The Art of Dredging
Dredging and shipping
The very basics of trailer dredging.
This article only wants to show the very basics of trailer dredging.
Example: a small (3000+ m3 trailer with one suction pipe and underwaterpump).
On the drawing above we indicated the three main components the dredge operator has to follow-up and control, to achieve maximum output:
- Draghead
- Dredgepump
- Hopper
Each of these components influence each other, in the following way:
1. Draghead
The draghead is that part of the system that is in touch with the seabed, excavating material, scraping it off the bottom.
Apparently, following parameters influence the "cutting production" of the draghead:
- weight of the draghead (indirectly controlled by swell compensator pressure),
- flow and pressure of jetwater,
- number of scrapers, pickpoints, area of cutting,
- ship's velocity,
- etc...
There is an optimum to be found in all that.
If that would be an "active draghead", the dredge operator has lots of extra possibilities to control the density and velocity of the mixture, "real time".
2. Dredgepump
This particular pumpgraph has a maximum efficiency at around 2.5 m3/s (by which you can calculate mixture velocity in m/s if you know the diameter of the velocity meter pipe).
The efficiency graph in this example is pretty flat, which allows us to operate the pump at between -say- 1.5 to 3.5 m3/s, which is a wide efficiency range.
Note: this is a graph for a dredgepump pumping water. When density rises, the whole thing becomes much more complicated.
So: first equation: the operator may adjust the draghead so that the resulting flow is close to the maximum efficiency as in the pumpgraph.
That sounds ok, and it's pretty easy to do; he has a lot of parameters to control that draghead.
Unfortunately; it's not that easy, because there is a third component in the equation:
3. Hopper
Once the mixture of sand and water enters the hopper, another process develops: soil particles settle down in the hopper, water flows out through the overflow.
Animation: soil particles settle down in a liquid, no horizontal flow.
The reality in the hopper is more complex than the animation in the movie above:
- There is horizontal flow in the hopper, from pump discharge towards overflow, and
- this flow is not constant.
- Soil particles are not homogeneous. We load a mixture of fine and coarse particles.
The settlement behaviour of soil particles is one of the least understood processes in dredging... Follow some of the links at the bottom of this article, to know more.
Sketch: loading in hopper.
The sandbed in the hopper rises during loading, resulting in higher mixture velocity (Q=v*A) over the settled sand.
Thus, sand will settle more difficult during the last stages of loading, and especially the fine material will be lost through the overflow.
Sedimentation with horizontal flow (more here).
Following diagram (Hjulstrom) shows what happens with particles, when in a horizontal flow.
Will the particles
- settle down (deposition, sedimentation),
- or simply be transported horizontally
- or will particles even be picked up from the sandbed (erosion) ?
In fact, every pipe operator knows this diagram from experience.
In the hopper, density of soil particles is high, and particles will hinder each other to settle down, this is "hindred settlement". So, the situation in the hopper is less favourable for sedimentation than this diagram shows.
See diagram above: in the first stages of loading, mixture velocity is low, almost all particles settle down. As the sandbed in the hopper becomes higher, free flow section becomes smaller, higher mixture velocity, less favourable settlement conditions.
(The yellow area's represent more or less the "normal" working area's for trailer dredgers; sandy non-cohesive particles with a particle diameter between 0.05 and 1 mm)
Every change of dredge parameters on draghead and pump (by the dredge operator) will influence the sedimentation rate in the hopper, thus influencing production output of the dredger.
Almost never do all the components (draghead, pump, hopper) match perfectly together, mostly by a combination of particle size distribution, particle form, dredging depth, etc...
That's where the dredge operator steps in. To dredge is to average, to compare, to make choices, to adjust, to comprimise.
Marc Van de Velde
Check out more movies on transport over the sandbed, illustrating the Hjulstrom diagram:
http://faculty.gg.uwyo.edu/heller/SedMovs/bedload.htm (7.8 Mb) and http://faculty.gg.uwyo.edu/heller/SedMovs/Dietrich.htm (11 Mb)
Some articles on hopper sedimentation:
http://www.terra-et-aqua.com/dmdocuments/Terra-et-Aqua_nr76_02.pdf
http://saas.ulb.ac.be/benelux2005/upload/182.pdf
http://www.terra-et-aqua.com/dmdocuments/Terra-et-Aqua_nr86_03.pdf
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