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

Don’t rock the boat, don’t tip the boat over

DOP Dredge ‘Roanoke’, Long Island, USA

We were quietly enjoying our dinner on a relaxed evening in our vacation. Suddenly, we were rudely disturbed by rumble and clatter from across the valley. For our eyes developed a rock slide. Just as sudden as it started it was already over. Perplexed, we were too slow to capture the event and put it on social media. Afterwards, I took some pictures of the rubble. As you can see, it was not even a proper rock slide, more the collapse of a retaining wall.

Retaining wall collapse, Sóller, Mallorca, Spain, June 21, 2018

Come to think about it, it was not the first collapse I witnessed. Back in 2006, I was visiting our DOP dredge at Roanoke on Long Island, NY in the USA. I had to do some measurements and general inspection. I was below decks connecting the data recorder to the drive system and had to check something with the dredge master. Just when I climbed on deck, he yelled at me to hold on. Immediately a torrent of water and sand was flung horizontally over the dredge. Some stones cracked a window in the control cabin. Within seconds a tsunami lifted the dredge for about a meter and we kept rocking until the reflecting waves in the pit eventually subsided.

DOP Dredge Roanoke with pit bank in the foreground, before it collapsed. Older bank collapses in the background.

That was one big bank collapse to me. A bank collapse is a known, although undesirable phenomenon in dredging1. It is a result of dredging methods, relying on the development of an active bank to produce a heavy slurry, that is sucked up. However, the sediment does not consist of a uniform block of sand. Usually, the sediment is deposited in different layers, each with their own geo technical properties. These result in varying propagation velocities of the active banks. When a ‘faster’ sand is under a ‘slower’ sand, the upper layer is not supported anymore and collapses. As the bank slumps down, it displaces an enormous volume of water and this often causes a tidal wave of its own. At Roanoke, the effects were aggravated by the fact, that the upper bank ran all the way to above water level.

Progression of an active bank and bank collapse

As this bank collapse can be expected when dredging with active banks and different sand layers, dredging companies are very keen on predicting these nasty consequences. Not only for the safe working condition of the crew, but also to prevent material damage and eventually for a stable and reliable delivered profile. Exactly this is what is being investigated by dr. Askarinejad in the Laboratory of Geo-Engineering at the Technical University Delft2. He has a beautiful rig, where exactly those conditions can be simulated and measured. With a neat trick he tips the whole test facility to form an instable bank. This makes the bank collapse on demand3.

Static liquefaction tank TU Delft (Credit: dr. A. Askarinejad)

Basically, this is exactly what we can demonstrate with the ‘breaching exhibit ‘ in our dredging experience4. Of course you are welcome to come over. For those who are not in the circumstance to visit us, you can also visit the National Dredging Museum as they now have a breaching exhibit of their own5.

Handover of our old breaching exhibit to the National Dredging Museum

References

  1. Breaching Process OE 4626, van Rhee, TU Delft
  2. Amin Askarinejad, TU Delft
  3. Statische liquefactietank , Delft Integraal
  4. Loose sand, how hard can it be?
  5. Baggermuseum krijgt model van Damen Dredging, Binnenvaartkrant

See also