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Trenchless Australasia – September 2013


In a project that saw the first use of horizontal directional drilling to pass below live processing facilities in Australia, Stockton Drilling Services were engaged the Longford Gas Plant to assist in the design and construction supervision of twelve horizontal directional drills for the Longford Gas Conditioning Project in Sale, Victoria.

THE DEVELOPMENT OF a new gas conditioning plant to the east of Longford Gas Plant’s existing processing facility required control cables and high voltage cables to be rerouted in and around the existing live crude oil and gas plants.


Stockton developed a strategy where the scope would be split into two distinct parts. The initial scope known as the Northern Alignment, would be packaged as seven drills to install conduits from manhole to manhole. These would be 125mm HDPE ducts and range in length from 150-270m. These works could then be undertaken by a small rig operator outside the main plant areas using the minimum required footprint.

The second scope, known as the Southern Alignment would be conducted from within the plant boundary with a drilled length of just over 1000m. The alignment would pass beneath existing infrastructure for Gas Plants 1, 2 and 3 and the Crude Stabilisation Plant. In total sixteen conduits for high voltage and earth cables would need to be fabricated and installed.

Stockton, along with Wood Group PSN developed the horizontal directional drilling (HDD) designs whilst Golders were engaged to carry out the geotechnical investigation. The design process commenced in late February and tenders were released just two months later. Contractors were then invited to bid on one or both scopes; contracts were awarded in July, with BTB Australia commencing the Northern Alignment in September 2012 and Geotech Drilling the Southern Alignment in November 2012.

A number of technical decisions were made by Stockton to optimise the number and size of bores required for the Southern Alignment. It was determined that a dedicated 280 mm diameter mud return line would be drilled first. This would remove the requirement for road tankers within the plant and allow guidance cables to be installed below ground for steering the remaining bores. Together, these measures would significantly reduce the impact on normal plant operations. The mud return line would also be a spare should any subsequent conduit be damaged during installation.

After considering whether the bores should be cased or uncased, it was determined that sixteen 160 mm diameter conduits should be installed uncased, grouped in four bundles of four. The conduits would be progressively ballasted with water during installation to minimise installation loads, to ensure the bundles remained in their crucial trefoil configuration. 


As part of the Northern Alignment, two of the parallel bores would pass beneath a service corridor containing numerous existing underground services, including the main incoming gas pipelines. The congested nature of the processing plant required ground penetrating radar surveys to be conducted at all entry and exit points to check for unmarked services. All services detected were then exposed and confirmed by potholing. By adopting HDD, no exposed trenches were dug in public spaces, including car parks, and it ensured separation was achieved to all known and potentially unknown services.

For the Southern Alignment, HDD was the obvious solution as it provided a quicker, cheaper, safer and more manageable option. An open trench through the existing infrastructure would introduce many ignition sources along the full alignment of the works.


Stockton utilised its state of the art inertia gyro to progressively map the installed pipelines. It is thought to be the first time this type of gyro has been used in Australia. The Stockton-conducted gyro surveys of each installed bundle provided third party verification of the exact position of each conduit. By conducting an independent gyro survey, the pipeline owner can mitigate potential future risk by confirming the exact position and bend radius of the installed pipe.


One of the project’s highest risks identified was the potential impacts from frac-out. Having mud returns come to surface within the pipe racks, service corridors and infrastructure of the existing plant would have caused major safety and operational concerns. This risk had to actively managed during design and construction to be reduced to as low as reasonably practicable. Hydraulic fracture modelling was undertaken for numerous bore profiles. This allowed for the optimum depth to be determined along with the preferred bottom hole assembly (BHA) configuration, pump rates and mud rheology.

During construction a pressure sub was included in the BHA to monitor annular pressure gradients and a qualified mud engineer was on both shifts to maintain the fluid design parameters. These safeguards and other including surface entry casing ensured all five bores were drilled without any incidents of frac-out.


By May 2013, all twelve bores housing the 24 conduits were successfully installed without incident and with minimal disruption to Longford operations. The Longford Gas Plant, who had previously never employed HDD, have since seen the benefits of trenchless operations and as a result BTB Australia will be returning later this year to install two further drills for raw water and stormwater pipeline upgrades. Stockton supervisors will again be employed on-site to provide the construction oversight.

Contact Us

Charles Stockton
Managing Director/HDD Specialist

+61 (0) 400 623 441
158B Gaudrons Road Sapphire Beach NSW, 2450, Australia

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