Category: CSD

Cutter Suction Dredge

The Ancient History of the Cutter Suction Dredge ‘10th of Ramadan’

Cutter Suction Dredge ‘10th of Ramadan’

Last week I was away to Egypt. I have some emotional ties with Egypt. It took me the better part of seven years, to deliver the cutter suction dredge ‘10th of Ramadan’1, to the Suez Canal Authority. This all started from the qualification tender, building it at the Port Side Shipyard in cooperation with DTC, eventually all the way through the guarantee period. At a certain moment, I was so occupied with this dredge, I even prepared a paper model of the vessel in my own spare time.

Presentation of the paper model of ‘10th of Ramadan’

This nerdy pastime served a practical purpose. The specifications of the construction and the requirements for the installed equipment, were very complicated and it helped me to understand the problems in the hull construction before they arose in reality. The environment of the Suez Canal demands some specific requirements concerning tank and space division. Furthermore, the SCA is very aware of the capabilities of the crew and demands corresponding considerations on equipment and systems. To fit this all in the prescribed box, was quite a puzzle. The normal operational environment of the dredge is maintenance on the shallow shelves of the Suez Canal.

Example cross section of the Suez Canal

The maximum box size, that made the design so tricky was determined by a certain lock to access the irrigation system along the Suez Canal. On the west side of the canal, water from the River Nile is diverted to the fertile land over there. Already in ancient Egypt, this was a much treasured area. Joseph gave this area than called Land of Goshen to his family to live there2. Senausert III, Pharaoh of Egypt (1874 B.C.) also saw the potential of this area and used it to established a connection, between the Mediterranean and the Red Sea by linking branches of the Nile to the Bitter Lakes3. Subsequent kings renovated and expanded the canal, that eventually became known as the ‘Canal of the Pharaos’4. Unlike the international importance of the current Suez Canal, the Canal of the Pharaos was mainly used for local transport of produce of the irrigated land and construction stones from quarries on the Red Sea coast to the monuments along the Nile.

Canal of the Pharaos (Credit: Wikipedia)

In fact, the ancient Egyptians might be called the inventors of dredging canals for transportation, preceding the Chinese by about 1000 years5. Parts of the canal are still there. They are now incorporated into the irrigation system, that the Suez Canal Authority has to maintain, next to the big Suez Canal. In a way, I feel honoured, that the dredge that was once only a figment of my imagination and became a reality, will one day keep this ancient piece of Egyptian history alive and preserve it for future generations. Well, that may be a bit presumptuous, but I still feel proud over this dredge and have fond memories of the people from the Suez Canal Authority I have worked with.

OK, I may have pestered my colleagues so much with my stories about Egypt and my little dredge, they even prepared a cover for a book for me to write about it on my work anniversary.

Cover page ‘Mark Winkelman as Egyptian’ (Artwork: Gert Kraaij)

References

  1. ‘10th of Ramadan’, DredgePoint.org
  2. Genesis 45:10
  3. Canal History, Suez Canal Authority
  4. Canal of the Pharaos, Wikipedia
  5. History of Canals in China, Wikipedia

See also

The Origin of Clay, When Dredging Becomes Sticky

Clay forming Fountain Paint Pot, Yellowstone National Park, Wyoming, USA

This mud pot gave me a revelation on the origin of clay. I was aware, that clay is a completely different mineral than sand. For starters, sand is based on silicon dioxide and clay on some complex aluminium compound. Sand is mechanically worn down rock, usually quartz. But I never got around to understand where clay came from. Here, a small sign at the side of the mud pot revealed a complete different mechanism: chemical alteration of rock by hydrothermal action.

Sign at the Fountain Paint Pot, Yellowstone National Park, USA

Now, it became clear to me, how all the funny properties of clay arise from this generating process. Unlike weathered sand, clay grains are nice symmetric hexagonal crystals. And these crystals grow under changing conditions for temperature, chemistry and pressure. Exactly the environment in these mud pots. The sulphuric acid leaches the chemicals from the rock matrix, in Yellowstone usually Feldspar, the water bubbles to higher levels, transporting and mixing the ions and cooling down along the way. Just like salt crystallises in brine, the clay shakes out like tiny particles, about 2µm. These flakes coalesce into a new sediment: clay1.

Hexagonal sheets of a clay mineral (kaolinite) (SEM image, ×1340 magnification) (Credit: Wikipedia)

The specific mineral of clay, e.g. kaolinite, is a hydrated oxide. And the hydrate allows the charge of these semi-ions to be moved around. As same charges repel and drive themselves apart, the edges and corners of the little crystal will become negatively charged. Now, there are a bunch of discs that have a preference to stick to each other like masonry. Between the discs, there is not much space making the water content low. But, one can add water and the sediment will swell, but there will still be contact between the ends and centres of the disc. Even with this spongy structure, there is still some consistency. It behaves like a plastic substance, you can deform it and it will stay like that.

The plasticity of clay can be measured by rolling the clay in a sausage and measure the water content at which it crumbles. That is a lower limit. An upper limit of plasticity has to be determined by testing the effect of shaking a bowl with clay. Both methods are described2 in ASTM D4318. The difference of water content between the lower plastic limit and the higher liquid limit is the plasticity index. The higher the plasticity index, the more difficult it is to cut this material. It is like cutting warm butter, material is moved around, but you are not severing chunks of the bulk.

Synthesis of clay and the relevant properties for dredging

Whenever you hear dredge people boast about difficulties in dredging, usually it involves clay also. The cutting itself, it is very hard to cut the material out of the sediment. When the chunks come loose, the chunks will stick to the cutter head and the will get completely smeared over and no new material can be cut or sucked up. After that, the clay chunks will tumble down the discharge pipeline. Under certain conditions, the chunks will snowball and form bigger balls. Finally, the clay gets at the reclamation area and will cause problems with the drainage. Remember, fines clog the pores between the grains and prevent the flow of drain water. And clay particles are very fine and they glue the bigger grains together.

Knowing the properties of clay, it is obvious, that normal cutting tools for sand dredging, do not work in a clay environment. Based on the special properties of clay, we once developed a special clay tool for a specific project3. And it worked4. It was fun. And it will be another story.

DOP pumps with special clay cutter head at the ‘Markthallen’ project in Rotterdam

References

  1. Metasomatism, Wikipedia
  2. Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils, ASTM D4318
  3. DOP150 creates underground car park, DopDredgePumps.com
  4. Prestigieus project Markthal Rotterdam vraagt om innovatieve oplossingen, Autograaf 42-p.8, MvO Groep

See also

 

A Simple Soil Sample Show

Demonstration at a workshop ‘Advise a dredging customer’

Recently I had to give a demonstration on one of the aspects of my work: investigate what type of soils the customer will be encountering, what type of dredging equipment is most suitable for that application and what will the production be under those conditions. At this demonstration, the purpose was, to show how I can make something simple very complicated in a short time. The challenge here will be to take you with me on this precipitous path to enlightenment.

Ingredients for a sand sieving demonstration

The example case was to examine two different soil samples. Each was artificially mixed, but representing a Particle Size Distribution of two different borrow areas. The samples are delivered in widely available standardised containers. Yeah, you recognised them: 1.5L soda pop bottles. As the samples were wet, they had to be dried. Normally, this can take two to three hours. Here we applied a magical temporal acceleration by employing a Calvin style cardboard box as an oven. The literal main ‘activity’ for the participants was to manually shake the sieve tower. Rest assured, in our own soil analysis lab we use an industrial automatic sieve tower. This antique specimen comes from our museum. The separated fractions from the sieves, then have to be weighed and finally plotted in a so called particle size diagram. Horizontally you can take a sieve diameter and vertically you can read what mass percentage will pass that mesh diameter.

Resulting Particle Size Distribution and key parameters

Although both samples have the same median diameter d50 (354µm), which is indicative of the general particle size, the distribution is very different. Sample A is almost totally the same diameter, where sample B has a broad distribution. A measure for the distribution is the Uniformity Index (d60/d10). Another important figure to be taken from this graph is the silt fraction. This is the percentage of particles that is smaller than 63µm. More parameters can be taken form this graph, but these are the most important for now.

Influence of particle size distribution on slurry transport

From the equation for the critical velocity, the uniformity index plays an active role. If the uniformity index increases, there is more fine material. Fines tend to increase the density and viscosity of the fluid. Consequently the resulting mixture behaves like a heavier fluid, carrying larger particles. Effectively, the resistance of a non-uniform sand is higher than for a more uniform sand mixture.

Influence of particle size distribution on cutter production

Another aspect of the dredging process is the ability of the CSD to excavate the material from the bottom. Here the uniformity index has the complete opposite influence. A uniform sand distribution will have a lot of voids between the grains. The particles will move easily over each other. When there are lot of smaller particles available, they tend to clog up the voids and bond the bigger particles in a gridlock. These sediments are very hard to excavate. If no geotechnical investigation is available, the PSD can help to estimate a SPT.
With the grains size and the SPT, the audience consulted our lovely looking assistant ‘Sandy’ for a first selection of the required CSD.

Short evaluation of a selected dredge at ‘www.dredgefinder.com’

References

Wilson, Addie & Clift, (1992), “Slurry Transport Using Centrifugal Pumps”, Chapter 5 “Heterogeneous Slurry Flow in Horizontal Pipes”

Youd, (1970), “Densification and Shear of Sand during Vibration”, Journal of the Soil Mechanics and Foundations Division, 1970, Vol. 96, Issue 3, Pg. 863-880

See also

Geotechnical investigation

Dredge Finder