International energy markets in recent years have seen significant growth in shipments of liquefied natural gas (LNG) between countries. One of the latest LNG import projects is under construction in Italy, off the west coast near Livorno and Pisa, in Tuscany, where a fixed, offshore ship terminal is to be installed just over the horizon and linked to shore by a seabed pipeline.

The terminal is about 22.5km offshore and will allow LNG carriers to moor. The LNG will be offloaded into a fixed, converted vessel that functions as a floating storage regasification unit (FSRU). The unit will gently warm the LNG to change it from a liquid to a gaseous state for supply by a 32in (81.2cm) steel pipeline to shore and into the national distribution network.

But the onshore section has construction challenges that need to be overcome to ensure the environmental protection and minimum disturbance to water channels and flat environments of the vicinity. Design of the project turned to bored tunnels as the preferred solution to hold the gas pipeline, which will be installed and cemented into place.

Denys is currently boring the run of tunnels, mostly between cofferdams in the edge of the main shallow channel. Four micro-tunnelling TBMs are being used and the excavations have been underway for a few months and are making good progress.

The entire project is to be completed and operational by mid-2011.

Project Development
The LNG project – the Offshore Tuscany Regasification scheme – is one of Italy’s most important development in strategic procurement of natural gas supplies for the energy sector, and follows supply difficulties and energy peaks in winter in some recent years. The demand for gas is rising due to increased demand for gasfired cogeneration, producing both electricity and heating water.

At full capacity, the scheme will be able to supply about 4 per cent of the country’s annual demand and helps to also diversify the ownership of the energy infrastructure and increase competition. Italy has two other regasification terminals, at La Spezia and Rovigo. Aside from the offshore LNG project, further regasification facilities are to be built in the country to help LNG supply about a third of the country’s natural gas.

Permits to build and operate the regasification terminal are held by Offshore LNG Toscana (OLT), which is a dedicated project company established by a consortium of energy and industrial groups—Iren (46.79 per cent), E.ON (46.79 per cent), OLT Energy Toscana (3.73 pe cent), which originally conceived the project, and Golar LNG (2.69 per cent).

The special purpose company plans to invest more than EUR 600M (USD 840M) in the scheme, which could see each LNG carrier shipping up to about 130,000m3— equivalent to approximately 80Mm3 of gas—or slightly more.

LNG does not dissolve in water and evaporates on contact without mixing or leaving residue or polluting. Neither is there residue remaining from any spillage onto the ground.

The project site was selected to be near Livorno for two reasons—good port facilities to support the offshore infrastructure, and the area is also close to some demand centres and so also grid connections. Preparations for the project began in 2003 with the scheme being proposed and followed by the first step in the approval process. In 2004, the regional council and national government approved the environmental impact status. Approvals for construction of the project came in late 2005 to early 2006, and some months afterward the authorisation for the onshore gas pipeline in Tuscany was secured.

Construction activity on the project began in 2007 with some preparatory activities and the following year the project moved ahead to acquire two key assets— the gas carrier ship (“Golar Frost”) that would be converted into a floating offshore regasification terminal and for occupation of the specified fixed area of the Mediterranean Sea for the refitted ship. The following year the permits came for laying the pipeline both offshore and onshore.

The six-year-old Golar Frost was fabricated by Hyundai Heavy Industries and is being converted in dry dock in Dubai.

Offshore works commenced in February this year with the Castoro 7 pipe-laying barge to bring the 29.5km long pipeline to shore by the end of the following month. Preparatory activities for the tunnelling works to hold the 6.5km long onshore section of the gas pipeline commenced a year ago.

Onshore Works
While OLT signed a contract with Saipem to install the offshore works of the fixed point mooring terminal and pipeline, it agreed a separate deal to construct the onshore section with Snam Rete Gas, an Italian gas distribution, storage and regasification company, and part of the energy group Eni.

The onshore section of the project calls for about 5km of construction below the floodway channel of the River Arno and the remaining 2km on dry land. The pipeline is being buried at a minimum depth of almost 2m, and at least 6m below the floodway to guarantee navigability.

A number of alternative solutions were considered for the onshore works to bury the 800mm gas pipeline. The first option examined was for a continuous cut and cover open excavation that would run next to the channel but the option was dismissed for reasons to do with permit and environmental approvals.

A variation on the proposal had the alignment in the channel with sheet-piled excavations allowing for the cut and cover construction over a total length of 7km. However, again for environmental reasons and potential pollution from disturbing the bed of the channel led to the rejection of this option.

The third option for burying the pipeline was to use directional drilling but this was dismissed for technical reasons—the polluted bottom of the channel would have to be pierced numerous times for the drives, and there was the risk of potential blow-outs from below the channel bed. A further block on this alternative was that there was insufficient holding space for the pipeline before it could be pulled-in to a bore done by directional drilling.

Finally, it was established that microtunnelling, as a later suggested possibility, would be the most effective and acceptable approach. Micro-tunnelling was seen as requiring fewer construction sites along the alignment and consequently less excavation through the polluted channel bed and also it offers better control of the front pressure at the face would reduce blow-out risk.

A joint venture between Denys and Max Streicher is undertaking the onshore tunnelling and pipeline works for Snam Rete Gas under a lump sum contract valued at around EUR 40M (USD 56M), which was awarded in September 2009. The construction programme runs to mid-2011.

Denys is mainly responsible for the micro-tunnelling while Streicher is focused on the construction shafts, pulling-in the steel pipeline and mechanical works, such as welding on the pipe.

The client is represented on the project by Italian oil & gas and energy sector consultant Enereco.

Tunnel
Denys is constructing 5km of tunnel below the floodway using micro-tunnelling for a succession of drives of varied lengths, and the tunnels are also of varying diameters to house the 800mm steel pipeline to carry the gas.

The tunnel construction comprises:
• Two drives of 1170m length for concrete tunnel pipes of 2000mm i.d. (2400mm o.d.)
• Two drives of 900m lengths for tunnels of 1800mm i.d. (2160mm o.d.)
• A drive of 430m long for a tunnel of 1800mm i.d. (2160mm o.d.)
• A drive of 450m long for a tunnel of 1600mm i.d. (1960mm o.d.)

Geology along the tunnel alignment, below the floodway canal, comprises soft soil mainly fine sand overlying clay with organic material, although there is a short section where the bore will pass through lime with mica.

The eastern section of the alignment, the furthest inland, is mainly through fine sand while the sections of tunnel closer to shore will be bored through clay with organics mainly, the upper part of the face meeting the fine sand above.

Groundwater level in the mostly permeable soil is at the surface level of the channel, itself generally 1m deep, and so the head of water over the tunnel is 7m.

Key aspects of preparation for the tunnelling works included the planning and security aspects, and site investigation.

The key people Denys has on the microtunnelling contract are: Geert De Wilde, senior project manager; Nicolas Solda, chief project manager and Dirk Derycke, division manager for tunnelling.

Micro-tunnelling
Excavation in the tunnelling works are being performed by four slurry shield TBMs, all Herrenknecht AVNs and each with three or four parts and the concrete pipes for the tunnels are being pipe-jacked into position behind them. Denys owned three of the shields and also bought a new TBM for the job.

The tunnelling works call for a number of sheet-piled and braced cofferdams with jet-grouted wall-strips in the foundations as lower struts. The concrete base slab is the working platform for the construction activities.

Shafts are constructed by sheet-piling from a pontoon near the side of the 60mwide channel, staged excavation and then jet grouting in the bottom before pouring the concrete base slab. The cement columns are 800mm diameter and overlap at 550mm centres and each jet-grouted line is spaced at 1.56m centres.

Six starting shafts for the TBMs are to be used on the project, each 16m long by 5m wide by 10m deep. There are also three reception shafts, which are 8m by 5m by 10m, or half the size on plan as the starting shafts.

Tunnelling works started in June, and the early drives are three stretches at opposite ends of the tunnel layout:
• One of the two 1,170m long, 2,000mm i.d. stretches—the section nearest the sea, between shafts marked P0-P13. It was the first drive and completed between early June and mid-August.
• At the opposite end of the layout, the 450m long, 1,600mm i.d. drive between P66-P53, which was completed in late September.
• Coming from the opposite direction to P53, the 880m long, 1,800mm.i.d. drive from P43 which started last month.

The remaining drives will work progressively towards the middle of the tunnel layout and construction of the next two starting shafts, P34 and P24, have been completed. The final drive is expected to be finished in March next year.

The shields are guided by a VMT automatic laser system. While that focus is traditional for tunnel, a separate vertical measurement focus at the starting shafts is also important to ensure that, should the water level in the channel rise drastically, the tunnels can be sealed off. For that purpose a pre-constructed cap is ready, should it be required.

Bentonite is used for the pipe-jacking of the concrete pipes with an automatic lubrication system. An additive was used to counter the salt water conditions. The mix design of the concrete pipes, manufactured by local firm ILCEV, is matched to the marine environment.

The pushing capacity of the pipes vary as follows:
• Concrete pipes 2,000mm i.d. (2,400mm o.d.) – 1,000t
• Concrete pipes 1,800mm i.d. (2,160mm o.d.) – 900t
• Concrete pipes 1,600mm i.d. (1,960mm o.d.) – 800t

The quantities and application of the lubrications depends on the soil, and the performance is judged by the jacking forces required during pipe placement. The tunnelling works use intermediate jacking stations—one every 140m, approximately, and are located between two speciallyfabricated concrete pipes. Nine pipes are installed in the longest drives.

The average advance rate achieved is 20m in 24 hours, based on the long first drive over the summer, says Denys. Ventilation is provided when drives reach a distance of 300m or before should it be required. A Korfmann-manufactured fan is placed in the launch shaft and a 200mm flexible hose in the pipe.

Spoil removal is by slurry pumps. The contractor is using desanding units reinforced with filterpresses where necessary. The filterpresses are located at the three launch shafts in the section of tunnel with clay along the alignment—the half of the job to the west, nearest the coast.

In total, the excavated material from the bores, shafts and trenches will be approximately 35,000m3—including the last leg of the onshore section, furthest inland. At the end of the micro-tunnelling stretch, this final 2km long section will have the gas pipeline buried at a depth of 1.8m by placing in shallow trench excavation.


Location of the micro-tunnelling project that is part of a pipeline project to bring gas supplied onshore near Livorno, Italy Plan and cross-section of a typical launch shaft in the canal for the micro-tunnelling works Denys is using four Herrenknecht AVNs on the micro-tunnelling contract TBM launch on a 880m long, 1,800mm i.d. drive from shaft P43 in September The drives in the west end are mainly through clay, those in the east are mostly in fine sand and the head is 7m of water Intermediate jacking points are placed every 140m along the drives