Dredge process – Page 8 – Discover Dredging

Hydrogen Sulphide: How Nature Uses it and Dredges Get Rid of it

Bison grazing near Mud Volcano, Yellowstone National Park, Wyoming, USA

Yellowstone National Park is not only renowned for its active geology. There is also an abundant wildlife, roaming free over the area. One would guess, the local wildlife would be accustomed to these interesting geological features. Or would be disturbed by all the tourist gathering at the literal hotspots in the park. Well, you might be surprised. At times, e.g. bison tend to flock around the same hydrothermal features that attract the tourists. You don’t notice it from the pictures, but when you are there, you’ll smell it instantly: rotten eggs. As a dredger, you know what this means: danger!

The odour of rotten eggs is caused by the toxic gas hydrogen sulphide1. At Yellowstone National Park, this gas is released by the fumaroles, mudpots and geysers2. The bison might not be aware about the danger of inhaling this vapour. But they do know, that all the bugs and parasites, that live on their skin have a lower resistance for the toxin and fall of their hosts. Well, sometimes the bison fall for their own trap and get intoxicated themselves3. That is one of the dangers of hydrogen sulphide, above a certain a threshold, your senses get numbed and you don’t recognise the danger anymore.

Gas bubbles expanding in the vacuum of the dredge pump

In dredging operations, hydrogen sulphide usually has a biological origin, rather than a geological. In seasonally warm water, algae bloom near the end of the summer and die some time later. The decomposing biomass can release hydrogen sulphide amongst methane and carbon dioxide. Quietly trapped in bubbles between the grains in the sediment, they get disturbed by the dredge and enter the suction line. It is only in the dredge pump, that these bubbles get expanded and wreak havoc to the dredging process.

Gas removal concept before the dredge pump

The trick is to remove the gas bubbles before they enter the dredge pump. Several systems do exist but usually the inspection piece is modified and separates mixture and gas. It is up to the rest of the system what happens with the foam that gets extracted. It might be blown overboard or properly re-handled responsibly to protect the environment. Either way, the dredge pump will be able to operate at a more continuous load, the mixture density increase and total production will be higher.

Example of production increase in relation to gas removal rate

A good write-up about the dangers of hydrogen sulphide in dredging4 can be found at ‘The Art of Dredging’. There is also an article about the application of degassing systems to lower the dangers of hydrogen sulphide5. Even as the vapour is released at a single location on the ship, you still have to be aware of what you are doing and operations have to be adapted accordingly.

My personal experience with this nasty gas is only limited to commissioning degassing systems6, not actually working with them over longer periods. Even so, I got my impressions of life aboard under these circumstances. There was one occasion, where just in the week before our commissioning of our delivered degassing system, there was a severe accident. During our commissioning trials we had several warning alarms and whenever we went outside we had to wear personal gas detectors. If you did not report within an agreed period, alarms would ring and a search party dispatched. So, I am happy for every degassing installation delivered. It saves lives and fuel and cares about the environment.

Example of a standard degassing installation from Damen Dredging Equipment

References

Graduation of Ben Sloof: Hopper Loading Model and Overflow Losses

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)

References

IADC Young Author Award for 1DH Hopper Loading Model of Jordy Boone

Jordy Boone receives the IADC Young Author Award at the CEDA Dredging Days

OK, one last time we will revisit the CEDA Dredging Days. There was so much to see and experience, that there could be some more posts about them. However, daily life already demands more of my focus and there are fun facts to tell about them also. The conclusion of this series is about a well-deserved IADC award to Jordy Boone. Both, because he did write a terrific article and we will read more about it in the Terra et Aqua journal soon enough.

So, why was his article and presentation so special? Hopper loading is one of the key process components in the production cycle of a trailing suction hopper dredge. And therefore there is a lot of interest in this subject. Countless articles and numerous experiments have been performed on this topic, resulting in a lot of different hopper models. Traditionally a literature review starts with Camp (1936). And there are a lot of models that build on that approach. Camp and the derived models are similar to the Lagrangian approach, where they follow the trajectory of a single particle.

Basics and relations of Camp and derived models

A whole different approach is to follow Euler and fix the frame of reference. The contents of the system has to be modelled as a continuum. Ground breaking work has been done by van Rhee. He modelled the hopper in a 2D environment and based a more comprehensive 1DV model on that. Others have followed up and so does Jordy Boone.

Other approaches don’t take any physics in their modelling, but consider the hopper as a process block in a control system. At the moment they are only useful for monitoring and controlling an existing system, they don’t have a predictive value, yet.

In this overview, we can see, that the 1DH Boone model is part of the Euler family. Normally solving multi-dimensional 2D and 3D Euler problems tend to be slow. Van Rhee already pointed this out and part of his PhD. thesis was the presentation of a more comprehensive 1DV model. Basically a column cross-section through the hopper. Mixture would be deposited on the bed and water flows up and out of the system. Where that model does lack the influence of the density current, Boone literally upended this simplification by using horizontal strata in the hopper. Here, the mixture section can incorporate the horizontal density current conditions. This will give correct mass and momentum equations. Vertical exchange processes are than calculated by closure relations.

On top of the cake, Boone also verified his approach in laboratory experiments and prototype measurements. As his manuscript is also well written and accessible, he rightfully received the IADC Young Authors Award. Keep up the good work Jordy, we hope to see more interesting work from you.