Graduation of Ines Ben M’hamed: The Strength of Clay in a Test Rig

Ines Ben M’hamed defending her graduation thesis
Ines Ben M’hamed defending her graduation thesis

Last week, Ines Ben M’hamed graduated with good grades on her bachelor thesis. She did a project with us at the Research Department of Damen Dredging Equipment in Nijkerk. The topic was to investigate the strengthening of clay when it is subjected to shear. This deformation is a common phenomenon when cutting clay and as such a contribution to my own PhD project1 and consequently improving our products for these applications. A common problem with clay is clogging up the cutter head, but it is also not completely understood why the clay is behaving as it does and how much power is involved for the various regimes.

Fully covered cutter head in sticky clay

The effects of deformation on the behaviour of clay are much more pronounced than e.g. sand or rock. Rock does not deform, it just breaks. Sand deforms, but as it basically only involves hydraulic and mechanical forces, it is much better understood. Clay particles have wider range of interactions. Next to the hydraulic and mechanical forces, they may experience: adhesion and cohesion, molecular forces, electrostatic charges and chemical bonding in the higher temperature ranges. The general effect is that as the particles in the original situation may have a weak structure, the external disturbance causes the particles to get jostled around and all the mentioned interaction get a chance to hook on to each other.

Shear strengthening due to organising particles
Shear strengthening due to organising particles

The result is, that the particles get oriented and therewith a better opportunity to bond. The effect is a strengthening of the shear stress. As this strengthening is dependent on the strain rate, it is this strain rate, that is of interest for the prediction of the cutting forces. There are many publications available on what the consequences are of the strain rate on the Specific Cutting Energy. A well known model is by Sape Miedema2.

Strain Rate Effect on the Specific Cutting Energy (Credit: SA Miedema)
Strain Rate Effect on the Specific Cutting Energy (Credit: SA Miedema)

The trick with this model is, it depends on this strain rate effect. The sole experimental data available is by Hatamura and Chijiwa3 in 1975. They tested one type of clay on the three governing parameters: static shear strength, dynamic shear strength and the strain rate. There hasn’t been hardly any further experimental investigation into this problem. And as we regularly receive samples and soil reports that we can not test on these properties, it is also hard to predict the performance of our cutter heads. So, we decided to build our own cutting test rig.

Design of Ines’ cutting test rig
Design of Ines’ cutting test rig

This cutting test rig resembles the specifications to the original test rig of Hatamura. This will allow us to verify the parameters in the model ourselves. We also prepared the design with various option to enable us to allow assessment of clay samples that we receive from clients and service engineers. We hope to provide our customers with additional service in this problem. Currently, the parts of the test rig arrived very late and Ines was not able to include the build in her project. Respect for the good grade she received for her thesis. However, the parts are there and provide and excellent opportunity for the next graduation student to do their project with our company. Who dares?

Available parts for the cutting test rig
Available parts for the cutting test rig

References

  1. Personal Announcement: Going Back To School To Cut Some Clay, Discover Dredging
  2. The Delft Sand Clay & Rock Cutting Model, SA Miedema
  3. Analysis Of the Mechanism of Soil : 1st Report. Cutting Patterns of Soils, Hatamura & Chijiwa

See also

 

Graduation Of Alex De Rooij: Pumps Actually Fly Like An Unusual Airplane

Alex de Rooij receiving flowers from Suman Sapkota for his graduation
Alex de Rooij receiving flowers from Suman Sapkota for his graduation

When you hire a carpenter, he repairs everything with a hammer. So, what happens when you ask an aeronautical student to solve some issues in a dredge pump? He models the pump as a badly behaving airplane. And with success, Alex de Rooij joined our company as a graduation student and recently graduated on the topic of ‘Numerical Study on NACA Profiles Sensitivity in Dredge Pump Impellers’.

The normal procedure for designing pumps is relatively straight forward. Set the performance specifications and try to hit that mark with the simulated behaviour from an iteratively improved design. This is well documented and I’ve been writing about this process before.1

Pump design workflow (inspired by Suman Sapkota)
Pump design workflow (inspired by Suman Sapkota)

One of the design parameters is the NPSHr. This is basically the amount of absolute pre-pressure the pump requires to operate.2 The system and operating conditions will result in a certain available pre-pressure: NPSHa. When the NPSHa drops below the NPSHr, the pump will experience cavitation at the low pressure side of the blade. The flow of the medium will be disturbed and the performance of the pump will abruptly crash. There is some warning. Operating near the NPSHr, there will be an incipient cavitation where the vapour bubbles start to form, but do not cause any issue. The imploding vapour bubbles may be detected audibly for the trained listener.3 Next will be a stage on the NPSHr, where the bubbles get larger and they loudly implode. At this stage, the pump sounds like it is tumbling nuts and bolts inside. These imploding bubbles will definitely cause damage to the impeller. At last, working below the NPSHr, the bubbles will become so large, they will block the passage between the blades. The result is an immediate drop in delivered head.

Explanation of NSPHr, NPSHa and cavitation
Explanation of NSPHr, NPSHa and cavitation

The flow of the medium through the impeller can be simulated in a CFD program. Specifically for impellers, you will need to model a rotating frame of reference. And that is the usual representation of the results. However, with mathematics being one of the most powerful tools invented by humanity, we can have an alternative view on these results. We can cut the impeller along a radial and stretch open the meridional passage and the blades to a row of foils. And that is exactly where our young aspiring engineer comes in. In literature, the blades in the impeller are modelled having a constant thickness. But, Alex has been investigating what the influence will be when we model the blades as foils. Selecting a proper profile makes the blade less sensitive to stalling.

Conversion of axial view to blade to blade view
Conversion of axial view to blade to blade view

Alex, thank you very much for your work here at Damen. We’ve learned the influence of certain profiles on the performance and geometry of the pumps. You have the right mindset to pass your time at the TU Delft and graduate successfully over there also. And whenever you have some days of the month left after you spend your allowance, know that we can give you a warm reception at our office.

Alex working at the reception desk at Damen Dredging Equipment
Alex working at the reception desk at Damen Dredging Equipment

References

  1. Our Interview About New Pump Designs In The Latest Damen Nieuws, Discover Dredging
  2. When does your pump suck? Discover Dredging
  3. Cavitation in a Water Pump and Valve, Mountain States Engineering and Controls

See also

Young CEDA Visits Damen Dredging Equipment

Group picture of the Young CEDA visit to Damen Dredging Equipment
Group picture of the Young CEDA visit to Damen Dredging Equipment

CEDA has always been an advocate of sharing and disseminating knowledge, with an attention to personal contacts1. Of course, there are the high profile Dredging Days2 and the WODCON3 level events, but once in a while there are also smaller events. As part of the smaller program, Young CEDA4 regularly organizes visits at projects and companies to bring together the new generation of our dredging community. This time, a group of young professionals visited our company Damen Dredging Equipment here in Nijkerk5 to get familiar where we are and what we do. Rick Bekkers, Jorgen Groeneveld, Suman Sapkota and Reinier de Vries where hosts of the event and organised, with assistance of the ladies of our reception and back-office, a nice program with presentations a yard tour and of course drinks and the proverbial ‘bitterballen’.

Rick Bekkers giving a presentation on DOP dredges
Rick Bekkers giving a presentation on DOP dredges

After Rick and Reinier gave a presentation on the premises and the products, there was a yard tour and a demonstration of our dredging simulator by Wouter Beekman. And I am always happy, that the visit to our Damen Dredging Experience was a great success again. We do have our own little museum to experience the difficult issues in the dredging process hands-on6. Literally! Specially for this occasion, the exhibits have been expanded with new informative screens, made by Judith Korver. In a few slides there is an explanation on what the visitor experiences by operating the exhibit.

Informative screen at the pipe length exhibit
Informative screen at the pipe length exhibit

Part of the experience is the staircase leading up to the third floor with the exhibits. In the staircase, there is a collection of wall covering pictures on the history of Damen Dredging Equipment. Personally, I think learning from history is essential to get inspired7. Back in the old days, people had to be very innovative to come up with solutions within their limited technological level. And Mr. de Groot, the founder of our local Damen company was very innovative8. He was the inventor of the double walled dredge pump9 and the soil pumping station10.

Double walled dredge pump(l) and soil pumping station (r)
Double walled dredge pump(l) and soil pumping station (r)

A lesser known fact by now may be, that originally Mr. C de Groot was a contractor. He was a true descendant from a Giessendam dredging family and tried his luck at the IJsselmeer werken of the 1930’s. As a basecamp, he leased a land lot next to the sea locks of Nijkerk. Later on, he had more fun on building his own equipment and eventually started a manufacturing facility more inland along the approach canal. You can still recognize the original footprint in the current urban landscape.

Arial view of the original area of Damen Dredging Equipment
Arial view of the original area of Damen Dredging Equipment
Arial view of the current area of Damen Dredging Equipment
Arial view of the current area of Damen Dredging Equipment

At retirement, Mr. de Groot had no successors and had to sell the company. After an intermediate period, De Groot Nijkerk was acquired by Damen Shipyards8 in 1988. Eventually in 2004 our company was rebranded to Damen Dredging Equipment and we proudly fly the blue colours of the Damen family.

Flags welcoming visitors at the entrance
Flags welcoming visitors at the entrance

References

  1. Our Mission, Vision and Strategy, CEDA
  2. About CEDA Dredging Days, CEDA
  3. WODCON’s, WODA
  4. Young CEDA Meetings, CEDA
  5. Young CEDA Site Visit Damen Dredging Equipment, Nijkerk, CEDA
  6. Experience the Dredging Experience, Discover Dredging
  7. Welkom in het Nationaal Baggermuseum, Nationaal Baggermuseum
  8. A long history in dredge building, Damen
  9. How to dress your dredge pump for success, Discover Dredging
  10. Book Review: En De Sé Wie Net Maer, Discover Dredging

See also