The boring project faced by Tracto Technik (TT) sought to renew an ageing gas pipe over a length of 2 km from the ‘Sollstedt Hell’ to the gas pressure regulation unit at the town of Sollstedt. The project also aimed to improve the storage capacity to regulate the availability of gas quantities by renewing and increasing the pressure step in the pipes from 25 bar up to 70 bar (PN 70).

Main Contractor E.ON Thüringer Energy (ETE), awarded the task of carrying out the construction component of the project to Boyen and Doyen, who in turn engaged Beermann Bohrtechnik GmbH of Riesenbeck to carry out the horizontal directional drilling (HDD) bore.

The HDD rig chosen to install the new bore was a Grundodrill, Type 25 N, manufactured by Tracto-Technik of Lennestadt, Germany, which is a unit that Beermann has been successfully using on boring projects since 2008.

A remote challenge

The first challenge faced by boring contractor Beermann was the drilling site for the project, which was situated on the high plateau, 5 km from the nearest village. The forest lane which led to the site was only just accessible over the final 300 m, providing a challenge for the contractor when transporting the boring equipment and truck safely to the jobsite. It took a whole working day to complete the jobsite setup.

Water to mix the drilling fluid for the bore was extracted from a small river known as the Wipper, which was 6 km away.

The gas pipe ran above ground in an area of steep slope with a gradient of 78 per cent, and needed to be replaced by a parallel underground bore over a total length of approximately 150 m. The distance between the HDD rig and the start of the slope was approximately 70 m. This made an entrance angle of about 42 per cent necessary to be able to meet with precision the planned bore exit point, which was at the foot of the steep slope at the end of the continuous straight line bore.

The ground conditions on the bore path comprised layers of both weathered and very solid limestone rock – up to soil classification seven. The pilot bore had to be prepared taking into account these ground conditions.

A mud motor could not be applied due to 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 was only marginal.

Boring through

Two pilot expansion bores, with 250 mm and 350 mm diameter reamers, followed relatively quickly with the runs taking just five minutes per drill rod length to pull through. The consumed drilling fluid 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 a suction/vacuum transporter. The fluid consumption was considerable at 150 L per running bore metre. 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, with an outer diameter of 200 mm, and 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 were reinstated at the welding point.

Four 500 mm diameter HDPE pipes installed along with the gas pipe, with a 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. It took around one and a half hours to complete the installation.

Sealing the deal

The bore annulus sealing was carried out the next day. To do this, the bore hole was lagged at the bore exit point to give the seal the security to withstand the strains of 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 had already excavated a 1.3 m deep trench up to the point where another method was needed.

All participants were impressed with the performance of the Beermann bore team.

According to the company motto ‘All lights set to green’, this proved to be an environmentally sound and economical project.