The Mont-Ogenne Project forms part of the renewal of the DN650 Mont-Ogenne-Camptort pipeline, known as “Le Lacal”, a gas transmission infrastructure owned by TEREGA. Following the detection of defects in the coating of the existing pipeline, TEREGA proposed the reconstruction of the first 8.7 kilometres of the line, from the Mont compression station to Vielleségure.

For DENYS, we carried out the Horizontal Directional Drilling (HDD) works required to cross beneath the Gave de Pau, one of the most challenging sections of the route, which required a specifically engineered solution. One of the project’s key features was the crossing of the Gave de Pau River, for which HDD was selected as the preferred installation method.

During the design phase, we assessed the possibility of carrying out a single drilling operation to install both pipelines. However, this option was ruled out due to its higher cost and the additional risks associated with pullback operations, and we ultimately opted for two independent parallel drillings.

We completed the two HDD drillings, each with a total drilled length of 464 metres.

Drilling works specifications:

🔵 2 HDD drillings of 464 m each: 1 × HDPE Ø160 mm PN16 and 1 × DN650 steel gas transmission pipeline.
📏 Horizontal length: 461 m per drilling.
🎯 Pilot bore: Ø251 mm (9 7/8″).
🔩 HDD casing 1: Ø508 × 10 mm, 32 m long.
🔩 HDD casing 2: Ø1,220 × 19.1 mm, 32 m long.
⚙️ Final diameter for the DN650 pipeline: Ø914.4 mm (36″).

For the drilling intended for the DN650 gas pipeline, we carried out three reaming stages, progressively increasing the bore diameter to Ø508 mm (20″), Ø711.2 mm (28″) and Ø914.4 mm (36″).

Every project presents unique challenges and, in this case, one of the main difficulties was linked to the ground conditions and hydrogeological characteristics of the Gave de Pau area.

Geological investigations identified ground comprising:

🟤 Silts
🪨 Gravels
🟡 Sands
⛰️ Low-permeability tertiary molasse bedrock
💧 Groundwater level: approximately 2 m average depth

These geological and hydrogeological conditions were compounded by the environmental sensitivity of certain sections of the project area, including:

💧 Wetland areas
🌊 Flood-prone zones
⚠️ Contaminated land

Before defining the final drilling alignment, we carried out additional topographical and ground-penetrating radar (GPR) surveys, together with a review of the available geological information, in order to identify existing utilities and gather the information required for construction.

To meet the project’s technical specifications, we designed both crossings using the Intersect methodology, working simultaneously from each end of the alignment.

To achieve this, we deployed two drilling rigs, one on each bank of the Gave de Pau:

⚙️ 200-tonne drilling rig, equipped with API 6 5/8″ FH drill pipe.
⚙️ 50-tonne drilling rig, equipped with 4½” IF drill pipe.

Pilot bores were drilled from both ends until reaching a meeting point located in the central section of the alignment. At this stage, the intersection of the two pilot bores was completed by introducing the 8½” drilling head into the 9 7/8″ bore drilled by the 200-tonne rig.

This methodology was applied to both crossings, beginning with the Ø160 mm HDPE conduit and subsequently the DN650 gas pipeline crossing.

The alignment design and the use of the Intersect method enabled us to undertake the Gave de Pau crossing from both banks simultaneously, coordinating the progress of the two rigs while maintaining precise control of the bore path throughout pilot drilling operations.

Once the pilot bores had been completed using the Intersect methodology, we commenced the reaming operations required to achieve the final diameters specified for each installation. Throughout the process, bentonite-based drilling fluids and recycling systems were used, with dedicated areas established on site for the collection and management of drilling muds and spoil.

For the DN650 gas pipeline crossing, successive reaming passes were carried out until reaching a final bore diameter of Ø914.4 mm (36″). After conditioning and cleaning the borehole, we completed the pullback of the DN650 steel pipeline using the 200-tonne rig.

The installation of the Ø160 mm HDPE conduit was then completed using the 50-tonne rig. As part of the final quality control process, a gyroscope survey was carried out inside the installed pipelines to verify the as-built alignment.

This completed all stages of the process, including pilot drilling, reaming, bore cleaning and pipeline installation, while maintaining control over both the alignment and construction conditions throughout the crossing beneath the Gave de Pau.

Selecting the most appropriate trenchless solution is essential to anticipate and overcome the constraints and challenges that may arise throughout a project.

In this case, Horizontal Directional Drilling (HDD) enabled us to address the specific requirements of the Mont-Ogenne gas pipeline renewal through two 464-metre underground crossings beneath the Gave de Pau, avoiding the need for open-cut excavation within the river channel.

This solution allowed us to complete the works while minimising direct intervention in the surrounding environment and adapting the construction methodology to the geological, hydrogeological and environmental constraints present on site.

As a result, we successfully installed the new DN650 gas pipeline and the Ø160 mm HDPE conduit, demonstrating once again how trenchless technologies can deliver technically complex crossings while reducing environmental impact.