When the material is loosened or pulverised, it is then sucked up and transported through a pipeline by centrifugal dredge pumps. A suction inlet located beneath the cutter head (known as the suction mouth) is connected by a suction tube directly to one or more centrifugal pumps. The vacuum force at the suction inlet sucks up the loosened material. The CSD will discharge the dredged material to the disposal site either via a floating pipeline to shore or by discharging it into a barge with a special loading system. The choice of which to do depends on the distance that the dredged material has to go to the disposal site and what is more economical. If the distances are too great, barges may be more cost-efficient than hydraulic pipelines.
HOW BIG ARE CSDS?
CSDs come in a variety of sizes and types with a total installed power ranging from 200 kW on the smallest dredgers to some 30,000 kW for the largest. The dredging depth depends on the size of the dredger. Smaller ones can dredge in less than
2 metres depth, whilst some of the biggest CSDs can reach depths of more than 35 metres. The minimum dredging depth is usually determined by the draught of the pontoon. Both large and small CSDs are important parts of the dredging fleets of the major dredging companies. For instance, medium-powered dredgers fall in the 10,000-15,000 kW power range but are often not self-propelled. Recently, a number of very large CSDs have been built – some measuring 130 metres long – with some of the largest having a total installed power of 24,000 to 28,200 kW. Amongst these state-of-the-art CSDs some are fully diesel-electric powered dredgers. They have uninterrupted power supply units (UPS) to feed the vessel’s computers and essential navigation/nautical equipment.
WHAT ARE THE DISADVANTAGES OF A CSD?
In general, the larger, most modern CSDs are generally self- propelled and can be mobilised over long distances to a project. They can also be easily relocated during the project. However, as said above, when at work the CSD is stationary with at least two side anchors that are necessary for the dredging process.
Because of these anchors they may obstruct shipping movement in a harbour or access channel. Therefore, since even self- propelling CSDs operate in “quasi-stationary” mode, they are particularly vulnerable when working in shipping channel.
They are also sensitive to wave conditions and rough seas. When working under offshore conditions with waves or swell, they clearly have more limitations than trailing suction hopper dredgers even if equipped with swell compensators. Smaller cutters are limited as to their dredging depths.