Trailer dredgers are equipped with centrifugal dredgepumps, to remove soft seabed materials.
Maintenance dredging often involves removal of mud, which may contain gas.
This gas may be hydrogen sulfide, carbon dioxide, or methane gas, but other gasses can be present. These gases are often the result of the bio-chemical breakdown of organic matter in mud, or originating from sewage outfalls.
Dredgers often operate in areas where natural gases are trapped or dissolved in the dredged material. As the soil comes to the surface and is exposed to the vacuum in the dredge pump, the gas releases, expands and reduces the density of the mixture. This can radically reduce dredging efficiency, with as little as 5% gas by volume in the soil, and can completely stop the flow of dredged material.
Centrifugal dredge pump.Pipe operators anticipate these problems by hovering the draghead(s) in the top of mudlayers, avoiding a high vacuum; but density of dredged material is notariously less with this method.
These problems has lead to the development of degassing systems. These systems are especially popular on smaller dredgers, involved in maintenance dredging in rivers and ports.
How does a degassing system works ?
The idea is simple: the gas has its highest volume where is the lowest pressure; this is: in the heart of the impeller of the centrifugal pump (color blue in this animation):
In this position in the dredgepump, a pipe is inserted, on which a slurrypump is connected. This slurrypump extracts a mix of gas and mud from the dredgepump.
There are lots of variations to this system, but we will describe a common system here.
Since jetwater is mostly not used when dredging mud, this jetwater can deliver the power for the extraction of the gas/mud mixture from the dredgepump.
A special inspection pipe in front of the dredgepump is used, with a "tongue" that reaches into the heart of the impeller.
Above: the special inspection pice; dredgepump to the right. The blindflange on top is replaced by the pipe with "tongue" , on the left photo. This tongue reaches into the centre of the impeller and removes gas, and mud.
In the system described here, (developed by Damen) a separation tank is connected with the gas-extraction pipe in the dredgepump. The separation tank is kept under vacuum by a set of ejectors, placed on deck.
Principle of ejector
In the separation tank, gas rises to the top of the tank, and mud sinks to the bottom.
The mud is often returned into the dredgeline via a mud-return-pump (or silt-pump), while the gas evacuates through the ejectors on deck; in an overflow tank.
Typical overflowtank on deck (right). To the left two ejectors, driven by jetwater. The ejectors keep the separation tank under a varying vacuum, steered by the levelsensors on the separation tank.This (DAMEN) system has two different sizes of ejector, which -in combination- can give four different configurations (all ejectors closed, small ejector open, large open, both open), to regulate flow.
The separation tank has (mostly three) level sensors, which sense the interface between gas and mud in this tank (photo left).
The value of the level in the separation tank is used in a PLC, that actuates the ejectors, switching on and off ejectors as the level gas and mud rises or falls in the separation tank.
Control-panel for a degassing system as described here.
The whole process changes so fast, that it has to be automated, switching the ejectors as the level-sensors demand.
Degassing systems make the dredgeprocess continuous, with much higher densities pumped. Claims that hopper density rises by 15% are in no way exagerrated.
A system as described above, also makes good environmental sense: all mud is retained inside the ship, and the draghead(s) can lower deeper into a mudlayer, causing less turbidity in the water.
Another possible setup, almost unique, is a degasser on an underwaterpump.
Twin gas extraction pipes on the suction side of an underwater pump. (Dredgepump to the right.)
The extraction pipes are directly connected to three ejectors, jetwater-powered. Nothing fancy here, all valves are manually operated; no tanks, no automation.
The gas-mud-mixture is immediately released in the aquatic environment.
And fish don't vote.
A more elaborate idea is this degasser retrofitted onboard the French 8500 m3 TSHD "Samuel De Champlain", provided with a single underwaterpump.
This ship is operated by the French "Dragages des Ports" and is a highly specialised maintenance dredger for major ports in France.
The underpipe carries a complete separation tank, and a mud-return-pump. The whole system works much the same as described earlier, for an inboard dredge pump.
Marc Van de Velde