Currently I am in Gdansk, Poland. Last week I had a CEDA event1 and next week I am at the Hydro 2018 Conference and Exhibition2. On both events I will report later. My colleague Saskia den Herder wrote a teaser for you3. Now, here, I had the weekend for myself and what better to do, than be a tourist, visit a maritime museum and write a blog about it. So, I will report you about interesting dredging exhibits I discovered at the National Maritime Museum in Gdansk4.
The National Maritime Museum comprises three major venues: the museum building itself5, the ‘SS Sołdek’6 and the old city ‘Crane’7. All equally interesting in their own way. Buy a combined ticket and you get the ferry between them for free. As general maritime museums go, they mainly focus on the history of shipping, shipbuilding and the interaction with the development of the city or country. Gdansk in itself has a very long history in shipbuilding, as the country was well forested for providing the building material for ships. In modern times, one might have heard of the ‘Lenin Shipyard’8, the birthplace of the free labour union ‘Solidarity’, which brought Poland out of the socialist led economy. And of course, where there is water, there are ships and where there are ships, there is Damen9,10.
Between the many models and pictures I found some about dredges indeed. This one seems to be a very first attempt at mechanical dredging. The power was provided by two real horses. There were some sort of scoops or blades drawn over a chute. The wooden blades excavated the soil from the bottom. Water was expelled through holes in the blades. The drained material could be loaded in barges for further transportation. Only after translation later on, I learned that in fact this was an example of a Dutch dredge!
I did find a picture and a model of a locally build dredge. It employs a German steam engine and was built on an oak hull. It already featured the classic iron buckets on a ladder. The development of the working principle did not change that much. The dredged material could be delivered to barges at the aft end for further transport.
Finally I came across this model. It is a grab dredge ‘Homar’11, built in 1971 and operated by PRCiP Sp.z o.o. (Dredging & Underwater Works Co Ltd) here in Gdansk12. OK, I don’t want to brag, but it looked vaguely similar to the one we saw when we were on a site visit with the CEDA to the Port of Gdansk13. We had a splendid view over the harbour from the port control tower. And there I already noticed they were doing some dredging works in the entrance channel. But for all what we could see, it could also have been its sister ship ‘Świdrak’. And that concludes a nice round up of dredging discoveries for the weekend.
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.
Yesterday, Suman Sapkota graduated at the University of Twente on: ‘Technical and Sustainability Analysis of Sediment Erosion of Impeller Blades of Dredge Pumps’. We are very proud he worked for us on this topic and graduated with a grade 8 for it. As such, he stands in a long line of Master students1, who graduate with an 8 or even higher2. We value good students and we like to work with them only if the work they deliver is useful for us. This requires an intensive supervision and the results are correspondingly. This does not guarantee a good result, but you can always try3.
Suman graduated on a topic covered by the chair of ‘Sustainable Energy Technology’4. The objective was to study the economic and environmental impact of worn wear parts. When wear parts have to be rejected too early, they increase their environmental footprint. On the other end, if an impeller is severely worn, the efficiency decreases and the environmental footprint increases also. Our question was whether it was possible to improve the design of the dredge pump for a longer lifetime by checking the wear rate of the improvement in a simulation. To understand the problem and answer the question Suman had to start investigating the wear process itself.
From literature he evaluated different erosion models. Basically, what is the effect when a defined particle impacts the concerned surface under certain conditions. And the conditions will be depending on the flow pattern and the influence of the fluid on the particle trajectories. The second research part was the investigation of this flow pattern and simulated with a CFD application. The flow pattern and the particle properties were combined. This way, the movement of the particles can be traced from inlet to outlet.
Particle tracking in CFD simulation of an impeller
Between inlet and outlet, the particles bounce against several areas of the blade. The bouncing can be counted and mapped. Together with the information of the impingement conditions and the erosion model, an estimation of the erosion rate can be made for different flow conditions and soil properties. The mapping can be transformed into an erosion density map. These maps can be checked for pump capacity and particle sizes.
With these maps, we can finally evaluate in advance where impellers will wear down and if we can improve their lifetime by modifying the design. Thus we can reduce their environmental footprint and contribute to a more sustainable business.
Suman, thank you very much for working for us. You were a pleasure to work with. We wish you good luck on your career and a happy life.