CEDA DMC Works On A Guidance Paper For Soil Investigation

CEDA Dredging Management Commission WG on Soil Investigation (Credit: CEDA)
CEDA Dredging Management Commission WG on Soil Investigation (Credit: CEDA)

Did you ever start a project and it turned out that the conditions were different than expected? Welcome to the dredging industry. One of the most underestimated preparations for a dredging project is the soil investigation. As this investigation is of the utmost importance for the dredging community, the DMC is preparing a guidance paper on this topic1, which we discussed last meeting (February 7, 2020, IMDC, Antwerp).

Working for a dredging equipment manufacturer, I am not much involved in the actual soil investigation. However, often our clients base their purchase of a specific type of equipment on the soil investigation and as such we are often presented with the reports on soil investigation. Based on these reports, we calculate the possible production for various types and advise the client for a dredge that will meet their requirements on the maximum production. most of the time we provide a good advise and the client is happy.

Off course there have been occasions where the performance was not as expected. Often because the report on the soil investigation was inadequate. Either the report did not contain all the details, or the investigation itself was lousy. Either way, rubbish in, is rubbish out. Just as an example, let me tell you what can go wrong, when the information is not representing the real circumstances.

One of our products are the so called ‘DOP Dredges’2. They are based around the versatile DOP pump. Basically, it a DOP suspended on an A-frame on a pontoon with a powerpack. The DOP can be lowered into the sediment and create a typical suction dredge pit. The production is more based on the rate that water can enter the bank face and the velocity that the banks recede. Our client provided us a Particle Distribution Diagram of the available sediment3. It was a nice narrow graded sand, but there was a considerable fines tail on the lower end. This was being dealt by the washing and screening installation. According to the client was this the sand characteristic from the whole pit. And what could be better? If you excavate all the material, you really know what is there, right?

Difference between expected soil conditions (left) and real situation (right)
Difference between expected soil conditions (left) and real situation (right)

Well no. As it happened, there were cohesive silt layers between the narrow graded sand layers. When dredging, they sucked at the bottom of the pit. Any silt layers gradually broke of and disintegrated by the eroding density flow. As the pit was created over a long period, the falling chunks of silt just slid down the slope, without causing any harm.

Enter: the new DOP dredge. It started in a new corner of the pit and initially had some trouble penetrating the silt layer. Eventually it managed to get through and started excavating a cavity below the silt layer. These broke of, burying the DOP. Without any possibility to recover the DOP, it turned into a very expensive anchor.

Risk of getting your DOP trapped in a cavity under the cohesive silt layers and the solution
Risk of getting your DOP trapped in a cavity under the cohesive silt layers and the solution

If the presence of these cohesive silt layers would have been known, we would have adapted the suction pipe for a deeper penetration. That prevents the DOP becoming covered and facilitates easier extraction. This story proves two things: 1. A proper soil investigation can prevent costly accidents and budget runovers. 2. A DOP can be modified to most requirements, when the circumstances are known.

Meanwhile, the DMC is preparing its guidance document to assist you in preventing problems like this. Follow CEDA for updates4.

Standard suction tube (left) and long suction tube (right)
Standard suction tube (left) and long suction tube (right)

References

  1. Dredging Management Commission, CEDA
  2. DOP Dredger, Damen
  3. A Sample of Soil Samples, Discover Dredging
  4. News, CEDA

See also

My WODCON 2019 Presentation: Launching Robotic Dredging

Me, presenting my WODCON 2019 contribution.
Me, presenting my WODCON 2019 contribution.

Yesterday, I gave my presentation at the WODCON 2019 in Shanghai1. The WODCON is the triannual world dredging conference, were everybody in the dredging industry meets and exchanges knowledge and ideas. Just as I mentioned in my New Year’s post, I sometimes like to delve into some old archives, get inspiration and hatch new ideas2. So did I for this presentation.

Overview of the ‘Ketelmeer’ (Credit: Google Maps).
Overview of the ‘Ketelmeer’ (Credit: Google Maps).

A seminal dredging project concerning environmental dredging is the ‘Ketelmeer’ clean up dredging project, resulting in the creation of the contaminated sediment storage depot in the artificial island ‘IJsseloog’3. As careful removal of the contaminated sediment required novel dredging techniques, the government challenged the dredging industry to test four innovative concepts. The results were evaluated by the institute now called ‘Deltares’ and published in a report4. The original conclusion of the report was, that the auger dredge was the best in reducing the turbidity. Later, the bigger auger dredge ‘HAM291’ was constructed and used to actually clean up the lake. With the knowledge and the experience of the auger we also developed a range of auger attachments for our DOP pumps5.

Traditional auger attachment for a DOP pump excavator combination.
Traditional auger attachment for a DOP pump excavator combination.

Reading the Ketelmeer report again, it occurred to me, that one parameter had not been properly accounted for: the size of the dredge. The auger dredge was by far the smallest dredge in the game. With a weighed scoring method, the dredges were also compared in size and installed power. The reasoning is that a bigger dredge has more interaction with its environment. Naturally, the environment gets more disturbed and turbidity levels should be higher for a bigger dredge. And the data was there to support this hypothesis. Smaller is better! Still, this does not undermine the initial results of the concept, as that was evaluated for turbidity per cubic meter. The bigger dredges also delivered more production. But when comparing dredges of the same concept might the smaller ones will perform better on turbidity. And this is in accordance with our experience. Every project where we’ve supplied these auger dredge units, the contractor and the client where surprised and happy about the achieved turbidity levels. Now, we know why: smaller is less turbidity.

The next step in performance might be reached by further decreasing the size of the dredge. However, the DOP is already as small as it is for a viable application on an excavator. The conclusion is to have an auger operating directly on the bottom: an unmanned submarine dredging machine!

Possible general arrangement of a robotic dredge submarine.
Possible general arrangement of a robotic dredge submarine.

This machine should navigate by itself and self-supporting. The wear parts of the auger should be exchanged by itself and solar panels can provide extra energy for extended missions. It has only a small hopper and discharge should also be done quick and automatically. An unmanned barge or even a dump truck trailer at the shore of the waterway can be replaced at longer intervals. Obstacles and other tricky spots can be alerted to a human supervisor for later intervention. One machine alone does not have an impressive production. The real power is in applying them in numbers. As we are standing on the brink of a revolution in robotics and artificial intelligence, this scenario may be not as farfetched as from your first impression. Imagine a whole school of these mechanical fishes cleaning up your waterway, while you sleep…

Working method of robotic dredge submarine (Credit: Judith Korver).
Working method of robotic dredge submarine (Credit: Judith Korver).

References

  1. WODCON, Damen
  2. New Year’s post 2019, Discover Dredging
  3. Ketelmeer project, Wikipedia
  4. Rapportage baggerproeven Ketelmeer. RIZA Rapport, 97.023, ISBN 9036950708
  5. DOP Pumps, Damen

See also

Sensing My Audience, Feedback On How To Drive DOP Pumps

DOP pump and power pack, ready for commissioning.

This website provides me a platform to share my knowledge about dredging as far as my experience reaches. Sometimes, I do training sessions with colleagues or clients. They give me the opportunity, as an academically educated dredger, to experience how our figments of imagination work out in real life. People from different countries and lifestyle share their experience with our products. I really enjoy those sessions, they give me feedback on my normal work. Some of the topics on this website are directly related to the topics I present on those sessions. One of the recurring issues might not be that obvious at first hand. So, this post results from this feedback, aptly concerning the feedback loop in a load sensing circuit in a hydraulic drive system, specifically for a DOP.

The most common arrangement for the traditional DOP drive is to have a separate (preferably co-purchased) hydraulic power pack1. The power pack we supply is specifically designed to drive the dredge pump in the DOP. This requires the extra investment of a hydraulic power pack. Whereas most hydraulic cranes and excavators, that can handle the DOP pump by weight, will also be able to divert some hydraulic power to an auxiliary connection or so called Power Take-Off or Aux PTO. Especially when the excavator is working with a standard suction head, not much motion and forces are required for other functions. So, naturally clients tend to propose to use their DOP driven on the hydraulic PTO. This could be possible, but only under very strict conditions. And here comes the advice after commissioning hundreds of those units: ‘Don’t use the load sensing feature of the hydraulic PTO!’ Why? Let’s study the hydraulic diagrams.

Hydraulic diagram DOP pump and power pack.

The standard arrangement of a DOP is quite straightforward. As long as you do connect them in the right fashion. By controlling the angle of the swash plate, you control the delivered flow from the hydraulic pump and the rotational speed changes accordingly. Any load changes at the dredge pump result in pressure changes in the circuit, without changing the pump speed.

Typically, excavators do have a different power characteristic. When freely raking the stick, they encounter little resistance at preferably a high speed to reduce cycle time. When they hit the ground, the resistance or load increases and they typically lower the flow to maximize power delivery. They do this by connecting the pump line to a control line that influences the angle of the swash plate. Effectively, this results in a feedback control loop.

Infographic feedback control loop in a load sensing circuit.

And here comes the trick. Feedback control loops are designed to be stable. But the varying load at the dredge pump, in combination with the masses in the hydraulic lines, the springs in the rubber hoses and the damping of the flow losses, make the sensitivity of the control loop irrelevant. The signal from the dredge pump load is lost and becomes out of sync with the required action at the LS port of the pump. Any random static will make the swash plate rattle and usually in an instant, the drive stops. Other than maybe a blocked dredge line, no harm is done and service calls to our colleagues2 will resolve the issue: ‘modern excavators will allow you to turn of the Load Sensing feature’. Or, purchase said dedicated hydraulic power pack.

Damen hydraulic power pack mounted on a crane.

I would like to acknowledge Wim Roeterdink for his advice and review of this post. He is an expert on hydraulics engineering and he is always available for assistance in your particular hydraulics problem.

References

  1. Hydraulic power packs, Damen
  2. Service, Damen

See also

Parker hydraulic pumps