NEW ENERGY NETWORK IS on the way to transport natural gas from the Caspian region to Europe. The EUR 4.5bn (USD 5.28bn) Trans Adriatic Pipeline (TAP) is the last leg of this ‘Southern Gas Corridor’. TAP, as the name suggests, transverses the Adriatic and it makes landfall near San Foca, a small seaside village in the Puglia region of southern Italy.

The reception plant for the TAP gas supply is approximately 9km from San Foca, close to the nearby town of Melendugno. The plant and pipeline are designed to receive gas at a pressure of up to 145 bar, but the planned delivery pressure is 75 bar. By early 2016, TAP had awarded the EPC contracts:

  • Saipem was awarded the offshore link between Albania and Italy;
  • Renco was awarded the Pipeline Receiving Terminal (PRT);
  • A JV of Max Streicher and Enereco was awarded the onshore pipeline in Italy, which will connect to the offshore section at the landfall
  • Saipem was awarded the 1.5km microtunnel, operating out of the Melendugno site

TAP Italy, the name for the Italian section of the pipeline, includes an offshore pipe that runs for 45km and leads from the edge of Italian territorial waters up to the microtunnel crossing the Italian shoreline, which itself stretches 1.5km, before joining an onshore pipeline that is 8.2km long. The connection of the microtunnel to the on-shore pipeline is 700m from the sea.

Excavation of the micro tunnel will be carried out with a Herrenknecht AVND2000AB drilling machine. It belongs to the category of closed, full face excavation machines with a hydraulic slurry circuit.

MARINE WORKS

“One of the main challenges is not the drilling itself, which will proceed well below the seabed, but the operations related to the exit point once the drilling is completed, as the microtunnel needs to exit off shore in the Adriatic Sea in a sensitive environment. We will implement a number of environmental and safety [initiatives] so that the environment won’t be affected,” says Gabriele Lanza, TAP project manager.

‘There will be buoys, which will constantly monitor the turbidity of the seawater during the excavation and backfilling operations at the exit point. In this way, we will know how much turbidity we are causing, and whether this is more or less than that produced by normal currents.

“We will evaluate the actions to be applied as per the measures included in the Environmental Monitoring Plan. For example, if we realise that we are causing too much turbidity, excavation works will have to be slowed down until an acceptable level is reached again.”

“We expect that the turbidity would be minimal, as the TBM will arrive at the exit point underground, and not above the seawater.”

Lanza adds: “Bear in mind that bentonite is clay, so at the last 30-50m before reaching the exit point, we will change to a type of mud with a biodegradable polymer to avoid any clouding effects. Then, even if somebody sees a cloud emerging into the seawater, it will only be existing material from the seabed. However, we would like to avoid any visual effects so as not to worry people. Thus, we are making every effort to do this project in as eco-friendly a way as possible.

“We are working a lot in terms of monitoring and preventative measures under construction as well as maintaining a high level of surveillance on drilling parameters.

Save the Cymodocea

“In 2016 we performed further analysis regarding the exit point of the microtunnel as it might impact Cymodocea [a species of sea grass],” says Lanza. “We realised that moving the exit point around 60m to the east and down to a depth of 25m, we could dramatically reduce the impact with Cymodocea. This was also confirmed by a dedicated survey carried out in June 2017 by professor Giandomenico Ardizzone from Rome University. We also proposed a series of temporary works, such as a temporary bulkhead to avoid any marine fauna in the proximity of the exit point. Thus, we sent an environmental report to the government proposing to move the exit point.

“We carried out stabilisation measurements of geostatic load, at the specific request of the Ministry of Environment, Land and Sea (MEIT) regarding the microtunnel exit point to minimise excavation impact and any interference with Cymodocea section. “We proposed to install a layer of (prefabricated) concrete mattresses and a geo-textile sheet covered with sacks of rocks as the tunnel approaches its subsea exit point.

“The idea is to increase the load above the cutterhead during the final sections to ensure a better face stability. The stabilisation load will be consistently spread over a length of 30m, along the TBM axis.”

This intervention is expected within the pit that will be excavated for the TBM retrieval without making any impacts for the whole project.

Where the TBM drive ends there is approximately 4m of cover. Here the emerging pipeline will need to follow the seabed’s natural profile. Lanza says, “any gaps between the pipe and the seabed could cause excessive stress which could damage the pipes. The design and the installation need to ensure a match between the ground profile and the pipe, within its elasticity limits.

“We need to carry out a transition excavation between the lowest point [of the tunnel] and the seabed.”

This transition section comprises a 90m-long trench at a 3-degree gradient, and then an embankment, about 140m long, that will accommodate the pipeline as it follows the seabed.

Lanza describes the work as similar to placing artificial reefs offshore. About 900m from the coast, ships equipped with bucket cranes will dredge and place material for the trench and embankment. This operation is planned for the autumn.

The microtunnel is lined with 300mm-thick precast segmental lining (see box, opposite) for a 3m o.d. and 2.4m i.d.; the 36-inch (914.4mm-) diameter pipe will be housed inside the microtunnel, which will be filled with seawater.

Lanza explains, the microtunnel not only allows the pipeline to avoid creating any environmental impact while crossing the coast line, but it will also be beneficial for collecting seawater at the Italian landfall area, which can be pumped into the pipe during testing operations. “After the hydrotest is completed, the microtunnel will be sealed at both ends keeping it full of seawater.”

SCHEDULE

Shaft works have been completed. From July to September works will be put on hold because of a summer restriction period. The area is a popular beach holiday destination.

The TBM is due to start work around the end of September.

“We expect that the mechanised drilling excavation might last 110 days, calculating 10-12m per day for 1.5km, but it depends what we will find underground. We can even accelerate the advance speed. We will work 24h per day,” says Lanza.

The project is scheduled to finish in January 2020.

GEOLOGY

Geological investigations have been conducted by TAP during the design phases. Lanza says they expect to mine through layers of medium to dense sand. “The micro tunnel profile has a slight immersion of 3 percent, and then it keeps sub-horizontal, it goes a bit up again within a geological profile of sand in medium or dense composition,” he adds. “According to the studies, we shouldn’t encounter any problems relating to the geology.”

MELENDUGNO LAUNCH SITE

The launch shaft is 10m long, 11m wide and 12.5m high. There is a crawler crane, which will lower down pre-stressed, reinforced concrete segments down to the tunnel, which are manufactured by Ilcev and delivered by sea.

“Even if segments are already waterproofed, the joint side is usually delicate, but we can still hydraulically seal up to 5 bars,” Lanza says. “That is more than the hydrostatic pressure that we expect to face. This on-going pressure during thrust can ensure that this gasket can adhere perfectly. In this way, the microtunnel will be fully waterproof.”

Lanza explains that an improvement to the initial design was to improve the watertightness of the launch shaft. “One of the first objections [to the project] was the impact of the starting shaft on an the existing aquifer,” he says.

“Thus, we switched to a design with 68 secant piles installed with a Bauer machine.

“After installing these piles and before building the shaft, we had to build the bottom plug by jet grouting with a Casagrande C16. High-pressure injections of concrete and water have been carried out to consolidate the subsoil, starting from the shaft base up to 7m of depth.

ENVIRONMENT

Almost 200 olive trees have been temporarily removed from the microtunnel area and they have been stored in a nursery in Melendugno. A netting canopy has been built to protect them from the Xylella pathogen, which is currently spreading through olive trees in Puglia.

These trees will be replanted at their original site once the project will be finished.