We have another bright new MSc. engineer in dredging technology1: Gijs ter Meulen. Tuesday, he presented and defended his thesis on the forces and production of a trailing suction head. For this thesis project he was working at our research and development department at Damen Dredging Equipment2.
Trailing suction hopper dredges have become the tool of the trade for modern dredging contractors. They are versatile, flexible and able to transport sand over great distances. They load their cargo in their holds, by sucking up the sediment from the sea bottom with a big trailing suction head. This head looks like an out of size vacuum cleaner head.
Usually, it is very difficult to comprehend what is going on in and around the drag head. There is some laboratory research done, but not all results are freely available. Other knowledge is solely based on the experience of well-seasoned dredge masters. I do have respect for the experience of dredge masters, but their stories are hardly usable for an academic model description. So, Gijs took on the challenge to piece together a model, that satisfies our curiosity and fits with the experiences.
For this project, he identified several steps, which we briefly touch upon here:
The processes and forces around the drag head3 were all investigated on their cause and effect.
A model was set up, where each process and their interaction with the others were identified.
One main process in the drag head is the jetting production. A powerful jet of water is injected into the soil and this erodes part of the sediment under the drag head4.
Another main process is the cutting production5. What is not eroded away by the jets, is removed by the teeth at the back of the visor.
As the contribution of the processes to the forces and the production is known, the total performance can be calculated.
Along the way, this gave us very useful insight in the capabilities of the drag head and the trailing system, all the way to the requirements for the propulsion. Now, we will be able to continue to improve our drag heads even further. Any other students who would like to participate in that project are welcome to contact us5.
Gijs takes a new step in his career path. He is going to work for a well esteemed customer of us, so we will see him around in the dredging industry. Thanks Gijs, bon voyage!
Last week, I was at a test site for the ¡VAMOS! project. It was in the abandoned Magcobar Pit1. Well, only in ancient times, people have been digging for silver at this location. In 1963 the site was opened for a large scale operation to mine Baryte2. A mineral used in the oil industry. After the veins ran out in the open cast mine, they continued underground, extending the tunnels almost to the next mine in an adjacent area. Then they had to stop operation altogether and the pit got filled with water from a small river that now enters in an attractive waterfall. So far the historic perspective of this site. Currently the forgotten mine is bustling with activity. So many people and equipment was brought in from all corners from Europe, it looked like a circus has come to town.
When I came there, the team had already set up camp and assembled the Launch and Recovery Vessel. The Mining Vehicle was ready to be deployed and we could commission and test the special drive we had provided for the project3. Another item we had to commission was the dewatering facility. Well, it is a fancy name for a dump area. Basically, it is a reclamation area, where we can collect the material that has been cut by the mining vehicle and was pumped through the discharge line to the shore. We had one pond for collecting the cuttings and one pond for containing the fines. Eventually all effluent water was skimmed trough an overflow box.
As there might still be some very fine material contained in the overflow water, we wanted to dispose the water at a lower level than where we were going to do the cutting tests with the mining vehicle. This would ensure clear visibility and an undisturbed background turbidity for the effects measurements. A submerged pipe line for dredging is a well-known component in the dredging industry. If e.g. a discharge line of a cutter suction dredge has to cross a busy fairway, the discharge line is submerged under water, so the traffic can pass without interruption.
There are several issues to pay attention to. Selecting the right diameter is the first to consider. You definitely don’t want to block the submerged dredge line. In order to reduce the critical velocity and increase the mixture velocity, the diameter of the submerged pipe is usually chosen a bit smaller than the rest of the pipe. Furthermore, you don’t want any air get trapped in the submerged line. Air inside the line will make the pipes float again, usually at the most inconvenient moment and probably damage the line. Positioning the submerged line can be done by actually having the air in the line slowly escape. The line will lose buoyancy and settle on the bottom. Injecting compressed air will float the line again.
Deployment of a submerged dredge line
The mining we are doing at the Magcobar Pit is solely for scientific purposes. The material we gather and sample will not be used. But, we hope our technology will revive some disused mines again to their former glory. At least get some people back at work. There are a lot of little villages that fully depended on the activity of the mine. The little town of Silvermines is still remembering those good old times with a little monument to commemorate a glorious past4.
Ben Sloof was one of the best graduate students we had here at our company. For his thesis1, he tackled a complex problem and managed to model this in a reliable simulation. Now he is nominated for best Offshore Graduate Student. Today, there will be a KIvI Offshore lecture evening with a ceremony to award the prize2. Once again3, Ben will deliver a capturing pitch on his thesis. So, let’s review what he has achieved.
At the chair of Dredging Technology of professor van Rhee, a lot of effort is put in describing the hopper loading in so called ‘Euler’ models. This is where you calculate the flow of the fluid and derive the flux of material that is carried within. Ben is standing on the shoulders of giants here, as by now there are a lot of models available4. We opted to use an existing simulation platform: OpenFOAM. One of the plugins for this open source program is DriftFlux, where the valuable grains are treated as a continuum fluid moving through the rest of the fluid. The extra effort of Ben, was to modify this DriftFlux plugin to account for settling and scour. This is in itself is already an unprecedented feat. Complicated by the unstructured calculations within DriftFlux and OpenFOAM. Nonetheless, after careful verification, he was able to perform interesting simulations of the hopper loading process.
After careful examination of the simulations, Ben started to see patterns in the flow. These set him on a track to build a whole new model. This new layer model credibly describes the process as well, without the complexity of a CFD simulation. As the development of a multi-fraction version of the OpenFOAM platform is still in progress, final verification is still pending. At least, the differences we see between the single fraction model and reality can be explained by what can be expected. It is open to further expansion with future research and can be used as a starting point for the next improvement.
And that is an insight worthy of extra appraisal: finally cracking the riddle of the sands settling in the hopper. We hope you will receive the prize. You deserve it.
Good luck Ben, we wish you all the best on your future voyages to unknown destinations. We are sure you’ll be blessed and a blessing, wherever you go.