Towing the caisson
Defining the conditions of transport for an unusual structure
Trapezoidal in shape, measuring up to 28 metres wide and 27 metres high and weighing 10,000 tonnes, the caisson is neither a ship nor a hydrodynamic object.
By its shape, size and weight it is a challenge for towing between the manufacturing site in Marseille, and the immersion site in Monaco.
Adding to the fact that the caisson cannot be easily be towed in water, it also does not meet any towing standards.
The first shipment in July 2018 was therefore a completely new event.
18 months of preparation were necessary to complete this operation in order to define and validate the method and the means implemented.
The first caisson leaving the port of Marseille. Photo: Bouygues TP
o carry out the first transfer followed by the other 17 parts from the end of the summer of 2018, it was necessary to work on the basis of existing nomenclature for towing large objects.
Thus, a long qualification phase began at the end of 2016 to establish the conditions for towing a caisson. The consensus reached between the companies involved in the construction project, the companies in charge of towing, the public authorities and the insurers, made it possible to define the operations and the normative framework that takes into account:
• transport safety for workers
• transport safety for the equipment
• transport safety for each caisson
• safety for other vessels that may cross the convoy’s route to Monaco.
Once established, the towing conditions were validated from an administrative and insurance point of view.
It was from this point that it was possible to calculate the dimensions of the towing resources by establishing the following elements:
• The number of tugboats needed
• The power of the tugboat
• The configuration of the towing
• The rigging of the caisson
• Conditions of the currents and weather criteria for navigation.
Towing a caisson between Marseille and Monaco
Before its departure, the caisson is equipped for its transfer. Shutters close the Jarlan chambers for wave and swell damping. These are huge 900-kilogram closing devices that are placed on the openings of the caisson. It is thus protected against water ingress that could occur during towing. Leak tests are carried out before departure.
The shutters (in black in the picture) will be removed later. Photo: Palais de Monaco
The normative work concluded that towing would be done with a single 89 tonne capacity tugboat (the capacity of tugboats generally observed in ports is 40 to 50 tonnes).
The vessel chosen is the VB Fos. This tugboat comes from Mexico and belongs to the company Boluda, a specialist known worldwide for this type of operation.
The VB Fos is one of the few in its category that can be mobilised for a whole year, which is the time it takes to transfer the 18 caissons between Marseille and Monaco.
The towing is done by means of a wire rope that measures 57mm in diameter.
It can extend over 800 metres, which is the maximum distance that can separate the tugboat from the caisson.
Photo © Eiffage Infrastructures
The cable is unwound from a winch on the VB Fos, to be attached to the caisson by means of plates sealed on the caisson and a bridle.
The attachment system is also installed on the opposite side of the caisson for safety reasons. In the event of a problem with the initial attachment points, the tugboat can use this backup solution.
In Marseille, one working day is required to install the entire rig between the tugboat and the caisson.
The navigation between Marseille and Monaco then lasts three days, at a speed of 1.5 knots.
Upon arrival in Monaco, the caisson is installed immediately, during the day.
This very precise timing promotes the rapid execution of all operations and limits the risks associated with changing weather conditions.
The distance between the tugboat and the caisson can reach 800 metres. Photo © Bouygues TP
The weather conditions for the entire manoeuvre is therefore essential to ensure the success of each crossing and the installation of each caisson in its final position.
The teams in charge of this phase (transport and installation) work closely with Météo-France and have a specific subscription to its services to establish their routing.
It is extremely precise and identical to the one offered to skippers for major offshore races.
Weather conditions must be perfect during the crossing. Photo © Bouygues TP
Throughout the navigation route between Marseille and Monaco, seven shelter points have been defined to cope with sudden weather changes or extremely localised climatic phenomena.
If the appropriate weather conditions are not met, the crossing is cancelled.
In addition to good weather windows, a clear route must be assured throughout the transfer.
It is the maritime prefecture that defines the points on the route of the VB Fos for each crossing by decree. It marks out one nautical mile around the convoy in which no one can enter. As with any towing, which in this case takes place day and night, specific markings and signals indicate the presence of the sea convoy.
It moves at a speed of 1.5 knots. The purpose of this low speed is to limit the forces generated by the caissons fluid resistance, but above all to perfectly control the momentum of the caisson and the speed generated by the tugboat.
The maximum travel speed is 1.5 knots. Photo © Direction de la communication de la Principauté de Monaco
Preparing the base for the caisson
Before the caisson can be installed, it was first necessary to prepare the surface of the underwater backfill built with 1.5 million tonnes of aggregate (see themed feature #3: ‘From quarry to sea, the construction of the backfill’), on which the caisson belt will be erected.
Initially, this backfill was vibrated and compacted, creating bumps and hollows with an amplitude of about one metre.
The caisson belt will mark out the future coastline of the Principality. Photo © Bouygues TP
Two phases followed, skimming and levelling.
Skimming consists of roughly flattening the top of the backfill to a depth of -20 metres. A grab bucket moves the aggregate from the top of the bumps to the troughs. At the end of the operation, the differences on the surface are less than 20 cm.
The leveller can then operate and level accurately.
The leveller is a 54-tonne, 30-metre by 15-metre metal structure, placed on the backfill and equipped with a large 15-metre wide horizontal blade. A fine aggregate, 20/40mm in size, is placed in front of this blade, which then ‘scalps’ the surface, making it perfectly smooth to within a 5cm tolerance.
For this work, four divers check the progress of the blade and the functioning of the leveller. It takes six days to level the base area of a caisson.
The leveller. Photo © Eiffage
The installation of the leveller in Monaco. Photo © Eiffage
Once the surface has been prepared, the caisson can be installed.
The approach manoeuvre to the designated location is carried out with two multicat vessels placed at the front and rear of the caisson.
These ships have azimuthal propellers that rotate 360°. They are therefore extremely manoeuvrable, which allows to manually adjust the positioning of the caisson, and offers both stability and manoeuvrability. This is decisive for adjusting its position to the nearest centimetre.
12th September 2018: Installation of the first caisson. Photo © SAM L’Anse du Portier
This is how the caisson is brought to its installation area, practically to its final location.
At this stage, the caisson undergoes a first ballasting to bring it less than one metre above the backfill.
The final installation is managed by six hydraulic winches placed on the caisson.
Six people are on board to deploy six cables all around the caisson to be immersed, which are then docked on the adjacent caisson already laid.
The caisson is equipped with topographic monitoring equipment. Like the driver and co-driver in a rally car, the topographical manager reads the data and gives, in real time, the position of the caisson to the operations manager who moves it by managing the tension of the cables.
Once the caisson is above its final position, it is ballasted. Photo © SAM L’Anse du Portier
Once the caisson is definitively stabilised, balanced 50cm above its final position, it is completely ballasted to make it descend onto the backfill. In the event of a difference between the target position and the actual position, as the ballasting is liquid, all it takes is to discharge the ballast water from the caisson to return it to float and resume the operation.
When the installation is validated, the water is replaced by 0/20 grain sand to make the caisson even more stable. This is the solid ballasting phase. To limit the risk of turbidity, the water is pumped out as solid ballasting progresses, respecting settling times to discharge clear water into the sea. After this operation, the final weight of the caisson is about 24,000 tonnes.
The final weight of the caisson is 24,000 tonnes. Photo © Bouygues TP MC.
The assembly of the caissons belt was completed on July 18, 2019.