Bit Tooth Energy

In an early section of these notes on high-pressure water and its uses, there was a review of some of the ways in which jet power could be assessed. For the most part the best way to see how changes in a system alter the way the jets cut is to run a simple cutting test and in many cases it will give the answer that is needed. However, sometimes it is important to go beyond the simple assessment of whether condition (a) is better than condition (b) and to try and explain why it is.

One of the early discoveries in trying to explain abrasive jet behavior was made by my colleagues Marian Mazurkiewicz and Greg Galecki who showed that, when creating an abrasive waterjet system in the conventional way, that a large amount of the abrasive was being fragmented in the mixing chamber of the nozzle.


Figure 1. Size distribution of garnet particles after being fed through the mixing chamber of an abrasive waterjet nozzle, AFR 0.6 lb/min, at 30,000 psi.

Before entering the chamber the particles had been screened to be very close to an average 200 micron size. After going through the chamber the average particle size (50%) was 140 microns, with roughly 25% of the particles being smaller than 100 microns, at which point we had found that the cutting performance gets significantly poorer.

The way in which we found the particle size, (and also assessed how fast the particles were moving after they left the nozzle) was to set the nozzle horizontally, and then to fire the jet down the center of a large plastic tube.


Figure 2. Plastic tube set up to capture the abrasive particles from an abrasive waterjet nozzle. (The nozzle is on the left of the tube).

Barriers were placed at 1-ft intervals along the tube, so that as the particles lost energy and fell to the bottom of the pipe they could be collected into the small dark blue containers under the tube, and then dried, sized and weighed. During a test the top half of the pipe sections are replaced, so that the jet is contained over the pipe length, which was just over 20-ft, and this was long enough to capture, within the length, all the abrasives from all the tests of abrasive waterjet nozzles that we carried out (which included all those commercially available at the time).

It was based in part on this test that we were able to understand why some abrasive waterjet nozzle designs work better than others, and also to begin to understand more of the mechanisms that drive the cutting process.

As the abrasive slurry system (ASJ) started to enter the American market we were thus ready to test the way in which it worked and to see if we could find any improvement, as had been reported by those who first used the system.

When we set the system up so that the ASJ system fed a nozzle in the same arrangement as with the AWJ system we got a surprise. Much of the abrasive was collecting at the far end of the pipe, and it was starting to poke a hole through the end piece.

Over time we extended the tube, and eventually moved it outside to ensure that we could capture all the abrasive in the same way as earlier.


Figure 3. The test set-up needed to capture all the abrasive when using an abrasive slurry jet rather than an abrasive waterjet. (The pipe is roughly 50-ft long).

Even at lower pressures the ASJ was carrying the abrasive much further than was the case with the conventional AWJ system, showing that the particles were retaining more energy over greater distances. In retrospect this is not surprising, since there is sensibly no break-up of the abrasive particles with the ASJ system during the mixing and acceleration of the particles.

This is because the particles are mixed in with the water before the water accelerates, and when it does the abrasive is already mixed throughout the jet, rather than trying to force its way into the jet, while at relatively slow velocity relative to the water. There is, as a result, no break-up of the particles, and larger particles will lose energy more slowly than smaller ones. (One of the findings of the AWJ tests carried out earlier).

In addition because there is no air in the ASJ jet stream there is no active component trying to disrupt the jet, and as a result the water remains coherent to a greater distance from the nozzle, and has, as a result, a much greater capability of transferring the energy to the particles to accelerate them to their full potential, given the design of the system.

There are various different ways in which this benefit can be illustrated, using lab data, but the clearest demonstration is to take a conventional waterjet system, and to run the triangle test and then, with the same amount of abrasive in the jet stream, and a roughly equivalent amount of water, to run the same test with an abrasive slurry system.

Figure 4. Comparison of the cuts achieved with an abrasive slurry system (upper) and an abrasive waterjet system (lower).

The picture shows that the two systems are cutting to sensibly the same depth, and the ASJ system is being operated at a quarter of the pressure of the equivalent AWJ system.

There have been many comparisons between the two systems in the time since this initial evaluation was made, and there is a rough consistency in the results that have been obtained, on the order of that shown in figure 4. The comparisons are not all equivalent to this, since the tests compare different attributes of the two systems, and, for example, there are additional advantages of the lower pressure ASJ system that can enhance the relative performance. One way, when one compares equivalent horsepower, is to increase the diameter of the ASJ nozzle. This allows use of a larger abrasive particle that, in turn, will give a further increase in achievable depth of cut.

Unfortunately the difficulties in achieving consistent performance with some ASJ systems, over long operating periods, has made it more difficult for this relatively new system to penetrate fully into the market place as yet.

When the Direct Injection of Abrasive jet (DIAjet) was first introduced to the general public, back in 1986, there was some initial skepticism as to the overall market potential for the system. Certainly, as the next post will discuss, the ability to transfer higher levels of energy from the pressurized water to the entrained abrasive with no particle fragmentation in the mixing chamber, had a number of advantages, that I will spell out below, but the long-term problem was to develop a method of sustaining a constant abrasive feed in the mix, critical to high precision cutting, while concurrently having a system that can run continuously both day and night. To my own knowledge there have been at least two different designs of ASJ system that have solved this problem, but the market was damaged by the early problems in sustaining continuous flow to the point that customers shied away from this advanced technology, even where it showed some considerable commercial advantage.

The first, most obvious advantage to this new tool was in the smaller cut width that it generated, relative to conventional Abrasive Waterjet Cutting (AWJ). And because DIAjet became known as the original BHRA technology, and there were competitors over the years, let me re-name the technology (as the WJTA did some years ago) as Abrasive Slurry Jetting (ASJ).


Figure 1. Cut size difference between ASJ and AWJ as an illustration. The cuts are in Plexiglas, with the ASJ cut on the left.

The reason for the difference in cut width is, as explained in the last post, that the volume of the cutting jet is cut by 90% when the air carrier for the abrasive is no longer necessary or present in the jet stream. This increase in jet “delicacy” can be illustrated by a small example, and a humorous competition between Don Miller and ourselves back some years ago.

Don was, as the technology evolved, one of the master players in moving the technology toward the micro-cutting market that, to this day, remains remarkably under-exploited.

Because the abrasive particles accelerate to a large extent with the water that is both the cutting and carrier fluid, the cutting ability of the jet is significantly less sensitive to the diameter of the nozzle than is the case with the conventional AWJ. And, because the waterjet is not disrupted within the mixing chamber as a way of helping mix the abrasive with the water, so the jet stream can be kept convergent away from the nozzle, increasing the range, as I will discuss further in a later section.

The high precision cutting, using a finer abrasive since the nozzle diameter is smaller, can create very delicate pieces. We had been asked to use our system to cut jewelry out of silver, since while the ASJ could cut this easily and quickly to the desired shapes (matching necklace and earings) trials with laser cutting had been less successful because of the high conductivity of the metal.

This led on to a demonstration of the precision of the cut that can be achieved. One of the early models cut with an AWJ had been of a dragon, it seemed to be a good idea to match this with a knight on a trusty steed.

Figure 2. Knight vs dragon, in this case the knight was cut with an ASJ, while the dragon was cut with an AWJ system.

Don Miller had put together his precision system in his garage, and was able to control the cutting ability with an on-off switch located upstream of the nozzle, using sliding diamond coated plates to ensure a seal, without the wear problems which are common when trying to valve an abrasive laden flow.

Figure 3. Don Miller’s cutting equipment with precision table. Don Miller).

The need for the rapid on-off design comes where a series of holes must be punched into the target metal in order to effectively create a screen, or similar device. Pratt and Whitney engineers had used a somewhat different concept, and had held the abrasive in a polymer. This is sometimes necessary when using the ASJ system, when the cut-off in flow to the nozzle occurs with abrasive in the feed line from the reservoir to the nozzle itself. If there is any significant delay before flow restarts then the abrasive will settle to the bottom of the containing pipe. When flow then restarts this initial plug of abrasive is picked up by water flow and can block the nozzle when it reaches it. The use of the polymer holds the abrasive in suspension to prevent this happening. (We have seen abrasive held in suspension for over a week, using relatively low concentrations of polymers such as those used to suspend fracking sand for the oil industry).

Figure 4. The diamond-coated valve used by Don Miller to control flow in an ASJ system. (Don Miller)

The risk of abrasive build-up is increased when the shut-off valve is directly behind the nozzle, or where it is mounted vertically, when a polymer is not used, but where the jet is cycling rapidly to drill a precise series of holes in a rapid sequence, then the issue of nozzle blockage doesn’t arise as much.

Figure 5. A grid of 85-micron diameter holes drilled at 2.5 holes/sec at a jet pressure of 10,000 psi (Don Miller).

When we discussed the relative scaling that could be achieved with the technique, Don’s answer to the thickness of the lance that we had given the knight was to put scales on his dragon.

Figure 6. The scales on Don Miller’s dragon. The picture width is around 1 mm.

I had promised to go back and put eyelashes on the horse, but somehow we never managed to find the time.

The delicacy and accuracy of the technique is in marked contrast to manufacturing techniques other than those using abrasive-laden water as a cutting medium. Not only is it possible to cut through metals and other materials without distortion, even with very narrow webs left holding the pieces together, but since there is no heat involved in the cutting process, the precision is retained over the cut and part, after completion. This is illustrated with Don’s construction of a butterfly wing through 150 micron thick stainless steel. (The scale on the illustration shows mm).

Figure 7. Detail of a butterfly wing cut by Don Miller, using his ASJ system (Don Miller)

In recent articles in this series I have written about the processes that occur as a high-pressure waterjet impacts on a surface and then begins to penetrate and cut into it. However, as I noted in the last post, one of the problems with using plain water as the cutting medium is that it can pressurize within the cut and exploit any surrounding cracks, to the point that the edges of the cut are cracked and fractured, often back up to the top surface of the material.


Figure 1. High-pressure waterjet cut along a sheet of Plexiglas, note the fracturing along the sides of the cut.

This is not usually desirable, and what is needed is a way of cutting into these materials, so that the cut edges remain smooth, and the risk of shattering around the cut line is much diminished. The way that is usually used for this is to add small amounts of a fine cutting abrasive into the waterjet stream, and use this to cut the slots in the material, with the water there to add cutting power.


Figure 2. Abrasive waterjet (AWJ) cuts through safety glass. Note that there are two sheets of glass with a thin plastic sheet attached between the two.

This can be of particular advantage if you are faced with trimming, for example, safety glass (as shown in Figure 2). Cutting and shaping this glass used to be a significant problem in the industry, since the presence of the plastic sheet, between the two glass layers meant that it was not always possible to get both to break to the same plane if scribed with a glass cutter. Failure rates of up to 30% were described, to the author, as common when the technology switch to AWJ took place. And with the abrasive in the water, the jet cuts through both layers without really seeing that there was a problem. (And complex contours can also be cut).

The combination of abrasive and high-pressure water has many advantages over existing tools. Among other things it removes the majority of the heat from the cut zone, so that in almost all cases the Heat Affected Zone (HAZ) along the edges of the cut disappears and the quality of the cut surface becomes, when properly cut, sufficient to require no further processing. This can lead to a significant savings in certain forms of fabrication.

There are many different ways in which abrasive can be added to a high speed stream of water, and Dr. Hashish illustrated some of these in the introductory lecture he gave at an early WJTA Short Course, as follows:


Figure 3. Some different ways of introducing abrasive into the cutting stream of a high-pressure waterjet (After Hashish, WJTA Short Course Notes).

The top three (a, b, c) involve mixing the abrasive and the water streams at the nozzle, while the fourth (d) is a relatively uncommon design that is used in cleaning surface applications, and the fifth has never been very effective in any trial that we have run. The sixth (e) technique has become known by a number of different names, but for now, to distinguish it from the more widely used Abrasive Water Jet cutting (AWJ) I will give it the acronym ASJ, for Abrasive Slurry Jetting. It has a number of benefits in different circumstances, and I will write more about it in future posts. In more recent alternative designs to that shown by Dr. Hashish the flow to the abrasive holding tank is more commonly through a diverted fraction of the total flow from the pump or intensifier.


Figure 4. Very simplified illustration of the circuit where abrasive is added to the flow from the pump/intensifier before the nozzle. Obviously the abrasive is held in a pressurized holding vessel – the optimal design of which is not immediately obvious.

When fine abrasive is added to a narrow waterjet stream, and that jet is moving at thousands of feet a second, there are a number of considerations in the design of the mixing chamber, and those will be discussed in future posts. But one early conclusion is that, if the jet is going to be small, then the abrasive that will be mixed with it will also have to be quite small, though – as will be noted in a future post – not too small.


Figure 5. The simplified and generic components of a mixing chamber that mixes abrasive with high-pressure water in an AWJ system.

There were a number of problems with the early systems, such as that shown in Figure 5, at the time that systems first appeared on the market, and I will write about some of these as the next few posts continue to focus on this subject.

There have been a number of different abrasives used over the years, and it depends on the needs of the job as to which is the most suitable in a given case. In some cases discriminate cutting is required, and so an abrasive can be chosen that will cut the desired layer on the surface, but not the material behind it. In other cases the target material is extremely tough, and so abrasive may be selected that will rapidly erode the supply lines and nozzle, but which can still prove economically viable in certain cases.


Figure 6. Various types of abrasive, that can include (from bottom left going clockwise) blasting sand, copper slag, garnet and olivine.

There are many different properties of the cutting system, and the abrasive which control the quality and speed of the resulting cut. Some of these will be the topic of the next few posts, others will be discussed in further posts at a more distant time, when we discuss different cutting applications and the changes in a conventional system that might be made to get the best results in those cases.

Abrasive properties are not just a case of knowing what the material is. There is a difference, for example, in cutting ability between alluvially mined garnet and that mined from solid rock. There is a difference between different types of the nominally same abrasive when it comes from different parts of the world, and there are differences when the shapes of the abrasive differ. Glass beads and steel shot cut in a different way that glass and steel grit, for example. So there is plenty to discuss as we turn to a deeper discussion of abrasive waterjet cutting.


Figure 7. Parameters controlling the cutting by an abrasive waterjet system. (After Mazurkiewicz)

Tonight the President will talk to the nation about the oil disaster that has been going on in the Gulf of Mexico for over a month. That will likely be the news story of the night, followed by the answers to the five questions that lawmakers have of BP. By that time I will also be starting a daily visit to the National Hurricane Center to see if there are any signs of coming problems. All of which being said, now might be a good time to talk about erosion, how it is changing the Deepwater Horizon well conditions, and why precautions about the flow increasing are probably wise. And I am going to recap bits of an old Tech Talk, as I do so. (It’s partly why they are there.)

To begin with a simple point – fluid (oil and gas) will only move from one place to another if something is pushing it. (Newton’s first law). For the fluid in the reservoir under the Gulf, this force pushing the oil out is the difference in pressure between the oil in the rock, and the pressure in the well. The pressure of the oil in the rock is 12,000 psi. When the well was drilled the pressure of the mud that filled the well was over 13,000 psi and no oil moved into the well. Just before the disaster the fluid in the well was changed from mud to seawater. This lowered the pressure of the fluid in the well below that of the fluid in the rock, a differential pressure now existed, and where there was a passage through which the oil and gas could flow, and they did. The question has always been – how much?


Gas flows more easily through cracks than oil, and the disaster was first evident when leaking gas reached the drilling rig, and then ignited. The BOP then, at least partially, functioned. After the rig sank, the riser also sank, bending the pipe just above the BOP. At that time there were reports that a Coast Guard ROV examined the underwater assembly and did not see any obvious oil leaks. A couple of days later the flow was suggested at about 1,000 bd, and this then escalated to 5,000 bd. As cameras began to publically monitor the outlet of the riser the estimates started to grow, but a not-well-publicized effort measured the flow out of the riser, and found that it was around 8,000 bd, with allowance for leaks, the overall flow was estimated to be perhaps 12,000 bd. Once the broken part of the riser was removed and a cap placed over the well, a significant portion of the escaping oil was captured and could then be measured as it flowed into the surface vessel recovering it. Those values are currently at around 15,500 bd. BP is currently planning on additional capture this week of up to another 10,000 bd, and preparing for a worst case scenario with a flow rate of 80,000 bd. These numbers vary a lot, and yet they could all be correct.

Why? Well its called erosion, and simply put, the oil and gas that are flowing out of the rock are bringing small amounts of that rock (in the form of sand) out with them. Rocks that contain lots of oil are not that strong and are easily worn away by the flow of fluid through them.

Let me make an analogy with soil. If I make a hill of soil, and leave it sit for a while there will be a number of rainstorms fall on the soil. Initially the surface will all erode relatively evenly under the diffused flow after the rain, but very quickly weaker parts of the soil will be removed faster and instead of a smooth surface, the soil will be selectively eroded and channels or rills will start to form on the surface.

Hillside rill channels formed as initially diffuse rainwater water flow concentrates and erodes channels

These are larger than the passageways around the individual grains of soil, and so it is easier for the water to flow in these channels, and so more water collects in them and moves through them. As it does, because the fluid can easily get around the soil particles, and this was a weaker area already, more soil is removed, and the channels get deeper. This is known as concentrating the flow and means that, over time the channels grow bigger, the fluid flows faster, and it has a greater potential for erosion.

This also happens when oil and gas start to flow from a reservoir. It is generally not a good thing to allow, since the sand is still in the oil and gas when it reaches the surface and it is expensive to get out – as well as causing the problems I am about to talk about. So to stop it the well is fitted with a screen when it is first opened. The screen holds the rock particles (sand) in place around the well, slowing if not completely stopping the creation of the channels.

But sometimes the deposit doesn’t flow very well, the oil may be heavier, or other reasons, and in this case allowing those channels to develop can help production. This technique is known as CHOPS – Cold Heavy Oil Production with Sand. (The report cited is a multi-chapter pdf). The impact of allowing the sand to flow with the oil and gas (to be produced in the terminology) is very significant.

Simplified change in production from a well with the sand being produced (upper curve) relative to conventional flow (lower curve). (The curves have been smoothed and only a sample of the “noisy” data is shown). Government of Alberta CHOPS Report

There is thus a very good reason, from the oil in the rock point of view, for the production to have been increasing the way that it has. And for it to increase to the levels that BP are taking precautions to capture. And because they cannot get access to the flow channels to restrain their growth and hold the sand until the relief wells are drilled, that increase may well be unavoidable.

Now to the second part of the puzzle, which is what that sand does to the flow passages. The DOE release of some of the documents from BP included pressure measurements at different points along the BOP as taken on the 25th of May.

Pressures along the BOP on May 25th (DOE )

Because of the erosion, values only exist transiently, so dates become important.

When I cut with an abrasive slurry system the jet flows through a nozzle that is about 1 mm in diameter (0.04 inches). With a pressure drop of 5,000 psi across the nozzle, that jet, once formed, will cut through casing steel in around 15 seconds. It will cut through ASTM – A108 steel to a depth of over an inch at a traverse speed of 1.5 inches a minute. (Cement is much easier to cut). In this way very small initial openings are very easily made wider, to allow a greater volume flow. (And for those who assert that changing material properties will stop erosion, one of my upcoming papers deals with what happens to diamond).

The pressures across the BOP don’t show the pressure drops that they did earlier in this disaster. At one time it was reported that the pressure below the BOP was in the 8-9,000 psi range and that above it 2,800 psi with the 2,250 psi pressure being that of the seawater outside the riser. At that time I calculated that a flow of 500 gallons per minute (17,000 bd) would only require a gap in the range of 0.5 to 0.7 inches in effective diameter to allow that flow, at that pressure drop.

The pressure below the BOP is now at 4,400 psi with a pressure drop of around 2,150 psi which will slow the erosion significantly – but not totally. For that pressure drop to have occurred, for the same flow rate, the effective equivalent diameter through the BOP need only increase by 0.1 inches to 0.8 inches, which is not a lot. Were the effective diameter to increase by only another 0.15 inches the flow would increase to 25,000 bd, and if the effective diameter were to double to about 1.7 inches, then the flow would reach the maximum capacity that BP will be able to handle of 80,000 bd . Given the steady erosion that the BOP is seeing, and that a slow rate over time still gets there, perhaps it is not foolish of BP to bring in that additional capture and storage capacity.

And since a picture sometimes helps, the following picture shows a single ASJ jet at 5,000 psi cutting the walls of the OmniMax Theater under the Gateway Arch in St Louis. We cut the full 15 ft of the new wall exposure with the system (to make the hole for the theater to be put into). The jet flow rate was around 5 gpm.


And this was the cut after a single pass, we were cutting about 18 inches deep on a cut (rock and cement cut a lot easier than metal – though we accidentally ran over a couple of those bolts and cut them right off).

Dolomite and chert cut with an ASJ in the walls of the Omnimax Theater under the Arch in St Louis. (Its all hidden behind the concrete wall now).

Now you will notice that this says nothing about those ideas such as that propounded by Dougr that the casing has been cracked and oil is escaping into the surrounding rock., and that the casing is becoming a lot weaker. There are two reasons for this, firstly if there was a crack, in the same way as with the BOP, then over time that would have been eaten away as oil, gas and mud flowed through it. Once a flow starts it will rapidly eat out a larger passage, as the above has demonstrated. Once that passage was created then oil flow through it to the surface would make it impossible to see what was going on around the well (look at the cloud above the BOP). In fact there are very clear pictures from under the BOP. This would seem to show that there is no oil leaking there at present.

The other thing to remember is that BP are planning on using the second LMRP cap effectively as a seal on the well. They could not do that if the upper segments of the casing were damaged, and I imagine that they have enough data from the Top Kill testing to reassure themselves of that.

The National Oceanographic and Atmospheric Administration (NOAA) has now tested water samples from oil plumes at three sites in the Gulf of Mexico, at varying distances from the Deepwater Horizon oil spill.

NOAA Administrator Jane Lubchenco said that the tests conducted at three sites by a University of South Florida research vessel confirmed oil as far as 3,300 feet below the surface 42 miles northeast of the well site. Oil also was found in a sub-surface sample 142 miles southeast of the spill, but further tests showed that oil is "not consistent" with oil from the spill.

Lubchenco said the water analysis "indicate there is definitely oil sub surface. It's in very low concentrations" of less than 0.5 parts per million. Additional samples from another research vessel are being tested, she said.

To place these samples in context, consider first the locations at which they were taken.

Locations of the oil sampling sites reported by NOAA The red star is the well, and the oil from the well was found at the two sites (surface and sub-sea) North of the well, while the green spot which marks the site South of the well which was contaminated with oil from another source.


The oil in the southern location may potentially come from a source which also generated the tar balls on the Florida keys recently. This was the map of natural seeps that I put up the other day.

Reported natural seep locations in the GOM.

It is germane to also note that the concentrations of oil in the plume are at a level of 0.5 ppm. In context that means that there is 0.5 cc (or roughly 0.4 gm) of oil in a cubic meter of seawater. This is not discernable to the naked eye. Thus when a news report (such as that from Sam Champion on the ABC World News) talks of the oil plume and shows the blobs of oil that he saw subsurface in a dive some weeks ago, it is effectively deceptive, since the correlation of the plume with that visual conveys the impression that the plume contains a high concentration of oil. In fact the water appears clear, and were this fresh water and the oil were instead Benzene, EPA would let you drink it. At this level, if it takes 3.43 grams of oxygen to biodegrade a gram of oil, then it will only require about 2.7 gms of oxygen to treat the cubic meter of seawater. Since oxygen is somewhat scarcer deeper in the ocean it may much slower that the 100 gm/cu m/day that I mentioned as the top rate in an earlier post But on the other hand it is not likely to take months.

This relatively short-life for the oil after it is dispersed contrasts with the remnants of the oil that was not dispersed, in the colder waters of Alaska, after the Exxon Valdez oil spill. Remnants of that oil still remain 21 years later, and can be found as emulsions, tar balls, and trapped liquid. The suggestion by Jean-Michel Cousteau that the oil should have been left untreated, so that it could rise to the surface and be collected by skimmers, does not recognize that in many conditions skimmers are only able to collect about 15% of the oil, and that in large volumes (as with the Alaskan example) oil, once it reaches the shore, can survive for decades. Better surely to break it into small droplets that are degraded and disappear. And in that regard one of the benefits of adding Corexit to the oil is that it both breaks it into these small droplets, and that in the process it reduces their chance of floating on the surface and contaminating surface dwelling fauna. Corexit even works in cleaning marshes.

In other current developments, the flow of oil from the LMRP and cap over the well, continues to increase.

For the first 12 hours on June 8th (midnight to noon), approximately 7,850 barrels of oil were collected and 15.2 million cubic feet of natural gas was flared.

That increased flow (achieved by reducing the choke on the outflow pipe) can be seen indirectly by comparing the current picture from the Skandi ROV 2 with that earlier.

The small triangular pieces at the bottom of the cap are now well clear of the plume, showing the reduced flow.

As a result this increased flow is exceeding the capacity of the existing fleet sitting over the well. Upstream Online is reporting that as a result BP is bringing a Floating, Production Storage and Offloading (FPSO) vessel and a shuttle tanker, the Loch Rannoch, from its station off Shetland to the Gulf. (The Loch Rannoch was involved in another BP accident, a collision that stalled production at the Schiehallion field at the end of last year.

“The crash happened when the 130,000 tonne tanker was docking to take oil from the 144,000 tonne BP platform for transfer to the Sullom Voe terminal, off Shetland. Its only hose-reel used for exporting the oil was damaged in the collision,” the newspaper reported.

And a BP spokesman was quoted as saying that the Schiehallion FPSO was not back in production yet. (November 3rd).

The Loch Rannoch is an 850,000 barrel shuttle tanker, that carried oil from the FPSO at Schiehallion to Sullom Voe in Shetland.


The FPSO is a different sort of vessel. This is the one at Schiehallion (And I don’t think it is coming since it still has an oilfield to service).

The Schiehallion FPSO

At a top speed of 14 knots, and having left last Wednesday, with a stop in Rotterdam, it may still be a while. That will free up the drillship to move on to other things, providing they have an FPSO by then.

And one last point in this series of shorter items that has filled the news today, I had mentioned using an ASJ to cut outwards from the inner pipe of a series of casings, but the illustration I gave earlier had the pipe cut from the outside. This one (the outer casing diameter is 26 inches) was cut from the inside.

(Courtesy ANT)

And this shows the relative sizes and where the cut was made. It is needed as one of the final steps in the abandonment of the well.

(Courtesy ANT)

Perhaps BP might use it when they finally abandon the well.
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There has been a lot of activity at the bottom of the Gulf today, not all of it immediately successful, but all working toward to current aim of being able to field the Lower Marine Riser package. That installation requires that the broken existing riser that connects to the Blowout Preventer (BOP) has to be removed. The bent riser has been exerting some lateral pressure on the BOP, and this might be relieved when it is cut off. To minimize the damage the first cut is therefore going to be further down the riser. UPDATE: At 10:45 pm the wire saw is cutting the riser. But at 12:30 am the wire appeared motionless in the slot and could be jammed. While they worked on it on Wednesday they have now given up and are switching to the Shear. I have put an explanation (short) of what happened and how they could have fixed it at the end of this post. END

The Lower Riser Assembly attached to the top of the BOP, the riser has folded over to the left.

I described the plan of attack in an earlier post, and what has happened, over the course of today has tried to follow that script. I say tried, because there have been a couple of glitches develop over the course of the day. The large shearing machine (apparently owned by BTI) appeared on the scene, and in preparation for its use some of the pipes surrounding the main riser (the choke and kill lines) were first cut away using a diamond saw.

10:04 am just before the pipe was severed.

At the same time that this was going on, the wire saw that would make the final cut on the riser had been brought down to the site. The riser assembly has been cleaned of extraneous pipes already, and the wire saw would fit about the flange and below the bend.


The wire saw was then located ready to make the cut.

10:30 am Wire saw on riser

It was now time for the shearing machine (which I’m going to call a Shear from now on) to fit around the riser and to make the first cut through the pipe.

First shear location

Unfortunately the first cut did not appear successful, although there was a cloud of oil and gas released, indicating that the riser was at least breached. There was a pause, and the Shear moved to a new location closer to the riser. Again it tried to shear through the nest of pipes, that now included choke and kill lines. It was not successful, and returned to the surface where it will be either repaired or replaced. (There may be some problem with the hydraulics since it should have been able to handle the job that it was given). It (or a substitute) will likely make an appearance again before long.

While waiting for the Shear the ROVs worked, either alone or in combination, to cut away more of the pipes that surround the riser in the section to be sheared,

Cutting choke/kill line that still contains mud

Well I went away for a couple of hours and thus missed the shear coming back and cutting through the riser. This was the moment (recorded on Youtube) when the two parts of the riser separated.


Just after that the wire saw was activated. It appears, at 10:45 pm, to be half way through the riser. I'll try and note when it it done.

Wire saw at 10:45 pm

In the meanwhile, a little calculation, based on reports that the White House has announced that the removal of the riser and drill pipe that are protruding from the Blowout Preventer (BOP) of the Deepwater Horizon well in the Gulf may increase flow by 20% when the riser section is removed. There are two immediately obvious reasons why this might be the case.

The first of these is that there is a small amount of oil that was leaking up through the drill pipe that extended beyond the broken riser. That flow was one of the first things capped in the remedial effort. It did not have much impact on the overall flow volume, since the flow merely backed up and increased the flow through the main crack in the riser, but there may be a small increment of flow when this channel is re-opened with the cut below the fold in the riser.

The greater change in the flow, however, will likely come because the riser and DP, while not providing much increased resistance, did raise the pressure on the downstream side of the BOP by about 500 psi. We know that though the pressure down at the formation was at around 12,000 psi up on the upstream side of the BOP it fluctuates in the 8,000 to 9,000 psi range. The higher resistance on the downstream side, reduces the pressure drop across the BOP by that 500 psi, and the flow rate will be reduced accordingly (the gap size through the BOP is assumed not to change).

However, if the pressure drop across the nozzle was at 6,000 psi in the current condition, (which with an orifice size of 0.6 inches, would give a flow rate of 512 gpm) then raising the pressure drop by 500 psi would only increase the flow rate to 532 gpm, or a difference of 4%. Which might suggest that there is something about the drill-pipe flow that was initially capped which we don’t know yet. Alternately it may be that they think that removing the bend in the riser might ease the forces on the BOP, relaxing the metal a little and increasing the orifice size. After all it has only to open up by another 0.05 inches to give the increase in flow that the EPA are predicting.

Oh, and I mentioned earlier that an ASJ system had cut through casing and pipes at the bottom of the North Sea. I had the orientation of that cut wrong (at least for the picture below) since in this case it was from the outside in, but I am aware of it being successful the other way. And so here is the picture of casing and cement cut by an ASJ. Sadly it was so long ago - around 23 years, that I can no longer remember exactly the pressure it was cut at, but I believe it was 5,000 psi.

ASJ cuts of casing from the bottom of the North Sea

UPDATE: The wire motionless in the slot at 12:30 am


UPDATE: The wire was apparently stuck for a number of hours and they may have changed the wire, and then restarted with a second cut. BP is still predicting that the cut will be completed today and the LMRP installed.

UPDATE 2 (5 pm Wednesday): Part of the problem was apparently according to a BP spokesman that the cut through the first half had dulled the blade, so that when they got it restarted it would not cut. (What we do in those circumstances, which are not uncommon with diamond blades, is to run the blade through firebrick, and this erodes the material into which the diamonds have been pushed, and sharpen it. Then we drop the cutting pressure a little.) However, BP's current answer is going to be:

The technician said that rather than trying again with the saw, the plan now was to use a large shear to cut the riser. The shear, which is about 20 feet long and nearly 10 feet high, was used to make an earlier cut in the riser about 50 feet from the wellhead. Because the shear will not make as clean a cut as the wire saw, modifications would have to be made to the containment cap that is to be lowered over the cut pipe. But the technician said that even with the switch to the shear and the modifications, he expected the containment cap could be in place by Thursday.