Dredge process – Discover Dredging

Dellewal Terschelling: Stunning Example Ancient Building With Nature

Panorama of the port of West Terschelling and constructed mudflat

For most of us, the summer holiday is already long gone. So for me also. Still there is an observation I made that I want to share with you. We have been sailing on the Waddenzee1 this summer. Sailing, anchoring, mudflat hiking2, counting seals and other animals. One of the highlights was a visit to the island of Terschelling. A lot has changed since I was there last time with our boat. It must have been at least 30 years ago. At that time, we had to moor against the quay wall wherever one could find a spot. Now, there is a modern marine in the back of the port, specially made for yachts. Although stacking the boats next to each other is still the standard.

Port of West Terschelling and the new marina

The new marina is more or less in the same area where we used to moor. From there you have an excellent view on a mudflat, that has been there been for ages3. When I was young, I couldn’t understand what its purpose was. Only that old fashioned Dutch flat bottomed boats were still allowed to anchor and stand dry. For us kids, it was an excellent place to muck about with our little dinghy and get dirty walking on the mudflat. We just enjoyed it was there. I still don’t know the exact name of the mudflat. I’ve seen it called ‘Dellewal’, although that seems to be just the quay side over there. Other names that I found on charts are ‘Oostelijk Ras’ and just ‘De Plaat’. Enjoying a nice sundowner and contemplating life and dredging in particular, I suddenly saw the purpose of the area: it works like a ‘Spuikom’4. I really don’t know how to translate this in English. So, let me explain what it is intended for.

Explanation on the working principle of a ‘Spuikom’

Basically it is a part of the mudflat that is located at the south of Terschelling. About 85 ha in surface area, it is separated from the Waddenzee by a low dam. Just high enough to the high water level in the neap tides. The dam has an opening at the back of the harbour, near the marina. I Noticed that the water outside the marina was rushing by and the water inside was practically standing still. I figured out that the rushing tide was used to flush the old port. The huge surface area stores a lot of water that has to pass the quay in the port. Effectively increasing the flow velocity there and reducing sedimentation. This certainly helps in maintaining a navigable depth for marine traffic. Moreover, as each tide the area is filled from all around the dam and mostly emptied through the port, there is a resulting nett transport out of the port.

Hjulström diagram for particle transport in streams (Credit: Wikipedia)

Voila, that is why the old islanders build those dams! Any other person would be satisfied with this plausible answer. Have pity on me, I can’t stop solving the riddles of the sands. Wouldn’t this be easier with a dredge? Apparently, near Terschelling, there is a sedimentation rate of 0.5 to 1 mm per year5, or about 600 m³ annually. With the dam, this has to be kept out of the port with the volume behind the dam. The average increase in flow is about 0.5 knots. According to the Hjuström diagram6, this will transport particles smaller than 10 mm out of the harbour. The stored volume has a potential energy as in a power dam of about 6.25 GJ. This is released twice each tide, resulting in a delivered power of 280 kW. Combined, this results in a specific transport power consumption of 4000 kW/m³/h. No contractor in his right mind will ever use a machine with such a performance. BUT: the energy is free and working flawlessly for at least 200 years. I still have to see a machine doing that. OK. We can step up the analysis even further. Drawing the 280kW continuously from the tide is eventually slowing down the rotation of the Earth. Just for those worried: each year, one day will be in the order of 10^-19 seconds longer…

The moon decelerating earth’s rotation by tugging on the tides (Credit: NASA)

References

Another Fascinating Exhibit To Help You See Through The Dust

New settling and sedimentation exhibit at the Damen Dredging Experience

Sometimes, explaining a part of the dredging process can be as simple as seeing through the mixture. In this exhibit we can demonstrate what happens beneath the surface of the cargo in the hopper1,2,3. Along the way, we explain some quirky behaviour in other phases of the dredging process, also. The exhibit consists of five tubes in a frame, that can rotate around a horizontal axis. In the tubes are various types of soils. Each with their own settling behaviour. The exhibit was recently added to the Damen Dredging Experience. One more reason to highlight it here.

Samples in the settling and sedimentation exhibit

One major part in the dredging process is the hydraulic transport of particles in a carrier fluid. Pickup and transport have been touched upon in previous posts4,5. Here we concentrate on the end of the process: settling and deposition. This can be either in a hopper or on the discharge area. In both cases you will only see the fluid surface during the process and at best the top of the deposited sediment. How the material came there, was deposited and stacked up can’t be readily seen. As the tubes allow these processes to be observed from the side, we can follow the events.

The exhibit can be started by upending the frame with the tubes. The material that sat in the lower end gets now on the top end. They all are released at the same time and we see immediately see the differences in settling velocity for the different particle sizes6. The gravel falls down within ten seconds. The sand is slower and the clay even has problems getting started. One nice observation is the mixture of soils. Against the height of the tube, the fractions in the sample are released simultaneously. Still, the fractions separate over the fall height and stack up again in their original order. This not only happens in the tube. In the hopper or the discharge area, a widely graded sediment will sort itself to the various fractions.

Table of irregular shapes (Source: Wikipedia)

Although for all the samples the particles are released simultaneously, you can still see a slight difference in settling velocity within each sample. This can be either due to slight variations in size that are possible within each mesh size used for sieving. Another cause for the differences might be the differences in shape. A perfectly spherical particle will have a faster settling velocity than an oddly shaped potato7.

Explanation of terminal velocity and hindered settling

And even then, the initial particles that fall down have a greater velocity than the particles in the bulk of the sample, even when having the same particle size and shape. This is due to the water flowing up around the particle. The upward flow is slowing down an adjacent particle. This interaction is called ‘hindered settling’. At high concentrations this can contribute to the efficiency of pipe line transport8. But for the settling it is really hindering the loading time.
At the very end of the settling, the particle gets deposited at the bottom, or on top of another. The water that is caught in between has to escape. This causes one last puff of fluid to flow upward and take the very find dust present upward. This happens with each particle that settles and causes the layer of dust to lift to the surface of the deposited sediment. So even when loading a cargo of gravel, you will always end up with a layer of dust on top. So, don’t judge the quality of the cargo just by the dust you see on top. Take a deeper sample or base your evaluation on the signals from the sensors from the screening tower.

Full cargo load of gravel, covered with dust. And the seagulls know that the dust layer also collects all the snacks

References

Graduation of Arend van Roon: Detecting Flow Regime And Optimising Transport Efficiency

Arend van Roon defending his graduation thesis

Our first happy event this year is the graduation of Arend van Roon. He recently graduated on a project in the slurry test circuit at our Research and Development department at Damen Dredging Equipment1. His research was an interesting investigation in the detection of flow regimes. It gives some insight in the processes involved in the transport of water-solids mixtures. Let me share some details from the background with you, as I think this might be helpful for your own operation also.

Overview of the Damen Dredging Equipment slurry pumping test circuit

At first sight, it is hard to imagine, how something heavier than water, the grains, can be lifted when the fluid is moving. Sape Miedema has written the standard on mixture transport in his book ‘Slurry Transport’, explaining his approach with the ‘Delft Head Loss & Limit Deposit Velocity Framework2’. Without going into the academic details, I will try to help you grasp the gist of the phenomena.

DHLLDV book (Credit: Sape Miedema) — Slurry Transport text book cover (Credit: Sape Miedema)

First the grains have to be picked up. When they are lying on the bottom of the pipe, they are fully immersed, surrounded by the fluid on all sides. The free fluid on top and the pore water between the grains under and on the side of the grains. Now comes Bernoulli’s trick. When the fluid in the pipe starts moving, he says that the local dynamic pressure decreases, while the static fluid in the pores remains at the same pressure. The pressure difference between the pressure in the pores and in the moving fluid, pushes the grains out of the bed into the fluid.

Grain pickup and suspension process explained

Once the particles are in the fluid are in the fluid, they should stay suspended, or they fall back into the bed. The driving force here is the turbulence in the fluid. Usually dredging slurry mixtures are turbulent. This turbulence causes the fluid to flow in eddies. These are little vortices that generally move in the direction of the flow, but in a moving frame of reference tumble in all directions. Mmh, as they rotate in all directions, why don’t they cancel each other out? Now, imagine being a particle yourself, surfing on those eddies. When it is in a fluid, it tends to sink with a certain settling velocity. Independent of the local movement of the fluid. This means, that on the downward side of the eddy, the particle has a higher total velocity than on the upward side. As the eddy is sort of symmetric, the particle dwells longer in the upward draft than on the downward fall. In this infinitesimal time difference, the eddy transfers some extra kinetic energy from the fluid to the potential energy of the particle. As this loss of kinetic energy is compensated by an increase in pressure (remember Bernoulli) carrying grains in a fluid increases the pressure loss in the slurry transport.

Flow regimes and excess hydraulic gradient requirements in dredging slurry transport (Credit: Sape Miedema)

This turbulence is in short the background of suspension in the slurry transport process. Depending on al the various governing parameters: densities, viscosity, diameters, velocities etc, the equilibrium of forces result in several different regimes in the slurry flow. Ranging from homogeneous, through stratified to ultimately a static bed. Each with their own particular pressure losses. And that is what we are interested in. On our dredges, we want to transport as much material to the least amount of energy3. We are constantly looking for improvements in our equipment and sensors to assist the operator in visualising and controlling the actual state of his process4. Thanks to Arend’s project and the promising results, we can set the next step in our product development.

Explanation on slurry flow conditions, critical speed and specific power consumption

Category: Dredge process

Dellewal Terschelling: Stunning Example Ancient Building With Nature

References

See also

Another Fascinating Exhibit To Help You See Through The Dust

References

See also

Graduation of Arend van Roon: Detecting Flow Regime And Optimising Transport Efficiency

References

See also