Innovative use of microtunnelling on Chevron Australia’s Wheatstone Project smashes industry records and minimises environmental effects.
The multi-billion-dollar Wheatstone Project is a flagship project in Australia’s liquefied natural gas (LNG) industry.
Developed following an extensive period of environmental assessment and community engagement – including liaising with local government, independent experts and local communities – the project’s planning and construction is being carried out with explicit attention to minimising impacts to the surrounding environment and native wildlife.
Recognising these important values, Chevron commissioned a technically impressive microtunnelling campaign at the project’s shore crossing – connecting the onshore and offshore segments of the development via a 1.2 km pipe jacked tunnel.
Completed in only eight months, the project achieved many milestones and sets a new standard for Trenchless Technology in the oil and gas industry.
Located near Onslow in the Pilbara Region, the Chevron-operated Wheatstone Project is now in its fourth year of construction.
At almost 60 per cent complete, first gas is expected by late-2016. Wheatstone’s onshore LNG and domestic gas plants will be fed from four offshore natural gas fields.
Gas will be gathered using subsea wells and manifolds and transported via flowlines to the offshore Wheatstone Platform.
The platform will partially treat gas and condensate and transport it to onshore processing facilities via the newly-installed 225 km trunkline (pipeline).
The 112 cm diameter trunkline, Australia’s largest diameter gas pipeline, passes through a 1.2 km long microtunnelled and pipe jacked shore crossing.
Called the “critical link” by a Chevron spokesperson, the use of microtunnelling provided an innovative and cost-effective solution to cross the shoreline by tunnelling under the coastal environment to reduce impacts.
The shore crossing begins behind the coastal dune system, extends seaward and terminates under the ocean floor.
For the crossing’s design and installation, Chevron tapped Australian companies including Perth-based subsea and pipeline consultancy Atteris for the concept development and front-end engineering design (tunnel), and Thiess Tunnelling – now Leighton Tunnelling – for the detailed design and construction of the tunnel.
Atteris was also involved with providing engineering support throughout the execution phase of the shore crossing, as well as for the trunkline and flowline phases elsewhere in the Wheatstone Project.
Upon completion at the end of 2013, the shore crossing set new benchmarks in Australia,. It is both the first microtunnelled shore crossing in the oil and gas industry and the longest pipe-jacked tunnel in the Southern Hemisphere to date.
The project had a very short duration of eight months from mobilisation to demobilisation, with daily advance rates of 55 m achieved in two ten-hour shifts.
“The microtunnelling method had never been used as a conduit across a shoreline to pull a trunkline through within Chevron, or indeed within the oil and gas industry in Australasia,” said the Chevron spokesperson.
Deciding on microtunnelling
During the early stages of the Wheatstone Project’s assessment process, a number of options were considered for the trunkline shore crossing, including open trenching, rock groyning, above ground shore-pulling, horizontal directional drilling (HDD) and microtunnelling.
“For each shore crossing option, senior Chevron decision-makers carefully considered Chevron’s key commitment to reduce potential impacts to the environment alongside the logistical, cost, schedule and technical considerations,” said the Chevron spokesperson.
Examples of such considerations included the size of the Wheatstone trunkline (44 inch), as well as the very shallow water depth along the Onslow nearshore area (a pipelay barge was unable to come within several kilometres from the beach, for example).
These factors steered Chevron to pursue microtunnelling for the shore crossing.
Microtunnelling was favoured over HDD because, compared to HDD, the technique was deemed less sensitive to difficult geotechnical conditions.
Other potential major HDD risks included the very large diameter open borehole (in combination with the length of an HDD), the possibility of borehole collapse, the volume of drilling fluid release and a complex interaction with offshore construction equipment.
“Based on these circumstances, microtunnelling was recognised as an innovative solution and the senior management team selected it as the preferred option,” said the company spokesperson.
The microtunnelling methodology for the shore crossing delivered a number of environmental benefits without compromising safety or constructability.
These benefits included avoiding a physical surface presence, reducing the need to access mangroves or beaches, greatly reducing both onshore and marine disturbance footprints, avoiding potential sediment disturbance activities in the highly dynamic and sensitive Ashburton Delta, removing the need for machinery on the surface of the shore crossing, and reducing risk associated with leaks and spills.
“In selecting the method for the shore crossing, the primary environmental considerations were the potential impacts on the Ashburton River Delta and its habitats, and the potential impact on coastal processes,” commented the Chevron spokesperson.
With the trunkline needing to cross the shoreline at the lagoon system at the eastern entrance to the Ashburton River, the decision to use microtunnelling technology was based on extensive research showing the environmental objectives of protecting coastal processes, the creek system, mangrove habitats and listed species could be achieved.
“As evidenced by the environmental monitoring results pre- and post-construction, the decision to tunnel under this area significantly reduced potential impacts to this environment, while still providing a solution that met Wheatstone requirements,” continued the spokesperson.
“Ultimately, the decision to construct a microtunnel underneath the sensitive chenier and lagoon system demonstrated excellent environmental outcomes.”
The concept and front-end engineering design of the shore crossing tunnel was performed from 2010-2011 by a combined Chevron/Atteris team.
Chevron subsequently awarded the microtunnelling contract to Thiess Tunnelling (Leighton Tunnelling) in January 2012, with work beginning onsite in April 2013. The tunnel was completed in October 2013 and the site was demobilised two months later in December.
On the overall Wheatstone Project timeline, the shore crossing tunnel was one of the earliest scope items for the project because it needed to be installed prior to the trunkline installation and during the weather window of May-November 2013.
According to Chevron, missing this 2013 weather window would have pushed the trunkline installation out by one year, which could have impacted the project time schedule.
The microtunnel execution team, led by Thiess Tunnelling (Leighton Tunnelling) used an innovative tunnelling method and shaft design which allowed the team to pipe-jack three pipes in a complete push, reducing the number of times service pipes had to be manually handled and improving cycle times – something relatively uncommon in pipe-jacking.
Additionally, by utilising the latest and most advanced microtunnelling guidance system, the tunnelling team was able to drive the tunnel to within 135 mm of the design location.
“This was an excellent result considering the specification requirements were to finish within 1,000 mm,” said the Chevron spokesperson.
Thiess Tunnelling (Leighton Tunnelling) also designed a special lifting mechanism for the rotation of the concrete pipes and the movement of the jacking pipes around the project site.
“The rotation of the concrete pipes was a safety risk that required an engineering solution,” continued the spokesperson.
“Together with the supplier, Thiess Tunnelling designed a clamp that meant that the concrete pipes could be rotated in a controlled manner without the use of multiple cranes.”
For the movement around site, a specific set of forks and lever was designed.
This allowed the concrete pipes to be moved around the site easily and be picked up and placed in a controlled manner, thus reducing the risk of working at heights and men next to moving equipment.
Following the tunnelling procedure, the team formed a partnership with Independent Offshore Solutions (IOS) to develop a new methodology for recovering the Herrenknecht-supplied tunnel boring machine (TBM) from the ocean floor. According to a Herrenknecht spokesperson, recovering TBMs from oceans is difficult due to their size, with the feat only achieved about a dozen times in the world.
A watershed trenchless project
“Completed within a short turnaround and with surgical precision, the scope and ambition of the Wheatstone Project’s shore crossing microtunnelling work illustrates what can be achieved when a collaborative approach brings together the community, government and industry to work on innovative and mutually beneficial solutions,” concluded the Chevron spokesperson.
“The innovative application of microtunnelling technology demonstrated that with the appropriate level of commitment, passion and responsibility, we can safely access and develop much needed energy resources while minimising environmental impacts and preserving important habitats.”
“The development of the entire shore crossing concept, including selection of microtunnelling as a method, defining the tunnel entry/exit points and alignment, design of the subsea exit pit, the pipeline within the tunnel, and the pipeline cathodic protection, are all ground-breaking innovations in their own right.”
Looking ahead, microtunnelling for shore crossings of this scale – and in such delicate environments – is now a proven technology and could be implemented globally, both by Chevron and the wider oil and gas industry.
Key Wheatstone shore crossing facts
Total tunnel length: 1,242 m
Tunnel pipe material: Reinforced concrete
External tunnel pipe diameter: 2.5 m
Internal tunnel pipe diameter: 2 m
Tunnel pipe segment length: 3 m
Number of tunnel pipe types: 6
Tunnel pipe segment weight: 12-16 tonnes