In May 2014, workers at the Corrib project celebrated tunnel breakthrough as ‘Fionnuala’, the project’s 500 tonne tunnel-boring machine, inched her way into the Glengad reception shaft to see daylight after 17 months underground.
At 4.9km, Shell E&P Ireland Ltd had just overseen the construction of Ireland’s longest tunnel and Europe’s longest gas pipeline tunnel. Corrib project director Roeland Borsboom noted that “successfully building the tunnel in a Special Area of Conservation was a remarkable example of engineering excellence”. “At every stage,” he added, “the team has adopted measures to minimise risks as well as construction and environmental impacts on the local community.”
With tunnel construction completed and over 25,000 concrete rings in place to maintain its structural integrity under Sruwaddacon Bay, it was time to start the next phase of the tunnel operation, which was the installation of the onshore gas pipeline and associated services.
Installation and welding
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The banksman checks the pipe placement[/caption]
As the skilled team of tunnellers departed Corrib, a new team of welders took up residence within the tunnel. The gas pipeline sections were pre-fabricated above ground into strings of 72 metres. A practised choreography using four excavators slowly and carefully manoeuvred each long string of pipework, lowering them into the Aughoose reception pit and placing them on flatbed trollies ready for transportation by locomotive train into the tunnel. Inside the tunnel, the pipe was levered in to position and aligned in preparation for the welders to connect each string.
With regard to testing/commissioning and grouting, ultrasonic testing is completed at every weld and once the pipeline installation is completed, hydro-testing will take place. This will involve filling the pipe with water to enable testing at over 504 bar, five times its operating pressure. Installation of the 20-inch diameter gas pipeline, control umbilical and related services is expected to conclude in the coming weeks.
When all testing is completed, the tunnel will be completely sealed by backfilling with cement bentonite. The grouting operation is expected to be completed during February 2015.
The finished pipeline will then be ready to transport gas from the Corrib field to the Bellanaboy Bridge Gas Terminal for processing. Once all other construction elements of the development are finalised, it is expected that the Bellanaboy gas processing terminal will produce first gas in mid-2015. At peak production, the Corrib field has the potential to meet up to 60 per cent of Ireland’s gas needs.
Reinstatement
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Lowering the 72m pipeline into the tunnel reception shaft at Aughoose[/caption]
Returning the land back to what it once looked like is an important step for minimising construction impacts. As the project moves towards first gas, reinstatement work at all the construction sites is ongoing.
In advance of tunnelling, some 120,000 tonnes of peat land was temporarily removed from the Aughoose and Glengad compound areas and this will be reinstated once back-grouting of the tunnel is completed. At Glengad, the landfall valve installation will remain in place for the duration of operations, but all other construction-site infrastructure will be removed – thus minimising the long-term impacts on the environment.