‘Fionnuala’s’ boring journey nearing its end
18 March 2013
The Corrib gas project is one of the most significant engineering projects ever undertaken in Ireland. It is now in its final phase of construction, which involves laying the onshore pipeline from the landfall site at Glengad to the Bellanaboy Bridge Gas Terminal.
The onshore pipeline is 8.3km long and 4.9km of this will be installed in a tunnel, most of which will run under Sruwaddacon Bay, in north-west Mayo. When completed, the tunnel will be the longest tunnel in Ireland and the longest gas pipeline tunnel anywhere in Europe.
The tunnel will have an external diameter of 4.2m, an internal diameter of 3.5m and will run at depths of between 5.5m and 12m. The building of the tunnel requires the use of a large tunnel boring machine (TBM). Following a long tradition of naming TBMs, the Corrib TBM has been named ‘Fionnuala’ after one of the Children of Lir, one of the legends most closely associated with the Erris region.
The TBM commenced excavation of the tunnel in early January 2013. The entire tunnelling operation, which continues for 24 hours per day and seven days per week, is expected to take approximately 15 months to complete.
Excavation of the tunnel follows one direction, starting at a launch shaft in the townland of Aughoose and running to a reception site in Glengad, where the offshore pipeline reaches land. The compound at Aughoose contains all of the services and materials needed for the tunnelling process. The compound has been designed and constructed to minimise any environmental impact.
Paul Hughes is the tunnel construction manager for Shell E&P Ireland Limited and he is pleased with how the work is progressing.
“It’s a very challenging project, because of the huge logistical side to it, but it’s also very exciting,” said Hughes. “BAM/Wayss & Freytag are the joint venture contractor delivering the tunnel and they bring with them the combined expertise of tunnelling specialists from Germany, as well as Irish staff with vast experience of the construction industry here.
“There were some minor mechanical problems uncovered at the start of commissioning the TBM. This isn’t unusual in a project of this kind, but now things are working well and we’re on schedule with the work,” he added.
The TBM for the Corrib tunnel was designed and built in Germany. The single shield TBM is 140m long, weighs almost 500 tonnes and comprises 14 sections. The 28-tonne cutter head drills under the bay using a combination of cutter discs, scrapers and buckets and requires two 400kw motors to turn it.
As well as excavating the tunnel, the TBM also provides support by putting in place segments of tunnel lining as it moves along. The rotation of the cutting head breaks up the ground in front of the TBM, loose material is scooped up by openings in the cutter head and the debris/slurry mixture is then pumped back to the surface for treatment or disposal.
Once the tunnel is completed, the 20-inch (50cm) diameter pipeline will be installed, together with a number of service and umbilical control lines. The pipeline and services will be tested before the tunnel is backfilled with a grout mix. The finished pipeline will then be ready to transport gas from the offshore pipe to the Bellanaboy Bridge Gas Terminal.
Approximately 950 staff are currently employed on this final phase of the project. First gas is likely to flow at the end of 2014/early 2015, with the Corrib gas field expected to deliver up to 60% of Ireland’s natural gas needs at peak production.
The gas field was discovered in 1996 by Enterprise Energy Ireland Limited, which was subsequently acquired by Shell E&P Ireland Limited in 2002. The project comprises a number of sections, with the final section being the construction of an onshore pipeline that will connect an offshore pipeline to the gas terminal at Bellanaboy Bridge.
Shell E&P Ireland Limited is the operator of the project; the other partners are Statoil Exploration (Ireland) Limited and Vermilion Energy Ireland Limited.