The desired production rate of a CSD will determine the design, size and type of soil the vessel can dredge. CSDs have a high accuracy and a continuous rate of production whether they are standard vessels or custom-made, whether they are being used for dredging rock or sand and gravel, or for construction and reclamation works or environmental dredging. The production capacity designed is directly related to the hardness of the material that the CSD is going to dredge.
Also when designing a CSD, the maximum and the minimum dredging depths must be considered since these influence the viability of the dredger. Often the need for a greater dredging depth leads to a pontoon with deeper draught and thus to a reduction in the minimum dredging depth. And obviously when dredging at minimum depths, the dredger or the pontoon must have sufficient clearance. When dredging in shallow waters, the ladder may also need to be adapted.
Other factors influencing the production rate, besides the type of soil being dredged, include the minimum and maximum width of the cut. This will influence the installed cutter head side winch power, the strength of the ladder, the spuds and the pontoon. Also the type of CSD used is dependent on the accessibility of the work site by water. In some cases only a smaller CSD will be able to reach a site.
WHAT IS A SPUD POLE?
CSDs are always stationary when they are working – even if they are self-propelled. To lock the vessel into a stationary position, a CSD generally has two spud poles. One spud pole (the auxiliary spud) passes straight through the vessel, whilst the other, the working spud, is mounted on a movable spud carriage, which can be moved lengthwise along the vessel or pontoon. Steel cables are used to move the ladder or cutter head from side to side, with the spud in the spud carriage as the Centre of each concentric circle that it describes.
Although the vessel is stationary, moving the spud carriage causes the dredger to move. This is known as ‘stepping’. In this way, the CSD describes an arc round a fixed point – the spud pole or working pole – and the radius of the arc is increased by ‘stepping’ ahead with the spud carrier. In many CSDs this pole is mounted on a movable carriage, the spud carriage. A second pole, the auxiliary spud, is set out of the centreline, usually on the starboard side of the stern of the pontoon. This auxiliary spud is used to keep the vessel in position, when the working spud is raised, and the spud carrier is moved back to its initial position. Since spuds are literally dropped into the soil, they have pointed ends to make sure that they penetrate the soil deeply enough to be secure. They are hoisted out of the seabed and that requires specialised hoisting wires and systems.