Graduation Gijs Ter Meulen: Drag Analysis And Model For Forces And Production

Gijs proudly presenting his MSc. certificate.

We have another bright new MSc. engineer in dredging technology1: Gijs ter Meulen. Tuesday, he presented and defended his thesis on the forces and production of a trailing suction head. For this thesis project he was working at our research and development department at Damen Dredging Equipment2.

Trailing suction hopper dredges have become the tool of the trade for modern dredging contractors. They are versatile, flexible and able to transport sand over great distances. They load their cargo in their holds, by sucking up the sediment from the sea bottom with a big trailing suction head. This head looks like an out of size vacuum cleaner head.

Typical drag head on a railing suction hopper dredge.

Usually, it is very difficult to comprehend what is going on in and around the drag head. There is some laboratory research done, but not all results are freely available. Other knowledge is solely based on the experience of well-seasoned dredge masters. I do have respect for the experience of dredge masters, but their stories are hardly usable for an academic model description. So, Gijs took on the challenge to piece together a model, that satisfies our curiosity and fits with the experiences.

Concise development of the drag head model by Gijs ter Meulen.

For this project, he identified several steps, which we briefly touch upon here:

  1. The processes and forces around the drag head3 were all investigated on their cause and effect.
  2. A model was set up, where each process and their interaction with the others were identified.
  3. One main process in the drag head is the jetting production. A powerful jet of water is injected into the soil and this erodes part of the sediment under the drag head4.
  4. Another main process is the cutting production5. What is not eroded away by the jets, is removed by the teeth at the back of the visor.
  5. As the contribution of the processes to the forces and the production is known, the total performance can be calculated.

Along the way, this gave us very useful insight in the capabilities of the drag head and the trailing system, all the way to the requirements for the propulsion. Now, we will be able to continue to improve our drag heads even further. Any other students who would like to participate in that project are welcome to contact us5.

Gijs takes a new step in his career path. He is going to work for a well esteemed customer of us, so we will see him around in the dredging industry. Thanks Gijs, bon voyage!

Gijs, good luck with your future career in dredging.

References

  1. MSc Offshore & Dredging Engineering, TU Delft
  2. Innovation, Damen Dredging Equipment
  3. Designing Dredging Equipment [OE4671], Vlasblom
  4. Sand erosion with a traversing circular jet, Robert Weegenaar
  5. The Cutting of Sand, Dredging Engineering
  6. Internships, Damen Dredging Equipment

See also

Ben Sloof Nominated For KIvI Best Offshore Graduate Student

3D simulation of a hopper loading process

Ben Sloof was one of the best graduate students we had here at our company. For his thesis1, he tackled a complex problem and managed to model this in a reliable simulation. Now he is nominated for best Offshore Graduate Student. Today, there will be a KIvI Offshore lecture evening with a ceremony to award the prize2. Once again3, Ben will deliver a capturing pitch on his thesis. So, let’s review what he has achieved.

Van Rhee and similar ‘Euler’ models

At the chair of Dredging Technology of professor van Rhee, a lot of effort is put in describing the hopper loading in so called ‘Euler’ models. This is where you calculate the flow of the fluid and derive the flux of material that is carried within. Ben is standing on the shoulders of giants here, as by now there are a lot of models available4. We opted to use an existing simulation platform: OpenFOAM. One of the plugins for this open source program is DriftFlux, where the valuable grains are treated as a continuum fluid moving through the rest of the fluid. The extra effort of Ben, was to modify this DriftFlux plugin to account for settling and scour. This is in itself is already an unprecedented feat. Complicated by the unstructured calculations within DriftFlux and OpenFOAM. Nonetheless, after careful verification, he was able to perform interesting simulations of the hopper loading process.

Concentration and Velocity

After careful examination of the simulations, Ben started to see patterns in the flow. These set him on a track to build a whole new model. This new layer model credibly describes the process as well, without the complexity of a CFD simulation. As the development of a multi-fraction version of the OpenFOAM platform is still in progress, final verification is still pending. At least, the differences we see between the single fraction model and reality can be explained by what can be expected. It is open to further expansion with future research and can be used as a starting point for the next improvement.

Introduction to the key components of the proposed new ‘Layer Model’ (1DV)

And that is an insight worthy of extra appraisal: finally cracking the riddle of the sands settling in the hopper. We hope you will receive the prize. You deserve it.

Good luck Ben, we wish you all the best on your future voyages to unknown destinations. We are sure you’ll be blessed and a blessing, wherever you go.

Setting sail to distant shores

References

  1. Graduation of Ben Sloof: Hopper Loading Model and Overflow Losses
  2. KIvI Evening: Johan Sverdrup Platform Installatie
  3. Hopper Loading: What Happens Beneath the Surface
  4. IADC Young Author Award for 1DH Hopper Loading Model of Jordy Boone

See also

 

Alternative Fuel Systems on TSHD Samuel de Champlain

Trailing Suction Hopper Dredge ‘Samuel de Champlain’

The Trailing Suction Hopper Dredge ‘Samuel de Champlain’ has been in the news. She will get a mid-life upgrade with new engines running on LNG1,2. This is good news for the environment as the vessel will have lower emissions of greenhouse gasses. And it is good news for Damen as we see this old lady back again. Already more than ten years ago, we had her also as a customer concerning a gas related retrofit3. That time I was involved in the development and commissioning of that particular system.

Degassing installation on board ‘Samuel de Champlain’

The most peculiar feature of this degassing installation on this ship was that it had to work on the submerged dredge pump. Even as the pump had plenty of NPSH available at that depth, the gas content at certain locations was able to choke the pump. The submerged location posed special requirements on the operational pressures and the dimension of the sludge tank.

Off course, the most simple solution would be to have some gas ejectors on the drag arm and just blow the foam from the degassing scoop overboard altogether. However, that mixture contains (possibly contaminated) silt also, which is unscrupulously released to the environment. The Damen system has some extra components, such as a sludge tank, where the foam is separated in silt and gas. And a separation tank, where the gas is extracted from the clean water. Gas is than released to the atmosphere and the water returned to sea. As the sludge from the sludge tank is injected back to the slurry pipes, it ends up in the hopper and is disposed of with the rest of the silt.

Diagram of a typical Damen degassing system

An often heard complaint is, that the gas is still released to the atmosphere. That is right, but with the other systems it is released in the sea and comes in the atmosphere in bubbles. On top of that, one could also look at the natural process of gas formation4. If it is not dredged the gas would eventually get released by nature itself. With the autumn and winter storms, the bottom gets disturbed enough to release the contained gasses and enter the atmosphere naturally. The dredge only releases the gasses in a concentrated form, where nature does this gradually.

Another question I was asked regularly (and fitting to the opening article): ‘Can we capture the gas to drive the engines?’ That in itself is a good question, as it disposes the gasses beneficially. The degassing installation on board is capable of removing 800 kg/h of gasses from the mixture. If it were LNG, it would provide about a quarter of the energy requirements of this vessel. But, it is not LNG, only part is combustible methane, the rest doesn’t burn or forms sulphuric acid. Yuk! Certainly not something to pour in your expensive engine. It might be easier to just flare the whole mixture4. At least the potent greenhouse gas methane is converted to the less severe carbon dioxide. Any thought anyone?

It was fun to contemplate on this during the return trip from the commissioning. It was already running late (which commissioning doesn’t?) and we were 20 miles out at sea. The captain didn’t want to stop the dredge and he put us in the dingy to return to port. With calm seas and a fast RIB, this was a thrill ride to remember.

Return trip after a job well done

References

  1. TSHD Samuel De Champlain to be converted to LNG in a European firs; Damen Magazine
  2. Damen Wins Contract for First European TSHD LNG/MGO Conversion; DredgingToday
  3. Retrofit Degassing Lifts Dredger Efficiency; Maritime Journal
  4. Gas flare; Wikipedia

See also