This week, there will be a lot of interesting presentations at the CEDA Dredging Days 2019 in Rotterdam1. I would like to draw your attention to one particular presentation that I was involved with at certain moments in the project, though not in writing the paper. Kudos to my colleagues Frank & Frank to write the interesting manuscript2.
The topic of the presentation will be the change of perspective for the concept of marine aggregate dredging. Historically, the marine aggregate dredging takes place relatively close to shore. With the depletion of the deposits and the increase in demand, other locations further out at sea are coming into focus. As Damen, with a heritage in the design of offshore operating vessels, it was a natural choice to cross breed the offshore supply vessels with the marine aggregate dredges. The resulting offspring: the MAD series of hopper dredges3. Frank de Hoogh will elaborate on the seakeeping abilities of this innovative design.
Other dredging related innovations are the suction tube and ancillary equipment, the screening towers and the process sensors. Of those, I have some personal anecdotes on the screening towers and the density sensor. For all other interesting stories, you’ll have to attend the presentation.
The screening towers are fundamental to the efficiency of the process. If the screening is improved, shorter dredge cycles are possible and a better product can be landed onshore. Also, if the requirement for the product change, the screens have to be adapted to the new specifications. Ideally, this changing has to be done at one unloading phase, otherwise you miss a complete dredge cycle. A lot of effort has been done to optimise the design. But the real test was to actually build, modify and test the complete screening tower, before it was even installed on the vessel. So, we had this construction right here at our doorstep for a thorough evaluation.
One other component, that I was even more involved with, was the non-radioactive density sensor. There are regulations in place to phase out nuclear density sensors4 and a lot of alternatives are available. Back in the !VAMOS! project5, we had the opportunity to test a unit of an electro tomography system. The results indicated a good reliability and a worthy replacement for the traditional nuclear sensor6. Because of the tomography picture, there was an additional benefit: we received an early warning on the slurry behaviour. We could actually see when we were too close to the deposition limit in the pipe line. This enabled us to work with higher densities at lower velocities, resulting in better efficiency and less wear. As the rough process conditions in the mining pit were similar to the marine aggregate dredging industry, we proposed to use this on this MAD also. How we further developed and tested this system is for you to hear and see at the presentation.
Due to the physical processes involved in slurry transport, the mixture does not behave like a normal Newtonian fluid. It is some non-linear viscous substance. At high speeds and low concentrations, it is similar to the carrier water. Slowing down, there is a certain critical speed, where there is a minimum hydraulic gradient. At that flow condition, the specific power consumption of moving a cubic meter of soil is the lowest. So, although working at critical speed is dangerous, it has its advantages: low fuel consumption and less wear. Actually seeing the mixture approaching this critical speed from the deposition is an interesting feature of this new density measuring sensor.
References
- CEDA Dredging Days 2019, CEDA
- Next generation marine aggregate dredger as platform for innovation and basis for fleet renewal, CEDA
- Damen unveils Marine Aggregate Dredger, Damen
- Regeling bekendmaking rechtvaardiging gebruik van ioniserende straling, Staatscourant
- Project ¡VAMOS! Let’s Go Real!
- Real time production efficiency based on combination of non-nuclear density and magnetic flow instrumentation, WEDA