Upon identifying the loss of containment, the mainline valves of the DN250 pipeline were immediately closed and alternative gas lines were put in place for the Toowoomba city delivery station. Temporary pipework and compressor modifications were completed to provide continuity of supply to meet contracted customer demand.
The first crew was then immediately mobilised to begin the process of stabilising the pipelines and locating the site of the defect. A gas detector provided the crew with a general location of the defect. The first challenge was to stabilise the DN400 pipeline, which was situated above the defect and was now the only functioning pipeline delivering gas to Brisbane. This involved placing concrete bags to build a support under the exposed pipeline.
Once the DN400 was stabilised, attention could turn to removing the land mass over the DN250 defect site. This area was extremely unstable and required continuous geotechnical assessment and particular work methods to avoid a further landslip. Once the land mass was removed the defect was located and inspected. It was determined that the defect and areas immediately upstream and downstream from the defect needed to be removed to ensure both damaged and weakened pipe was removed.
On 26 January 2011, the DN250 pipeline was blown down between Oakey and Withcott to enable a cold cut of the defect. Cold cutting is where the gas is excavated from the pipe before the cut is performed. A field bend was constructed for the new pipeline section, the section was aligned and welded. Once the weld’s integrity had been certified, the pipeline was purged and re-pressurised incrementally, and gas leak surveys were conducted down the escarpment each time the pressure was increased.