Cutting – Discover Dredging

Personal Announcement: Going Back To School To Cut Some Clay

Learning early or later in life, studying is always a joy when you make it practical

‘Never too old to learn’ is my motto. Everyday I look around me and I wonder how this beautiful world fits together. Whether it be the stars in the sky, the waves at sea or life as we know it, there is always something to be learned about it. At school, I was not a great pupil, but I was always curious to learn more. For my master thesis at the Delft University of Technology, I investigated the performance of a dredge and made recommendations to improve its operation1. As the project was more focussed on mixture forming (and turbidity) and the redesign of the auger head, there was no attention for the soil mechanics involved in the cutting process.

Fully covered cutter head in sticky clay

Now is the time to get that straight. In my daily business, I came across several projects where the clay cutting was a real problem. This was one of the triggers that sparked my interest in sticky clay and made me pursue a more detailed investigation into this nasty stuff. I am very grateful my management was willing to grant me time to go back to the university and start a PhD project with professor Cees van Rhee to learn more about clay.

Synthesis of clay and the relevant properties for dredging

Clay is a completely different material than sand or rock. Those are either plastic and non-cohesive or elastic and cohesive. Clay is the worst of both worlds: plastic and cohesive. It can be described with certain soil parameters as e.g. undrained shear strength and internal friction angle. The failure model is based on Mohr’s circle etc. But those are all continuum approaches2. When you zoom in to the particle level of clay, a whole new world opens up. I already wrote about the interesting particle interaction in a previous post3.

Boltzmann strain rate function in clay cutting

It appears, that the consistency, deformation and failure of clay is related to the tiny electric charges distributed over the platelet crystals. The movement along the charges needs energy. The model to describe dislocation energies along electric charges has been studied by Ludwig Boltzmann4,5. His model governs a wide range of applications, ranging from cosmology to particle physics. I really plunged into the deep end of science with just simple clay. It already took some time to get my head around the concepts involved. Slowly it dawns on my what possibilities there are to improve our understanding of the cutting of clay and possibly to improve our products eventually.

Gallery of my dredging professors (l) prof. de Koning, (m) prof. Vlasblom, (r) prof. van Rhee

My ‘old professor’ de Koning was a proponent of ‘thinking with your hands’6. Professor Vlasbom encouraged me to graduate on a practical problem and also my current professor van Rhee suggested to do some preliminary experiments with sticky stuff to get some feeling about what I am going to study. Of course I took some clay home to play with it. But the best suggestion was by my colleagues, who thoughtfully gave me stroopwafels7. The ultimate representation of sticky non-Newtonian stuff between layers of latticed disks.

Fresh supply of stroopwafels for practice and celebration

References

Which Teeth Will Survive The Cut? Adapting Your Selection

After my last post1, I received some comments and questions about the actual products we are applying in our cutter systems for our CSD’s2. Indeed, from a pure physical perspective, last post cuts to the heart of the processes, but does not explain our design of the working tool that makes a cutter suction dredge do its work.

Over the years, there has been a lot of development in this tool. Originally, suction dredges were plain suction dredges, working in non-cohesive sand. When the soil was more cohesive than could be dug with the standard suction dredges, attaching a mechanical device for loosening the ground enabled the suction dredge to work in this environment. From this original concept, the cutter head was already recognisable as a crown with teeth on a back ring and a suction mouth in the centre. From there, a lot of experimentation was done, but ultimately it all came back to this concept. Although modern cutter heads have a vastly improved performance and lifetime.

The cutting process in a modern cutter head is a combination of the rotation of the head and the swing of the dredge. The teeth describe a compound path of translation and rotation and each individual tooth has its own set of cutting parameters for depth and angle varying over time. Moreover, the combination of teeth on the different arms, allow for a staggering of the teeth that each tooth cuts fresh material and optimising the use of the teeth and spreading the wear. This results in a complicated geometry of the arms and a intricate pattern of the teeth.

Teeth system with adapters (left) teeth system direct on arm (right)

Once a cutter design has been chosen, there is still some tuning possible. Normally, the teeth are fitted on adapters and there are several teeth types available for a certain adapter. Pick points, Chisels and Flares. Most productivity can be expected from the wider teeth. However, the penetration of the wide teeth is less. So, for harder material you want to select narrower teeth.

Adapter system (left), teeth range with adapter (top), teeth range direct on arm (bottom)

Wear is also an issue3. And as the teeth are in direct contact with the fresh material, the wear rates can be severe. The disadvantage of a teeth and adapter system is that that are quite big. So, less teeth fit on an arm, reducing production on average. As most of our CSD’s are working in more gentle sands we selected a cutter system, that provides more teeth to engage in the action, increasing production. As these teeth are fitted directly on the arms, there are no adapters that wear also. Consequently having no adapters, simpler arms and dirt cheap teeth result in a low investment low OPEX cutter system. Although you might have to check the state of your teeth more often, in the end you spend less money on a cubic meter produced.

Teeth can be worn down to the root. Also they are not wearing evenly. Usually, they last longer on the outside, near the back ring. You might consider using different tooth forms over the arm. Experience and practice, will guide you in selecting the best combination. In line with the previous post, the analogy will be on the table. Just as you select different teeth for your fork, you can select different teeth on your cutter depending on the dish being served.

References

Experiencing The Cutting Edge Of Dredging Technology

Exhibit on cutting forces at the Damen Dredging Experience

Ever tried to eat peas with a knife? Didn’t fare well, right? Sorry, this post will not help you. It is just more entertaining doing so. And you might remember this week’s lesson.

After all the other posts, I would like to continue our tour along the exhibits of the Damen Dredging Experience. We’ve seen at “the Bank” that by gravity and hydraulic action the sand can start moving towards the suction mouth. Another well-known mechanism for collecting the sand is by cutting. This mechanism is primarily used in the cutter heads of CSD’s of course, but also in the trailing drag heads of TSHD’s.

Be aware, that the gravel sample in the exhibit is an artificial sediment, specifically designed to be porous and demonstrate the “rolling peas effect”. Naturally occurring gravel sediments have a wide range Particle Size Distribution and will have virtually no gaps between the grains. The smaller grains will lock the bigger grains in place and it will be more difficult to move them.

In this exhibit you will see two different types of sediment. Wet fine grained sand and very coarse gravel in a jar with a handle. When you rotate the blade in the gravel, you will notice a rather high cutting force, that remains relatively constant. The water can easily move through the pores. Rotating the blade in the jar with sand, is very hard at first. But as soon, as there is a chunk of sand dislodged, the cutting force is decreased dramatically. The decrease can be explained by the fact, that once water has entered the shear plane, it cannot dissipate back in the pores and will lubricate the chunk moving over the bed. This is a phenomenon, sometimes encountered with ploughs.

Under pressure in densely packed sediments

The theory of cutting sand is largely explored by dr. S.A. Miedema1. He wrote an extensive book2 on this subject and anyone interested in the details is encouraged to read it. Again, the basis for cutting sand is the dilatancy of sand, just like in the previous exhibit. The grains are moved and the water has to be forced in the pores. As the blade moves at a certain speed, the sand exerts a horizontal cutting force on the blade. Force, times speed is power. At this speed and cutting height you have a certain production. Power divided by production yields a Specific Cutting Energy3, which is a parameter for how much effort it costs to cut 1m³. The SCE is largely governed by the undrained shear strength and the angle of internal friction and is different for every type of sediment. They are measured with a Cone Penetration Test4,5. In order to estimate the production of the dredge, we really need to know these parameters. If they are not available, maybe you can receive the results from a Standard Penetration Test6,7.

Basic explanation of the theory of sand cutting

From the equations, you can derive that for a hard material, the SCE can be quite high. Consequently, with a known installed cutter power, the production Q will result quite low. From this perspective, there is no upper limit in the hardness of the soil, anything can be cut. It is just, that the resulting production might be too low for a viable business case. In this respect, it is always difficult to say the maximum hardness of the soil the CSD can cut. Usually, the increased vibrations and unsteady process will limit the productivity in such circumstances.

Example of a cutter production for a CSD

This provokes a nice practical experiment for you at home or when you have you have to entertain guests at dinner: have a nice recipe with big peas and fine grained rice8. Serve the peas and rice separately and notice the variation of effort to stir the ingredients separately. Then, mix a portion together and notice the increase in cutting force. For enhanced realism, add some sauce. Exclaim your amazement to your perplexed table partners and explain that you are not playing with your food, but are on a study assignment for your work. Bon Appetit!

Tag: Cutting

Personal Announcement: Going Back To School To Cut Some Clay

References

See also

Which Teeth Will Survive The Cut? Adapting Your Selection

References

See also

Experiencing The Cutting Edge Of Dredging Technology

References

See also