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Installation of a gas pipeline through rock in Northern Ireland

10.01.19 - In Crossgar, a small town about 25 km south of Belfast in Northern Ireland, the staff of Bóthar Drilling Ltd, St. Mullins, faced a major task at the end of 2017: As subcontractors of McNicholas Construction, Elstree, they were to install two ND 125 (SDR 11) gas pipelines with a total length of 185 m, using the HDD method. The client was the gas distribution company Phoenix Natural Gas from Belfast. Not a big deal at first glance - but a real challenge at second glance due to the prevailing soil conditions. In addition, a river had to be undercrossed along each bore path. Other companies had already failed in similar construction projects near the planned HDD bores and were unable to successfully complete the bores they had begun. A soil survey had come to the conclusion that a glacial deposit with massive bedrock was to be expected at a depth of approx. 2.5 m for both HDD operations. And that was not all: below this rock layer lies slate-like sandstone. In order to be able to successfully execute the bores, Bóthar Drilling immediately obtained support from Tracto-Technik UK Ltd. After weighing up all the marginal conditions, the HDD experts decided on the so-called "King of Rock", the Grundodrill18ACS, which is especially suitable for drilling in alternating soils and solid rocks.

The most difficult conditions

The route of the first bore with a length of around 105 m ran along a country road with a single lane in both directions. When planning the drilling profile, a minimum depth of cover had to be maintained with regard to the depth position for undercrossing the river. According to plan, the launch and target pits were positioned within the area of the roadway, therefore one lane of the road was shut down for the construction work and traffic was controlled with a traffic light system.

After the Grundodrill18ACS was unloaded from the flat bed trailer, the experts from TRACTO-TECHNIK UK directed it by remote control to the desired position in the immediate vicinity of the previously created starting pit. The Bentonite mixing system was transported by the truck which had pulled the flat bed trailer with the drill rig to the job site. Since the river water had a pH value of seven, it could be used directly for producing the drilling fluid.

The Grundodrill18ACS was equipped with a bit for drilling through the bedrock. This was initially intended for drilling down to horizon beneath the rock layer. After 66 m (22 rods of 3 m each), the drill head reached the softer ground. Now the drill head had to be replaced. For this purpose, Tracto-Technik employees gradually pulled the drill rods back into the starting pit and disassembled them. The hard drill head (HDH drill head, type 1) was used for the remaining hole. After 108 m (36 rods), this reached the target pit with pinpoint accuracy and was replaced there by the Ø 160 mm backreamer, which pulled in the HDPE gas pipe while simultaneously expanding the borehole. Overall, the gas pipeline was successfully installed within two days and a net drilling time of slighly less than 15 hours.

Success all along the line

It was now clear to everyone involved that the second bore which also included a river crossing, should be carried out with the Grundo-drill18ACS as well. This approx. 80 m long bore was completed within the record time of less than 12 hours. All in all, the Grundodrill18ACS could press home its advantage convincingly, where other HDD systems had to struggle with problems so far.
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