The relocation of the AGL gas network pipeline, the primary gas feed to the northern suburbs of Sydney, was required when the Road Traffic Authority (RTA) announced plans to widen the bridge as part of the Lane Cove Tunnel Project.

Several options, including reinstating the pipeline infrastructure on the new expanded bridge were considered with a 650 m horizontal directional drill eventually selected as the preferred method.

Agility Project Engineer Lloyd Edmunds said the HDD option offered a unique method to relocate 750 m of the 250 DN pipeline with minimal impact on the surrounding area.

“The HDD solution was more economically viable than other solutions, and offered less risk in terms of potential environmental impacts and service disruptions,” said Mr Edmunds.

Article continues below…

“We were dealing with an old river bed that has deep alluvial deposits, and drilling into it could have resulted in an environmental incident.”

Agility engineers inserted the drill into the eastern bank of the river, under 650 m of water before emerging on the western shore. The process has been likened to the non-invasive ‘key-hole’ surgery common in the medical profession.

The HDD required extensive front-end engineering and geotechnical assessments. Contractor AJ Lucas Drilling drew on past experience with complex HDD’s and engineering support, working closely with Agility to ensure a smooth operation.

Mr Edmunds said “AJ Lucas assisted in ensuring the bore path, pipe and coating were designed to withstand stresses from the installation, in-situ stresses from the minimum bend radius of the bore path of 210 m, the operating pressure of 3,500 kPa and the combined residual stresses. Conventional trenching would have been more disruptive, generating dust and noise pollution. It is also space demanding and requires more plant and equipment.

“Instead, the project construction zone involved only two compact sites: the drill rig site including mud pumps and recycling facility on the eastern side of the river, and the pipe string and welding area on the western side of the river.”

The drilling bore path was designed with extra depth and curvature to reach the lower sandstone layer and avoid the alluvium, located directly under the river bed. The extra depth also reduced the risk of what is known as ‘frac-out’ of bentonite during drilling operations.

The HDD also minimised impacts on local traffic in an area surrounded by major arterial roads. “In using HDD we avoided closure of nearby Epping Road, a major arterial with 75,000 car movements a day. Conventional methods would have resulted in continual disruptions, including closures and delays, to this road over the life of the project,” said Mr Edmunds.

The project, now fully operational, took ten weeks to complete and was delivered without environmental incident and on time and on budget.