters from the orifice, so that expecting some of the smaller nozzle sizes (say 0.005 inches) to cut cement more than half-an-inch from the nozzle may be an exercise in futility – and raising the jet pressure in that circumstance is unlikely to fix the fact that the target is simply out of range.

So, with those caveats, here is the result that was obtained by Puchala, Lechem and Hawrylewicz*:


Figure 2. The effect of nozzle angle on cutting performance in removing concrete (*Puchala, R.J., Lechem, A.S., and Hawrylewicz, B.M., "Mass Concrete Removal by High Pressure Waterjet," Paper 22, 8th International Symposium on Jet Cutting Technology, Durham, UK. September, 1986, pp. 219 - 229.)

Nevertheless it is clear that there is much better performance where the jets are inclined at an angle between 25 and 35 degrees to the normal to the target surface. This is reflected in the improved efficiency of cutting (as shown by the second line in figure 2, showing a more significant change with angle than is evident from the depth of cut measurements). In all cases we have found that the jet angle needs to be 15 degrees or greater to make sure that wall taper does not occur.

Correlating the rotation speed against the traverse speed of the head over the surface to find the optimal cutting performance is a little more difficult, and should generally be assessed for given concrete targets with a short test run, before the major effort is undertaken. One reason for this is the wide range in performance that can be found with different cements. We have worked with cement that was sufficiently weathered that it could almost be removed using one’s hands, on the one hand, and the new cements that contain silica fume, or small wires or fibers pose a different and more difficult challenge on the other.

This also holds true over setting the advance rate of the nozzle assembly into the slot after each pass (where the head is cutting in a series of passes to penetrate the slab). Here the advance is going to be controlled in part by the size of the aggregate, though it should be noted that even with little apparent progress the nozzle assembly should be advanced after each pass, since this exposed a fresh layer of cement to attack, and this will lead to more aggregate release and help in clearing the cut.

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Sunday, August 10, 2014

Tech Talk - Rig Counts in the Middle East

In recent posts about the situation in the Middle East, I have noted the need for Aramco to increase the number of drilling rigs that it must use, since it is now looking for natural gas in their tight sand deposits rather than finding the large reserves that they had hoped in the shale reservoirs. It is interesting in this regard to plot the number of rigs that have been working in the Middle East.

Getting the overall data from Baker Hughes the rig count can be plotted, over time, to give the following:


Figure 1. Rig Counts in the Middle East (Baker Hughes)

If one looks at the trend for the last twelve months, it has remains on a fairly consistent upward trend, following that of the longer time interval plot of Figure 1.


Figure 2. Recent trend in Middle East Rig count (Baker Hughes)

Back in the days of The Oil Drum, Euan Mearns and I had this concern, which occasionally surfaced, about these numbers. From my early post on the subject which noted that back in 2005 the KSA were running around 20 rigs, which would not be enough to get them the production they were claiming to need in the future, to Euan’s in 2011, the topic was revisited regularly over the time that the count steadily mounted as the Kingdom had to drill an increasing number of wells just to keep production at around the same overall level.

I am using the KSA as the example, given the large volume of its production relative to that of the others in the Middle East, but as the numbers show, the trend toward increased drilling rate to create enough productive wells to sustain production as the larger volume wells dry up is starting to become a steadily more frantic race across the region.

Rune Likvern used the phrase “Red Queen” in discussing the overall long-term need of the companies in the Bakken to have to drill an increasing number of wells, with individually reducing production, in order to remain in place with regard to overall production. As the production from the Bakken now exceeds a million barrels a day it may seem foolish to be predicting this “squirrel cage” view of the future, but the rig count up there is still running at around 190 rigs, which is not enough to sustain future growth for long, given that access to the sweet spots is limited, and they are beginning to run out of new sites.

So it is in the Middle East. The rig count numbers are mounting steadily, it is reported that there were 88 rigs drilling in the country in October 2012. Last year this rose to 170, and the number is expected to rise to 210 by the end of this year.

Aramco have done remarkably well, over the past decade, in developing new technologies to harvest the attic oil left around the tops of the major producing formations such as Ghawar, as the main body of the fields begin to be exhausted. But the problem with these secondary rig operations is that they were directed at the smaller pools around the field, rather than tapping into the major volume, and thus they had an expected and finite life. That life is starting to come to a close. Just as, when sucking a thick milk shake through a single immovable straw, when it stops drawing fluid, there is still a fair amount left in the cup. But as you move the straw around and slide it up and down the sides, the amount that you recover gets less, and it takes greater and greater effort to get it, to the point where you quit and discard the carton. And that is where the Middle Eastern oilfields are beginning to find themselves.

The high-quality light oils of the mainland are rapidly running out, and the remaining fields with the promise for sustaining Saudi production at around 10 mbd for the next few years, are the heavier sour crudes from the offshore fields such as Safaniya and Manifa. At the same time there is a need to reduce the increasing amount of oil (now at 3 mbd) being consumed in country, with the hope that this can be replaced by domestic natural gas. But those hopes are being reduced as the shales are found to be less productive than anticipated, and hopes are now switching to the slower production that can, hopefully, be achieved from the tight sands – but at the cost of an increased number of wells, inter alia.

This is the writing on the wall for global oil production, and in the short-term it will be neglected. Increasing the number of rigs will, in that interval, increase the number of wells that will produce, even though the volume from each well will be less, and the overall life of the wells will similarly reduce, as higher production techniques tap into smaller fields.

But we are now on the treadmill in the squirrel cage, or, as Rune would have it, we have wrapped ourselves in the cape and crown of the Red Queen, and must run faster and faster just to stay in place. (There are additional concerns since, as an example, Manifa could not be brought on line until there were refineries built that could process that crude, and so the options for increasing production beyond the capacity of refineries to absorb that increase is a futile exercise).

There will soon come a time when the gain from the overall increase in new wells will not match the decline in production from older wells, particularly if the effort to “run faster” is restricted to only a few players (Russia for example is not yet putting the effort and investment into increased drilling rates in order to sustain their overall levels of production, and given the age of their major fields are likely now in terminal decline).

Ouch!

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Saturday, August 9, 2014

Waterjetting 24b - Cleaning and Cutting concrete - a cautionary tale

The control of cut depth is one of the more difficult aspects of using high pressure waterjets in places where the aim is not to cut all the way through a part. The ability of an abrasive jet to continue cutting beyond the expected target depth can first be evident to an operator when they leave the jet running, but stop the motion while they go and do something else. On their return they discover that the jet has cut, not only through the part, but also the bottom of the cutting tank, and in some circumstances also into the concrete floor beneath it. Honesty compels me to admit that the table in my old lab had at least one (repaired) hole in the bottom and a concrete mark to show where. I know of a least one very prestigious university with a waterjet that has the same sort of feature (they actually did it before we did).

Cut depth control with a plain waterjet is a little easier, since the water will run out of energy – or the jet structure can be tailored to control its effective range more easily than with the higher density abrasive particles.

Life becomes a little more complicated where the traverse speeds are slower, where the bottom of the slot will become very irregular as the cutting jet tracks backwards and forwards as the nozzle moves at a steadier pace. Henning has divided the cut section into three zones:


Figure 1. The division of the cutting edge into three zones (Henning et al 18th ISJCT)

The fluctuating patterns if the jets are cutting down to zone three make it more difficult to retain control of depth, which is most easily achieved if the cutting is restricted to zone one and the abrasive is restricted to primary impact , without the additional cutting that comes where the jet and particles bounce further down the cut, as shown in the pictures on the right of figure 1.

Restricting the cutting depth in this way (and reaching the required depth of cut with multiple passes) works quite well for abrasive jet cutting of different materials and is the technique often used in milling pockets into a variety of materials, as discussed earlier.

There are, however, some risks to this in the use of plain jets, particularly when working with target items that are made up of different materials – such as concrete. One of the problems was identified fairly early on, in the use of high pressure jets to clean surface runways at airports.

The aim for jet use on runways is to remove the surface coating of rubber that is laid down on the tarmac when planes land and in that first instant of contact as the wheels come up to speed, a small amount of rubber is moved from the tire to the pavement. However, if the jet parameters for cleaning this surface layer are not picked correctly then the jet will remove not just the rubber, put also some of the cement from around the aggregate particles in the surface.

The problem that this raises is that the cement is rough, while the pebbles of cement are usually smoother (since the often come from river deposits). Thus if the cement around the surface exposure of the pebbles is removed, a smoother surface is left on the runway. This is not good, since the point of the rougher surface is to provide friction that will slow the plane down, and the polished surface removes that traction.

The pressure of the jet can be adjusted so that, at the point where it is hitting the cement it no longer has the power to remove it, but this is a value that is going to change with the pump operating pressure, the nozzle diameter, and the standoff distance between the nozzle and the runway. It will also vary with the type of materials that are in the runway itself, so it is very smart to try some test runs at different control values before going onto the field to do the actual removal.

Concrete properties change quite a lot from place to place. In some of the earlier work that was carried out on showing how jets could cut through concrete, tests were carried out at an airfield in the southern United States. For the purpose of the tests cuts had to be made through the pavement, so that pieces of it could be easily removed.

Our approach was similar to that used when we cut the walls at the University using a rotating waterjet on a small carrier (though as memory serves this was a modified riding lawn mower) to traverse back and forward over the cut, moving the nozzle down each time.

The problem that we ran into was that we wanted to cut a slot that was about 2 inches in width, which we had presumed would be wide enough to liberate the pebbles and give access to the deeper parts of the slab. Unfortunately in this case the pebbles that had been used in making the concrete were more than two-inches in size, and so when there were parts of these sticking out of each side of the opening there was not enough of a gap between them to get the assembly into the slot and to deepen the hole, without a lot of adjustments.

It was possible to cut through by making the cut slot wider by making a second, adjacent cut, and with the jets cutting down about 2 inches into the material on each pass, it was possible to work down through to the bottom of the slab, although the large size of the aggregate meant that the nozzle path itself had to be at a greater distance from the wall than we had planned. The combination meant that it was not nearly as rapid an operation as we had anticipated. (The traverse rate was about 2 ft/minute, which was much slower than expected to allow the jets to undercut the larger pebbles). Much more material had to be cut out of each slot in order to achieve full cutting through the slab and this slowed the cutting process – plus there was the time needed to work out how best to change the cutting patterns on site so as to make the process work at all. (And the pebbles were a quartzite aggregate so that even increasing the jet pressure would not have effectively cut them, without adding abrasive to the mix, which was not – at the time – a viable alternative).

The point in mentioning this is that, while the job seemed initially to be a relatively simple one, because we did not know enough about the target material we were caught off-guard when it turned out to differ from our assumptions. We have been caught that way a number of times. We were asked at one time to demonstrate precision cutting of a piece of metal – assumed it would be no more than two-inches thick, and set up a cutting time based on that assumption, and then were faced with a block of eight-inch thick Hastelloy. Which we did cut, as requested, but it took some changes in the cutting plan, which had not been built into the day’s schedule. Asking those few extra questions, in both cases, would have saved us some embarrassment and time.

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Sunday, August 3, 2014

Tech Talk - fracking tight sand and shales

The recent news that Saudi Arabia has not found natural gas to be as available as it had thought from its shale deposits, and is shifting to exploring for gas in their tight sand formations has not caught a lot of attention. But it is worth considering some of the aspects of this – and hence this post.

The Oil and Gas Journal define tight sands in this way:
The term tight gas sands refers to low-permeability sandstone reservoirs that produce primarily dry natural gas. A tight gas reservoir is one that cannot be produced at economic flow rates or recover economic volumes of gas unless the well is stimulated by a large hydraulic fracture treatment and/or produced using horizontal wellbores. . . . . . . . . Tight sands produce about 6 tcf of gas per year in the United States, about 25% of the total gas produced.
The production of natural gas from tight sands is different to that from shale sources, for, as the Canadian Center for Energy notes:
Initial production rates are low; however, productivity is long lived.
To understand why this is, and why this differentiates from shale production requires a little bit of an explanation.

Back in the time that the plants were growing that went on, over time, to produce the oil and natural gas we now extract, the surrounding ground was typically soil, with intrusions of sand as the tides and rivers shifted. As succeeding layers of ground covered these layers, so the soil turned into shale and the sands into sandstone. Sand particles tend to be more round, and so as the particles packed together the gaps between the grains provided space for the oil and gas to collect. Clay particles, that went on to form the shales, on the other hand are flatter and pack together more tightly. Further, while the sand formed continuous layers of material, with relatively few cracks, the shale, on the other hand cracked as it was compressed, dried and was squeezed – simplistically in the same way as the lake beds crack when they are exposed to the sun after a drought has driven off all the water.

The result of this is that the fine sands that formed into the tight sands have relatively low permeability – fluids within them find it difficult to get through the very narrow passages and driving pressures have to be high to overcome the frictional forces along these passages. At the same time, because there are relatively few natural fractures in the rock, artificial cracks have to be driven through the rocks, in order to provide enough large passage ways for the gas to escape. It still flows relatively slowly through the rock around these cracks before it reaches them, and this is why the production is relatively low, but continues over time.

On the other hand the shale structure holds much of the oil/natural gas in a different way. Firstly there are the natural fractures within the rock, which tend to run into one another, and which hold much of the fluid that is released early in the production of these formations. Secondly the rock immediately around the fractures has been itself fractured somewhat during the formation of the layer, and has a higher porosity (i.e. holds more fluid) and permeability than the central segments of each of the larger pieces.

To simplify the situation therefore, prior to hydro-fracking the natural gas in a tight sand is permeated relatively evenly throughout the rock (given the variety of the conditions that control rock structure over even relatively short distances). On the other hand the natural gas in a shale is preferentially concentrated in the natural fractures and the rock adjacent thereto.

So now a horizontal well is drilled along the rock, and, at intervals pressurized to drive fractures into the surrounding formation. The rock structural differences mean that the sandstone and the shale will respond differently. And a slight qualification at the beginning of the explanation, because of the length of the feed lines to the injection point where the fracture will start, the pressure rises relatively slowly on the rock, rather than with the sharp pressure spike from an explosive blast, which produces multiple fractures. (The use of a small shaped charge to induce the fracture by driving a small hole into the rock, will similarly only produce a single fracture extending out from the well bore for each charge).

Contrary to the pictures that are shown by oil and natural gas companies on how fracking cracks grow out into the rock, the cracks will not branch out under their own normal development. Crack splitting only occurs at relatively high energy transfer rates, when the crack speed reaches the maximum, and the crack has to bifurcate to absorb the input energy, those conditions normally will not exist at any distance from the wellbore.

What happens in the case of the shale formations is that the pressure opens the natural fractures in the rock and as these link together the pressure can open them over a significant volume of the rock, giving the equivalent spread of fractures away from the wellbore as the initial pressurized joints intersect others as the sand and fracking fluid migrate along the joints into the rock. Opening up these passages and giving access to the relatively higher porosity rock along their edges allows a relatively high rate of flow for the initial volume of natural gas which is found in these zones. But as they deplete, and the natural gas has to travel from the more central part of the shale blocks, so there is the dramatic drop in production which has been remarked by many (around 65% drop within the first year in some cases). Production continues to fall as the more easily produced volumes drain through the intersected fractures, and the residual production falls to a slight fraction of its initial value.

In contrast, the tight sand formation will flow in a different manner. Because of the low number of natural fractures and the lack of permeability changes along the joints, much of the channeling through the rock has to be put in place by the fracking process itself. Because there is much less collection of fluids into natural fractures (though there is still some) the initial production rates are not as high as they are from the shale formations, since the overall induced fracture density and dispersed penetration is not as pervasive. On the other hand once the fractures are in place, although permeability is low, if the formation does initially produce at an acceptable rate, it is more likely that the well will continue to flow with a lower, but steadier flow, than the shale well, and for a considerably longer time.

Thus the news that Saudi Arabia is now moving to try and develop their tight sand fields suggests that they will need to carry out much more extensive development in order to get the initial volumes of natural gas that they will need, but that once these wells are producing, that they will last longer than they would in the shale fields.

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Saturday, August 2, 2014

Waterjetting 24a - Cutting concrete - 1

There are a number of differences that take place when high-pressure waterjet operators change from a lower pressure, higher volume flow rate to one where the jets are operated at a higher pressure, with a smaller jet size. One way of illustrating the difference is in the way that the jet will interact with concrete, and that is the theme of this particular article.

Concrete is made up of two different material types, there is the cement and there is the aggregate.


Figure 1. Slot cut into concrete, showing the pebbles of the aggregate (brown) that are held in place with the finer cement (grey)

In an earlier post I wrote about the use of waterjets to remove damaged concrete from bridge decks and garage floors. In this short series the focus is going to be more on cutting through the concrete for whatever reason that it is necessary. It is the reason, however, that will likely help select the best way to cut the material.

In a typical concrete the cement paste is considerably weaker than the pebbles that make up the aggregate. Using the compressive strength of the material as a guide that of the cement may, for example, be less than a tenth of the value of that of the aggregate. And yet, when repairs are to be made to the concrete, or when pieces must be cut out, the systems are generally designed to cut through the harder aggregate.


Figure 2. Conventional approach to cutting through concrete.

The system that is used has to be capable of cutting through the hardest material in the mix, and that is usually the individual aggregate particles. (We will cover the rebar in the mix in a later post).

The slot to be created, is often not that critical in itself. For example we needed, at one time, to insert an opening in a series of concrete walls. Because this was done in the center of a university campus, the benefits of the relatively quiet waterjet cutting over jackhammers and other means of removal were significant, as was the amount of time required for set-up of the equipment. But one immediate aspect of the job was that the outlines of the hole that had to be cut were not that critical.

This is because, after the hole was to be cut, then carpenters would install a frame to hold a door, and they needed some space at the edge of the hole for adjustments, so that the tolerance on the cut was roughly plus or minus half-an-inch which covered the size of the aggregate particles.

This meant that it was not necessary to cut through these pebbles in the wall, but rather it meant that the system could be designed purely to remove the softer phase of the concrete, the cement, without needing the pressure to cut through the harder aggregate.


Figure 3. Concrete schematic showing where water jets have removed the cement (central white zone) from around the aggregate (darker blocks).

If all the cement is removed from around a piece of aggregate (Figure 3) then there is nothing holding it in place, and so the force of the waterjet (if that is used for the removal) will be enough to lift the pebble out of the slot. As a result the slot can be created at a much lower pressure than would be the case if the pressure had to be adjusted to cut through the aggregate.


Figure 4. Schematic of a slot created in concrete through removing the cement from around the aggregate particles without the need to cut through the aggregate.

The edges of the hole are not as smooth as they would be if the cut were made through the pebbles, but on the other hand the rough nature of the surface means that any later infilling of the slot with fresh concrete will have a rough surface to bond to so that the adhesion between the two layers will be much greater than that from a conventional repair.

Because the jets do not have to cut through the aggregate the cuts can be made a t much lower pressure (in the case of the University walls at less than 10,000 psi). This makes it easier to build relatively simple equipment at low cost to do the job. Back when this particular series of cuts were made it was not possible to buy reliable swivels that would allow the jets to spin and cover a larger area of the slot surface. Instead Dr Clark Barker, who designed the tool, used a four-bar linkage to allow the jet to sweep out an oval path on the wall, with the overall platform for the system mounted on a shop lifter.


Figure 5. Simple tool used to slot concrete. The high pressure hose is connected to the cutting lance on the rhs of the picture. The lance is held in a pivot at the back of the beam, and caused to oscillate through the rotation of an off-center connection to the wheel at the front of the beam. Drive to that wheel is through a chain from a motor that is not shown. The orange frame is a conventional shop lifter.

The connection to the driving wheel shown in Figure 5 could be adjusted, as could the position of the wheel along the beam, in this way adjusting the width and height of each orbit of the lance.


Figure 6. Slot cut through an 11-inch thick concrete wall using an orbiting waterjet.

The exposed rebar was cut later, using a cutting torch. A number of walls were cut in this fashion, and though the slots went through the walls in each case, the jet was large enough (around 0.05 inches diameter) that it was able to rebound within the cut and undercut the pebbles and remove them without the jet being directed directly at the cement under the pebbles.


Figure 7. Slots cut through a concrete wall using a high-pressure waterjet. Note aggregate pebbles are sticking out of the cement.

The walls were cut through to a height of about six-feet in less than an hour of cutting time, though there was some additional time needed to move the cutting platform up to cover the top of the slot. The nozzle was moved into the cut after each two passes, with the assembly being slowly raised over the cut length, using the shop lifter, and then lowered again before moving the lance into the slot. Changing the distance also changed the angle of the jet to the cut surface, and helped in getting the jet under any of the pebbles still attached to the concrete.

I’ll continue on this topic next time.

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