Dredge-pump ship

Trailing suction hopper dredger is a ship that has a full sailing capacity used to maintain navigable waterways, deepening the maritime canals that are threatened to become silted, to construct new land elsewhere or to replace sand eroded by storms or wave action on the beaches. This is made possible by large powerful pumps and engines able to suck sand, clay, silt and gravel.

Operation
Properties
A trailing suction hopper dredger is self-loading/unloading and sometimes equipped with a pressurized discharging system.

Loading
From the side of the ship one or two suction pipes descend to the bottom of the seabed. On the end of the pipe a so-called trailing drag head is connected. This head is comparable to a large vacuum cleaner and is trailed along the seabed. In the head there are nozzles connected to a high pressure water installation that are capable of loosening the material on the seabed. Due to lower pressure in the pipe, the material will be sucked inward and discharged in the hopper.

The vessel should always have a positive speed over the ground. It is possible to regulate the density of the sucked substance. If the head is lowered, more material will be sucked but this risks damaging the installation because the material can get stuck inside the pipes. Once the mixture is loaded inside the hopper, the substance will sink and the water is discharged overboard, saving on storage space. A trailing suction hopper dredger can only suck relatively loose substance because the steel teeth are not so big. Harder substances such as rock or ironbased rock must be destroyed by a cutter.

Discharging
There are several ways to discharge a TSHD. The most common way is dumping the material.

    Dumping - This is done by simply opening the doors that close the hopper. Once opened the substance will simply drop to the seabed because of gravity. The hopper will never be completely empty because there will always be some residue and water left inside.
    Pressing - High pressure pumps will pump water inside the hopper to loosen or even liquefy the substance, which can then be pumped via long hoses over great distances (f.i. ashore)
    Rainbowing - This is the same principle as pressing, but instead of transporting the substance it will simply be blown away. This is often used to create land.
    Crane - It is always possible to discharge the load with a crane, but this will be a time consuming process.

Equipment
A trailing suction hopper dredger is equipped with the following equipment:

    One, or more, rearward extending suction pipes
    One or more, Dredging pumps to create an under pressure in order to suck the substance into the hopper
    Transportation tubes to transport the substance from the pipes to the hopper
    An overflow to discharge the redundant water overboard
    Degassing installation to extract any possible gas from the substance in order to reduce damage and increase fluency

Functions
A dredging vessel and particularly a TSHD is mostly used for

    Maintaining the depth of a harbor, canal or waterway.
    Constant delivery of sand for land reclamation or beach nourishment.

en.wikipedia.org

Dredging is the operation of removing material from one part of the water environment and relocating it to another. In all but a few situations the excavation is undertaken by specialist floating plant, known as a dredger. Dredging is carried out in many different locations and for many different purposes, but the main objectives are usually to recover material that has some value or use, or to create a greater depth of water.

Description
Dredging is the form of excavation carried out underwater or partially underwater, in shallow waters or ocean waters. It keeps waterways and ports navigable, and assists coastal protection, land reclamation and coastal redevelopment, by gathering up bottom sediments and transporting it elsewhere. Dredging can be done to recover materials of commercial value; these may be high value minerals or sediments such as sand and gravel that are used by the construction industry.

Dredging is a four-part process: loosening the material, bringing the material to the surface (together extraction), transportation and disposal.

The material can be brought to the surface by suction or mechanical means.

The extract can be disposed of locally or transported by barge or in a liquid suspension in kilometre long pipelines. Disposal can be to infill sites, or the material can be used constuctively to replenish eroded sand that has been lost to coastal erosion, or constructively create sea-walls, building land or whole new landforms such as viable islands in coral atolls.

History
Ancient authors refer to habour dredging. The seven arms of the Nile were channelled and wharfs built at the time of the pyramids (4000 BC), there was extensive harbour building in the eastern Mediterranean from 1000 BC and the disturbed sediment layers gives evidence of dredging. At Marseille, dredging phases are recorded from the third century BC onwards, the most extensive during the first century AD. The remains of three dredging boats have been unearthed; they were abandoned at the bottom of the harbour during the first and second centuries AD.
During the renaissance da Vinci drew a design for a drag dredger.

Purposes of dredging
Maintenance: dredging to deepen or maintain navigable waterways or channels which are threatened to become silted with the passage of time, due to sedimented sand and mud, possibly making them too shallow for navigation. This is often carried out with a trailing suction hopper dredge. Most dredging is for this purpose, and it may also be done to maintain the holding capacity of reservoirs or lakes.

Land reclamation: dredging to mine sand, clay or rock from the seabed and using it to construct new land elsewhere. This is typically performed by a cutter-suction dredge or trailing suction hopper dredge. The material may also be used for flood or erosion control.

Capital dredging: dredging carried out to create a new harbour, berth or waterway, or to deepen existing facilities in order to allow larger ships access. Because capital works usually involve hard material or high-volume works, the work is usually done using a cutter suction dredge or large trailing suction hopper dredge; but for rock works, drilling and blasting along with mechanical excavation may be used.

Preparatory: dredging work and excavation for future bridges, piers or docksor wharves, This is often to build the foundations.

Winning construction materials: dredging sand and gravels from offshore licensed areas for use in construction industry, principally for use in concrete. This very specialist industry is focused in NW Europe, it uses specialized trailing suction hopper dredgers self discharging the dry cargo ashore.
    
Contaminant remediation: to reclaim areas affected by chemical spills, storm water surges (with urban runoff), and other soil contaminations, including silt from sewage sludge and from decayed matter, like wilted plants. Disposal becomes a proportionally large factor in these operations.
    
Flood prevention: dredging increases the channel depth and therefore increase a channel's capacity for carrying water.
    
Fishing dredging is a technique for catching certain species of edible clams and crabs. In Louisiana and other American states, with salt water estuaries that can sustain bottom oyster beds, oysters are raised and harvested. A heavy rectangular metal scoop is towed astern of a moving boat with a chain bridle attached to a cable. This drags along the bottom scooping up oysters. It is periodically winched aboard and the catch is sorted and bagged for shipment.

Other
Harvesting materials: dredging sediment for elements like gold, diamonds or other valuable trace substances. Hobbyists examine their dredged matter to pick out items of potential value, similar to the hobby of metal detecting.
    
Beach nourishment: this is mining sand offshore and placing on a beach to replace sand eroded by storms or wave action. This enhances the recreational and protective function of the beach, which are also eroded by human activity. This is typically performed by a cutter-suction dredge or trailing suction hopper dredge.
   
Peat extraction: dredging poles or dredge hauls were used on the back of small boats to manually dredge the beds of peat-moor waterways. The extracted peat was used as a fuel. This tradition is now more or less obsolete. The tools are now significantly changed.
    
Removing rubbish and debris: often done in combination with maintenance dredging, this process removes non-natural matter from the bottoms of rivers and canals and harbours. Law enforcement agencies sometimes need to use a 'drag' to recover evidence or corpses from beneath the water.
    
Anti-eutrophication: A kind of contaminant remediation, dredging is an expensive option for the remediation of eutrophied (or de-oxygenated) water bodies; one of the causes is like mentioned above, sewage sludge. However, as artificially elevated phosphorus levels in the sediment aggravate the eutrophication process, controlled sediment removal is occasionally the only option for the reclamation of still waters.
    
Seabed mining: is a possible future use, recovering natural metal ore nodules from the sea's deepest troughs.

en.wikipedia.org