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


  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



Loose Sand, How Hard Can it Be?

Breaching exhibit at the Damen Dredging Experience

Did you ever tried to build a sand castle? Probably yes. Felt frustrated it always collapsed unexpectedly? At least I did when I was a child. But it took me an academic study to know why. Lucky you. You just have to read this blog post and experience a moment of enlightenment. So, this is good moment to stand up and get some coffee. You will enjoy reading it more and remember my lecture every moment you take a sip.

The second exhibit in the Damen Dredging Experience is an installation, which we call: ‘the Bank’. Usually there is some mechanical or hydraulic action, that will cause the sediment, to become unstable. In this exhibit, we can turn the little wheel at the lower right corner. The first thing you will notice is that at the higher end of the soil surface, the grains will slightly move and start to tumble down along the slope. Where the activation of the particles start at the slope is called the active bank.

Breaching the bank and density flow

The effect we would like to demonstrate, is that different soil types, do have different behaviour in this process. There are three different soil types, from course to fine, from the front to the back. The finer material at the back seams to stay the longest at rest. This is due to a phenomenon that we call ‘dilatancy’1. If a stack of grains is sheared, they have to hobble over the tops of the layer below.

Under pressure due to dilatency on a shear plane

When the grains do hopscotch over each other, they require more space to do so. Effectively the pores increase in volume and the total sediment expands. The extra space cannot be accommodated for by expanding water, it has to be replenished. The extra water has to come from the outside. But the grains themselves are in the way and form resistance to the incoming water. The resistance causes a differential pressure under the ambient pressure, commonly known as ‘vacuum’. And grains under vacuum tend to cling together and form chunks. This happens mostly, when the pores are small, or when the grains are small. Exactly what you can see in the exhibit.

Once the sediment is loosened from the active bank, it rolls down the slope, it behaves like a dense fluid, driven by gravity2. When the slope becomes less, or the running fluid encounters resistance, the sediment will settle again at the so called ‘passive bank’.

Outflow of density current and sedimentation

Here the reverse process happens, the water has more trouble getting out of the suspended flow and run longer. The passive slope will be flatter at finer grains than in more coarse material.

Both processes can be identified in e.g. the DOP3. It is usually suspended on a wire and lowered onto the seabed. Powerful jets excavate a small pit where the suction head takes up the suspended material. The walls of the pit become unstable as an active bank. The loose material flows into the pit. This turns into a continuous process and the active bank, runs away from the suction pit.

Breaching and density flow in a DOP process

Now, it is immediately evident, why DOP pumps have this characteristic suction pipe. It fits snugly in the pit and has the least resistance for the incoming density flow. Another benefit of the suction tube, is that if the bank collapses on the DOP, the suction pipe can be extracted without too much trouble. Extracting a pump from under a collapsed bank imposes the same trouble as creating a passive bank: suction due to dilatancy.

So, your sand castle collapses when water enters the pores. A demonstration of grains becoming as strong as a concrete block by under pressure is a well-known household phenomenon: vacuum packed coffee. Now, you will think of this, whenever you open a new pack of coffee.

Vacuum packed coffee is stable due to under pressure in the pores


  1. Dilatancy, Wikipedia
  2. Density current, Wikipedia
  3. DOP pumps, Damen

See also

Options for Repairing Parts That Ought to be Replaced

Patching up a pump casing as crisis management

Summer is arriving. We’ve had lots of sunshine and thunderstorms with rain these days. For sunshine, you protect yourself with lots of sun screen with a high protection factor. If it rains, you put on a raincoat and wellies. So, if you want to protect what is dear to you, you cover it with the appropriate cover. Hmm, if your pump gets eroded by your mixture, you cover it with a protective layer. Right?

So, let’s see what options we have? Common solutions to protect the wear parts of the pump are:

  1. Vulcanise a rubber film over a new cast wear part. Usually, the pump parts are designed to receive an additional layer of several mm to a couple of cm. Astoundingly, the soft rubber, lasts longer than the hard alloy. This is due to the elasticity of the rubber. Impacting particles are bouncing back into the fluid and don’t damage the metal1. There have been several developments, where polyurethane2 can be a viable alternative to rubber with the same protective principle. Rubber and polyurethane have to be applied by specialised companies under controlled conditions. One warning though, rubber can be cut. When dredging shells or coral, the rubber is sliced to pieces and the flow peels away sheets of rubber or PU.

    Wear resistant elastic collision of rubber and PU
  2. Instead of the flexible rubber, also hard chemical compounds have been developed to be applied as protective layer. This layer can be applied on a virgin part, that can accommodate the layer by design, just like the rubber cover. Or it can be used to restore an already worn down part and extend the lifetime that way. The pastes have been engineered to be applied in the field: wet, rust, salt and dirt. As it easily applied, many owners are very fond of this solution. Although lifetime is not extended as much as the first option.

    Wear plate restored with chemical coating
  3. Another process for new builds and restoration is hard-facing3. On a suitable base alloy, the vulnerable surface is cladded by welding a layer of very hard metal onto it. The hard-facing can be much harder than the sand particles (Mohs remember) and that the added material is brittle as glass doesn’t matter as it is carried by a much more ductile base material. As the base material is also softer, once the hard layer is away, the wear rate will become unexpectedly rapid. Especially as such local wear spots tend to eat through the material, due to the increased turbulence. Special care should be taken, when the wear part is not completely covered in hard-facing, but only partially covered. The discontinuities are hot spots of wear. Discontinuities can also appear from the hard-facing itself. The different material properties of cladding and base material cause to be vulnerable to check cracking and flaking4.

    Warning: make sure you’re wear part can be welded, or it will crack

All three solutions are labour intensive. especially vulcanising and hard-facing takes many hours. And applying the wear paste has to be done more often. Even if labour is cheap in your operation, you still have to take it into account in your spare part strategy. More so, if you only rely on just one wear part and don’t have one on stock and the dredge is idle, you’ll lose a lot of money on income.

Another consideration might be the environment. An all metal wear part can be recycled easily. Rubber, PU and paste can be a pain to get rid of, responsibly.

Off course, I am an engineer at the manufacturing side and the above perspective may be biased, but I like to be proven wrong. Until then, I would rather purchase a durable all metal wear part, than go to the trouble and costs of the extra handling. Whatever your final wear part strategy, it should revolve around having the correct spare parts at the right time at the right location.

Spare parts on stock


  1. Elastic collision, Wikipedia
  2. Synthetic applications, W&TS
  3. Hardfacing, Wikipedia
  4. Frequently asked questions about hardfacing, The Fabricator

See also