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

How DOP Pumps Developed and Entered the Digital Age

First application of a DOP pump

Recently, my daughter asked me: ‘Dad, what good is it to know your history?’ And I answered: ‘Dear, if you don’t know your past, you will not understand the world around you.’ And the world around us is changing rapidly. The most recent change in our dredging world is the launch of the DOP web shop1. The ultimate entrance into the digital age of a well proven pump. For those young people that only know how to order online (or others interested in dredging history): long before webshops were around, customers and suppliers had a direct relationship with each other.

In the early ‘90s, when Ballast Nedam received the contract to build the railway tunnel near Schiphol Airport2, they had a real tough challenge. The ground water level near Schiphol is very high. Any hole there, fills up rapidly. Using sump pumps to remove the water from a building pit would be useless. To prevent collapse of the sides, there was already sheet piling in place, supported by braces to carry the side load. The space between the braces was too small for a long reach excavator. And the area under the braces too low to work from pontoons. Moreover, the foundation pilings where already in place and they should not get damaged by the excavation with a crab crane.

Construction site of the railway tunnel at Schiphol Airport

At this point, Ballast Nedam contacted their supplier De Groot Nijkerk for a smart solution. Ballast Nedam wanted a small self-contained dredging machine, that would fit between the braces and remove the sediment hydraulically. In a real Gyro Gearloose fashion, De Groot Nijkerk managed to patch together a contraption to prove the concept: ‘the first DOP pump’ (of some sort). It consisted of a normal dredge pump and a submerged jet pump in the same frame.

Proof of concept for a DOP

The tests were successful and the prototype was turned into a production model. The main difference being that the bearing and the dredge pump were designed with a mechanical seal to remove the gland water installation. This mechanical seal required some development on itself, as standard mechanical seals were too fragile. The newly developed seal was of real dredging proof quality. The product was successfully used and word spread around the Dutch contractors about this nifty little dredging machine. As a result, the new DOP was introduced in 1991 and started a career of its own.

Introduction of the first standard DOP on the market

As customers were very original in creating their own solutions for their specific problems, this single product slowly evolved in a whole line of products and options3. For as long as I remember, there was this picture in the product leaflets, that the customer could use to configure their own tool. Hence the slogan: ‘Your job, our tools.’

Typical selection diagram of DOP options

Over time, the range has been reengineered and thoroughly standardised. Due to this standardisation, the sales could also be standardised. Thus, the natural consequence: the webshop. Here you can experience online convenience with personal service.

Real DOP’s on display. Buy them at https://dopshop.damen.com/

References

  1. Damen DOP shop
  2. Schiphol Airport expansion, Wikipedia (Dutch)
  3. The DOP® submersible dredge pump and the possibilities for the contractor, DPC December 1998

See also