Physical change

15 April 2008

Assembly hall seen from the North, with the gantry crane

Assembly hall seen from the North, with the gantry crane

Installation of heavy equipment for a large scale scientific project at the CERN laboratory in I Europe required input from V nstallation of heavy equipment for a large scaleSL Heavy Lifting from the beginning.

A super conducting particle accelerator is being installed near Geneva by CERN, the European particle physics laboratory, in its 27 km long circular tunnel. It is for an experiment called “Large Hadron Collider”(LHC). To house this project, a cavern 97m below ground level and a 20 m diameter vertical access shaft have been built near the village of Cessy in France, 12 km north of Geneva in Switzerland.

At 52 m long, 26 m high and 25 m wide, the cavern has the size of a cathedral. It will accommodate the CMS (Compact Muon Solenoid) detector, which weighs 12,500 tonnes and measures 22 m long by 15 m in diameter. The detector consists of 15 predominantly circular segments. It has already been assembled and tested in an assembly hall constructed above the access shaft. The individual detector segments weigh between 250 and 1,920 tonnes.

For the lowering of the segments (also called loads or elements) into the cavern, CERN had already made a decision in the planning phase of the assembly hall: The lowering would be by means of a fixed gantry crane, to be installed outside and on top of the hall. A comparison of the features of several possible equipment principles led CERN to specify hydraulic strand jack type lifting and lowering units as the only acceptable solution.

CERN invited companies active in the field of moving heavy and exceptional loads by means of hydraulic strand equipment to pre-qualify for the job. Five companies or groups of companies fulfilled the criteria given by CERN and received the tender documentsat theendof 2003.

In scope

The scope included the concept, detail design, provision on a rental basis, erection, operation and disassembly of the portal crane; in addition, the design and fabrication of all custom built accessories (ancillaries) needed to hook up the 15 loads. tender documents included concept drawings that showed how these anciliaries could look. It was, however, clearly written that the responsibility for the flawless fitting and performance of the anciliaries would be with the lifting contractor.

The interfaces in the handling of each of the 15 segments were defined as follows: CERN staff moves the segment onto the mobile concrete slab that closes the access shaft. The lifting contractor installs the anciliaries, attaches its lifting strand cables and lifts the load by a small distance, just to allow the concrete slab to be moved laterally and hence to open the shaft. The lifting contractor then lowers the element 97 m to the floor of the cavern, releases the cables and disconnects the strand anchorages and anciliaries. CERN then moves the load on air pads into a parking position, to make space for lowering the next segment.

After review of the offers and clarification of technical details, CERN decided to award the contract to VSL (Switzerland) Ltd. The contract was signed in late 2004. The subsequent detail discussions and checks that had to be made on site showed that the fact VSL had decades of experience in this field of activity and its offices are near Berne (less than two from site) was mutually advantageous.

In this context it should be mentioned that the concept and design of the anciliaries for the six segments were called end disks. Each of these tall and slender elements stands on a huge steel cart. The hook up is only possible at this level, about 7 m below the centre of gravity; the suspended loads are, therefore, unstable. For this reason, CERN provided two pairs of large tubular sections in the upper part of each disk, to serve as attachment for stabiliser devices. Due to space constraints, the concept and detail design of these devices, which embrace the lowering strands, turned out to be a genuine challenge.

The CMS detector is a prototype. During the assembly phase, the start date for the lowering work had to be adapted several times to the reality. Due to this, the assembly of the gantry crane occurred not in one move, but in several stages.

Portal crane

The portal crane has 28 m span and 25 m free height under the main beams. It consists of two pairs of lattice towers that VSL had used before for other lifting work, mainly overseas, and two interconnected welded steel girders 3.40 m high. The CMS segments have different hook up geometries that vary by several metres in plan, therefore, the position of the four lowering units must be adjustable by horizontal jacking in two directions.

This jacking is achieved by two main platforms that sit on the man girders and which can be jacked longitudinally. On each one of these platforms, there are two secondary platforms that allow lateral adjustment. Each platform supports a VSL strand unit SMU-580 and its motor-driven strand coiler drum. These strand units have 550 mm piston stroke and work with 55-off seven-wire strands of 15.7 mm diameter and an ultimate capacity of 2 8 tonneseach.

For the heaviest element, this results in a safety factor of 3.20 for the strands. Each strand bundle is 130 m long and weighs 10 tonnes, hence the question of where to put all these strands is a significant one. The motor-driven coiler drums developed by VSL have been used on several large bridge projects in Asia. They allow a controlled storage of the strands in a limited space.

The control and monitoring equipment was installed in a small container on the main girders, between the weather houses. Forces in the four strand cables were controlled via the hydraulic pressure in the jacks. This, however, did not give any information on whether or not the load was hanging horizontally, as it should, or slightly inclined. Therefore, the level condition of each segment being lowered was controlled using two inclinometers (two for redundancy reasons). They were fixed to the segments and their their respective outputs were relayed to displays in the control room.

The steel strand cables passed through four large existing openings in the assembly hall roof. quirement that no rain or snow should at any time penetrate the hall was was achieved in two ways. One was using two storm proof weather houses enclosing the equipment platforms and protecting the jacks and coilers from the weather and the strands from corrosion. The other was to seal the space between the movable platforms and the roof using a system of four mobile frames and square mantles made from tarpaulin.

The electric hydraulic pumps are situated on separate platforms, adjacent to the weather houses. During lowering work with hydraulic strand units, energy has to be led off in the form of heat. Installing the pumps in the open air helps the cooling of the hydraulic oil. In this case, where there was continuous lowering over several hours, additional oil coolers had to be fitted to the pumps.

Law of the land

The site is in France so the gantry crane had to be designed according to French standards andre tested before commissioning. This included a static load test with 125% of the heaviest load, a total of 2,400 tonnes. For this purpose, the four strand cables were anchored in the concrete slab that closes the shaft and which was additionally ballasted with several hundred tonnes of steel blocks. After this, a dummy load made of an existing steel frame and steel blocks was lowered down to the cavern floor and subsequently lifted up again, to check the performance of the strand equipment and the respective controls.

The lowering of the CMS elements started in November 2006 with two compact HF segments, each weighing a relatively modest 250 tonnes. Before the end of the year, two of the tall and slender end disks weighing 310 and 880 tonnes were loweredinto the cavern.

To mid-March 2007, six other segments were lowered, among them the central barrel YB0, at 1,920 tonnes the heaviest of all segments (www.khl.com Latest news, Giant going down at CERN, 28 February). Following schedule in the hall and in the cavern, the programme for the five remaining elements includes two periods of several weeks with no lowering activities. The last segment, an end disk of 310 tonnes, will most likely touch the cavern floor in early September 2007.

Once the anciliaries are properly fixed and the cables hooked up, the opening of the shaft and the subsequent lowering of a segment, independent of its weight, takes about 10 hours. One full lowering step of 500 mm takes three minutes; the actual lowering speed is of the order of 10 mm per second. At this relatively low speed, dynamic effects in case of, for example, a sudden emergency stop are barely measurable. In any case, they are only a fraction of the 15% that had to be accounted for according to the applicable standards.

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