It is hard not to be spellbound by the 15.87m diameter TBM being disassembled at the Herrenknecht headquarters in Schwanau, Germany. Technical acceptance of the machine, the largest ever built at the factory, took place at the end of August and now, in late September, an enormous white, rail-mounted gantry crane sits above the 4,800t earth pressure balance machine, systematically removing components as it is prepared for transportation to its ultimate destination in Italy.
Herrenknecht technicians seem to blend in to the pipework as they carefully deconstruct the TBM, which has been purchased by Italian contractor Pavimental S.p.A to build the 7.5km Santa Lucia tunnel through the Apennines as part of the renewal of the Autostrada A1.
The incredible scale of the machine makes it hard to imagine a bigger one. But the Santa Lucia TBM has a larger cousin. In May 2015 a 17.6m Mixshield began boring the Tuen Mun-Chek Lap Kok Link in Hong Kong for the Dragages-Bouygues joint venture on the first section of a 5km tunnel connecting Hong-Kong’s airport to the mainland.
As fascinating as it is to learn about the development of Herrenknecht’s largest machines and see Europe’s largest TBM being taken apart, Tunnels & Tunnelling has another item on its agenda focussing on the opposite end of the tunnel boring size spectrum. “We have been focussing on downsizing, although the market goes both ways of course,” says Michael Lubberger, senior product manager for pipelines in oil and gas at Herrenknecht. He points to one of the most recent developments, the company’s smallest ever horizontal directional drilling (HDD) rig. Existing models have traditionally been at the maxi or mega range with a pull force of up to 6,000kN but the latest model is a smaller 80t machine. The new HK 80 CK has a crawler base (C) and the K denotes its “Kompact” size. It uses a 20ft (6.1m) drill pipe compared to the standard 32ft (9.8m) and a number of measures have been implemented to ensure that the rig has a smaller footprint and lower transportation requirements. For example the fluid pump is small enough to be fitted on board at the factory, and the use of a Siemens PLC system for steering control has significantly reduced the number of electrical components.
Its compact size makes it ideal for pipe placement in urban areas says Lubberger. “It takes around half a day to set up and saves a lot on transport costs. The first machine will go to a site in the Pyrenees at the end of October,” he says noting that expected installations would be HDPE pipes and power cables and key markets are Europe and the US.
Development and design of the new machine took around six months and was a response to growing demand in the market, as was the development of a new range of HDD down hole tools also released this year. “We saw that there was a need for better hole opening and cuttings transportation,” says Lubberger pointing to a steel barrel-shaped tool sitting in the yard. “This is just back from its first job. We supplied it to the contractor as we wanted to see how it performed on a real worksite. It was used in hard limestone and flint so it was an abrasive environment.”
The new tool is the down hole jet pump, which has been specifically designed to increase the efficiency of the boring process by improving the removal of excavated materials and therefore reduce the risk of what is known on site as “frac-out”.
This is where the bentonite-based drilling fluid pumped in to maintain the hole and remove excavated material, builds up in the bore creating an over-pressurised environment. If the overburden above the bore is less than the pressure generated by the fluid in the hole then the bentonite suspension will inevitably seek to restore the balance by finding an alternative path, which is likely to mean a blow out at the surface. As well as removing the potential for pressure build up during excavation, the new down hole jet pump also maintains a very clean bore hole, which is designed to make subsequent pipe placement faster.
To date the traditional method of cleaning an HDD run has been using a barrel reamer, which is pulled through the borehole after the initial path has been opened with the pilot bore and a cutter head has made several passes through the bore to open it up to its final required diameter (see box). “The down hole jet pump works differently. Instead of pumping fluid through the borehole it acts as a down hole vacuum cleaner,” says Lubberger explaining that the rotation of the barrel and its unique shape causes cuttings to fall into the pump where they are then pulled inside the drill string and pumped out towards the mud cleaning equipment on the surface. It comes in a range of sizes between 20 and 70 inches. “The advantage is obviously that the cuttings can be much larger and the borehole will not be pressurized as you are not pushing mud through it anymore,” says Lubberger.
“With the conventional barrel cleaning run technology cuttings of up to 10mm can be transported if the mud properties are well aligned. But mostly you just get sand out of rock drills, the bigger cuttings will stay at the bottom of the borehole,” he says. However the new down hole jet pump can transport cuttings of 35mm.
Testing Times
As Lubberger points out the new jet pump has been tried and tested. In Malmö, Sweden, a Herrenknecht HK150C rig was used for a 260m drive through hard limestone with sections of flint. The contractor BAB Röryryckning AB needed a very clean borehole due to the tight dimensions with only a small annulus between the bored diameter and the 40inch (1m) HDPE pipeline that was to be placed as a protective pipe ahead of the final steel district heating pipework. Lubberger says that the contractor was surprised by the volume and diameter of the material that emerged. “Cuttings of up to 50mm in length were discharged. Larger cuttings had been crushed by the down hole jet pump into smaller pieces or pushed into softer sections of the borehole,” he says noting that the tool can also be set to flushing mode when crossing softer parts, which it encountered on both sides of the crossing. This avoids the grid of the pump clogging up,” he says.
Another innovation is that the down hole jet pump can be used directly behind a newly-developed cutter called the “full face hole opener”. The cutter reams from the initial pilot hole to the final diameter in a single step, which also has the advantage of maintaining the bore through the centre of the pilot hole.
One of the challenges for HDD operators making multiple passes to gradually increase a pilot bore diameter, is maintaining the centre line of a bore as an offset often develops between the axis of the drill string and the axis of the reamer heads. According to Herrenknecht its single pass full face hole opener reduces the tilting of the reaming head and therefore the potential for damaging bending loads on the drill rod.
Designed for rock the hole opener has larger bearings than traditional cutting tools allowing it to withstand up to twice as much pressure and therefore allowing greater penetration and it can cut through rock with a hardness of up to 350MPa. Herrenknecht points out that one of the biggest advantages of the hole opener is that by using a single cutting tool over a single pass major cost and time savings are gained.
The full face hole opener head itself can house a range of roller cutters including milled tooth (MT) bits in softer rock and tungsten carbide inserts (TCI) in harder ground. Both have been put to the test on the Flanagan South Pipeline project in the US where contractor Laney Directional Drilling of Houston, Texas, working for main contractor The Enbridge Company, used the HDD method with the full face hole opener for two river crossings. In the first location, near Independence in Kansas, the 36 inch (DN900) steel petroleum pipeline had to cross 536m beneath the Verdigris River. Boreholes dug at 40m depths either side of the river revealed alternating, thin, horizontal shale and limestone layers with varying strengths up to a maximum of 108 MPa. The alternating layers made it challenging to maintain the proper line and grade of the borehole for which the pilot bore was created with a 12.25in TCI Tricone drill bit in combination with a drill motor. After the pilot hole was drilled, reaming to the ultimate 48in (1219 mm) diameter was carried out by the 48in full face hole opener in just one step. It was equipped with a set of TCI roller cutters designed for very hard formations along with a 47in centring device, which was attached behind the hole opener and increased the concentricity of the reamer head.
Installation required low pull forces and was carried out without complications. The roller cutters were used for a total of 197 hours and were sent off the Herrenknecht’s Seattle facility for inspection. The body structure of the roller cutters and the tungsten carbide inserts showed little wear so the roller cutters were fitted with new seals and bearings to be on the safe side.
The reamer body structure also showed relatively little wear, and only in the area of the replaceable front centraliser was hard facing re-welded for the next borehole, which would also prolong the life of the front centraliser.
Crossing rivers
Further south the same pipeline needed to cross under the Arkansas River in Cleveland, Oklahoma. In this location the ground conditions were mainly sandstone with layers of shale and limestone. The rock strength was around 55MPa and the layers were moderately broken with an RQD index of 50-60 meaning that there was a moderate risk of an unstable borehole because rocks from the borehole wall may fall into the invert.
The presence of shale layers increased the risk of clogging the reamer head with material. One special feature of this borehole is that the reception side is about 130ft (40m) higher than the launch side. This means that for over a distance of about 755ft (230m) the borehole is dry. The horizontal layering of the different formations meant that the long horizontal part of the borehole could be run through the more stable sandstone as compared to the first project. With the better stability, the sandstone seemed the best layer to drill in. However, it was feared that the abrasiveness of the sandstone due to its high quartz content could cause high wear on the roller cutters and reamer body.
After the pilot hole was completed Laney’s own 3000kN HDD drilling rig was set up on the higher side and the 48in (1.2m) full face hole opener connected to the drill string on the lower side.
With the lower compressive strengths of the sandstone and limestone the decision was taken to use a set of milled tooth (MT) cutters on the hole opener, as the greater tooth protrusion of MT cutters of around three times more than tungsten carbide insert protrusion of the TCI cutters, means that in softer ground it should make faster progress, theoretically allowing more than two and a half times higher penetration per revolution.
Again the installation went smoothly and despite the abrasive section of sandstone, the wear on the body structure of the reamer head was only moderate. After the second project, however, the front and rear centring units needed to be replaced. The MT cutter bodies were worn out and could no longer be used for another project.
The river undercrossing has shown that the MT roller cutters work well and that the reamer head runs very concentrically down hole. It has also been found that the nozzles in the reamer head have an important role to play in reducing the clogging of the reamer head as the configuration of the 20 different nozzles was altered as the bore passed through sticky shale sections.
Tooling up
That the new tools are performing well is good news for Herrenknecht, which has given its HDD range a special focus over the past 18 months acknowledging the massive global requirements for renewing and installing new utilities.
Other utility placement methods developed by the firm include the Direct Pipe system which combines microtunnelling and its Pipe Thruster technology to place pipes of between 0.8m and 1.5m; its well known AVN machines used for pipejacking diameters of 0.4m to 4.2m; its auger boring machines and of course the full range of TBMs, which have been designed for tunnels of between 1.5m (Single Shield) up to potentially 19m (Mixshield and Multi-mode).
Although it is yet to be built designs for a 19m TBM are complete and development continues at the cutting edge of large diameter tunnelling reinforcing Lubberger’s point: development is indeed happening at both the small and large diameter ends of Herrenknecht’s tunnelling world.
