Monday, September 26, 2022

Three Challenging Microtunnels Completed

The TransMed and GreenStream pipeline project, currently under construction, will transport gas from Algeria and Libya to Europe

The TransMed and GreenStream pipeline project, currently under construction, will transport gas from Algeria and Libya to Europe, via Sicily. As part of the project, a new gas pipeline was required to be installed using microtunnelling techniques in the Sicilian town of Nicosia, Italy.

To maximise the accuracy of the drive it was decided to utilise VMT’s SLS-Microtunnelling LT as the navigation system for the Herrenknecht-manufactured machine. The contractor for the works was Ing. La Falce SRL, with this project being the first time this contractor had utilised VMT navigation equipment.

With all three drives on the project utilising VMT guidance systems, the third and final drive of 308 m length broke through on 16 September 2021. The first drive of 375 m length commenced on 1 May 2021, with breakthrough successfully completed just 27 days later on 28 May 2021. The second drive at 581 m length commenced on 14 June 2021, breaking through on 22 July 2021

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Challenging Route

The drive profile of the 1.6 m diameter pipeline had a challenging vertical curve of radius 2,000 m, with Drives 1 and 2 having steep inclines at the reception shaft end of the drives (+249 mm/m on Drive 1, +213 mm/m on Drive 2). Tunnelling was made particularly difficult by the steep inclination and hard rock surface.

An environment of hard rock, sometimes combined with other materials, means that the tunnelling machines operate under high pressure and high contact force conditions. Steep inclinations cause the squeezing of the pipes between two forces in that whilst the machine attempts to push pipes up from the start shaft, gravity pulls them back down.

In this case, this combination of conditions generated micro-stresses in different parts of the pipe string which were occasionally released. The resulting vibrations caused drastic and uneven back-and-forth movement of the pipe string, with the jackhammer-like movements increasing with each metre of advance of the drives. To address these conditions, reinforcement was made on the TCA bracket. Although the VMT system still faced significant vibrations, the stability of the system was greatly improved, and excavation was able to continue as planned.

In addition, advance surveying works were required to get a precise location of the pipe axis in elevation. Here, additional preparation and accurate flow during the surveys contributed to the successful system performance.

Drive 3 presented its own challenges, demonstrating the variability that can often be experienced in underground conditions. Whereas the ground material of the first half of the drive consisted mostly of quartz, an abrasive material which can grind on the cutting wheel, the second half of the drive consisted mostly of clay, causing the periodic stalling of the machine.

There were congratulations all-round for the VMT’s customer, partners and the project engineers involved in the successful completion of these challenging routes.

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