Powerful images from sea catch the eye for many reasons and uncommonly so when related to tunnel projects. Images that have done so recently relate to key milestones in works involving bored tunnels performed offshore on separate projects – in Kuwait, where an outfall tunnel’s tunnel boring machine (TBM) was recovered from the seabed and lifted out of the water; and, in the UK, where the latest intake head for the cooling tunnel system at Hinkley Point C power station was lowered in.

SEABED RECOVERY OFF KUWAIT

Following a successful pipejack bore to place an emergency sea outfall as part of the Umm Al Hayman wastewater treatment project, in Kuwait, the Herrenknecht TBM AVN2200AB used for the tunneling was recently recovered from the sea bed in a complex operation off the coast.

The contract for the tunnel works was awarded by WTE Wassertechnik GmbH to a joint venture of Al Hassanain and Joseph Gallagher.

Recovery of the 14m long, 140 tonne TBM was a challenge from where it finished its drive approximately 10m below the seabed. The surrounding sand was excavated by airlifts to expose the machine and then the TBM was raised using a combination of air bags and a barge-mounted crane. The conditions of the work were difficult with winds and high temperatures.

After the floating package was towed to shore, on a journey of approx 40km to a jetty beside the Al Zour power station, the TBM was lifted out of the sea and onto dry land using a 350 tonne crawler crane. The last part of the project at the outfall will involve offshore installation of a diffuser.

LOWERING INTAKE HEAD AT HINKLEY

Balfour Beatty recently has completed the third and final offshore tunnel for the Hinkley Point C nuclear power station’s cooling system under the Bristol Channel in the UK. The project is the UK’s newest nuclear power station in more than 20 years.

Three Herrenknecht TBMs built a total of 8.8km of tunnels for the offshore cooling system – two 3.5kmlong, 6m i.d. intake tunnels and a 1.8km-long, 7m diameter outfall. The tunnels are 33m below the seabed and are completed by fitting their respective intake heads and outfall heads.

Energy company EDF said most of the head structures for cooling tunnels on the project are in place – both outfall heads have been positioned already and two of the four intake heads are now in place. The largest heads are 44m long, around 8m high and weigh in at just under 5,000 tonnes. The intake heads as well as the outfall heads are concrete structures made to a specially-designed nuclear classified mix. To cast the intake heads, the concrete pours were broken down into nine main pours based on shape, complexity and other factors.

Two floating cranes, working in tandem with a combined lifting capacity of 7,300 tonnes, were used to position the latest intake head for the cooling tunnel.

The task is made more challenging as Bristol Channel is a high tidal range environment.

Balfour Beatty has three major packages of works at Hinkley Point C, including tunnelling and marine, electrical and overhead power lines.