The specification for this underground drilling included: final hole diameter 1.75m, with a tolerance of +50mm and -5mm; start position within 25mm across and 50mm along tunnel centre-line; maximum transport height for the drilling machine of 3.5m; verticality within 2:1000 allowing a maximum wander of 18mm over the depth of hole; maximum ‘banana’ of 16mm off the centre-line; maximum wall steps or hole roughness of 10mm.
To give additional challenge to Drillcon’s solution, the customer wanted the 190MPa granite to be drilled with the minimum of disturbance and to remain totally dry. Each of the 12 holes was to be completed in 43 hours and a drilling rate of over 1m/h should be achievable.
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A quick calculation showed that the flushing system must be capable of handling more than 100kg of rock per min. Another ‘rule of thumb’ suggested that a thrust of around 200tonne would be necessary to achieve the specified progress.
So who was the client with this incredible specification and what drilling machine, and which flushing method, could possibly tackle the job?
Time to waste?
The safe management of radioactive waste is a problem facing governments and power suppliers, world-wide. Svensk Karnbranslehantering (SKB) is the Swedish authority charged with carrying out this task and at its Aspo Underground Hard Rock Laboratory all aspects of the various methods proposed are tested.
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By GlobalDataThe particular scenario for this project says that the spent fuel will be put into copper canisters and placed into holes drilled in the rock, in a repository some 400m below the surface. Compacted bentonite rings will surround the canisters and the 5m o.d. access tunnels will be backfilled with a mixture of crushed rock and clay.
The copper prevents movement of radioactive substances whilst the bentonite cushions the canister against any ground movements. The bentonite will also swell to prevent ground water invasion and the corrosive elements that it may contain. In the event of failure of the copper, the bentonite will act as a filter and, along with the rock, prevent or seriously slow the transport of any radionuclides.
The time for this waste to become ‘safe’ is measured in tens of thousands of years. In that period it is possible that new technology will offer a better method of disposal, an alternative use for the waste or a method of making it safe. SKB’s principle is that everything has to be reversible – it must be able to remove the canisters, safely, in years to come. All of this has to be done with robot machinery.
The purpose of these test holes is to enable SKB to have a ‘dress rehearsal’ for burial and retrieval of the canisters and to carry out a study of canister, bentonite and rock behaviour. No radioactive material will be involved at this stage.
Past experience for Drillcon
Other nuclear waste disposal authorities are carrying out parallel research or sharing in one another’s programmes. In 1994, the Finnish authority Posiva Oy contracted Drillcon to carry out similar work at their underground laboratory at Olkiluoto. On this occasion Drillcon used a Subterranean 005 raise-boring machine to drill a 12.25in. (311mm) pilot hole and opened this out with a 5ft (1.5m) blind hole reaming head.
Although successful, the project turned up several problems that would make the technique unsuitable for Aspo. The raise-boring machine needed 6.2m of headroom and was limited to around 60tonne thrust. Although the holes drilled achieved the accuracy specified by Posiva, the blind drilling technique offered no means of monitoring or controlling the drilling to the tolerances set by SKB.
One new technique, which Drillcon developed successfully in Finland, was vacuum flushing. Here a vacuum pump was attached, via a ‘rock bin’, to the flushing swivel. The rock bin takes out the majority of the cuttings leaving the cyclones and filters in the vacuum pump to remove the fines and dust. Success of the system depended upon the design and position of the bit nozzles, both in and out, and the resistance of the pipework to the considerable abrasion of the rock-laden air stream. Experience in this medium gave Drillcon, and its subcontractor Disab Vacuum Technology AB, much confidence in their approach to the SKB specification.
Joint venture,
It soon became clear to Drillcon that the only type of equipment that could tackle this project had to be continuously steerable. It also needed to be a ‘down-the-hole’ drilling machine with all the thrust and torque being applied directly at the cutter head rather than via drill rods. A TBM had to be the answer, but who made one that went vertically downwards?
For their answer Drillcon approached Boretec which, since its take-over of Robbins, has become one of the largest manufacturers of TBMs as The Robbins Company. Its Miti-Mole mini TBM (SBM) looked as though it could be modified to fulfil the required role. The principal additions would be as follows:-
mucking and required drilling tolerances;
umbilicals;
The commercial arrangements were that SKB would part rent and part purchase the SBM and its launching system. Drillcon would operate the machine and provide all consumables together with the flushing system and Robbins would supply technical support. The contract was to drill one ‘learning’ hole followed by the 12 test holes. This ‘learning’ hole was to enable Drillcon and Robbins to debug the machinery, train the operators and tune the whole system to the performance required by SKB. No time penalties would attach to this stage!
The Robbins SBM 1.8 was delivered to Aspo in October 1998. Its basic specification was as follows:
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“It soon became clear that the only type of equipment that could tackle this project had to be continuously steerable – straight down” |
The SBM itself weighed 20tonne and was 3.10m long. The cutter head was dressed with 20 Midi disc carbide cutters on Wedge-Lock saddles mounted to give 30mm spacing between carbide rows. As hole diameter was critical and cutter changing in the hole impossible, the outer row on the head was double tracked. There were also two gauge cutters on the outer row.
SKB required two cutters, one each on bottom and gauge, to be fully instrumented. This data collected included disc torque and thrust together with full saddle forces and the rotational position of each disc in the hole. Data transmission from the cutter head was via a radio link to the control cabin.
Other instrumentation gathered pressure and extension in each feed and stabiliser cylinder together with total torque, thrust, penetration rate and weight of cuttings. All of this could be monitored by the driller and was continuously recorded throughout. The recordings were made on a workstation well away from the rig and SKB scientists will assess these results for years to come.
The ‘learning’ hole
The major items to be debugged during this period concerned the vacuum mucking system. This involved the complete redesign of the vacuum nozzles and pipework back to the swivel and the eventual doubling of the vacuum power.
Although Drillcon had some experience in this from Finland, the operation of cleaning a 1.5m diameter, pilot shaft, down reamer was clearly very different from that of a 1.75m full-face cutter. What resulted was a pair of rubber faced replaceable radial conduits that together swept the whole face area in half a revolution. These led to two 150mm diameter nozzles close to the cutter centre leading to the suction swivel at a shallow enough angle to prevent serious erosion. Inlet air came through five 70-mm nozzles in the cutter face.
The suction hose from the swivel back to the rock bin had an internal diameter of 204mm and was specially lined with erosion-resistant material. It was vital that other pipework contained no sharp bends as these were very quickly ground away by the rock stream. This was particularly true at the inlet to the rock bin.
The original vacuum pump from Disab was exchanged for a 235-kW unit capable of producing 8600m3/h at a depression of 0.8bar. Drillcon soon discovered that there was a limit to raising the lifting power by increasing flow rate because this decreased the air density and therefore lowered the lifting capacity. This unit was eventually run at about 70per cent power to produce a most effective, trouble-free mucking system.
Other, inter-related, problems sorted out during this learning curve included:
Production holes
The collaring work had to be carried out in both drill-and-blast tunnels and within a 5m o.d. tunnel, drilled by another Robbins TBM. This latter had a steel roadway laid in the invert, giving headroom of 4.2m, and 2m diameter pre-cut holes at the drilling positions. The drill-and-blast tunnel was given a concrete floor to offer the cutter head a smooth start. Robbins had built the launching platform onto a double-axle semi- trailer, which was towed into position by a tractor.
Hydraulic jacks then moved the collaring tube precisely over the drilling position where it was either bolted to the floor or, in the TBM tunnel, braced against the crown with the thrust reaction frame. Drilling was started with low weight, slow revs and the stabiliser pads lightly extended inside the collaring tube.
The vertical laser projector, bolted to the tunnel crown within the thrust reaction frame, focused its beam on a ‘target’ on the cutter head bearing support. CCTV displayed this image at the driller’s position. It had been Robbins’ intention that the SBM should be steered by adjusting the flow and pressure to the relevant thrust cylinders. Drillcon very soon found that directional control was far easier using the front stabiliser pads. It produced a smoother wall and, by appropriate adjustment of the rear pads, could more easily maintain verticality. The extension of all eight stabilisers was recorded as drilling stopped to add the next length of casing.
Casings
The casings were 1m long and ‘D’ shaped, with bolted flanges on each end. Outside the flat part of the ‘D’ were channels to guide the umbilicals between the flat and the wall of the hole. The first casing, weighing 920kg, was inserted between the thrust pad and the SBM top flange whilst it was supported by hydraulically operated locking pins.
Once all the flange bolts were tightened, cylinders on the thrust pad lifted the SBM for the locking pins to be withdrawn and then lowered it back to the bottom of the hole. All eight stabilisers were then returned to the positions recorded above and drilling was then restarted.
The biggest single contribution to rate of penetration, despite the rock hardness, was the replacement of the carbide cutters by discs. It has been suggested that the highly effective flushing system took away the usual advantage of carbides when working on mucky faces. In this instance the discs always had a clean rock face to cut.
Effective drilling time over the 1m stroke varied from 175min down to 42 min. On seven occasions one stroke was drilled in under an hour. The best average over a complete hole was 77min/m.Once the teething problems had been sorted out with the ‘learning’ hole, the flushing method turned out to be a most effective system requiring a minimum of attention.
The start and stop positions of the holes were never more than 12mm off line in any direction, less than half the tolerance. Verticality at best was within 1mm and at worst 13mm; no hole exceeded the ‘banana’ limit of 16mm. Only the first hole failed the roughness requirement. This was solved in subsequent holes by replacing the carbide cutters on the gauge with discs.
The 43 hour drilling time limit, agreed between Drillcon and SKB, was only exceeded once, on the first production hole. This was directly due to a breakdown in the rock bin emptying system. The average time to drill one hole was improved over the contract to a best of 18.3h.
What next?
SKB will take several years to carry out its evaluations of the proposed method of spent fuel burial and recovery. It is suggested that by 2010 they could be ready to start building a deep repository . The Aspo work is strictly limited to a ‘dress rehearsal’. This assumes that SKB gains government permission and the people of Sweden generally support it.
It will therefore be at least 12 years before any ‘production’ drilling can take place. At that stage it is proposed to drill 400 holes as a pilot scheme, representing around 10% of the current volume of waste. A further 3600 holes will be drilled if the pilot is proved to be successful.
