When travelling on a 1,300-foot-high bridge on a mountain known in German as “Sollstedter Hölle,” which, in English, means “Sollstedt Hell,” one wonders whether this name was going to prove to be a good omen given the job at hand.
The drilling site for the project in question was being operated by boring contractor Beermann from Zeitz, Germany. The site was situated on the high plateau, approximately three miles from the nearest village. The forest lane that led to the site was just about accessible over the final 1,000 feet, with the contractor just being able to transport the boring equipment safely to the jobsite, including the truck. It took one whole working day to complete the jobsite setup.
Water to mix the drilling fluid for the bore was extracted from the small river known as the Wipper, which was four miles away. The water was pumped into tanks and transported to the jobsite using farm tractors.
The reason for the boring project was the need to renew an aging gas pipe over a length of more than a mile – from the Sollstedt Hölle to the gas-pressure regulation unit in the town of Sollstedt. Furthermore, the storage capacity to regulate the availability of gas quantities also was to be improved by this renewal, and the pressure step in the pipes was to be increased from 25 bar up to 70 bar.
The main contractor, E.ON Thüringer Energy from Erfurt, Germany, awarded the task of carrying out this construction project to Boyen & Doyen from Erfurt, which, in turn engaged Beermann Bohrtechnik GmbH of Riesenbeck, Germany, to carry out the HDD bore.
The HDD rig chosen to install the new bore was a GRUNDODRILL, Type 25 N, a unit which Beermann had been successfully applying on boring projects since 2008. The rig already had reached an impressive 3,800 operating hours prior to this work. Operator Lutz Dietze says, “The longest bore we had to complete was more than 1,500 feet on a project in Hamburg; the thickest pipe we have pulled in with the machine was a 20-inch-diameter long-distance energy pipe.”
In the area of a steep slope with a gradient of 78 percent, the gas pipe ran aboveground. This section was to be replaced by a parallel underground bore over a total length of approximately 500 feet. The distance between the HDD rig and the start of the steep slope was approximately 230 feet. This made an entrance angle of about 42 percent necessary to be able to meet precisely the planned bore exit point at the foot of the steep slope at the end of the continuous straight line bore. In comparison to most projects where the bore entrance angle usually is at 10 percent to 24 percent, this bore angle was very acute.
The ground conditions on the bore: Layers of partially weathered and partially very solid limestone rock, which often were clearly visible on the surface of the steep terrain. The pilot bore had to be prepared taking into account these ground conditions. A mud motor could not be applied, due to the very tight working conditions and the inclination of the bore. Therefore, the use of an aggressive boring head with special hard metal bore tips seemed to be the only solution. A depth sonde (with 90 feet of depth capacity), from specialist steering systems manufacturer DCI, was fitted for the monitoring and steering. The cover on the steep slope was almost 85 feet deep in parts. This caused the signal to be relatively weak, and, at times, it could only be described as marginal.
“This was a real challenge,” relates Beermann’s Kai Winkler. “However, with our many years of experience, and the application of the most modern boring technology, we were confident of carrying the bore out as planned. Also, the signal got stronger after the critical phase of the bore, confirming that we always were on course.” The pilot bore was completed precisely on target.
Two pilot bore expansion bores, with 10-inch and 14-inch-diameter reamers, followed relatively quickly, with the runs taking just five minutes per drill-rod length to pull through. The drilling fluid consumed was collected at the target point, and passed on to a second, deeper pit at the access road. From there, the drilling fluid was disposed of by use of a suction/vacuum transporter. The fluid consumption was considerable. After each expansion bore, the backreamer was pushed back through the borehole in order to compress and rinse out the bore spoils from inside the bore at its deepest point. This cleaning process was vital in order to prevent the product pipe getting stuck when it was finally pulled in. After the final cleaning process, the bore was thoroughly cleaned and made ready for installation of the product pipe.
Some 12 8-inch OD steel pipes (with a PE and reinforced glass fiber coating) were welded together, x-rayed and checked for tightness with water at 100 bar. Then the PE and reinforced glass fiber coating was reinstated at the welding point. Installed along with the gas pipe were four 2-inch-diameter HDPE pipes – pulled in to host the control cable, and also for the filling of the annular space, which is necessary after the installation.
A Bagela cable winch pulled the pipe string uphill to the borehole. The pipe installation process began as soon as the final backreamer had been connected. About 90 minutes later, Dietz announced the completion of the installation.
The bore annulus sealing was carried out the next day. To do this, the borehole was lagged at the bore exit point, giving the security that the seal would withstand all strains during the filling process. The filling of the annular space along the HDPE pipe gave the gas pipe a secure position and bedding, preventing corrosion of the outer pipe surface, and stopping the penetration of water into the annular space.
From the bore exit point, the pipeline installation proceeded using the open-trench method. A milling cutter already had excavated a four-foot-deep trench up to the point where it could no longer be applied.
All participants were impressed with the performance of the Beermann bore team. The company’s motto is “All Lights Set to Green,” and this proved to be a really environmentally sound and economical project. ND