Offshore Urban Extension - Towing and Installing Caissons
The eighteen caissons, manufactured in the port of Marseille, are currently being towed and submerged in Monegasque territorial waters. These superstructures, which each have sides that measure 28 metres and a weight of 10,000 tonnes, must be positioned at a depth of 20 metres with accuracy of 10 cm to create a protective belt for the future Portier Cove eco-district. We take a behind-the-scenes look at this crucial stage of the construction work that began on 12 September 2018.
Three caissons have already been transported to Monaco. There will be a total of 18 by next summer. As and when they arrive in the Principality, they are submerged, and will eventually form the new coastline of the Monegasque territory. Due to their shape, size and weight, these concrete and steel mastodons represent a real challenge, both with regard to towing them from the manufacturing site in Marseille and submerging them in Monaco.
Caissons are not easily towable or manoeuvrable, because they were not designed for navigation. For this reason, it was necessary to design the entire protocol with a view to carrying out these specific operations. The first delivery in July 2018 was therefore a completely unprecedented event, requiring 18 months of preparation. The mission has been successfully completed, making possible a relatively standardised procedure for the other caissons from now on.
A tugboat with a power of 89 tonnes
The caissons were specially equipped to be transported to the Principality. They are towed by a single tugboat with a power of 89 tonnes, using a 57 mm diameter wire cable. The cable can extend to a length of up to 800 metres, the maximum distance between the tugboat and the caisson. This cable alone weighs 9 tonnes.
Navigation between Marseille and Monaco takes three days at a speed of 1.5 knots. Seven shelter points were identified on the route, in case of sudden changes in the weather or extremely localised climatic phenomena.
When they arrive in Monaco, the caissons are submerged on the same day. This very precise timing enables all the operations to be undertaken rapidly. In this way, the risks associated with changing climatic conditions are limited.
Adjusting the position to the nearest centimetre
The final position for the caissons is on an underwater embankment constructed from 1.5 million tonnes of natural aggregate, with the upper part at a depth of 20 metres below sea level. Two phases prior to installation then follow respectively: smoothing off the mound and levelling.
Smoothing the mound consists of roughly flattening the surface of the embankment. A cable dump truck moves the aggregate from the top of the mounds to the gulleys. At the end of the operation, thanks to this infilling, the gaps on the surface are less than 20 cm.
The levelling machine can then start operating to level the surface accurately. It consists of a 54-tonne metal structure equipped with a large 15-metre-wide horizontal blade placed on the embankment. A fine aggregate is placed in front of this blade, which then "scalps" the surface, making it perfectly smooth. For this work, four divers check that the blade is moving forward correctly and that the levelling machine is operating as it should. A total of six days' work is required to level the base area of a caisson. After this operation, the gaps between one point and another on the surface of the embankment will be no more than 5 cm.
Once the placement surface has been prepared, the caisson can be installed. The approach manoeuvre to the designated location is carried out with two multicat vessels, positioned at the front and rear of the caisson. These machines are equipped with azimuth thrusters with 360° rotation, and are therefore extremely manoeuvrable. They enable the position of the caisson to be managed manually, allowing for more stability and manoeuvrability of the caisson. This is crucial for adjusting its position to the nearest centimetre.
The caisson is thus brought to its installation area and almost to its final location. At this stage, the first ballast is put in place, to position the caisson less than one metre above the embankment. The caisson is equipped with a topographic monitoring device - by reading the instruments, the exact position of the caisson can be shown in real time to the person in charge of the manoeuvre, who can then move the caisson by controlling the tension of the cables between the two multicat vessels.
Once the caisson is stabilised definitively and balanced 50 cm above its final position, it is completely ballasted, to lower it onto the embankment. In the event of a discrepancy between the target position and the actual position, as the ballasting used is liquid, ballast can simply be removed from the caisson to re-float it. The operation can then be resumed.
Once the installation of the caisson has been checked and approved, the water is replaced with sand, to give the caisson even more stability. This is the solid ballasting phase. To reduce the risk of turbidity, the water is pumped gradually, respecting the settling times and discharging clear water into the sea. After this operation, the final weight of the installed caisson is approximately 24,000 tonnes.
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Government Communication Department