A trailing suction hopper dredger (TSHD) is a type of vessel that can maintain shipping waterways, deepen sea channels that are in danger of silting up, build new land elsewhere, or replace sand washed away by storms or waves on beaches. This is made possible by large, powerful pumps and motors that can suck up sand, clay, silt, and gravel.
Loading
From the side of the vessel, one or two suction pipes are lowered to the bottom of the seabed. At the end of the pipe, a so-called drag head is attached. This head is comparable to a large vacuum cleaner and is dragged along the seabed. The head has nozzles connected to a high-pressure water installation, which are able to loosen the material on the seabed. Due to the lower pressure in the pipe, the material will be sucked in and discharged into a hopper.
The vessel must always have a positive velocity relative to the ground. It is possible to adjust the density of the substance being sucked in. If the pressure is lowered, more material will be sucked in, but this can damage the installation, since the material can get stuck inside the pipes. After loading the mixture into the hopper, the material will sink and the water will be discharged overboard, which saves storage space. A hopper dredger with a hopper suction head can only suck in relatively loose material, since the steel teeth are not that big. Harder materials such as rock or iron based rock must be broken with a chisel.
Discharge
There are several ways to unload TSHD. The most common method is to dump the material.
- Dumping - This is done by simply opening the doors that close the bin. Once opened, the material will simply fall to the seabed due to gravity. The bin will never be completely empty because there will always be some residue and water inside.
- Pressing - High pressure pumps pump water into the bin to loosen or even turn the material into a liquid, which can then be pumped through long hoses over long distances (such as to shore).
- Rainbowing - This is the same principle as pressing, but instead of transporting the material, it is simply blown away. This is often used to create land.
- Crane - It is always possible to unload the cargo using a crane, but this will be a lengthy process.
