A trailing suction hopper dredger (TSHD) unites dredging, transport and discharge in one single vessel. As a result, the dredging cycle of a TSHD is split into four phases: sailing empty, loading the hopper, sailing to the discharge area and discharging the load to shore, after which the cycle starts again.
The first step in the dredging cycle is sailing empty to the area to dredge. In most situations, this means that the TSHD sails from the discharge site to the dredging site.
Once the TSHD has arrived at the loading site, the suction arm(s) are lowered off the side of the vessel. The dredge pumps are activated and the dredging begins. When dredging, a mixture of soil and water is pumped into the hopper. At a certain moment, the maximum load of the hopper is reached, and it is filled with sand or gravel and water.
To load more soil, the surplus of water is flushed overboard through the overflow. The overflow is not always used, especially when contaminated materials are dredged (e.g. with maintenance dredging). Most of the particles in the soil will settle in the hopper, but a small part of the sediment flows overboard together with the water. The lighter the soil, the higher the overflow losses are. The losses also tend to increase towards the end of the loading process, especially in lighter sands, so a hopper will never be completely filled with soil.
Loading characteristics of a TSHD depend on the capacity of the dredging installation, the soil to be dredged, the dredging depth, the size of the hopper and the overflow losses that occur during loading. The loading time for a normal TSHD is around one hour. For a gravel dredger, this can be much longer.
After loading, the suction arms are raised and the TSHD sails to the discharge area. Sailing times may be short, but can also be very long as is frequently the case during offshore gravel operations or in some land reclamation projects. In case of long distances between the dredging area and the discharge area, the overall production is strongly influenced by the sailing time, as is the dredging cost per m’ or tonnes of dredged material.
TSHDs have three methods of discharge. Based on the aim of the dredging project, the project phase and the installed discharge equipment, discharge takes place by:
- Dumping: a normal TSHD discharges its dredged material by dumping through bottom doors – or a comparable bottom discharge system. In case of sand dredging with a conventional TSHD, discharge via the bottom doors takes approximately five to ten minutes.
- Pumping ashore: using the vessel’s own pumps, the sand or gravel is pumped ashore via a pipeline connected to the bow coupling. Pumping ashore time depends on the circumstances, like soil characteristics and discharge distances, but takes around 1.5 hours (for sand). In the case of offshore gravel dredging, the material is generally brought more or less dry to the shore. Special equipment is required and the time for unloading is much longer.
- Rainbowing: this technique is used in shallow locations or ashore, for instance in land reclamation and beach nourishment projects. The sand from the hopper well is discharged to a certain location using the rainbow installation. The name rainbowing refers to the arc shape in which the slurry comes out of the installation. To empty the hopper by rainbowing, even more time is needed compared to pumping ashore.
After the discharging phase, the TSHD will return to the dredging area to continue the dredging cycle.
Production of a TSHD
The total time required for one dredging cycle of a TSHD is called the cycle time. The effective production of the dredger is calculated by dividing the hopper load by the cycle time. The overall production is highest when the hopper load is as high as possible and the cycle time as short as possible.