Vol 21 No 5 (04-2012)
Horizontal Directional Drilling bore through a steep slope to renew an ageing gas pipe over a length of 2 km

With a need to renew an ageing gas pipe over a length of 2 km from a mountain to the gas pressure regulation unit at the town of Sollstedt, a Horizontal Directional Drilling project (HDD) was completed in Germany. In the project, the storage capacity to regulate the availability of gas quantities was also improved by this renewal and the pressure step in the pipes was increased from 25 bar up to 70 bar (PN 70).

The drilling site, a mountain known in German as "Sollstedter Hölle", which in English means “Sollstedt Hell”, for the project in question was operated by boring contractor Beermann from Zeitz, in Germany. The site was situated on the high plateau approximately 5 km from the nearest village. The forest lane which led to the site was just about accessible over the final 300 m, with the contractor just managed to transport the boring equipment safely to the jobsite, including the truck. It took one whole working day to complete the jobsite set up.

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 had already reached 3,800 operating hours prior to this work. Operator said: “The longest bore we had to complete was over 480 m on a project in Hamburg; the thickest pipe we have pulled in with the machine was a 500mm diameter long distance energy pipe".

In the area of a steep slope with a gradient of 78%, the gas pipe ran above ground. This section was replaced by a parallel underground bore over a total length of approximately 150 m. The ground conditions on the bore path comprised layers of partially weathered and partially very solid limestone rock, up to soil classification 7, which was often clearly visible on the surface of the steep terrain. The distance between the HDD rig and the start of the steep slope was approximately 70 m. This made an entrance angle of about 42 % 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 is usually at 10% to 24%, this bore angle was very acute.

The pilot bore had been 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 28 m depth capacity), from specialist steering systems manufacturer DCI was fitted for the monitoring and steering. The cover on the steep slope was almost 25 m 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. 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 were always on course”, said Beerman. The pilot bore was completed precisely on target.

Two pilot bore expansion bores, with 250 mm and 350 mm diameter reamers, followed relatively quickly with the runs taking just 5 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 a considerable at 150 L per running bore metre. Water to mix the drilling fluid for the bore, was extracted from the small river known as the Wipper, which was 6 km away. The water was pumped into 10m3 tanks and transported to the jobsite using farm tractors.

After each expansion bore the backreamer was pushed back through the bore hole 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 cleaned thoroughly and made ready for installing of the product pipe.

Some twelve steel pipes of outer diameter of 200 mm, with a PE and reinforced fibre glass coating, were welded together, x-rayed and checked for tightness with water at 100 bar. Then the PE and reinforced fibre glass coating was re-instated at the welding point. Installed along with the gas pipe, were four HDPE pipes of 50 mm diameter and 4.6 mm wall thickness were 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 bore hole. The pipe installation process began as soon as the final backreamer had been connected. The weight of the pipe string was calculated to be 5.7 t. The tensile load was an average of 9 t, well below the reserve load of 25 t available to the boring rig. About an hour and a half later the operator announced the completion of the installation.

The bore annulus sealing was carried out the next day. To do this, the bore hole was lagged at the bore exit point thus giving the security that the seal would withstand all strains during the filling process. The filling of the annular space along the HDPE pipe using the filling mixture “Drillmix” 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 had already excavated a 1.3 m deep trench up to the point where it could no longer be applied. All participants were impressed with the environmentally sound and economical project.

Courtesy acknowledgement: TRACTO-TECHNIK GmbH & Co. KG for providing the above information and photos.