A look at the final stages of construction of Corrib Gas pipeline tunnel

Finalising works on the Corrib Gas Pipeline Tunnel, under Sruwaddacon Bay in north-west Mayo, marked the completion of the longest tunnel in Ireland and the longest utility tunnel in Europe. Some 3.5m internal diameter, 4.2m outer diameter and measuring 4.9km it is approximately 300m longer than the Dublin Port Tunnel. The tunnel is part of the Corrib Gas Project, which is now in its final phase of construction. The Corrib gas field was discovered in 1996 by Enterprise Oil which established Enterprise Energy Ireland Ltd to bring the Corrib field to production. Enterprise Oil was acquired by the Shell group in 2002 and Enterprise Energy Ireland Ltd changed name to Shell E&P Ireland Ltd. The Corrib Gas Partners are Shell E&P Ireland Ltd, Statoil Exploration Ltd and Vermilion Energy. The gas field contains an estimated one trillion cubic feet of gas, making it Ireland’s largest gas find after Kinsale Head (1.5 trillion cubic feet). At peak production it has the potential to meet up to 60 per cent of Ireland’s gas needs for the next 15 to 20 years. The Corrib development has four distinct parts:

The offshore wells and subsea facilities
The offshore pipeline from the wells to the landfall at Glengad
The onshore pipeline between landfall and the gas terminal (which includes the Corrib Tunnel under Sruwaddacon Bay)
The Bellanaboy Bridge Gas terminal

The 4.9km long Corrib tunnel was built on behalf of Shell E&P Ireland Limited (SEPIL) by a joint venture between BAM Civil and Wayss & Freytag, both operating companies of Royal BAM Group. The tunnel was built using a slurry shield Tunnel Boring Machine (TBM) designed and manufactured by Herrenknecht AG, one of the largest manufactures of TBMs in the world. TBMs are usually christened and, in keeping with this long-established tradition, the Corrib TBM was named ‘Fionnuala’ after one of the Children of Lir, a legend closely associated with the Erris region. BAM Civil (formerly Ascon) is the largest civil engineering and public works contractor in Ireland while Wayss & Freytag, one of Germany’s largest construction companies, specialises in the construction and development of tunnels using TBMs. Founded in 1875, Wayss & Freytag has developed many of the techniques used today in tunnel construction worldwide.

Tunnel advance from one direction only

To facilitate the tunnelling works, temporary compounds were established in Aughoose, where the TBM commenced excavations, and in Glengad where the TBM was recovered from the reception shaft upon completion of the drive. Tunnel advance was from one direction only with the compound in Aughoose set up to accommodate all the necessary equipment and materials required for the tunnelling process. The compounds were designed and constructed to minimise the environmental impacts of the project, particularly on Sruwaddacon Bay, which is a designated conservation site, a candidate Special Area of Conservation (SAC) and proposed Special Protection Area (SPA). Peat removed during construction was stored on site for subsequent reinstatement works, buildings and equipment were limited in height and painted specific colours to reduce visual impact, acoustic fences were erected around the compounds to reduce noise emissions and all surface water was collected and treated on site prior to being discharged. The TBM, a mixed face slurry shield TBM, which was capable of boring through both hard and soft material, was designed and manufactured in Germany over a period of 12 months. During an extensive site investigation programme, it had been established that approximately 25 per cent of the tunnel drive would be through rock, with the remaining 75 per cent through sands and gravel. As such the TBM had to be capable of being able to deal with both these geological scenarios. Fionnuala, the Corrib TBM, consisted of a 28t cutter head connected to machine cans and shield with a further 14 individual trailers carrying the various support and backup services for the machine. The entire TBM weighed in excess of 500 tonnes and once fully installed measured more than 140m in length.

Cutter head used combination of cutting discs, scrapers and buckets

The cutter head at the front of the TBM used a combination of cutting discs, scrapers and buckets to advance through the rock, sand and gravel it encountered while advancing. The material, once excavated was pumped via a pressurised bentonite slurry circuit back to the surface using a series of pipes installed inside the tunnel. On the surface the excavated material was separated into various grading of material using two no. separation plants and filter presses and later disposed. A naturally occurring, inert clay called bentonite was used for lubrication and cooling of the cutter head and to assist in the transportation of the excavated material from the tunnel advance to the surface. As the TBM moved forward, a series of 1.2m wide concrete rings made up of five no. interlocking concrete segments and one keystone were erected. These concrete rings were installed inside the TBM shield and as the TBM moved forward they formed the lining of the tunnel. Once in place the void between the concrete ring and the excavated face was pressure grouted to ensure that there were no voids between the tunnel lining and the natural ground outside the tunnel. In total, more than 25,000 individual concrete segments, manufactured in Ireland by Shay Murtagh, were used to line the tunnel. Transport of personnel, equipment and material to the tunnel was done by rail with specially designed locomotives and rolling stock procured to suit the size of the tunnel. Due to the length of the tunnel, a ‘California Switch’ was installed at the midpoint which allowed the use of two trains inside at any one time. The TBM operated 24 hours a day, seven days a week and was manned by three interchanging crew of 20 operatives, half of whom worked underground and half of whom worked on the surface. The assembly of the TBM commenced on site in October 2012, with the machine being ready to launch in January 2013. Tunnelling works continued in earnest until completion in May 2014, when the TBM broke through into the reception shaft in Glengad. Following breakthrough, the TBM was dismantled in sections and removed off site.

Works on tunnel fit-out

Following removal of the TBM, works commenced on the tunnel fit-out, which involved the installation of 5km of 20” gas pipeline, 10km of 250mm dia. HDPE wastewater lines, 18km of 180mm dia. HDPE back grout lines, 10km of signal cable and 25km of fibre-optic cables. The sub-contract for the mechanical installation was awarded to Murphy International Ltd which carried out detailed design works for the pipeline installation in-house in line with the principles established by the JV for the use of the tunnelling infrastructure. Detailed design for this phase of the works sought to utilise plant and equipment that was on site and available for the works. In this regard, all the tunnelling rail infrastructure was retained and tunnel locomotives were used for the transport of material. Segment bogies were used for the development of the pipeline working platforms and the previously installed electrical equipment (substations and transformer) that were used for the tunnelling works were modified to be made suitable for use during welding works. Design works included calculation of the load support necessary for the various brackets installed in the tunnel to accommodate the various services, analysis of the allowable maximum pull lengths of the pipe strings in the tunnel, calculation of the allowable stresses and bend radii on the umbilicals and design of a means of monitoring the pull forces during installation.

Stress analysis using Finite Element Analysis

A full stress analysis of the gas pipeline in the tunnel during testing was also carried out by Murphy Land and Marine’s specialist design team ProTech, using Finite Element Analysis. Pipeline design works were reviewed and verified by JP Kenny on behalf of SEPIL. Prior to the completion of tunnel construction, mechanical works commenced on site with the fabrication of 72m long pipe strings which would later be transported and lowered into the start shaft/ramp and brought into the tunnel. Preparation of the pipes in this manner ensured that the time spent installing the pipe in the tunnel was reduced and the amount of welding works that had to be carried out minimised. Following the demob of the TBM and the tunnel specific equipment, the mechanical fit-out of the tunnel was carried out over a period of five months, and like the tunnelling works, fit-out was carried out on a 24/7 basis. Upon successful completion of the service installations, a rigorous testing and commissioning period followed whereby all the installed services were tested and approved. Once testing works were completed the tunnel was ready to be backfilled. Detailed design works for the tunnel backfill was carried out by ARUP with input from BAM-ST (part of the BAM group from the Netherlands) and the JV. A large part of the design development for the tunnel backfill centred on the material being used for the backfill and how it behaved. Based on previous experience of material used in the backfilling of a similar tunnel where the grout material was required to be pumped 2.5km, Roadstone Wood were selected as the preferred supplier and worked alongside BAM-ST to develop and test an acceptable mix.

Properties tested included ripening time, viscosity, bleeding and curing time

Properties tested included ripening time, conductivity, density, marsh viscosity, absolute viscosity upon mixing and whilst in motion, anticipated slope when delivered, bleeding, electrical resistance, curing time and strength. Of paramount importance during the mix development was the curing time of the material, which needed to remain fluid for up to 72 hours. Trials during development included a scale test of the discharge of the material to an open channel and observation of the material performance at various times during the curing process. With this information a computational fluid dynamic (CFD) model was developed and executed by ARUP’s New York office on three-dimensional and two-dimensional scales to evaluate the tunnel backfill process with the proposed grout mix. Other associated design works involved the sizing and specification of the pipework and pumps, pressure drop calculations, development of pump operating envelopes, failure mode analysis and the development of a standby and spare philosophy. Backfill design works were reviewed and verified by DeLaMotte & Partner GmbH on behalf of SEPIL. As with the pipeline installations, the design for this phase of the works again sought to utilise plant and equipment that was on site. Booster pumps used in the slurry circuit during TBM advance were used to pump the grout during tunnel backfill operations. Storage tanks, set up on site originally for the tunnelling works were used to store surface water, which was then recycled and used in the grout manufacturing process. Over a period of one month a total of 47,000m³ of grout material was batched on site and pumped into the tunnel. Again, works were carried out on a 24/7 basis to ensure continuous pumping and reduce the risk of blockages to the grout lines. Grouting operations were competed in January 2015. BAM/Wayss & Freytag was delighted to be involved with the Corrib project and is proud of the innovative technical solutions that were developed over the course of the tunnelling works to achieve a successful completion of the works. The completion of the tunnel and its fit-out has seen a significant step taken towards ‘First Gas’ which is expected to flow from the Corrib Gas field in 2015. Shell has launched a campaign to showcase the many benefits that have been delivered by the development of the Corrib gas field. Read more here Author: Pat McAndrew, BAM Contractors Pat is a chartered engineer with more than 20 years experience in heavy civil engineering works. He has worked with BAM Contractors for more than 14 years during which time he managed the award winning Cill Ronan Harbour Development on Inis Mor. Pat managed the BAM Contractors elements of the JV works which included the shaft construction, M&E installation and tunnel backfill. He is a former graduate of NUI Galway.