GLNG Narrows Crossing Design

Benefits

The Gladstone Liquefied Natural Gas (GLNG) project utilised specialist expertise from Atteris to successfully and cost-effectively deliver a major waterway crossing for a 42-inch gas transmission pipeline. This project has set a benchmark for the industry, with the same engineering concept now being used globally for major waterway crossings in onshore pipeline routes.

Background

GLNG incorporates the development of coal seam gas resources in the Surat and Bowen Basins in south-eastern Queensland, Australia, a 420 km underground gas transmission pipeline and a two-train liquefaction and storage facility on Curtis Island, Gladstone, Queensland, Australia.

When planning the pipeline, GLNG faced a very challenging waterway crossing named “The Narrows”, between mainland Australia and Curtis Island.

Work Performed by Atteris

GLNG engaged Atteris to undertake the engineering design and support the construction phase for the Narrows Crossing. The selected concept for the crossing involved a 4.4 km long, 3.4 m diameter segmental lined concrete tunnel to serve as a conduit for the 42 inch diameter pipeline and accompanying fibre optic cable.

Atteris performed the following work on this project:

Concept development of the waterway and mudflat crossing, by assessing crossing concepts including open cut and cover, horizontal directional drilling and tunnelling; tunnelling was selected as the most cost-effective option for this crossing
Front End Engineering Design of the 4.4 km long and 3.4 m diameter concrete tunnel and 42-inch diameter gas transmission pipeline through the tunnel, including addressing complex engineering aspects, such as
- Pipeline mechanical engineering design
- Deciding whether to water-fill the tunnel or not
- Pipeline installation engineering
- Design of tie-ins to overland pipeline
Detailed design of pipeline, including CP system, tie-in spools; and provision of owners engineering to the GLNG team for construction engineering and on-site construction supervision (detailed design of the tunnel itself was performed by the tunnel contractor).
The spools design was optimised to ensure adequate allowance was made for the spool to flex under expansion, pressure and thermal loading, while taking into account the uncertainties related to the foundation stiffness, and minimising the spools footprint required.