This -specialised- article deals with hopperflushing systems in 20.000+ m3 hoppers.
Pumping ashore a load of sand from a hopperdredger, involves mixing the sand with water in the hopper, to make it "pumpable".
Mostly, discharging is done in one, two, or even three passes, whereby the first pass is often called "bulk discharging", with maximum density. Following passes are strictly cleaning passes, with far lower density, so lower output. Depending on cycle time production, the last passes may be skipped.
Datalogging of shore discharge production of a megadredger. In 90 minutes appx. 18.000 m3 is discharged ashore. Blue line is density (t/m3), red line discharge velocity (m/s). In the first 60 minutes, average density is 1.5 t/m3, this is the so-called "bulk"-phase, followed by two shorter "cleaning-up" phases, where density drops considerably.
This mixing proces is done by a system of jetpumps and jetnozzles.
The design of this hopperflushing-system and the position of the nozzles in the hopper are of utter importance to the efficiency of the whole discharge proces.
The jetwatersystem offers mixing capacity to the discharge proces. If this mixing capacity is inadequate, the discharge production will be less than optimal. One important design criterium must be that the jetwatersystem should be able to mix enough sand and water to get the discharge pumps running at their maximum output, usually in the first "pass" only.
Design of the nozzles affects range, impact zone and delivered energy. Left: larger nozzles, right, smaller nozzles.
Little research has been done in recent years on the positioning of jetnozzles in the hopper.
Up till now the layout of these has been a copy / paste operation from smaller hopperdredgers, in the 10.000 m3 hopper volume range.
However, since the advent of the "jumbo" dredgers (year 2000), it became clear that, due to errors of scale, a simple copy of earlier 10.000 m3 hopperdredgers hopper-jetsystems won't do the job in 20.000+ m3 hoppers.
"Jetnozzles offer mixing capacity to the proces of pumping ashore."
Here is why:
1. Jumbo's (and mega's) have wider hoppers. Thus a standard design hopperjetnozzle does not have a sufficient range to vector energy over a longer distance, up to midships.
"Jetnozzles vector a certain amount of energy towards a "target-zone" in the hopper."
2. They have deeper hoppers, allowing the sand to compact more in the deepest layers of the hopper, due to pressure.
3. Larger dredgers run -on average- longer cycli, and are -often- more exposed to wave action, in open sea coditions, allowing the sand to compact more in the hopper. The ship -while saling- acts like a giant vibrating compactor for the sand particles in the hopper.
The capacity of the jetpump-installation (maximum flow, pressure) is a compromise in the whole design of the ship, between the requirements for dredging and discharging, with power limitations.
The jetnozzles -however- have only one function: to fluidise and mix sand and water, towards maximum density, preferably such that the discharge dredgepumps are fed in their maximum output range.
Both hoppernozzle design, and their layout in a specific hopper is still unresearched and often not well understood.
This video is a simple presentation of the process of sedimentation, and -later- compaction between sand particles. The volume of sand decreases, the density inceases, as the spaces -the pores- between the individual particles become smaller.
Angularity ("How round is the sand particle ?") is a parameter for cohesion of he sand. Round sand particles have smaller common friction area's and the sand wil be less cohesive than with more angular shaped particles (left, above).
Sand, consisting of particles with more or less the same diameters, will also contain lots of pores inbetween the particles, filled wih water.
In well graded sand, the smaller particles will take up space between the larger particles, filling up the pores, and leaving no water in between the particles. The sand will turn into a semi-solid mass, with high friction between the individual particles.
Left: restload in a hoppersection with water-saturated sand; the sand mass has lost all cohesion and becomes "quicksand", acting almost ike a fluid. (Gerardus Mercator, 2002, Singapore). Sand dredged off Ramunia shoals, Malaysia.
Right: restload in a hoppersection where the hopperflushing system cold not break up the cohesion of sand particles. Sand became a solid mass during transport. The material is hard to penetrate, even with a pencil. (Gerardus Mercator, 2002, Singapore. The only difference with the first situation: this sand was dredged on the other side of the same sandbank, where particles were more angular, on average.
This process of compaction and dewatering is highly accelerated by:
Discharging sand with centrifugal pumps, is a proces that involves fluidising the sand by adding water.
Fluidised sand, or "quicksand"
This added "process water" will often force its way through aquifers in the sand mass, and even break through the surface of the sand.
Added process-water is divided in two categories:
1. Non-mixture forming process water (like water added in the selfempty channel, or through the hopper discarge line)
2. Mixture forming process water, added through hoppernozzles, putting energy directly into the sand mass, and fluidising it
Water of type (2) is always to be preferred above type (1), because type (2) process water will lead to a higher discharge density, thus production.
Marc Van de Velde