• Offshore extension

From the bowels of the earth to the depths of the sea: the long route of the “stones”

13 February 2018 - 17h18

Almost a million and a half tones of aggregate will be needed just for the phase of preparing the backfill for the foundations of the future caissons. Produced in Châteauneuf-les-Martigues, they have been transported by sea at a rate of one return voyage every 71 hours, a total of around 50 round trips.

Everything begins at the end of a small winding road at the top of a hill. It is here, at the Jean Lefebvre quarry, that the 20/180 block-size distribution material (that is, between 20 and 180mm in length) has been prepared since April 6th 2017. Opened in 1973, and covering an area of 156 hectares, this quarry is one of the largest in the region producing solid limestone rocks. With the arrival of the order from Monaco, all the quarry’s operations have been reviewed. Working for the ArcelorMittal Group, concrete mixing plants, large public works and construction companies and others, the quarry has an average annual production capacity of between 1.5 and 2 million tonnes. The extra quantities produced for Monaco’s requirements, 1,450,000 tons, mean a significant increase in its activity! Numerous facilities have been installed specifically for this purpose, and funds have been allocated, and staff have been recruited.

First stage: production

From the belly of this hill, one hears a deep rumbling sound. As happens every day, blasting is taking place to break the rock up into thousands of unequal-sized pieces. An excavator then takes over, and unloads the blocks that have been regurgitated by the Earth into a crusher. Feeders fill special machines, dump trucks, which then store the blocks, which are now smaller in size, in a dedicated area. It is now time for the cleaning process, which consists in removing the clay from the aggregate, washing, screening (separating the materials according to size), and loading trucks. The dump trucks, which have now been reloaded, take the material to a kind of belt known as a conveyor, which takes the blocks to a giant innovative drum resembling a washing machine – the scrubber – where the clay is removed from the “pebbles”. The material moves on to a jigging machine, and then to another conveyor that takes it to an enormous 250-tonne silo. Without stopping, the trailer-type lorries feed a curious kind of sweeper. When they arrive beneath the silo, they are automatically weighed when empty. Each is then filled with 30 tonnes of material, again automatically, in less than a minute, before giving way to another loading. Over 50,000 lorries will leave the quarry over about 10 months.

Transport time…

At about 40 kilometres from the Jean Lefebvre quarry lies the port of Fos-sur-Mer. There is a vast area at the entrance to the mineral terminal, one part of which has been dedicated to the Monaco offshore extension project. Everything is meticulously organized here. On the left, there is an immense pile of 30,000 tons of material that has been unloaded by the lorries arriving from Châteauneuf- les-Martigues. 30,000 is a number we will also see again, because it is no coincidence that the loading capacity of the gigantic 200-metre-long Simon Stevin is also 30,000 tons. This first pile of aggregate is classified as reserve stock. It will only be loaded on to the steel colossus for its last voyage to Monaco. From a logistical standpoint, in fact, a paralysis of the transport of material between Châteauneuf and Fos, such as a strike that impacts road access, has been provided for. In order not to immobilize the Simon Stevin and to provide a little oxygen, this reserve stock permits a round trip to be made by the ship with no outside intervention, explains Guillaume Robe, the Deputy Head of Department at Bouygues TP.

…and loading time

Now, in mid-January, a few metres from the reserve stock, another “pile” is growing in size significantly. While the ship is en route, the lorries continue to transport the material. A rapid calculation of a primary school mathematics lesson type: the Simon Stevin loads 30,000 tons of material. Each lorry carries 30 tonnes, so one thousand round trips between Châteauneuf and Fos are needed to fill the Simon Stevin. Another not especially complex little calculation: now that we know the loading capacity of the ship and have clarified the loading rate – 1,500 tons an hour – we arrive at a total of around twenty hours to load the Simon Stevin completely. This is a rhythm the lorries cannot achieve, which is why the trips continue while the ship is between Fos and Monaco, thereby increasing the so-called advance stock. And on the subject of the Simon Stevin, here it comes, pointing its bow towards us. It is supposed to be at the quayside at 3 p.m., but it is slightly late on this 17th of January because of the especially windy conditions, with winds that have reached over 100 kilometres an hour. At 4 p.m., it begins its mooring procedure. Everything goes very quickly, following a well-oiled procedure. The two hoppers located near the reserve stock receive the material brought from the so-called advance stock by enormous loaders of around 600 horsepower, whose bucket volume could hold several Renault Twingos (see photos), and by the trailers that are constantly arriving from the quarry. The material is carried to jigging machines on two parallel conveyors. Two operations take place there: the material is first washed by high-pressure jets (see below), and then the 4/20 type material is separated out. Further along, a de-sander enables the 0/4 (classified as sand) to be extracted. At the end of this phase, therefore, only the required 20/180 remains, which, once again on conveyors, is carried to more hoppers, which discharge the material into two “grasshoppers” (oblique conveyors). At 4:51 p.m., these conveyors discharge the first stones into the belly of the floating colossus. The operation will not be completed until the early afternoon of the following day. During these twenty hours of loading time, excavators inside each of the ship’s two holds divide the materials without stopping.

A unique ship

Designed especially for this type of activity, the centre of the Simon Stevin is equipped with a pit, into which lengths of pipe with a diameter of 1 metre are placed. These pipes, which are ten metres long, are assembled as needed depending on the depth of the seabed so that they can discharge the 20/180 as near to it as possible, thereby, together with the previous washes of the material, limiting the resulting turbidity as far as possible.

Georges-Olivier Kalifa

Photos © GOK

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