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

Hydrogen Sulphide: How Nature Uses it and Dredges Get Rid of it

Bison grazing near Mud Volcano, Yellowstone National Park, Wyoming, USA

Yellowstone National Park is not only renowned for its active geology. There is also an abundant wildlife, roaming free over the area. One would guess, the local wildlife would be accustomed to these interesting geological features. Or would be disturbed by all the tourist gathering at the literal hotspots in the park. Well, you might be surprised. At times, e.g. bison tend to flock around the same hydrothermal features that attract the tourists. You don’t notice it from the pictures, but when you are there, you’ll smell it instantly: rotten eggs. As a dredger, you know what this means: danger!

The odour of rotten eggs is caused by the toxic gas hydrogen sulphide1. At Yellowstone National Park, this gas is released by the fumaroles, mudpots  and geysers2. The bison might not be aware about the danger of inhaling this vapour. But they do know, that all the bugs and parasites, that live on their skin have a lower resistance for the toxin and fall of their hosts. Well, sometimes the bison fall for their own trap and get intoxicated themselves3. That is one of the dangers of hydrogen sulphide, above a certain a threshold, your senses get numbed and you don’t recognise the danger anymore.

Gas bubbles expanding in the vacuum of the dredge pump

In dredging operations, hydrogen sulphide usually has a biological origin, rather than a geological. In seasonally warm water, algae bloom near the end of the summer and die some time later. The decomposing biomass can release hydrogen sulphide amongst methane and carbon dioxide. Quietly trapped in bubbles between the grains in the sediment, they get disturbed by the dredge and enter the suction line. It is only in the dredge pump, that these bubbles get expanded and wreak havoc to the dredging process.

Gas removal concept before the dredge pump

The trick is to remove the gas bubbles before they enter the dredge pump. Several systems do exist but usually the inspection piece is modified and separates mixture and gas. It is up to the rest of the system what happens with the foam that gets extracted. It might be blown overboard or properly re-handled responsibly to protect the environment. Either way, the dredge pump will be able to operate at a more continuous load, the mixture density increase and total production will be higher.

Example of production increase in relation to gas removal rate

A good write-up about the dangers of hydrogen sulphide in dredging4 can be found at ‘The Art of Dredging’. There is also an article about the application of degassing systems to lower the dangers of hydrogen sulphide5. Even as the vapour is released at a single location on the ship, you still have to be aware of what you are doing and operations have to be adapted accordingly.

My personal experience with this nasty gas is only limited to commissioning degassing systems6, not actually working with them over longer periods. Even so, I got my impressions of life aboard under these circumstances. There was one occasion, where just in the week before our commissioning of our delivered degassing system, there was a severe accident. During our commissioning trials we had several warning alarms and whenever we went outside we had to wear personal gas detectors. If you did not report within an agreed period, alarms would ring and a search party dispatched. So, I am happy for every degassing installation delivered. It saves lives and fuel and cares about the environment.

Example of a standard degassing installation from Damen Dredging Equipment

References

  1. Hydrogen sulfide; Wikipedia
  2. Mudpots at Yellowstone National Park; US NPS
  3. Poison gas kills five bison in Yellowstone; Bozeman Daily Chronicle
  4. H2S (hydrogen sulphide); The Art of Dredging
  5. Degassing systems for dummies; The Art of Dredging
  6. Retrofit degassing lifts dredger efficiency; Maritime Journal

See also