Deepwater Oil Spill - Tropical Storm Bonnie -1a

I have not been over to the ROV sites for a while, since the rigs are preparing to close, but (h/t to JamesRWhite) I have to say that if I were running a pressurized line that had the leak that is now evident in the Hos ROV 1 feed, I would be seriously thinking about doing something to alleviate the problem. (Such as pumping in mud to alleviate the driving pressure).

Leaks on the fittings around the wellhead

And if I didn't do something there were two members of my staff who would have bent my ear until I did.

Deepwater Oil Spill - Tropical Storm Bonnie -1

With all activities shutting down around the Deepwater well due to the projected path of Tropical Storm Bonnie, as it has now become, it is appropriate to quote the Press release from Admiral Allen today.

Due to the risk that Tropical Storm Bonnie poses to the safety of the nearly 2,000 people responding to the BP oil spill at the well site, many of the vessels and rigs will be preparing to move out of harm's way beginning tonight. This includes the rig drilling the relief well that will ultimately kill the well, as well as other vessels needed for containment. Some of the vessels may be able to remain on site, but we will err on the side of safety.

As I stated earlier today, I have directed BP to continue with the well shut in procedure while the work to kill the well is temporarily suspended. I have also directed BP to take measures to ensure the vessels operating the ROV's are the last to leave, and the first to return in order to maximize monitoring of the well. Monitoring of the site during the well integrity test remains one of the government's highest priorities.

While these actions may delay the effort to kill the well for several days, the safety of the individuals at the well site is our highest concern. We are staging our skimming vessels and other assets in a manner that will allow us to promptly re-start oil mitigation efforts as soon as the storm passes and we can ensure the safety of our personnel.

Kent Wells briefing on Thursday had little to add to this, though there was a little emphasis on noting that the Admiral had given permission for the well to be kept shut in.


And just to remind us that there are other things that the natural cycle of the Earth can bring to our attention, there is a little more earthquake activity north of Iceland today, that we have seen in a while. But is remains relatively deep, at the moment.
UPDATE: While this is offshore Northern Iceland there have been 3 subsequent earthquakes of 3.0 or more magnitude in the area (where the yellow star is) since I posted this. Stars are for 3.0 or larger, dots are for smaller ones and quite common along the rift line.

Deepwater Oil Spill - There will now be a slight intermission (pause)

The approaching tropic system that has been mentioned in earlier posts has now not only caught the attention of the folks at the Deepwater well, but has moved them to action. Because of the length of time that it takes to disconnect the systems and then move the vessels out of harms way, BP decided to insert a storm packer, or plug into the relief well and has gone ahead and put it into place. (From Kent Wells briefing on Wednesday afternoon). This will allow them to disconnect the drilling platform from the well and to move it out from the site if necessary. Before the rig could set the packer it had to withdraw all the drill pipe from the well, though it would use some of it to set the packer, which was put into the well 300 ft below the seabed.

The sequence of events that Mr Wells had defined, and which appears to have won the approval of the review panel and Admiral Allen, was that the relief well would have to be cased, before the static kill of the well was attempted. There is a concern that, with the relief well only about 4 ft from the original well, in a condition where the relief well has only rock walls without a liner, the risk of possible wall failure in the relief well was too great. This is not a judgment that it is easy to argue with without much more information on the actual geology at the current bottom of the relief well. If the rock is intact, and relatively competent, given that the well is still above the zone where the leak has likely damaged the rock, then this may be somewhat overcautious, but if there is any risk of weak rock, and of communication between the two wells then it may be a valid precaution. Although it should be noted that the original well is supposed to be still sealed with casing and a liner at this level in the well – since the RW was supposed to run the last casing while some 50 ft above the end of that liner.

But with the static kill now on hold until after the “weather” storm has passed, and the drillship brought back on site (if it has to move), the well re-established, and the plug removed, and then, after checking the well, taking time to run the casing, cement it in place, and check the cement quality after insertion (something that will be a priority into the foreseeable future) it may be some time before the static kill is implemented.

There are a number of different ways the well can be temporarily plugged, but in general a packer is used. This is a device that contains a section with a flexible rubber sleeve (see below). The packer is lowered into place, and the packer inflated (you might think of it as similar to blowing up a bicycle tire) so that the packer section fills the well bore, and stops fluid from leaking. (The full procedure for installing one version of such a packer is given here).

Diagram of the parts of a storm packer(From Packers and Service Tools Inc )

There is an alternate system made by Weatherford described here. This has three sealing sections rather than the one shown above.

After the storm, and with the rig relocated, the drill string can re-attach to the top of the packer, deflate the rubber section, unsealing the well. The packer is removed and the well can restart. The removal of the packer is not without risk, and accidents can happen. But with that packer in place, the relief well operation is on hold, and so is implementation of the static kill, in BP’s eyes. Given that it will take 3 -4 days to re-establish the well and run the casing, the end of the operation is now moving inexorably into August.

There is one additional worry however, and that is that the current seal on the well is being allowed as a test condition. It is possible, and Admiral Allen alluded to this in his press conference today, that the well will need to be re-opened before the vessels disperse ahead of the storm. With all the connections to the floating risers, and the dispersant tanks not having been connected up and tested, this may lead back to a spillage of the full flood of the oil into the Gulf, until such time as the vessels return and re-establish control after the storm has passed. (The weather one, not the political storm this decision is likely to raise). That action will come down, as other things have, to the judgment and decision of just one or two individuals who will decide whether to leave the well shut-in or to re-open it.

The leaks in the system are, at the moment, very slow, though not insignificant, since they are pointing out points of weakness in the system. Can they be left for a week to ten days, without deterioration? – That is a judgment call. And it requires an assessment of what the consequences of a failure would be, relative to the oil invasion that will come with opening the valves.

The storm will affect other activities associated with the spill. Crews that were skimming the oil have been laid off, and some of the boom may also be moved. It will be interesting to see how the newly dredged islands hold up in this weather.

Deepwater Oil Spill - time is not our friend

There is a certain frustration in hearing some of the officials who act as spokesmen for the management team handling the spill from the Deepwater well in the Gulf of Mexico. Their evaluation of the situation is bound around a full collection and compilation of the existing evidence, a comprehensive and contemplative understanding through a scientific explanation of the causes of whatever anomalies and other behavior that is not following the model anticipated, and subsequently then working out the best steps forward and determining the potential benefits relative to alternative approaches. Such an attitude works well in a scientific laboratory, where whether the results are available tomorrow or next week only really matters if there is another lab in the world that is working closely along the same lines as you are. (And if the work is relatively topical that is often the case). Unfortunately this relatively leisurely approach to making progress is not nearly as compatible with a situation where a high-pressure piece of equipment is showing signs of leakage, and where there is the possibility that, within the week, equipment is going to have to be withdrawn from the site because of the imminence of a hurricane.

The imposition of an ultimately superior layer or more (depending on how much the White House is actually involved in decision making) of evaluation and decision making can do little beyond stretching the time over which decisions are made, eating up the time that is available for action, before the current gentle weather window closes. Now it may be that the current tropical depression will not solidify into a problem (I’ll let wiser heads in those topics answer that question), but even if this one does not, there will come a time when one will, and the working interval is shrinking.


Some of the worries about seeps in the vicinity have now been put to rest, in his brief yesterday Admiral Allen noted:

The first one was to see pitch about three nautical miles (Ed. Note amended to kilometers) from the wellhead itself. We do not believe that is associated with this particular well integrity test or the Macando well.

Similarly the bubbling from the sediments around the well have not been seen as something to worry about, although the material ejected is being tested. (It proves very difficult to get a meaningful picture of this). There is, however, one leak that is due to the well, and that is in the equipment that is sitting on the well itself.

Let me just tell you right away, because this happened overnight, as you know, we had a – a connector piece of equipment that we established in to allow us to put the capping stack on. These are the three rams that are associated with the capping stack. This is a schematic of those three rams. The leakage is occurring in a flange just located right about here, and there is hydrate formation appearing on this side of the capping stack as we move forward.

We do not know, but we do not believe this is consequential at this time, nor is – doesn't appear that the hydrate formation is inhibiting any operation of the capping stack. This is something we will continue to monitor as we move forward.

He noted that

it is the collective opinion of the folks that are talking about this that the – the small seepages we are finding right now do not present, at least at this point, any indication that there is a threat to the wellbore. . . . . . . There is a – there's actually a metal gasket in the flange, rather than a rubber (one). It's actually a metal – metal seal in there. And that appears to be the source of (the leak). But we don't know if it's consequential to the operations of it. It's not a huge leak, but it is causing the formation of hydrates.

(Ed note I corrected some transcription errors). The lack of concern seems to focus on the possible stratification of the fluid in the wellbore, and the concentration of any sand, which could cause problems if rapidly released.

Now that in itself is somewhat revealing, since one of the things that I have discussed in the past is the concentration of sand in the fluid flow, and that, when the fluid gets to a pressure differential of 2,500 psi or more that sand will erode metal and anything else in its way, as it flows out. With the sensible admission of the presence of that sand, what BP intend, apparently and if necessary, is to bleed the pressure down sufficiently slowly that the current segregation within the well, with the lighter gas-related hydrocarbons rising to the top, can be maintained until the pressure differential is low enough that the sand would no longer cause much erosion if caught up in the fluid. (Whether this would need to take the “several days” that Admiral Allen suggests is, perhaps, debatable.

There are a couple of problems with that. The first is that the sand is not in a single size range, but likely goes all the way down to sub-micron in size. The smaller particles don’t settle out that easily and thus are likely to be present to some concentration in the fluid throughout the well. Which raises the second problem which is that particles do cause erosion if they are moving over a surface at relatively high speed (caused by the pressure differential). In a much earlier post I discussed this and the effects that it might cause.

In my other life we have dealt with the problems of having abrasive get into high pressure fittings, and the leaks that result. Leaks tend not to fix themselves, and get bigger over time. Expecting that they might not change over the next month, while the odd hurricane might pass by, and the relief well completion gets postponed, is not a reassuring path to take.

In Kent Wells review on Tuesday he was, similarly to Admiral Allen, complacent about the leaks.

And then in terms of the couple of gas leaks that you probably observed on the BLP and capping stack. Those are just coming from places where we have what we call (metal) seals. Those are small leaks that are as a result of gas. Those connections have been tested to very high pressures in the case of the capping stack we actually tested it to 15,000 PSI with water and with no leaks, and it’s just when we – we probably got a gas bubble that’s formed up there and that’s why we have that very slow leak. It’s nothing that we’re concerned about.

At those pressures and temperatures, the gas is still liquid and still capable of carrying sand with it.

The potential for injecting mud to kill the well, which is getting more of a hearing at the moment, could be the way forward. Once mud in any significant volume is introduced into the well, through existing lines initially designed just to do this very thing, then the pressure at the top of the well will decline. This lowers the differential pressure across any leaks, lowering the flow and extending the time period before they may fail.

But, in regard to doing this “top kill”, Admiral Allen noted

We now have a closed system, so there's back pressure. And so the question is is there enough back pressure there where you could do basically more of a static rather than a dynamic top kill, where you could put mud in. That might suppress the hydrocarbons.

There's been some discussion about whether or not that might be possible. We're looking for BP to give us an idea of whether or not that it's possible, how they would do it. And we'll react to that when we receive it.

And BP themselves does not have a sense of urgency about moving forward with the process. From Kent Wells:

And then in terms of the static kill – and once again, I want to reinforce, no decisions have been made yet on proceeding forward with that. But we are continuing with preparation and planning. We continue to get equipment lined out, what we would want to do, making sure that we will have the right equipment out there to do it, writing procedures, starting to get procedures approved.

At the same time, we’re doing testes (sic) with scientists, challenging the way we’re thinking about this, what we’re doing, so we’ve got parallel paths going on that’s leading towards somewhere ideally in the next day or two that we’d be in position through unified command to make a decision whether we’d go forward with that.

He may take a couple of days to make an animation showing how it will work. Essentially however it involves reversing the flow down one of the kill lines (originally set up to allow mud flow into the well) which are now being used to allow oil to flow out of the well and up to a service vessel. From Kent Wells:

Now, one of the things we do need to do is we need to make some changes on the Q4000 to change it from its ability to contain oil and turn it back around into the pumping facility. But that does not take us very long to make that change and of course we’ll always have the ability to change back if at some point we need to do that.

It will, likely, take much longer for management to decide whether or not it should proceed. And the weather window continues to shrink.


Oh, and from the Admiral’s brief, in case you missed it.

The Discoverer Enterprise is no longer on station.

Deepwater Oil Spill - bubble, bubble oil and trouble

UPDATE on this post. Although there has been some additional discussion, and the test has been allowed to run another 24-hours, not much else has changed over the past 24 hours, apart from knowing that the anomaly on the BOP is a slight leak on the flexible joint. I am going, therefore, just to add to the earlier post, to give an update on the overall situation.

I rather suspect that we will know a lot more about the behavior of the sediments and matter at the bottom of the Gulf within the next year or so than we have learned in the past hundred years. I am looking at the view from the Skandi ROV 2 at 10 am on Monday, and it is looking at a patch of mud that is bubbling a little, though over a relatively significant area (that of the camera illumination). There is no trace of oil venting and flowing upwards (and a fish just swam by) so there will be, no doubt, some samples taken, and, over time, we will learn what is the cause.



There were other views, from different ROVs that seemed to show clouds of something, but the definition was poor and it was not clear that this was not mud that the ROV itself has stirred up. This has been the case several times today, in watching the video, though there were, in the seep area, shots of small drops of oil heading up to the sea surface.

Given the debate that is developing between BP and the panel that advises Secretary Chu and Admiral Allen, the redirection of the thought process to include another attempt at a top kill, brings in a whole pile of new matter to be used in those discussions

As the day continued there has been clarification of the remarks that both Admiral Allen and Kent Wells have made in the past, as well as an update on the relief well progress, and the resurrection of the idea of possibly doing a top kill. Looking at the Kent Wells conference at 5 pm he began by reporting on the status of the relief well:

Our first relief well, the total depth is at 17862, that’s our casing point. We’re four feet horizontally from the Macondo well at 2.8 degrees and we’re looking directly at the Macondo well. So we’re absolutely perfectly positioned. The team is feeling very good about how they’ve set this well up.

They’re now in the process of what we call opening the hole. So they’re drilling the hole a little bit bigger diameter and then on Wednesday, Thursday we’ll run casing and cement is in place and there’s some testing to do followed by the drill out and ranging runs

The pressure in the well itself has rise to over 6810 psi and is rising at about 1 psi per hour. This lower pressure than the pressures originally estimated makes it possible to reconsider the top kill option. This is where, by feeding mud into the top of the well through the kill line, while the well is shut-in, the mud fills up the well. (The oil and gas are pushed back into the formation). Then should they be able to fill the well up with this mud, the weight of the full column of it, down the well, would be high enough to balance the pressure of the oil in the formation. At this point, rather than the well being shut in, by the cap, it becomes killed by the mud pressure on the flow. There is no longer any concern about pumping the mud in at any high rate of pressure, since the flow is already stopped. Instead the mud flow and pressure can be set to a slightly higher pressure than currently is in the well, and then slowly increase the flow to fill the well, without bringing the pressure to such a high level as to further compromise the well integrity. The injection would be followed with cement, to seal the well at the top of the underground part. This would later be followed by the well intersection by the relief well, and an injection of cement at the bottom of the well.

There are three areas where concern has been raised over the possibility of oil escaping the well below the sea bed and migrating back up to the surface. This is why the ROVs are located around the well monitoring the sea bed itself. There are, as noted earlier, patches where the sea bed is evidently bubbling (in that you can see where the bubbles pop out of the mud). But there is no sign of gas or oil then slowly rising to the sea surface from the bubble action. It may, therefore be something like a field of clams sitting below the surface and aspirating and then spitting out some of the sea water. This action is not at the moment of concern, BP has checked the fluid coming out of the sediment and it is running at around 15% methane, which could just arise (according to Mr Wells) from biodegradation in the mud below the sea bed.

There is a natural seep some 3 miles from the site, this hydrocarbon flow has been tested and is not related to the Deepwater Spill. And so the only other area of concern is a very small leak coming out from the seal in the flexible joint (which, if you remember was straightened before the new cap was installed). The leak, at the moment is very small, and not of that much concern. However if the leak starts to get bigger, and then turn into a stream, it may pick up some of the sand that is reported as being a concern from being in the BOP assembly. This will then, at the pressures anticipated, be enough to erode out the leak to an unacceptable size within a couple of hours. For now, however, it is very small, and not continuous flow, and so can be viewed with less concern, relative to other issues.

The leak was detected in a flange between the top of the well and the rams that regulate flow up the main bore.

Video footage is showing some hydrate build up on the outside of the stack and scientists believe a small amount of oil and natural gas is leaking out.

Allen said the leak is not expected to hurt performance of the device and is not seen as a threat to its structural integrity.

Deepwater Oil Spill - should oil flow restart?

At the end of last week BP began the testing of the Deepwater well cap, closing all the valves and stopping the flow of oil and natural gas into the Gulf waters. With this cut-off in flow, the volumes to be collected at the surface are rapidly diminishing around the well, and the use, albeit controversial, of the dipersant at the same time as more of the oil was collected, means that the amount making it to the shore has also already diminished. So now the question becomes, does BP restart the collection process by re-opening valves to the surface vessels? It also opens the questions as to how much of the preventative work now being brought up to speed, is actually going to be needed?

The debate as to whether or not to re-open the well is illustrated by the comments by two of the main characters. In his Sunday brief Doug Suttles noted the success of the new cap, and the fact that there is no evidence of leakage from it. He had noted that the oil in the reservoir is hot, but by monitoring the temperature at the new cap, they had seen, over time, internal temperatures fall to those of the surrounding sea. This would indicate that hot oil is not still reaching the cap, and that fluid flow in the upper sections of the well has ceased.

At the same time the slow but steady increase in pressure within the well indicates that it has integrity, and is able to withstand the build-up in pressure as fluid accumulates around the well down at the level of the initial reservoir. Nevertheless BP are continuing to monitor and run seismic surveys to make sure that there are no surprises.

On the other hand Admiral Allen sent a letter to BP on Sunday, that raises some new issues.

My letter to you on July 16, 2010 extended the Well Integrity Test period contingent upon the completion of seismic surveys, robust monitoring for indications of leakage, and acoustic testing by the NOAA vessel PISCES in the immediate vicinity of the well head. Given the current observations from the test, including the detected seep a distance from the well and undetermined anomalies at the well head, monitoring of the seabed is of paramount importance during the test period. As a continued condition of the test, you are required to provide as a top priority access and coordination for the monitoring systems, which include seismic and sonar surface ships and subsea ROV and acoustic systems.

When seeps are detected, you are directed to marshal resources, quickly investigate, and report findings to the government in no more than four hours. I direct you to provide me a written procedure for opening the choke valve as quickly as possible without damaging the well should hydrocarbon seepage near the well head be confirmed.

It seems that those who argue that there are possible leaks from the well into the surrounding sediment have found at least one politically powerful ally.

The phrasing of the letter is, however, a little odd – the “the detected seep a distance from the well and undetermined anomalies at the well head” section raise questions as to – what seep, at what distance? And what about “undetermined anomalies” if they aren’t determined are these the “unknown unknowns” we have been warned about in the past? And as comments have noted, there is the question of the legality of re-opening a well, and deliberately restarting to pollute the Gulf.

The press release that the Admiral also issued today expresses concern over the possibility of a sub-surface leak.

Work must continue to better understand the lower than expected pressure readings. This work centers on two plausible scenarios, depletion of oil from the reservoir and potential leakage caused by damage to the well bore or casing.

While we are pleased that no oil is currently being released into the Gulf of Mexico and want to take all appropriate action to keep it that way, it is important that all decisions are driven by the science. Ultimately, we must ensure no irreversible damage is done which could cause uncontrolled leakage from numerous points on the sea floor.

Do I detect the hidden hand of Dr Chu in that penultimate sentence? I notice that the option of cross-flow is not specifically mentioned as one of the alternatives, particularly near the reservoir, and I get the impression that it is only in the near surface that there is concern about leaks.

There is a second concern with the decision to re-open the well which makes this issue a bit of a hot potato. Whoever makes that decision, and BP seem to have made sure that it is the Admiral who must visibly make it, will be the individual that starts the oil flow back into the Gulf – and that won’t be popular.

Admiral Allen recognized that the flow would be restarted in his press release on Saturday

When this test is eventually stopped, we will immediately return to containment, using the new, tighter sealing cap with both the Helix Producer and the Q4000. Additional collection capacity of up to 80,000 barrels per day is also being added in the coming days.

Kent Wells, in his brief the same day noted that

if we do decide at any point either during the remainder of the test or following the test, that we want to open the well back up initially we will have to blow it back into the Gulf for some period of time, relevantly short period of time to bring the pressure down on the well so that we can then go in to our collection systems namely the (Q port) valves and the Helix Producer.

While I am not totally sure of the reason for the longer term period of oil release, there have been rumors of a three-day period, there is a relatively simple explanation as to why the pressure in the well has to be released before flow can start back up the riser lines to the vessels on the surface. If the valves between the well and the risers are opened with the well at pressure, then that pressure is immediately transferred to the fluid in the line, and a hydraulic shock, similar to that known as “water hammer,” will propagate down the fluid line. Although water hammer is usually seen when a valve suddenly shuts in a pressure line, the same sort of effect can occur when a sudden pressure pulse is applied to the fluid in a line of pipe.

The most dramatic example of that which I have personally encountered was when we were first removing explosive from a casing using a high-pressure waterjet lance, and the flow channel blocked. The resulting bang initially caused us to think that the explosive had reacted. But the round was still there and it was only when we looked at the hose, which had split in several places, and had both end fittings fail, that we realized what had happened. Having a similar failure in a hose carrying oil from the seabed to the surface would create a much greater problem and one much more difficult to fix than ours, which was working in the same sort of pressure range as the fluid contained in the well.

But the pressure can be lowered relatively rapidly over the course of time (a matter of minutes not days, in the same way that the flow was cut-off to the Gulf) so there may be some other issues that are not yet being made public. After all with the cap holding some intermediate pressure, it is not necessary to vent fluid into the Gulf, as flow is allowed to the surface collection vessels, in a condition that would lower the well pressure from the current levels without putting oil into the water.

The Earliest Longwall - coal mining before the 1830's

Earlier I have written about the large amount of coal that was often left to hold the roof up while miners excavated the coal from rooms offset from main tunnels, with the rooms themselves being extended to create an intersecting set of passages. But even where the pillars left between the original tunnels are later removed as the mine retreats the working faces back towards the main shafts and exits, a significant amount of coal can be left.

About 250 years ago this was a clear problem in the Shropshire coalfields of the United Kingdom. At that time underground mining was usually carried out by crews of men and boys, where the coal was first removed by undercutting the coal seam manually with a pick, to a depth of about 3 ft. The bulk of the coal was then broken down to this slot and the fragments (ideally about 4-inches in size) were shoveled and hand-loaded into pit tubs, to be hauled away. A good day's work was about 20 tubs, and I have described this method and how it evolved, in earlier posts.

However, even as early as the 17th century (Economic Development of the Coal Industry 1800 – 1914 Brian R. Mitchell p 71) a different method of mining began. At first it was known as Shropshire mining because of where it started, but it later became known as “longwall mining”. The advantages, even then, of the technique were obvious. They included a greater percentage of larger coal (easier to sell), simplicity of working and ventilation, better roof control and a greater production of coal from the workforce, perhaps as much as 30% higher. In particular it was a cheaper method of mining and it allowed a much higher level of production from an area by concentrating the activities of the miners, and focusing the transport.

I just came across the book that appears to have been the first proposal for the more modern version of its use. (The Miners Guide – being a description and illustration of the principal mines of coal and ironstone in the counties of Stafford, Salop, Warrick and Durham”, by Thomas Smith 1836. And so I thought I would begin this short sequence on longwall mining with a description of how the technology first evolved, from that book.


The method was one that gradually evolved from initial headings that were mined by two separate working teams, the first being the holers and the second the brushers..

When the coal was of good quality, and high, then the process was to undercut the coal to a depth of 3-ft, with a man being able to undercut a length of about 22.5 ft a day. He would then cut vertical cuts to the same depth along the edge of the heading, which in the illustration below would be about 30 ft wide – depending on coal and roof quality. There were a group of these men who initially worked the face, and then moved on. They were followed by the brushers, whose job would be to break out the bulk of the coal from the face, and load it into tubs. They would also support the roof with timber props, as this was needed and the coal was removed. This was conventional room and pillar. But it left a lot of coal in the pillars.

Plan view of room and pillar or "on the square" mining.

Initially it was in thinner coal seams that “the long way” was developed as a way of getting almost all the coal out.

First, as with conventional mining, gate roads are dug out to the edge of the property (back in those days this was about 300 to 600 feet) with the direction going down the dip of the seam from the shafts at A and B, which are about 20 ft apart and some 7 ft in diameter. These roads were 6 - 9 ft wide and full seam height. Air passages or thirls were driven between the gate roads to help ventilate them as they were driven (the “a” passages). Cross-connecting tunnels between the gateroads were then driven, near the edge of the property.

In those days it cost around 0.35 to 0.4 English pounds (Ep) per yard, with workers being paid 0.225 Ep per day including candles and drink. The thirl would cost around 0.15 to 0.2 Ep per yard to drive. (An area up to 60-ft in diameter would be left unmined around the shaft area to hold it up).

Once the edge of the property had been reached then a section of the mine, some 90 ft long, would be mined with six miners each taking some 15 ft and holeing the coal. This was undercutting the face, to a depth of 3 ft, over each stint, and it would take a day, with the each miner also cutting a vertical slot at the edge of his section, so that it was held only by the coal at the back of the panel. The sections were mined on either side of the gate roads, moving towards the common middle of the “panels” being mined.

The holers were then finished in that section and moved to a different section, and a second set of miners mined out the rest of the coal, known as brushing the coal to the 3-ft depth. At the same time, since they were removing all the roof support they would put in timber props to hold the roof up, and would also construct small pillars or cogs, that were made from stone, fine coal, and other refuse, when they felt they were needed. In this way the white strip shown in the diagram below, at the back of the mine was extracted first, with the sections progressing first laterally out to the adjacent gate roads, and then back towards the shaft. While it takes 6 men to hole the 90 ft face, it would take only 3 men to brush and cog it. (And they would use small charges of gunpowder to help if the coal was not easily broken out). The difference from conventional room and pillar can be seen in the small size of the cog pillars that were left, as mining progressed. In this case, from one gate road to the next, with the mining face parallel to the gateroads and retreating from one to the next.

Plan view of Shropshire mining, the mining faces were parallel to the gate roads, and the dotted lines show the way that the tracks would be laid to get the tubs in and out.

Wooden tracks were laid along the gate roads, and then bent to pass along behind the face, to allow a horse and boy to collect the tubs as they were loaded, and then to replenish the men with empties. The costs for this method of mining, which was known as broaching, was given as:

(note that there are 12 pennies (d) in a shilling (s) and 20 shillings to an English pound of the period. And an English pound is now worth roughly $1.50). At that time the market for coal was such, that the mine owner would expect to get the following for the coal (with the price based on size).

A profit, at best, of just under 0.10 Ep per day, per working section.

The technique, was still quite dangerous, since the expanse of roof that the miners worked under got larger as the excavation moved away from the gate roads, and the cost of moving rock and dirt into the workings to build the cog pillars would have been significant (as would the time taken to assemble them).


Thus a new method was proposed, and the initial description is as follows:

For getting out the coal by long work, the pits A and B are sunk, as in the other case, at a distance of six or seven yards from each other; and the main gate roads driven to the boundary of the work at C and D, properly thirled with the openings for temporary use. From the ends of the main gate roads branches are cut, at right angles, to E and F, along the boundary line of the proposed area to be cleared, so that the mine may be said to be headed in the form of a Roman T, the roads E C, and D, F presenting the faces of the coal, which are to be worked homewards, or towards the pits. Simultaneously with the traverse gate roads, an air head ef is driven at a distance of three or four yards, with its thirls, which are closed in succession as the work proceeds. . . . . .The necessary roads and heads being completed, the work of getting commences; in order to which, the miners hole one yard under along the entire faces of work EC and DF which may be each from 50 to 100 yards in length according to the extent of the area to be cleared. Cuttings are then made at proper distances, to the height of five or six feet, or to a convenient parting, and the coals are brought down, turned out and drawn away along the gate roads. Cogs or pillars are then constructed of the waste and slack, to support the upper measures.

The holeing and cutting then proceed another yard in width, and then another; still clearing away the coal and supporting the roof with cogs, till the lower measures are drawn out, to the width, along the under face of 8 or 10 yards. By this time the over-hanging measure have, by their gravitating force (sic), sunk and bedded themselves on the cogs, pressing them down to a sort of continuous floor of what is called gob, or compressed and compacted slack. This is assisted by the use, as experience may dictate, of timber, which is taken away when the working of the stage above commences.

This is the first description I have found for what we now call longwall mining. By turning the mining face so that it advanced into the solid and away from the opening left, the overlying roof was able to bridge over the working area. This considerably improved roof control, and made it a much safer method of mining. In presenting the method the author notes that the cost of large coal, using room and pillar mining, which is the top method described, worked out to be around 0.118 Ep per ton mined. When the costs were worked out for the long way, the mined cost was found to be 0.105 Ep per ton, giving 0.013 Ep (3.25d) benefit.

However the increase in the volume of coal produced (and thus the royalty yield per acre) doubled to 2,046 EP per acre.

At the time that the book was written, it was a method just beginning to be developed, and the presentation was as much a proposal as a description of something in place. How it turned into the most productive of underground mining methods, in the course of the following 180 years will take another post or two to describe.