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ISFOG 2025: Commissioning The Test Rig And Reporting To Academia

Fully covered cutter head in sticky clayThis week, I will be presenting my paper1 about the initial experiments on the test rig at the 5th International Symposium on Frontiers in Offshore Geotechnics (ISFOG 2025)2. I will be there in the breaks to explain my poster3 in the lunch breaks. For my audience not present at the symposium, I can highlight the most interesting parts here. I presume, most of you are aware of the operation of a Cutter Suction Dredge and also know about its problems when working in clay. The clay will adhere to the teeth and arms and clog the cutter head. This leads to interruption of the project and in consequence: time and cost overruns. Also, the production itself is difficult to calculate. This is why we at Damen Dredging Equipment started the CHiPS project with the TU Delft4 to investigate the process, improve the estimation model and optimise the design of the cutter head for operation in clay.

Forces involved in the cutting of clay

Forces involved in the cutting of clayFor this purpose, we constructed a linear cutting test rig. Last post about the graduation of Fabian Kruis has more on the results of his thesis5. In the ISFOG article, we wrote about the design and performance of the rig and the opportunities it provides for further research. The design criteria for the rig as was laid down in the assignment for Ines Ben M’hamed were6:

Identifying the main parameters influencing the cutting forces and the cutting regime.
Designing general arrangement for testing linear cutting models.
Capture the signals for force and deformation.

The developed test rig was inspired by the model described by Hatamura and Chijiiwa7. The blade is attached to a linear moving trolley, cutting through a block of clay mounted in a frictionless moving soil bin. The reaction forces on the box are measured. and images of the grid printed on the side of the clay block are captured with a GoPro camera of later evaluation with PIVlab®. A set of 30 experiments was defined according to the Buckingham-PI method as presented at the CEDA Dredging Days last year8.

General arrangement of the linear cutting test rig

Next to the cohesion and adhesion, the tensile strength of the clay had to be measured to obtain a consistent result. We could confirm the linear relation between cutting depth and the cutting force as predicted by existing models from literature. As we were using modern techniques for capturing images, we were able to accurately measure the displacements with the PIVlab® application. The good results are due to the novel printing technique developed by Fabian Kruis, to apply a grid on the side of the clay sample. One remarkable result is, that most models for the calculation of the sliding forces, only take adhesion into account, but measurements indicate that the external friction cannot be neglected. This appeared in the measured shear angle, which was much lower than the shear angle predicted by existing models.

Captured deformations in a vector field. Note the differences in shear angle

The experiments yielded a treasure trove of measurements, we are still analysing them. e.g. We noticed some strange reversal of the vertical cutting forces. And we are interested in the transition from one cutting regime to another. Those results will be presented in my next journal paper. In the mean time I am watching all those captured movies over and over again. To me it’s very inspiring and I like to share an example.

A slow motion movie of a clay cutting experiment (ASMR)

References

Graduation Fabian Kruis: Modelling Friction In Clay

Fabian Kruis presenting his graduation research

Fabian Kruis graduated on his master thesis at the Delft University of Technology on a project for my PhD research1. He investigated the cutting behaviour of plastic clay. As it was the first time we are now actually using the test rig designed by Ines2, he first had to do was a lot of trouble shooting for commissioning the test rig. Spoiler alert: the cutting forces were much higher than expected and the linear drive was not strong enough to cover the whole range of experiments we’ve wanted to do.

The cutting forces involved with cutting of clay are acting on all four sides of the simplified chip. On the outside, there is the barometric pressure of the surrounding water. On the far end, there is an unknown and hard to determine force from the rest of the chip that is not in contact with the blade anymore. At the shear plane, there are the normal force, the internal friction and the cohesion. At the blade, there are the normal force, the external friction force and the adhesion. The sum of these last three forces will give the cutting force we are looking for, as they make up the required cutting power on the drive. But they can only be calculated, once the other forces are known.

Overview of all the forces involved with the cutting of clay, acting on the chip

Fabian’s assignment was to have our own experience with the cutting of clay and check whether the models used in the dredging industry have any reliability in predicting the cutting forces. checking whether all assumptions and simplifications were justified. e.g. Plastic clay does have similar properties and behaviour as a fluid. And a fluid does not have an internal friction. Consequently, clay should not have an internal friction also. Right? When there is no internal friction, there can’t be an external friction either. Right, right? Fabian tested these assumptions by actually performing shear tests on internal and external planes.3

Explanation of internal friction for solids, fluids and clay

At least for the clay we used in this research, he already found that the assumption for ‘no friction in clay’ is not valid. Consequently, this had knock on effects on the rest of the cutting force calculation. We did find a different behaviour, the shear plane was off and the cutting forces were indeed much higher than expected. It is now up to me to use Fabians results and model modifications to implement into my own research. As a matter of fact, I used part of his thesis to write an article and hope to present this soon. I’ll keep you update on those developments.
As we are very satisfied with Fabian’s work and him as a person himself, we offered him a position in our team at Damen Dredging Equipment in Nijkerk, which he happily accepted. So, next to progress for my research, we have a new colleague. Welcome Fabian, thank you!

Fabian signing his MSc. certificate. — Fabian signing his MSc. certificate sitting in the ‘dredging chair

References

CEDA Industry News: Snap Back Story out of the Old Stone Box

Cutter wheel dredge ‘Sylvia’ at work on the TIWAG Langkampfen power dam reservoir

You might have read the CEDA Industry News1 publication of September and the LinkedIn announcement2. Or, have been present at Session 10, The Story of Safety3 of the CEDA Dredging Days 2024, where I initially recounted my story of misfortune. For the uninitiated, I’ll shortly recount it over here, although the focus will be more on the design problem back than.

Me telling this story at the Safety Session of the CEDA Dredging Days 2024 (Credit: CEDA)

When I was fresh out of university, working for my previous employer, I was assigned to advise during the commissioning of a bucket wheel dredge in Austria. Here at my website, I’ve already mentioned a very peculiar problem we had with starting the dredge on Monday morning4. The whole dredge was a funny contraption, trying to fulfil the specific requirements set by the customer. The task of the dredge was to clean out the sediment caught in a silt trap5 in front of a power dam to prevent the material flushing through the delicate turbines. Furthermore, the fine sediment should be removed from the system to be sold for beneficial use, but the larger boulders that came along should be rejected from the system. Unless they were very large and stay in place for forming the liner of the silt trap.

General modes of siltation at the usual location in a reservoir

The proposed method was to use a bucket wheel to gobble up the larger stones, but leave the even bigger ones down at the pit floor. Additional advantage of the bucket wheel is that they tend to have a very low resuspension rate which is beneficial for reducing the turbidity in the river and save the dam turbines. However, the larger stones taken by the bucket wheel will not pass the dredge pump. A proven solution to filter out stones for the dredge pump is a so called stone collection box. They come in various executions, but this one was a large cylindrical vessel with a cover on top. Inside was a cage that connected to the inlet and outlet of the vessel. When the cage was filled, the lid was lifted by the crane and the attached cage came out. The cage was swung over to a barge alongside and the stones were unloaded through a trap door. Sounds straightforward, right?

General arrangement of the ladder and suction system on bucket wheel dredge ‘Sylvia’

The thing is, when you have anything in the suction pipe section, it will create a pressure loss and the pressure in front of the dredge pump becomes lower. Or higher vacuum, as you wish. This is already a problem for an inboard pump when you are working at sea level, even more so when working at altitude. The NPSHr is easily reached. In order to have enough NPSHa, even when the stone collecting box was filling up, it was decided to have a submerged dredge pump for increasing the pre-pressure. Consequently, the stone collecting box would also be placed on the ladder, in front of the dredge pump.

Explanation on NPSHr and NPSHa in mountains and submerged, including stone box

All in itself no problem. The disadvantage is that a submerged stone collecting box should be very strong, big and heavy to withstand the under pressure, although there is very little room around the ladder, the ladder itself becomes quite heavy and bulky and the submerged frontal area increased dramatically, increasing the total drag of the dredge in the fast flowing river current. To mitigate this, the ladder winch, the service crane and the 25 ton forward wire winch were all stacked over each other, so each could do it’s job independently. Although none of us ever did a collision check in the design phase of the dredge. In actual operation, they all were in each others way. When lifting the cover of the stone collecting box, it got caught by the forward wire. When lifting it further, it slipped of the side by the strong drag forces on the wire. I was to first to experience our negligence and got hit on my head when I was to eager to inspect the contents of the box. I blacked out, fell in the frigid water in the ladder well and woke up washed up on the ladder. My colleagues rushed me to the hospital, where I ended up between all sorts of winter sports injuries. Fortunately for me, I am able to recount my story and share it here and through the CEDA Industry News.

Annotated overview of the accident site where the wire slipped from the cover onto my head.

ISFOG 2025: Commissioning The Test Rig And Reporting To Academia

References

See also

Graduation Fabian Kruis: Modelling Friction In Clay

References

See also

CEDA Industry News: Snap Back Story out of the Old Stone Box

References

See also