Waterjetting 27d: Drilling at a fixed diameter

In the last post I described how we initially came up with a simple design for drilling through material, using an axially aligned jet and a larger jet offset to one side at an optimal angle of around 20 degrees.

One of the problems with the use of this design is that the outer jet has to remove all the material in front of the nozzle during the time that it rotates around and advances the distance of the incremental feed rate. If it does not then there is a significant problem. Consider the case where the drill penetrates through a layer of limestone, while drilling otherwise in sandstone.

Figure 1. Sectioned waterjet drilled hole through a sandstone:limestone:sandstone sandwich of rock.

Note that although the hole does not deviate as it goes through the harder material since, unlike conventional drills, there is no mechanical contact between the high-angled rock and the nozzle assembly. But the hole reduces in size. If the hole reduces in size below the diameter of the nozzle holder, this will not contact the rock until it has passed behind the plane of the reaming jet. In other words the only way the blocking rock can be removed is to back the nozzle along the hole so that the reaming jet can hit the material blocking progress.

Figure 2. Drill passage blocked by protruding rock in the path of the nozzle body, but behind the cutting plane of the inclined jet.

One way to ensure that this is not a problem is to advance the drill at a slower rate, with the rate of penetration controlled by the ability to cut the hardest rock that the drill will pass through. The problem with that approach, and concurrently that of setting a fixed advance rate, is that, at the same advance rate and rotation speed, the drill will drill through different rocks at a different diameter. While this can be an advantage, in a limited number of cases that I will discuss in a later post, in most cases it is better if the hole is at a relatively constant diameter.

So how can we solve this problem?

One approach taken in Australia was to change the design and location of the cutting jets. Rather than have a single jet cutting out to the perimeter of the hole, two jets were used, but crossed over the axis and cut on the opposite side to their location. This had an additional advantage over the initial design in that, when drilling longer holes (and this went on to drill horizontal holes that ranged up to a kilometer in length IIRC) the head was balanced and so did not wobble and get out of alignment because of the force imbalance.

To overcome the problem of drilling at too small a diameter additional reaming jets were placed on the front of the nozzle assembly, so that he hole would be reamed to the diameter needed to allow the support hose access.

Figure 3. The addition of a pair of reaming jets. Note that offsetting the two front nozzles will also allow them to put a torque on the front part of the nozzle, which can therefore be self-rotating from the left hand of arrow A forward.

But the problem is not completely solved with these changes, since should any rock protrude into the hole in the distance A, so that it hits the larger diameter that follows, again it is not possible for the reaming jets to cut this rock without backing up the drill.

There is another problem, in drilling horizontal holes where the hole diameter can vary. Consider that if the drill goes into a softer material then, at constant advance (ROP), the hole diameter becomes larger. As the drill moves over this larger hole it will be riding on the floor of the hole, and thus the front of the drill will tip forward into the floor of the larger hole. This will incline the drill downwards, and so the hole will no longer be of constant alignment, but rather will gradually, over distance, tip increasingly downwards.

It is therefore critical that the hole be drilled at a relatively constant diameter (allowing for some hole roughness). How to achieve this? The answer is to put a gaging ring or collar of the required hole diameter, in the cutting plane of the rotating jets.

Figure 4. The use of a collar at the front of the nozzle to ensure the hole is cut to the right diameter.

It itself this isn’t sufficient to give the hole a constant diameter, since there is still the problem of drilling through materials of differing resistance. To overcome that problem we put a spring at the back of the drill, with a contact switch to a valve on the feed to the hydraulic motor powering the drill advance. Thus the drill would start to rotate, and the motor would increase the speed of advance until the collar bumped up against the rock. At that time the spring would compress, the contact switch would close, and the advance would momentarily stop. The drill would rotate around and remove the obstructing rock, the spring would expand opening the flow to the motor, and the drill would move forward. It may sound as though it would be a stuttering advance, but when we tried it in a mine you couldn’t tell that the mechanism was working, apart from the hole being of constant diameter, and by watching the spring. It drilled at between 7 and 12 ft a minute in an aggressive sandstone.

Figure 5. The drill assembly used underground. The hydraulic advance motor (it pulls the drill forward using the chain drive) can be seen under the drill sash (the red and grey bar – painted in 1 ft intervals).

In a normal drilling operation when a drill intersects a previously drilled hole at a shallow angle, then the second drill will follow the path of the first hole, and cannot drill through the opposing wall at that shallow angle. (We know this from experience having broken two drill steels trying while excavating the OmniMax Theater under the Arch in St. Louis). But with the waterjet drill we were able to make to second drill cross the intersection.

Figure 6. Photo down one drill hole, showing the point where the hole intersected a second, and crossed without deviation.

Hopefully there is now enough background so that next time I can talk a little more about the effects of borehole pressure on drilling performance.

Waterjetting 27c - Drilling nozzle design

In discussing how stress affects the ability of waterjets to drill rock, I have discussed the effect of the stress in the ground on drill performance, but before discussing the effect of the borehole pressure it is perhaps best to spend this post talking about the simplest drill bit design.

The diameter of the first jets we used to cut into rock were about 0.04 inches in size, with the nozzle holder used to hold the nozzle on the end of the supply pipe being at around an inch in diameter. As a result, if the jet was to cut a path into the rock, it would have to rotate around the face of the rock ahead of it, removing all the rock ahead of the assembly, and allowing the head to advance.

Figure 1. Original concept of a waterjet drill used to penetrate sandstone.

Of course, back when we first did this in the 1960’s the swivels weren’t available to allow us to rotate the high pressure line, and so we rotated the rock samples instead.

Figure 2. First holes drilled at the University of Leeds. Note the central cone.

Because the jet had to penetrate across the diameter of the hole, so as to remove the cone ahead of the tool, and since the jet would only cut around 2.5 times the jet diameter in width at any one time, the rate that the head could move forward was limited to a maximum of 0.1 x rotation speed (rpm) in inches/minute. And, because the rotation speed controlled the depth which the jet cut into the rock, the rpm had to be kept down to ensure that the jet cut to the full required diameter on each pass. The top speed we could achieve, even in relatively soft sandstone, was around 4 inches a minute.

One of those fortunate accidents that sometimes befalls research folk then occurred. I had asked Jim Blaine, our machinist, to make a new design, with one jet pointing forward and one off to the side, intending that the two be offset. However, due to a misunderstanding, he drilled the second, smaller hole along the jet axis, while offsetting the angled jet to cut further out from the diameter. Since the nozzle was built we proceeded to try it.

Figure 3. First dual jet nozzle design.

Because the axial jet removed the central core, we could offset the inclined jet so that it needed to cut a shorter distance in order to reach the required gage for the hole. That meant that we could rotate the nozzle faster, which in turn meant a faster drilling speed, much faster.

Figure 4. Hole diameter as a function of rate of penetration of the drill (in meters/minute), for two outer jet angles, and two rotation speeds.

Note that in the above figure, with a 30 degree outer jet, spinning at 970 rpm we were able to drill a hole at a speed of roughly 280 inches/minute instead of the previous 4 inches/min by adding only 25% more water to the bit with the second orifice.

As mentioned above, the limit on the advance rate was the depth which the jet cut into the wall, and the amount of rib between adjacent passes that the jet cut would leave.

Figure 5. An early hole drilled into Berea sandstone, at a slow advance rate, using a 10 ksi jet pressure.

Figure 6. Hole drilled into Berea sandstone at 970 rpm, 225 ipm advance rate, with a 15-degree inclined jet. Note that the hole perimeter has the equivalent of a thread cut into it.

It is pertinent to make a small observation over the advantage of that slightly roughened outer wall to the borehole. One of the ways in which miners hold up the roof while they are working underground, is to insert rods (known as roofbolts or rockbolts) into drilled holes placed in the surrounding rock. To improve the grip between these bolts and the wall, miners will also often insert packages of glue into the hole to fill the gap between the bolts and the rock wall.

Unfortunately when the hole is drilled with a conventional mechanical drilling bit, the walls of the hole are left relatively smooth. This means that the bolt has a poorer grip on the wall, and is more easily pulled out of the hole. The US Bureau of Mines ran anchorage tests for different rock wall finishes.

Figure 7. Effect of hole roughness on the anchor strength (US Bureau of Mines)

Conventionally a larger hole, with greater bearing surface, would give a stronger anchorage. This is shown by the greater load carried by the hole drilled with the 1-3/8th bit, over that drilled by the 1-1/4 inch bit. But both of these were smooth walled, and the bit drilled at 1-inch, with a roughened wall had almost three-times the pull strength even though of smaller size.

The roughness of the hole can be controlled by adjusting the feed rate, relative to the rotation speed, both as a function of the jet pressure, nozzle diameter and outer jet angle. It turned out, through experiment, that the optimal angle for the jet was at around 22.5-degrees, depending on the type of rock in which the drill was working.

The effect of rock properties plays a very significant role in the performance of the drill. And it was very easy, early in the program, to show that the important rock parameter was not the compressive strength of the material. To show this we drilled through prepared samples of an Indiana limestone and a sandstone, both of which had approximately the same (uniaxial) compressive strength. The advance rate was kept constant, as was the rotation speed, as the drill penetrated from one rock into the other, and then the hole was cut in half (as were the samples shown above).

Figure 8. Hole drilled from limestone into sandstone.

Although the hole maintained alignment, drilling straight forward through the steep interface between the two rocks (a problem with some conventional drills) the hole diameter changed dramatically.

How we changed the design to maintain hole diameter, and, at the same time, adjusted for changing borehole depth will be discussed next time.

Waterjetting 27b - Drilling rock under stress

Last time I opened discussion on the topic of cutting a material that contained high levels of stress. This is a more common situation when working with rock, since – as a general rule of thumb – the vertical stress on a rock increases by 1 psi, for every foot deeper one goes into the earth. Thus, for example, if one goes down around 700 feet, the depth of a number of coal mines, then the background pressure on that rock is some 700 psi due to the weight of the rock that is pressing on it from above.

Now I should also mention that this is only a general rule, because, over the millennia, the rocks move, are split by earthquakes, overlain by volcanic eruptions and many other events that make that generalized statement less accurate for any given location. And one factor is that, if there weren’t such movements, then the natural horizontal stress on the undisturbed rock would be about a quarter of the vertical stress (the ratio is known as Poisson’s ratio, though usually derived for the resulting strain on the material, rather than the driving stress).

What one often finds, when these values are measured, is that the horizontal stress is higher than the above simple calculation would suggest. Which is a long way of saying that it is often difficult, without making a measurement, to know exactly what stress a rock is actually undergoing when found underground. But if some of the rock is removed (because it contains valuable ore) then the stress field redistributes, and some of the simpler assumptions come back into play. And we found that out when we drilled these holes:

Figure 1. Oval holes drilled into a lead-bearing sandstone;

You can see that we were drilling oval holes. The drill we were using used two high-pressure (10,000 psi) waterjets that were rotating at constant speed as we fed the drill into the rock. (And I’ll discuss the drill design and other stress effects in the next post). The small dark spots in the rock are galena, and as I will discuss in some future post, we were able to separate the galena from the sandstone at the drill, in part because of the way the waterjets penetrate, as I will discuss below.

Figure 2. Waterjet drill penetrating sandstone at up to 12 ft/min.

The region of the mine we were working in was around 700 ft. deep, and had been previously mined. Roughly half the rock volume had been removed, over a relatively large surface area, so that the pillars that were remaining were carrying roughly twice the load that they were before mining took place. On the other hand, since the rock on either side of the pillars had been removed, the vertical load was all that was acting on the rock within the body of the pillar, where we were drilling the holes. So very crudely the vertical stress, before we started drilling was around 1,500 psi in the rock.

Now, to explain why the holes are oval rather than round, consider that a waterjet works by getting into the cracks that exist in the rock, pressurizing the fluid and causing the crack to grow until it meets other cracks that together free a small piece of the rock mass. In this case the rock is made up of grains of sandstone and galena which have boundary cracks around each particle. By growing the cracks using this process, the rock is broken out into the individual grains of sand and galena.

But when the rock puts pressure on the rock, so the cracks are squeezed closed, and the water finds it harder to penetrate into them. This happens to the rock on the sides of the hole. As it is being formed, the load that was being carried by the rock being removed transfers to the rock on either side of the hole. Because the load is vertical this means that the jets find it harder to penetrate the rock on either side of the hole, and the horizontal diameter of the hole is therefore less than it would be otherwise.

Figure 3. Lines showing equal stress magnitude around a hole drilled into a rock loaded vertically. (This is purely representative and does not carry a scale, the lines are of diminishing intensity as they move away from the hole.)

On the other hand, as the load from the overlying rock moves out to either side of the hole, it comes off the rock at the top and bottom of the hole, and those cracks get larger, and were no longer being squeezed shut. As a result the jets found it easier to penetrate into the rock, and the vertical diameter of the hole is thus larger than it would be otherwise.

Put these two together and the result was that the jet drilled holes that were oval in shape, as shown in Figure 1.

As one way of making sure that this was really the cause of the change in hole shape, we used the waterjets to cut a slot around the perimeter of a part of the rock in the pillar. By making a horizontal cut above the slab that this outlined, we removed the vertical loading that the rock was seeing due to the overlying rock.

With no external loads on the rock, from either direction, it was as easy for the jets to cut into the rock in all directions, and, as a result, the holes that the jet drilled were round.

Figure 4. Round holes drilled in unstressed rock near the block of holes shown in Figure 1.

Again, while the effects are much larger when shown in cutting and drilling rock, the effects would be similar if we were cutting material that was under other internal stresses and which were then cut by a jet in a shop or other surface facility.

In the above case we were working in a mine where there was free access to the rock, the situation changes if we had been trying to drill down from the surface, and that will be the topic of the next post. In passing it should be noted that the waterjet drill was not only quieter, but also less powerful and smaller than the existing mechanical drill, and it could drill the rock faster.

Figure 5. Comparison of mechanical drill (upper) and the waterjet drilling equivalent (lower) on a drilling rig underground in a lead mine.(You need to look closely to see the drilling rod that is shown in Figure 2.)

The Chilean mine rescue continues with 4 men out.

I am sitting watching the rescue of the miners from the copper/gold mine in Chile. Four of the miners have been brought to the surface so far. These were some of the fittest of the men trapped, to make sure that if, in the early stages of the use of the travelling cage, that they could handle any mishaps. And after three rescue personnel rode the cage into the mine, it is now being lowered empty.

The event is being handled with some skill by the Chilean government, they had a camera feed from the bottom of the mine. Thus we saw the rescue cage arrive at the bottom of the mine, and the first of the rescue team get out to help with getting the men out. The trip down takes just over ten minutes, then the harness which the man in the cage wears (which carries an oxygen feed, and also can provide a support since it is hooked to the top of the cage) is removed and passed to the next man to be rescued. He is already wearing a coverall that was made to his measurements, and a jacket (given the move from the heat of the mine to the cold of the desert night at the surface). After donning the harness he is fitted into the cage, the door is closed, and the cage slowly lifted back into the shaft. (It is long enough that it does not come completely out at the bottom and at the top and bottom has spring-loaded wheels that roll on the walls as it moves up and down). It takes fifteen minutes to get to the top, and the rider has a headset so that he can talk to the surface crew while ascending.

So far there has been no glitches in the process, the President of Chile is on hand to greet the miners, and the second one out presented him with a rock from a bagful that he had brought out for the top folk present. And after greeting the dignitaries and a couple, at most, family members the rescued men lie on a gurney and are taken for medical examination. (The fourth man to ride to the surface had only been working in the mine for 5 days and was from Bolivia).

In the chat that the program hosts carry on to fill in the time that the cage is travelling, they talked about the debt that the world owes to miners. These men went on shift on August 5th, working in an old (over a hundred years old) and over-mined deposit and were trapped for 69 days. They are talking about changes that will come about as a result of this, but so often a month after the rescue, it fades into history without much change. Over a hundred years ago in Northern England virtually the entire male population of the mining village of Hartley were killed when the only shaft into the mine was blocked. Yet here this mine had only one effective exit. The second nominal exit, up a ventilation shaft, had no ladder all the way up, and so was not available, though the miners did try it to see if they could get out.

In this case the rescue was helped by the mining and drilling community from around the world. Three different machines were used to drill the holes for the rescue shaft. The speed with which it was done hides the complexity of the job that was achieved. Bear in mind that when the rescue began it was expected to take until Christmas.

There were only two T-130’s in Chile, (the machine that drilled the successful shaft) and there were only certain sized pipe sections available, and so it was a case of working out how to get it done with equipment that had never been used for this before. There were a couple of times when it seemed they might not be able to do it, there was a pause of some four days because of problems with the bit in trying to drill the hard rock, but they worked through it. And they were successful., even though they had to drill around the winding drilled hole that had been drilled first to intersect the rescue area. It required that they bring in the best crew that they could find, and that included flying one driller in from Afghanistan where he had been working drilling water wells. It is likely the most difficult job that they have faced, and this has largely gone unremarked in the media.

Given that the technology was successful there is some talk of developing a permanent set of equipment that can be moved to the site of any future mining disaster, and building on the lessons learned with this event, be better prepared to create access to anyone trapped. This sort of effort will continue to be necessary. Miners work at a considerable distance, normally, from the access shaft through which they enter and leave the mine. In most operations the area around the shaft is left un-mined so that it supports the shaft walls and holds their integrity. This was another safety precaution apparently abandoned in this instance. But it means that miners have to travel some distance to get to the working area, and when disaster strikes they may be a long way from that safe passage out.

Mining will thus remain a dangerous occupation, perhaps even more so in the future. For just as we are now seeing, as the ASPO conference last week noted, the approach of peak oil, so we are also approaching peak minerals. As with oil, the need for future supplies means that smaller and more difficult and dangerous deposits will be worked. In order to save on cost risks will be taken, and men will be trapped and die. It is, sadly, a price that the bulk of society seems quite willing to pay. Fortunately in this case that price does not have to be paid, but unfortunately in too many parts of the world it is still being paid on far too often a basis. And as the need for miners and the minerals and fuels that they produce continues to grow it is hard to see that situation changing much. The 1,500 journalists who are in the desert, without decent accommodation and amenities will soon leave, I would not be surprised, after a while, to hear that miners were back, working in much the same conditions as before. The world need and the money that it will be willing to pay will be incentive enough.

Deepwater Oil Spill - patiently waiting, Chilean drilling, and coal in Europe

Well, although the Admiral said that the intersection of the relief well with the Deepwater Horizon well would occur within 24-hours of his last press conference, and we are now past that, there has, as yet, been no word of the current status. BP noted that at 11:30 am on Thursday morning (Central) that DDII, who has the BOP on the failed well, has pulled its diagnostic tools from the well, until the interception is completed. Now the nature of those tools has not been explained, they were, if my memory serves, looking to fish out the dropped drill pipe that had been held by the shear rams in the original BOP. However, it isn’t clear whether even that operation had been completed. Remember also that if the drill pipe is sitting on the cement rather than in it (a function of when it fell), there is not likely to be enough space for any tools to get down to do anything inside the casing (such as perforate it to create a circulation path).

While the BP site notes that the DDIII – which is drilling the relief well – is conducting a ranging run, one should remember the Admiral noting that with the new method they are using they can do the ranging without pulling the drill string back the way that they had to do when they were further away. (It should also be noted that it would not be unheard off for the drill to miss on the first attempt).

It could also be that the well has been intersected, but that the condition of the wall of the well has created an unexpected problem (bear in mind that the relief well is trying to drill into the open hole section of the well, where there is no casing or cement liner on the wall, and the production casing is likely set away from the well wall.)

There are a number of things that can be postulated, we will just have to wait for some official word before we can find out what is happening. Even the ROV camera feeds can find nothing more interesting to show than poking the new BOP with a rubber hose.

Apparently I was wrong in my understanding of what is going on with the rescue of the Chilean miners. I had interpreted the intersection of a drill with a rock bolt as meaning that one of the smaller bores, similar to that which found the miners, had been drilling another small access hole for sending down supplies (such as the empanadas and steaks they will get this weekend to celebrate the 200th anniversary of the country).

However it turns out that there are a couple of other large drilling units on site that are now trying to reach the trapped miners. The one that had the broken bit that took a week to fish from the hole, is being drilled using a T-130 drill, which is reaming one of the supply holes already drilled from 5-inches to 12-inches. That step in the process should be perhaps completed by this weekend, but then the shaft will have to be widened in a second step out to the 28-inch diameter that the miners need to get out.

Reaming the hole in two stages has some advantages, since it does not force the large volumes of debris down the small initial hole,, where larger fragments might jam and stop the advance. The drill is already at the 1,640 ft mark, out of the 2,067 ft needed to reach the miners. The Strata 950 has only reached 1,050 ft.

Meanwhile a third option, using a oil platform drilling rig (Rig 422) has arrived and been set-up. It might be possible for this to drill a larger hole faster, and it is being located so that it has to drill a slightly shorter distance (1,958 ft) to reach the refuge. It can drill at between 65 and 130 ft a day. Apparently the roads over which the parts had to be carried were so bad that several of the haulage trucks arrived on site with flat tires.

And it is worthy of note that the European Union has decided that the last coal mine in Hungary will have to close at the end of this year. Since the mine feeds a local 240 MW power plant, which will immediately begin disassembly, this has implications for both the heating of the community this winter, and for future local unemployment. The argument has been made that because coal mining is subsidized (it used to be in the UK, that coal was subsidized, in part to keep the miners in employment, after the mines were closed unemployment in those regions of the UK was very high for a significant number of years). The mine is currently burning biomass with the coal, in order to reduce pollution, but that is no longer a sufficient justification for its continued operation. It supplies 5% of the energy used in Hungary.

This should be contrasted with plans in the UK to build a new 1.6 GW coal-fired power station at Hunterston in Ayrshire. It is a plan that has led to strong opposition but in all these debates, it is not clear if the reality of alternate supplies at the levels needed has been adequately considered. Given the strong objections from some locals over the siting of wind farms in South-West Scotland (farms it is actually difficult to see, as I have found), one wonders what source of magical power is expected to replace the diminishing contributions from North Sea oil and gas.