Graduation – Page 4 – Discover Dredging

Ben Sloof signing his Master of Science degree

Do you remember Ben Sloof? Our young bright graduation student presenting at the Young CEDA Pitch Talks? Last week he graduated (with honours!) on ‘Numerical modelling of overflow losses and flow in Trailing Suction Hopper Dredgers’. You may have noticed that hopper loading was already a topic for previous posts about Ben Sloof and Jordy Boone at the CEDA Dredging Days. It results from the great interest in this part of the dredging process. And it still is a very dynamic field of research indeed, as in a short time frame various new models are presented. Who would ever think that dredging technology is a boring business? We are already dredging for ages, so what innovation could ever contribute to the progress of dredging? Well, I’ll explain what this new kid on the block found out now.

Overview of a hopper loading process with OpenFOAM simulation

Due to time constraints on a graduation project, Ben was limited to work on only a part of the hopper loading process. He worked only on the internal interface between mixture and sediment. Moving free surfaces of a filling hopper or a lowering water table from a reduced overflow height were left out of the scope. Still, it quite accurately describes the observed phenomena in the hopper loading process. We see a negative buoyant jet, we see the jet scour pit, recirculation and mixing in the fluid body and a ‘clear’ top layer which carries away the fines as it is skimmed away through the overflow weir. Deviation from the measurements can be explained by the difference in single fraction particle size.

Introduction to the key components of the proposed new ‘Layer Model’ (1DV)

With the observations from the CFD, Ben Sloof made his new ‘Layer Model’ with specifically a single fraction of particles. Then, he made an effort to correct the ‘Layer Model’ for physics of multiple fractions, with not too much improvement. Later on, he introduced a ‘fiddle factor’, that magically happened to be spot on. Further research and development of the model is currently under way to build a better foundation for this. Who knows, even Einstein had to introduce the ‘Cosmological Constant’ to fit his Theory of General Relativity to reality. Years later he was proved to be right1.

The performance of the models by van Rhee and Boone were already discussed2. Now, the validation of the Sloof ‘Layer Model’ is also included. Only after adjustment for the multi fraction in the results, the model nears the performance of the van Rhee model. Difference here is calculation speed. Sloof manages to estimate the hopper performance in seconds. This is why he rightfully received his degree with honours. He improved the speed AND accuracy of the calculation in a method, that would normally take the better part of a PhD study. I am looking forward to his future achievements.

Although this graduation assignment really helps us in refining our design process and gave some insight in the loading and overflow process, it still is an academic exercise. And any scientific model is just as good as the assumptions that were made at the beginning. In reality, the circumstances might be very different from what the model assumed, resulting in quit some different performance.

Heavy weather dredging (Retrieved from YouTube 18/10/2012, unknown source)

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Above is a typical view from the bridge of a trailing suction hopper dredge. I had to visit this dredge, because the operator had problems loading the hopper. As the valuable cargo of sand is always covered beneath a layer of water, they had no idea what the problem exactly was. Although the waterline was dangerously high, the delivered pile of sand on the quay was relatively small. After a couple of dredge cycles, measuring the loading curve and sounding the surface of the sand beneath the water layer, it dawned to me, there were in fact two problems. One, the dredged sediment was much heavier than the hopper density would allow. Two, the distribution in the hopper was not very efficient and on the return trip home, most of the sediment was washed out again. The first problem took some time to convince the owner, but finally the argument came through. The last problem was very difficult to imagine for them and as I had no proof or calculations to show them, they could not visualize the faulty loading process. Luckily, they accepted the proposed modifications and could make some profit again.

Ever since that incident, we were looking for some way to model the hopper loading more credibly with better visual reporting. Simultaneously, this could also improve our design and layout of our hopper dredgers. One of the best models we could use, were those by Van Rhee1, or by Miedema and Vlasblom2. As we have close contact with the Delft University of Technology, we asked them for a graduation project to improve these calculations. Eventually, Ben Sloof applied for the assignment and proposed a very interesting approach to solve this mystery.

Whereas the existing models rely on analytical models with at most a few well defined areas, Ben indicated, that there was also the possibility to address this with a regular CFD solver. He would have to modify the various components to work with particles in the fluid, but he was convinced he could pull it off. As this implies a lot of coding and knowledge of fluid dynamics, this was a tall order and certainly took some time. Now, he is almost ready to graduate. As this is a very interesting topic for everyone, he will take part in the Young CEDA pitch talks3 at the CEDA Dredging Days. His presentation on a ‘Re-engineered model to optimize the settling of material in the hopper’ will certainly be interesting to attend.

Tag: Graduation

Graduation of Ben Sloof: Hopper Loading Model and Overflow Losses

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See also

Hopper Loading: What Happens Beneath the Surface

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See also