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

Graduation of Ben Sloof: Hopper Loading Model and Overflow Losses

Ben Sloof signing his Master of Science degree

Do you remember Ben Sloof? Our young bright graduation student presenting at the Young CEDA Pitch Talks? Last week he graduated (with honours!) on ‘Numerical modelling of overflow losses and flow in Trailing Suction Hopper Dredgers’. You may have noticed that hopper loading was already a topic for previous posts about Ben Sloof and Jordy Boone at the CEDA Dredging Days. It results from the great interest in this part of the dredging process. And it still is a very dynamic field of research indeed, as in a short time frame various new models are presented. Who would ever think that dredging technology is a boring business? We are already dredging for ages, so what innovation could ever contribute to the progress of dredging? Well, I’ll explain what this new kid on the block found out now.

Overview of a hopper loading process with OpenFOAM simulation

Due to time constraints on a graduation project, Ben was limited to work on only a part of the hopper loading process. He worked only on the internal interface between mixture and sediment. Moving free surfaces of a filling hopper or a lowering water table from a reduced overflow height were left out of the scope. Still, it quite accurately describes the observed phenomena in the hopper loading process. We see a negative buoyant jet, we see the jet scour pit, recirculation and mixing in the fluid body and a ‘clear’ top layer which carries away the fines as it is skimmed away through the overflow weir. Deviation from the measurements can be explained by the difference in single fraction particle size.

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

With the observations from the CFD, Ben Sloof made his new ‘Layer Model’ with specifically a single fraction of particles. Then, he made an effort to correct the ‘Layer Model’ for physics of multiple fractions, with not too much improvement. Later on, he introduced a ‘fiddle factor’, that magically happened to be spot on. Further research and development of the model is currently under way to build a better foundation for this. Who knows, even Einstein had to introduce the ‘Cosmological Constant’ to fit his Theory of General Relativity to reality. Years later he was proved to be right1.

Comparison of van Rhee, Boone and Sloof

The performance of the models by van Rhee and Boone were already discussed2. Now, the validation of the Sloof ‘Layer Model’ is also included. Only after adjustment for the multi fraction in the results, the model nears the performance of the van Rhee model. Difference here is calculation speed. Sloof manages to estimate the hopper performance in seconds. This is why he rightfully received his degree with honours. He improved the speed AND accuracy of the calculation in a method, that would normally take the better part of a PhD study. I am looking forward to his future achievements.

Although this graduation assignment really helps us in refining our design process and gave some insight in the loading and overflow process, it still is an academic exercise. And any scientific model is just as good as the assumptions that were made at the beginning. In reality, the circumstances might be very different from what the model assumed, resulting in quit some different performance.

Heavy weather dredging (Retrieved from YouTube 18/10/2012, unknown source)

References

  1. Cosmological Constant Confirmed
  2. IADC Young Author Award for 1DH Hopper Loading Model of Jordy Boone

See also

Overview of hopper loading models by Ben Sloof
Nice report with an overview of the various hopper loading models

Dredging Engineering (lectures)

Dredging Engineering (papers)

Damen Standard TSHD

Damen MAD