Sea Outfall Saint-Jean-Cap-Ferrat realized under limited space conditions

The community of Saint-Jean-Cap-Ferrat on the Côte d’Azur between Nice and Monaco extended the 50-year-old sewage system with a new 110m long pipeline. The entry point for the Horizontal Directional Drilling operation was located at the “Pointe de la Causinière“, which was situated directly next to a narrow coastal road at the foot of a hill topped with luxury mansions. Jobsite setup required maximum precision of the team of the French construction company RPS – and accuracy of the mobile Herrenknecht Crawler Rig HK150C (thrust/pullback force 1,500kN, max. torque 70kNm) in the narrow and winding alleys of the coastal town. Navigating the Herrenknecht Crawler Rig to the jobsite was a precision job. The rig was brought in pilot-bore position in April 2007 to build the pipeline from ashore into the sea.

At the “Pointe de la Causinière“, the seafloor is covered with corals. For environmental reasons, the cuttings therefore could not be removed with bentonite. Instead, they were ejected with compressed air. The geology – first, brittle and porous folded rock consisting of clay and limestone, and then weathered rock and sand formations – represented a challenge for both the contractor RPS and the drilling technology. The first 10 meters of the bore were lined with concrete to avoid settlement and make cuttings removal with compressed air more efficient.

The planned pipeline alignment with a steep entry angle of 22 degrees could be handled without problems by the Herrenknecht rig and the jobsite installation that had been specially adapted. In HDD drilling, the drill bits are changed after pilot bore to expand the drill hole to its final diameter. A tricone drill bit (Ø 200 mm) was exchanged with a hole opener (Ø 864 mm), a hard rock drill bit, to expand the pilot hole at a depth of 36 meters below sea level. The disassembled drill bit then floated to the surface with an airlift bag. The hole opener was lowered into the sea in the same way, where divers connected it to the drill string. The Herrenknecht Crawler Rig HK150C with a thrust and pullback force of 150t then expanded the 110m long drill hole to its final diameter of 864mm. After pullback of the product pipe, the construction works were completed on July 4th, 2007. A challenging Sea Outfall project, characterized by limited space conditions, a steep entry angle, and unstable geology, was successfully completed by the construction team using the flexible and efficient Herrenknecht Crawler Rig.

Contact information:
Herrenknecht AG
Head of Corporate Communications
Achim Kühn
Schlehenweg 2
77963 Schwanau
Phone: +49 (0) 7824 302-540
Fax: +49 (0) 7824 302-473
eMail: pr@herrenknecht.de
Web: http://www.herrenknecht.com