BACKFILLING BATA AIRPORT DOCS BY CUTTER SUCTION DREDGER: CSD
Parameters involved in the choice of dredging type
Disc cutter suction dredgers : (CSD)
Description:
This model has a cutter at its suction head. This device rotates and enables materials to be broken up, which simultaneously enables them to be pumped.
The force of the cutter, reinforced by anchoring on the pile, enables rocks with considerable resistance to be broken up. By way of an example, the largest models can easily get through rocks which have a resistance equivalent to that of a very good concrete.
2 models of disc cutters of different sizes
It is therefore possible to fit a cutter disc on a suction dredger. However, it is not possible to use this combination when extracting compact and resistant materials, as the superstructure of the suction dredger is not sufficiently reinforced to withstand the stresses imposed by this type of operation.
The addition of the cutter to the suction equipment has considerably increased the judging possibilities of the suction dredger. It can be used both for new works and for maintenance dredging, and for very varied sediments. The dimensions of cutter suction dredgers range from small standardised or removable units up to large units capable of working in exposed stones and at a maximum depth of 30 m.
There are different types of cutter, and a choice of tour to be implemented depends on the nature of the materials (rocks, silt, sand etc.) and the solution chosen for their excavation. Some of them, such as closed crown cutters, in effect enable dilutions to be limited which could prove good within the context of onshore management, for example. It is, in addition, possible to change the teeth on the crown depending on the type of material to be dredged.
Once its structure has been dismantled and it has been mixed with water, the dredged material is sucked up through the draghead, passes through the suction pipe and built-in pump to then be evacuated to the discharge pipe work. The concentration of the mixture varies from 10 to 40% depending on the properties of the products dredged. With regards to units capable of breaking down the structure of the rock, it sometimes happens that another specialised unit picks up the products (mechanical dredging).
Parallel to the disc cutter dredger, there is another form of cutter dredger capable of carrying out rock excavation work: bucket-wheel dredgers. These cutting devices are largely used within the context of quarry operations but have not until now found favour with international dredging order givers. Their efficiency has, however, been illustrated many times over since these devices give full satisfaction within the context of mining extraction operations which imperatively require good productivity.
These dredgers generally speaking have 2 anchorage piles on the port side and starboard side at the stern of the craft. These machines operate by sweeping the working area, resting on one of their anchorage piles. Lateral movements are carried out using winches and cables connected to anchors or mooring posts. When the extraction of the materials has been carried out over the entire relevant arc of a circle, the opposite pile is embedded and the supporting pile is lifted, which allows the cutter to move forward a sufficient distance to extract sediment once again over the next working arc. The dredger moves, as it were, by walking on these piles.
There are models which have sliding or tipping piles which facilitate the positioning and movement of the dredger over its working area. In addition, some dredgers are fitted with hydraulic jacks and articulated drag arms which enable the cutter to be positioned very accurately. These improved models are particularly suitable for dredging under small crafts or in a confined zone.
Once pumped, the sediment is discharged:
- Into large-capacity barges. This solution is, however, tending to disappear, because of the significant dilution of the materials (around 5 to 7 volumes of water for one volume of sediment in situ) which manifests itself in overflow phenomena and results in significant discharges in suspension around the barge. This technique is still used for the transfer of sandy or gravel-type material which are able to settle rapidly in the barges;
- To compartments built onshore, using pipes. Discharge may be carried out over several kilometres by installing relay pumps if necessary. The building of compartments is customised according to the nature of the materials to be discharged there in the area to be backfilled.
Generally speaking, suction dredgers used for dredging operations involving loose sediments are not self-propelled and are therefore moved using service vessels. On the other hand, dredgers of this type used for direct dredging and backfilling operations, because they are devoted to operating in an open environment, use their own propulsion system.
Benefits and limitations:
Small-sized dredgers are generally modular and can easily be dismantled and transported by road. This enables these tools to operate in places that are remote and difficult to access. Larger-sized units are, for their part, generally self-propelled and therefore can be transferred over a zone in a self-contained manner.
Progress made in terms of geolocation also applies to stationary dredgers. The very dredging principle of these tools coupled with GPS accuracy gives them formidable accuracy on the vertical plane. When dredging is finished, the seabed is generally flat and does not have any depressions or bumps.
These dredgers are, in addition, sensitive to the presence of solid waste. Various debris can coil around or get blocked on the cutter, which manifests itself in erratic shut-downs which are detrimental to extraction output.
The exclusive use of this type of machine when dredging does not enable the blocks and waste that may be found in port sediment to be brought up (as opposed to mechanical dredging).
Big units equipped for rock breaking can operate on all types of material, while smaller-sized stationary dredgers with lower output are only effective on silt and fine sand. Large units are, in addition, often self-propelled and are closer to standard ships which means that they can easily operate in an open environment which is not the case for dredgers that do not have a propulsion unit.
This is also one of the other advantages of these techniques, since the great variety of stationary dredgers available on the market means that different types of situation can be tackled. The largest models can operate in an open and exposed environment over areas where the materials to be dredged are by nature more compact (rocks, compacted sand, etc.) while smaller-sized dredgers can work easily under small crafts and in small recesses which are preferential sedimentation areas for fine sediment.
Operational depth is also limited by the length of the drag arm. Generally speaking, the more imposing the size of the extraction unit, the greater the depth the drag arm will reach. It is, in addition, possible to extend the drag arm in some models, provided that the pump has sufficient capacity.
Output is in general high and varies between 50 and 5000 m³ per hour depending on the dredger and the material. They can also discharge over a limited distance (between 500 m and 2000 m) depending on the power of the pumps and the nature of the materials. To overcome this restriction, it is however possible to use relay pumps (booster pumps) which enable discharge distances to be increased (distances are doubled, generally speaking).
