Friday, October 31, 2014

Waterjetting 26d - Range, position and rewards for jet assisted cutting

In the earlier posts in this chapter I have discussed the problem of getting the nozzle of a waterjet system close enough to the tool:target contact that the jet retains enough power to be effective. At the same time the jet must strike within roughly 1/10th of an inch of that contact to be effective in helping with the cutting process. In the figure below, for example, the jet that comes from the nozzle ahead of the pick will initially strike in that region, while the jet at the back (right) of the pick box will not.

Figure 1. Potential positions for jet nozzles around a conical pick

There is one other consideration, perhaps more relevant in a rock cutting operation than in a metal cutting one, and that is the issue of tool wear. In the above situation while the rear jet can never hit the critical zone, the one at the front of the tool will lose effectiveness as the small carbide cutting cone wears and moves the crushing zone back under the pick shoulder. As an improvement consider the situation shown below:

Figure 2. Simplified schematic showing a high-pressure waterjet hitting the contact between a cutting tool and the underlying rock.

In this case when the tool is sharp then the jet is striking the rock just in front of the edge of the tool, and the performance is enhanced. Further, as the tool starts to wear, so the jet impact on the rock begins to move further forward of the tool contact. But because the face of the tool and the jet are almost parallel the slight change in distance is relatively insignificant.

By the same token, if the rearward jet in the first example had been moved so that the jet struck just under the back of the pick it would still have been able to remove the crushed rock, even as the bit wore. One way to improve the effect of the jet is to spread the water flow by making the jet into a fan or conic spray, this can be effective:

Figure 3. Reduction in thrust with lower pressure fan jets (after Hood)

Again the bit is cooled, keeping it sharper, but also even at the lower pressure if the rock is removed as soon as it is first fractured then it does not crush and then re-compact under the bit.

However higher pressures work better, both in terms of overall rate and in terms of the efficiency of cutting, based on British data.

Figure 4. Change in cutting performance with increasing jet pressure (after Morris 1985)

Given therefore the need to bring the jet to the crushing zone in as powerful a form as possible, one suggestion has been to bring the jet down through the center of the cutting pick.

Figure 5. Nozzle located above the contact point, but fed through the pick body. (After Fairhurst).

The problems with doing this are several. In the particular example shown the orifice is pointing the jet into the rock some quarter-of-an-inch above the crushing zone and this is too far away for the jet to achieve maximum benefit. Further as the tool will wear, so the contact surface will move back further away from the jet, further losing the assistance and failing to be able to remove any of the crushed rock as it is formed.

There are practical problems, however, when (as has been done in Russia) the orifice is brought closer to the tip of the tool. One of the difficulties is that whenever the tool is then used without the jet operating at pressure, then crushed rock will enter the nozzle and within a very short distance plug it with compacted fines.

It is then, frequently, not possible to use jet pressure to get that material out of the nozzle, (particularly when the pump is supplying several orifices on a cutting head). Without the water the tool rapidly erodes, because of another weakness in the design.

For when the orifice is placed within the lower tip of the tool, the volume of the orifice is removed from the bulk volume of the cutting bit, making it much more susceptible to wear.

As long as the jet is brought up to pressure first, and the tool only then brought into contact with the rock or other target, then the tool performs well. Unfortunately (as operators are human and thus prone to the occasional error) cutting heads have often been brought into contact with rock without the jets being at sufficient pressure, and the benefits of the jet assist are thus eliminated due to this loss in nozzle clearance.

There is a corollary to this, in that, as jets began to be used more frequently on cutting heads, the amount of water spraying into the working zone became both a source of irritation and a considerable unnecessary loss in power, given than the cutting head tool only makes contact with the rock for a small fraction of the rotation around the shaft axis.

Figure 6. Roadheader with jet assist working at the Middleton Mine in the UK

To reduce the volume of water, control valves were set into the flow channels so that water was directed at only those picks that were in contact with the rock. The problem with programming this is that, depending on where the head is around the profile of the tunnel, so the arc of the head that the picks are cutting on will change.

But the benefits, where all these different factors are considered in the design and operation of the machine are considerable. As a very rough statement, the cost of a machine will increase more than linearly as it’s weight is increased. In order to cut harder rock without jet assistance, the picks must be pushed harder into the rock, and this thrust must be resisted by the friction exerted between the floor of the tunnel and the base of the machine – usually treads. Thus harder rock requires that conventional machines be heavier. However, when jets are added to the machine that power cost is removed, as the thrust levels are reduced. Thus smaller (and more mobile) jet-assisted machines can cut more effectively than their conventional counterparts.

Figure 7. Introduction of heavier machines to mine harder rock, until the advent of the waterjet assisted machine in 1980 (after Morris)

The savings in the reduced cost of the machine (saving $500,000) more than covered the cost of the high-pressure waterjet equipment (around $100,000).

Hood, M., A Study of Methods to Improve the Performance of Drag Bits used to cut Hard Rock, Chamber of Mines of South Africa Research Organization, Project No. GT2 NO2, Research Report No. 35/77, August, 1977.
Morris, A.H., "The Development of Boom-Type Roadheaders," Seminar on Water Jet Assisted Roadheaders in Rock Excavation, Pittsburgh, PA., May, 1982.
Fairhurst, C.E., Contribution A L'amelioration De L'abbatage Mecanique De Roches Agressives: Le Pic Assiste Et Le Pic Vibrant, Doctoral Thesis, L'Ecole Superieure des Mines de Paris, October, 1987, 221 pages (in French).

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Tech Talk - more on volcanoes, peak oil and slow transitions

Many news agencies are following the slow inundation of the Hawaiian town of Pahoa, as lava from Kilauea inches into the small town at the rate of around 15 ft an hour. It is a slow death to parts of the community since the lava started moving in June and the flow has travelled over 24 miles on its way to the sea. Not quite as prominent in the news is the continued outflowing of lava in Iceland where the flow from Bárðarbunga has now covered just over 25 square miles, and the threat from the outpouring of sulfur dioxide continues to move around the island as the wind patterns change. As the energy of the eruption falls, there is concern that less of this is getting into the upper atmosphere, causing higher concentrations in the lower layers of the cloud. The volcano is putting out about 35,000 tonnes a day more than all the industries in Europe. Safe concentrations are considered to be around 500micrograms/cu. m. while levels as high as 21,000 micrograms have been measured.

Figure 1. Gas cloud threat from Bárðarbunga on October 31 (Icelandic Met Office)

It is only the high winds of the Icelandic winter that dilute the gas below the threat to individuals. And yet the earthquakes in the caldera persist with events above level 5 still occurring almost daily. There were 200 yesterday, with ten being larger than magnitude 4.

Figure 2. Earthquakes in the Bárðarbunga region of Iceland in the last 48 hours. (Icelandic Met Office)

The water in the caldera is melting at an estimated rate of 2 cu. m/second with hot magma residing under the originally half-mile thick glacial cap.

While these events are generating hundreds of megawatts it is not in the form of useful energy at this point, but despite the disappearance from the headlines of the Icelandic event, it still has the potential for much greater societal impact than does that in Hawaii. But it will happen more slowly (at least until the potential eruption when the icecap is penetrated.) And sadly it is this demonstration of the short-term focus of the news media and the need for dramatic pictures that again bring me to the analogy of these events to what is happening with Peak Oil.

As noted in an earlier post, the EIA have pointed out that the current glut in oil availability and thus the fall in gas prices correlates inversely with the increase in production from Libya. Their OPEC governor has pointed out that the current global oversupply is at around 1 million barrels a day. Libya has recently produced about 800 kbd of this, and while OPEC as a whole is not worried out the imbalance (since they are projecting that global demand will rise this much over the next year), he would like to see current production curtailed by 500 kbd to get the price back over $100 a barrel.

It is this marginal supply of around half-a-million barrels a day which is now the level of volume that can transform us from having too much to not enough. Which goes back to the remarks that Charles Dickens put in the mouth of Mr. McCawber:
'My other piece of advice, Copperfield,' said Mr. Micawber, 'you know. Annual income twenty pounds, annual expenditure nineteen nineteen and six, result happiness. Annual income twenty pounds, annual expenditure twenty pounds ought and six, result misery. The blossom is blighted, the leaf is withered, the god of day goes down upon the dreary scene, and - and in short you are for ever floored. As I am!'.
Our sixpence, it would appear, is now at around that 500 kbd. OPEC will not increase production much above current levels, in fact it is hard to see where they could anticipate being able to do so. Libya remains threatened by worsening violence, which has been approaching the El Sharara oilfield and it remains questionable as to whether they can continue to sustain production.

The other big question mark remains Iraq. How far the Kurds can increase production up through the pipeline to Turkey remains a question. They have recently announced that the new pipeline is carrying 240 kbd and if the logistics can be put in place the volume could well increase. Problems however with contractors, making the necessary field connections and the nearby conflict will likely combine to slow that progress.

If both sources of supply continue to produce, and even increase a little more than at current levels then the global surplus will still be eaten up by increased demand over the next year. The short-term drop in prices (which may well extend over the winter) will gradually disappear as the surplus reduces. And in so far as the current drop in prices discourages new investment in costly alternate places, even if only in the short term, that cannot but help OPEC as supplies tighten in the future, and that competitive oil is not in place in the market to reduce the consequent price increase.

The short-term loss therefore may well, before long, be returned in higher prices in the summer and towards the end of next year. Such a projection assumes that the recent increases in US production will slow down, and that seems to be a reasonable assumption, given the changing price structure and the lower returns on wells drilled outside the “sweet counties.” One can only drill so many wells where production is rewarding, before the land gets full.

In the short term the drop in prices will also encourage demand, helping to build back what had been a falling away from earlier OPEC projections of demand growth. It will be an interesting year, and perhaps one that will change faster than the slow but steady changes that the volcanoes are having on their local communities. But if so it may still be too slow for the media to closely follow, since many of the controlling events take place away from media attention and occur without, often, immediate visible impact.

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Saturday, October 25, 2014

Waterjetting 26c - Cutting tool shape

When it was first discovered that high-pressure waterjets could significantly improve the performance of mechanical cutting tools, whether in machining metal, or in cutting rock, it was anticipated that this would have a broad-ranging application. This has not been the case, and the reasons are varied, depending on the application, but quite often they relate to the way in which the mechanical tool was expected to work. The examples will, again, come from rock cutting, but also apply when cutting or machining other materials.

Figure 1. Three common types of picks used in cutting into stone for driving tunnels, or for cutting and mining coal.

The initial work of Mike Hood, in cutting quartzite, had used a relatively simple flat-faced bit that was dragged across the rock surface at a known depth of cut and directing s single, or pair of jets to cut along the line of contact between the rock and the carbide was relatively straightforward.

Figure 2. Locations of the jets for Mike Hood’s initial tests on improving jet performance. (Dr. Hood)

Getting the jets to cover the full zone of contact and rock crushing was critical to achieving the best results for the tests, and proved also effective when the cutting tools were tried in the field.

Figure 3. Relative normal forces on a cutting bit with change in the position of the jets assisting in the cutting of rock (Dr. Hood). Note that the machine stalled at 4 mm penetration without the jet assist (the black line).

The cutting picks that are more commonly used in softer rock, shown in figure 1, are not quite the same shape, nor have quite the same purpose. Early trials were with the forward attack pick, which through the early 1980’s was the most common design used.

Figure 4. Laboratory trials with a jet added to a forward attack pick

Rather than having a flat face, this pick has a wedge-shaped front face. This is so that, as the pick cuts into the rock, so the wedge shape pressing into that groove will put a high lateral load on the rock on either side of the cut, causing it to shear off the solid. Those chips can be seen to the front right of figure 4.

Where the jet cuts into and removes the crushed rock under the front of the bit, this allows the bit to make a deeper bite, and this, in turn, makes it more likely that the tool will make larger chips. This is not an unflawed benefit, particularly if the jetted slot now extends a little deeper than the tool.

Figure 5. Illustrating the wedging action of the tool in creating lateral chips beside the tool.

As the chips get larger so the force required to break them from the solid increases, and the actions do not occur symmetrically on each side of the tool. As a result the lateral loading on the tool becomes more significant, and because the process of chip forming and breakage is cyclic so there greater fluctuating forces make their way through the drive train back to the driving shaft and motors.

With most machine designs these fluctuating loads are, however, reduced in overall magnitude, because of the reduced forces needed to move the pick forward, without having to deal with the crushed material under the pick, which the water jet has removed, providing it is within about 1/10th of an inch of the cutting tool.

Achieving that positioning becomes a little more difficult with the transition to a radial pick, however there were additional problems with that intermediate design, particularly in harder rocks, where they wore out at rates as high as 7 picks per foot of advance. This led to the development of the point attack pick, as shown in figure 1.

This pick design has become the most popular for use in mining machines over the past 20-years. The round shape of the tool and shaft are designed so that, as the pick wears it will rotate in the holder, and this will spread the wear evenly around the tool, making it last longer – and in the case mentioned in the last paragraph a change to this design reduced pick costs to around 1 pick per foot of advance. But there are a couple of problems with adding waterjets to this tool.

Figure 6. Point attack tool geometry (Goktan and Gunes)

This geometry makes it very difficult to bring in a waterjet to hit the right point at the rock:tool contact, because of the double cone at the end of the tool. While the nozzle can be positioned so that it can direct a jet into the right point (for example by being at the point where α is in the figure) the problem arises with the size of the nozzle mounting block, and the small size of the jet, where a large number are being used to cover all the picks on the cutting head and total flow volume is limited. To fit the nozzle block means that it must be at a greater standoff distance from the point (perhaps four or five inches), while the small orifice size means that the effective range of the jet may be no more than an inch or two.

The change in pick design and the difficulty in adding waterjets to the new tool therefore led to a discontinuation of the trials of the combined system. This was unfortunate since the forward attack picks initially cut better than the point attack, but wore out more rapidly – hence the change. But with the addition of the waterjets the tool lifetime, and sharpness, was increased in some cases more than five-fold times, while the other benefits – such as the ability to use smaller machines to carry out similar performance – made capital investment less. But these events occurred at the wrong time, as the coal market was entering one of its down cycles as the developments were being made, and the technology was therefore not adopted.

I will conclude this small chapter next time, by addressing one of the answers to the problem of getting water to the point attack tool.

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Monday, October 20, 2014

Waterjetting 26b - Jet positions to help bit cutting

The addition of a high-pressure waterjet to the leading edge of a sharp tool can make a considerable difference to the performance of that tool. I have discussed this a little in two earlier posts, the first of which was an introduction to the topic, and the second highlighted the problems of getting the nozzle close to the active contact zone so that it can be effective.

In this post I will discuss the benefits that this jets can create in the performance of the machine. The discussion is largely focused on rock excavations, since that is where most of the basic and applied research was developed, but, as I also mentioned previously, this benefit can also be gained if the jets are added to machine tools that are cutting into metal – even metals that are otherwise hard to machine.

The idea of pushing a sharp(ish) tool into rock to break it out goes back to the deer antler picks used to pry flints from chalk some thousands of years ago. But it worked, although the picks are now made of metal and powered by machines. The shapes have also changed over the years.

To make an effective cut requires two different sets of forces be applied to the picks. The first of these is the one that pushes the pick into the rock and gives it the depth of cut that is needed. (I’ll call this the Normal or Thrust Force, since it acts perpendicular to the surface being cut). The second is the force required to pull the tool along the face, this is often referred to as the Cutting or Drag force. Neither are very constant in rock (as opposed to metal cutting) since the rock will chip around the but as it moves forward, which frees and blocks the passage of the tool as it moves.

As I mentioned last time, pushing the tool into the rock will cause the rock under the tool to crush, and then re-compact, if the particles aren’t removed. Thus the most effective time to remove them comes as the tool first breaks them free from the solid. This also saves the energy that would otherwise go into not only further crushing, but also re-compacting the particles. Once they are re-compacted and compressed they become harder to remove and help increase the friction on the tool that cause it to heat, and weaken.

But if the particles are effectively removed, then the region under the bit is washed free, there is less confinement on the remaining rock, and it becomes easier to break.

Figure 1. Crushed rock under an indenting tool. (Richard Gertsch)

Figure 2. Crushed rock under an indenting tool, with the tool removed and a 10,000 psi jet fired at the contact point after removal. Note that there is still some crushed rock that was not removed.

Figure 3. Crushed rock removed during crushing by a jet pointed under the bit as it indented the rock (basalt) (Richard Gertsch)

The impact on the forces that the bit sees can be dramatic. In the early tests of drag bits in cutting the quartzite rock that holds the gold veins in South Africa, Dr. Michael Hood took a tool that normally stalled out under full load, when it was cutting into the rock to a depth of 4mm.

Figure 4. Normal forces on a bit (in KN) without jet assistance (black) where the machine stalls at 4 mm penetration, and with jets at different locations along the cutting face.

Mike tried a number of different locations for the jets at varying points over the face of the drag bit. Initially he used higher pressure merely as a way of getting enough water to the bit to keep it cool, but quickly saw that the performance was greatly improved. As Figure 4 shows, the normal force pushing the bit into the rock was considerably lowered, even when the depth of cut into the rock was increased almost three-fold, with the best location for the jets showing that the machine retained considerably capacity for cutting.

Similar results were obtained with improvement in the cutting forces seen in pulling the bit down the face.

Figure 5. Change in cutting forces with high pressure jet applications to a cutting tool in basalt (Mike Hood)

Again the machine stalled with a depth of cut of 4 mm, without waterjet assistance, and cut to more than 11 mm depth with power to spare with waterjets in the optimal location. This was found to be at the corners of the cutting tool, since in this location the jets were confined by the uncut rock on either side of the tool, and thus rebounded to cover the entire line of contact between the bit and the rock.

Figure 6. Optimal location for the jets on the drag bit for cutting South African Quartzite. (Mike Hood)

For the jet to work most effectively the water must continue to remove all the crushed material from under the bit as it is created. Where the rock is already fractured (as it may be because of natural ground fractures or high stresses on the face because of the depth at which mining takes place) then the confinement of the space around the tool can be less and this reduces the ability of the water to spread along the face of the tool and remove all the crushed rock as it is formed.

Others have also looked at the position of the jet relative to the cutting face, and sometimes, especially in harder rock, where the jet can intersect broken rock above the cutting tool, it may be better to bring the jet into the crushed zone from behind the bit.

Figure 7. Changing bit performance with change in jet pressure at three jet positions relative to the bit. (After Ropchan, Wang and Walgamott).

A slightly different experiment was tried by French investigators who tried locating waterjets around the carbide inserts of a drilling bit. Part of the problem with such bits is to ensure not only that the nozzle is close enough to the crushing zone as to remove the rock, but also to make sure that the nozzle is close enough to the surface that the jet retains enough power. In this particular case, by drilling a small hole through the carbide tool, the investigators were able to bring the two tip jets to the point that they needed, with enough power to be effective. This is shown by the ability to achieve a rate of penetration (ROP) which was more than double that of a conventional bit, with only conventional cooling, for the same amount of thrust force.

Figure 8. Change in rate of penetration with change in jet location on a drill bit.

I’ll return to this topic next time.

Hood, M., A Study of Methods to Improve the Performance of Drag Bits used to cut Hard Rock, Chamber of Mines of South Africa Research Organization, Project No. GT2 NO2, Research Report No. 35/77, August, 1977.
Ropchan, D., Wang, F-D., Wolgamott, J., Application of Water Jet Assisted Drag Bit and Pick Cutter for the Cutting of Coal Measure Rocks, Final Technical Report on Department of Energy Contract ET-77-G-01-9082, Colorado School of Mines, April, 1980, DOE/FE/0982-1, 84 pages.

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Monday, October 13, 2014

Tech Talk - Pessimistic Talk in a time of surplus

The oil markets are concerned that there is too much oil currently available on the market, and that, as a consequence, oil prices may continue to tumble. Saudi Arabia is reportedly telling Reuters that it is happy with prices that may fall as low as $80 a barrel. As I mentioned the other day, some of this has to do with market share, and the KSA increasing production, and thereby seeking to weaken the likelihood of investment in other places, in turn ensuring their share holds up, not just now, but also down the road. The effect on gas prices has been rapid, with prices in parts of Missouri down to $2.65 a gallon – about a dollar less than I was paying only a week ago.

The effect will also have the benefit of a boost to the economy, which of course can’t hurt in the run-up to an election. But in the longer term it is hard to see how this boost can be sustained for more than a year. In the last post on this I mentioned that, outside of the US, Russia and KSA global oil production had dropped around 3 mbd over the past couple of years. Yet increased production (KSA raised production by 100 kbd in September as part of a total 400 kbd increase from OPEC overall) has, for now, been able to match and surpass this in order to meet the global demand. OPEC continues to expect that demand will increase by a million barrels a day this year and 1.19 mbd next. They further expect that the increased production to meet this will be met from outside the cartel, with the gain declining from 1.68 mbd this year, to 1.24 mbd next year, holding OPEC production to a decline of 300 kbd from the current 29.5 mbd. Simplistically the gains are maximized in increased production from the United States (880 kbd); Canada (250 kbd) and Brazil (190 kbd). They are anticipating a slight drop in Russian production, as part of an overall decline of 80 kbd for the FSU countries.

Part of the problem in projecting the balance revolves around estimating the production from Libya, Iraq and Iran (LII). Libya has reported raising production back to around 800 kbd, but some of that comes from the Shahara field, which was still involved in factional fighting, even as it came back on line at some 20% of normal. The three countries produce around 7 mbd (Iran 3 mbd, Iraq 3.2 mbd; Libya .8 mbd) so that the fluctuations in their production and sales can have a very significant impact on the global oil market, and the prices that are paid – but they function within OPEC, and it may be that the current drops in price are reminder that the big dog in that trailer is KSA, currently running at around 9.7 mbd.

It is foolish to try and predict, over the immediate short-term, how the fighting in Libya and Iraq will progress. Similarly it is hard to see how relations with Iran will change, potentially easing sanctions and allowing them to sell more product into the global market would upset the current balance in trade, and could, in the short-term, increase the glut and lower prices.

But supplies from those outside the cartel and the Americas are continuing to decline. That is not going to change. The rates may fluctuate a little (though the current drop in prices is not going to encourage large scale investment in declining fields) but the overall trend is steadily downward. And it is within that picture that potential changes in the production from the three LII countries have to be placed.

Figure 1. Libyan oil production through September 2013. (EIA)

Yet, as the fields have brought oil back to the market, there is a concurrent fall in global prices, as the EIA note.

Figure 2. Recent oil production from Libya and the price of Brent Crude (EIA)

Pre-conflict Libya was producing over 1.6 mbd, it recovered to 1.4 and is now struggling at around 0.8 mbd. But the prospects for the levels of peace required to sustain even that level do not seem promising. The conflict is worsening and seen as spiraling out of control.

Moving East to Iraq, despite the use of air power, the situation in the North is not improving, although the Kurds have now a pipeline to carry oil up into Turkey that is not controlled by the Islamic State. While it is still a matter of debate how much oil they will be able to sell, they hope that, by the end of next year they may be able to pump as much as 1 mbd, up from the initial 0.1 mbd when the pipeline went on line. At the same time, in the South, the oil fields lie some distance from the conflict, and there seems little threat, at the moment, to the plans to increase production, and move the majority of the oil to the coast for export. It is, therefore possible to foresee an increase in Iraqi production of perhaps a million barrels a day in the next couple of years. Is it likely? It is hard to say. Factional fighting is always hard to predict, and the willingness of those involved to use explosives makes it even more of a problem to predict what will occur, given the vulnerability of pipelines to attack.

Predicting how Iran will change is similarly conflicted, in that it is hard to predict the behavior of those who control the country, and in turn impact oil exports.

But putting this within the context of OPEC, I suspect that overall production will not fall much outside of the current volumes that the MOMR are predicting – which is sensibly overall stable output over the next year or so. And if that is the case, then I would, as mentioned last time, expect to see that the global surplus of oil supply over demand will gradually disappear over the next year, with the impact becoming evident once we reach the summer of 2016. It would be nice to be wrong, but I think it unlikely.

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Friday, October 10, 2014

Waterjetting Technology - 26a More on waterjet assisted cutting

When mankind first began cutting out flints to make the tools and weapons that helped make primitive life more successful they often used either bone antlers or stones from the river as the tool to cut into the chalk or other host rock that held the flint. For thousands of years as rock was excavated for broader use, including making building stone, the rock continued to be cut manually, and it has only really been in the last hundred and fifty years that manual picks have been replaced with power driven machines. However, in great part, the machines have had to be made larger and heavier than they might need to be because, in large part, unlike the pick swung by a miner, the machine cannot selectively attack the rock that it is facing, but must cut along a foreordained path.

Figure 1. Conventional tool path in cutting concrete, the tool has to cut through both the hard aggregate pieces as well as the softer cement.

The tools that cut through the rock mechanically must, therefore, be able to cut through all the different materials that they are likely to encounter. Where the rock is like a concrete, with hard and soft parts, then the tool must be able to cut through the hard (aggregate) as easily and fast as it removes the soft (cement) phase if the machine is to maintain productivity. When I wrote about cutting concrete, I pointed out that this “brute force and ignorance” approach to getting through material was expensive in the time that it took to make a hole, and in the energy that had to be expended, both combining to make the overall process itself more expensive than it need be. That cost includes not just the costs of the process itself, but is also less obvious in that the machine itself has not only to be bigger, but because it also typically sees a wide range in force applied through the cutters to the drive mechanism, it also has a shorter operational life because of these fluctuations. (One shearer model saw such failures within six months of start-up).

There are a number of different ways in which the sensible application of high-pressure waterjets can improve cutting performance, lower machine size and cost and provide a win-win situation. But there is a need for caution, since the small size of the waterjets that are often used is much below the scale of many other parts of the machine, and, as a result, the precision with which the jets need to be applied can often be neglected.

In an earlier post on this topic, I discussed how a mechanical tool will crush the rock over which it passes during cutting. This crushed rock confines the bit, and is often re-compacted so that frictional forces rise, and the temperatures can be high enough to soften tungsten carbide.

Figure 2. Crushed rock under the impact of a mechanical pick. The size of the indentation relative to the size of the crushed rock is evident.

If the jet is to be effective it has to be directed into the cut at the point where the crushed rock is being created, so that the jet can remove the broken pieces as they are being formed. It is this critical location of the jet relative to the bit:rock contact that is often missed by those who have tried to apply this technology in the years since Dr. Mike Hood first demonstrated the benefit.

A number of experiments over the years showed that if the jet is more than about a tenth-of-an-inch (2.5 mm) from the point of the pick where it enters the rock (or the edge of the tool if it is a broader shape) then the jet will not be able to reach and remove the crushed material. This is particularly true when the rock being cut is, as in the above figure, a basalt, which the jet of water cannot normally penetrate at pressures of 10,000 psi. thus, if the jet does not reach the crushed material then the energy put into its creation has been wasted.

There are two parts to that last statement. They deal with all three planes in which the jet lies, relative to the point of pick contact. There is the relative position at which the jet hits the rock, where it is critical that it hits just where the rock is being crushed, and then there is the distance of the nozzle from that contact point. The latter point is one that I have also written about in earlier posts, but which can also be neglected when engineers are designing systems. There are a number of papers (which seemed to be at a peak at the 8th International Waterjet Symposium in Durham, UK in 1986) where this distance was set incorrectly (values up to 0.3 inches and above were reported) and it is not, therefore, surprising that some investigators found that the results were not as good as expected.

Figure 3. Jet cutting at the front edge of a pick (Front cover of the 8th International Symposium on Jet Cutting Technology, BHRA, Durham, UK, Sept. 1986)

If the nozzle is too far from the rock contact, then the pressure of the jet will have fallen to a pressure that is too low to be effective. This has been a less obvious problem to overcome, since to many observers the jet seems coherent with distance, but, given that jet flow is often divided between a number of different nozzles on the cutting head, the individual orifice sizes can be quite small (perhaps 0.01 inches in diameter). If the effective jet throw distance is 125 diameters, then the range of the jet is 1.25 inches. Yet in a number of applications, because of difficulties in fitting the nozzle in place, the orifice can be placed more than 2.5 inches from the rock contact. Again the result of this is to make the jet sensibly ineffective.

The jet has to be put into the right place, and with the correct amount of power, if it is to be of any use. Sometimes that can mean that the nozzle is placed behind the pick (so that it can be protected by the pick from the rock, yet can be brought close enough to the crushed zone that it can penetrate it from behind. This is a little more difficult to achieve, since the precision of location is a little more difficult.

Others have tried feeding the jet down through the pick, and I will explain some of the benefits and problems with this as I continue on this theme next time.

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Monday, October 6, 2014

Tech Talk - The Price of Power, and its consequences

The changing colors of the leaves carry the message that winter will soon be here, and so it is time to stock the yard with wood to carry us through until spring. In Missouri I just found wood, cut to the length I need, and stacked, for $110 a cord and (since it has to be cut) it will arrive next week. Only the chimney then needs a quick sweep, and we’ll be ready for another season. (We burn just under a cord of wood a month, and this keeps the electricity bill sensible).

At one time the wood was insurance, in case of an extended power outage (and we had one that lasted three days, one winter) but we enjoy the heat from the tile stove, and so it is now part of our life. And with the continued risk of a loss of power, the insurance remains comforting.

Driving back from Maine a couple of weeks ago, gas prices fell over $0.25 a gallon along the 1,300 mile trip, another benefit of living in the Mid-West. But at both ends of the drive, the impact of fuel prices continues to slow economic growth, as it does nationally. Gail Tverberg has written of the inter-relation between the economy and fuel prices, most recently on Monday. However we disagree on one point, since she anticipates a potential significant drop in oil prices, which I do not.

In the early days of The Oil Drum I remember walking through the streets of Denver to a meeting with two other contributors, and suddenly realizing that I, the more technically based of the three, was by far the most pessimistic. Increasingly I am realizing that while this pessimism has not ameliorated, the current relative abundance of oil and gas in the United States has given many folk an undeservedly complacent view of the next few years.

Ron Patterson recently pointed out that if one discounts US production, the rest of the world has seen a decline in production, with non-US production now down around 2 mbd from its all-time peak. (If one also removes Saudi Arabian and Russian production from the mix, the decline gets closer to 3 mbd). Now to assume that this is totally due to a loss in production capacity would be a mistake. Saudi Arabia continues to adjust the volume of their production to try and keep global prices relatively stable, dropping production by 400 kbd in August. In the immediate short-term that was not enough for their purpose, and they are now lowering price a little, perhaps in order to sustain their market share. The cuts were in the range of $0.20 to $1.20 a barrel). Although it could also be a way of trying to sustain global growth at a time of weakness.

Figure 1. Global Production without including the United States – as plotted by Ron Patterson. I added the trend line at the end of the top plot.

These flutterings at the margin however don’t help my concerns, because they are focused only on the short-term, and don’t consider the overall situation. If the production from the rest of the world is declining at around 700 kbd, and Saudi Arabia will only produce to a maximum of 10 mbd, and Russia appears to be in that plateau that precedes decline, even without the loss in funding that recent US Government mandates will impose, then that leaves the growth in US production as being the only source to match both the decline in global production, and the continuing demand for more oil which together total around 1.7 mbd. And US production projections, even at their most optimistic can’t do this, even for one more year.

Figure 2. Projected Growth in US production (EIA)

The mathematics are, of course, not absolute numbers but remain somewhat flexible. There could be a sudden cessation of conflict in Libya and full production might return; all conflict might end in Iraq and production development might surge at the investment opportunity; and sanctions might disappear against Iran – but somehow I don’t see any of these happening.

The argument of the Cornucopians, that one can either find a substitute for the fuel in some other resource, or that technology will suddenly become available to allow unanticipated levels of production from the existing reserves and resources is, perhaps why I – knowing a fair bit about the technology – am more of a pessimist than many others.

The analogy that I use may be a little crude – but you can’t have a baby in a month by making nine women pregnant. You can’t create new technology out of thin air by suddenly investing a few billion in a bunch of scientists pulled from lists on the Internet either. There are not that many folk who are sufficiently expert to be useful, particularly in the fields that relate to the production of fossil fuels. Many of those who do exist are, like me, coming to the end of their professional lives, so that the skill sets and knowledge bases that they have built are disappearing. Many of the doctorates that we see today are based more on computer modeling than on hands-on experimentation and engineering. And unfortunately the knowledge that we have about the nature of the rocks at depth, their behavior and how to change the way in which they yield their fluids still leaves a lot to be desired, when it comes to validating the models that are produced.

But even if such new technology were developed it would take decades to see it adopted in sufficient volume across the world that it would have a significant impact on global fuel production. It was for this reason that, back in 2005, the Hirsh Report discussed the need for a twenty-year lead-time to develop new technology that could replace our needs for fuel. The time that they suggested that we had available now is beginning to seem very optimistic, while the moves to ameliorate the problem have been judged less critical and thus no longer receive the attention and funding that the have in the past.

And so, when the crisis comes, and this is increasingly likely to come in the next two years, there will be no good answers, just tightening supplies and rising prices. This is perhaps why I am beginning to think that the next President of the United States still may well be, despite all the gaffs, Brian Schweitzer.

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