January 2018 – Discover Dredging

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

Hiking Through the Norris Geyser Basin and the Risks for Your Dredge Production

Our Norris Geyser Basin hike in the Yellowstone National Park, Wyoming, USA

Smell is said to have the strongest memories. And usually it happens, that one smells a certain whiff and your mind is instantly transferred to the happy days of childhood where your grandma makes your favourite pie. So, what does it say about me, when we were hiking trails over the Norris Geyser Basin in the Yellowstone National Park, the fumes from the geysers brought me back to the soil laboratory where we bake the soils for analysing and sieve tests?

Overview of our cute little soil mechanics lab

Just like the baking process to dry soil in an oven, the sediment in the Norris Geyser Basin is heated by the hot ground water underneath. The hot water in the basin or in the sample leach silica and calcium from the grains and evaporation transfers those scents to your nose. Also, as the silica and calcium reach the surface, they cool down and get deposited on the outside. In the oven, the calcium will form some white spots and there is a thin crust of just a few grains thick. In the Norris Geyser Basin everything turns white and the crust is much thicker. Still, the crust is relatively brittle and accidents do happen when people stray from the indicated trail and sink through the crust and get cooked in the underground steam1.

Warning: Dangerous Ground (Credit: US National Parks Service)

Calcium cemented sand can sometimes be found in a dredging project too. There it is of some nuisance, as it makes soil reports unreliable and causes some unpredicted difficulties for the operation. The calcium glues the grains together and the grain size appears to be bigger. As smaller grains are more effected, the real particle size distribution might be much wider than anticipated. So, thorough shaking and pounding of the sample is important before sieving.

Effect of calcium bonds on apparent and actual particle size distribution

If you only had a survey for the actual or relative density, you may have estimated, that there is rather course material in an open (loose) structure. During dredging, you might find the bank is not free flowing, but comes down in chunks. You might even run into problems of a bank collapse. On the other side of the pipe line, the bonds will have been broken up by the cutter and the dredge pump. The reclamation area is surprisingly filled with lots of fines in the Particle Size Distribution. And as the fines clog the pores between the bigger particles, they hinder the drainage of the reclaimed land2, you may have problems getting the required relative density and bearing capacity. Bank collapses and an insubordinate reclamation area are better averted. Check the local geology and be vigilant on the soil samples for calcium cementation.

The Norris Geyser Basin in Yellowstone is a very special geological place, with cementation due to hydrothermal activity. However, cemented sand and its descendent, sandstone can be found anywhere. Normally we would encounter cemented sand from a marine and biological origin. e.g. Deltas, Beach and shore face sands, Tidal flats, Offshore bars and sand waves, Storm deposits, Submarine channels and fans3. Pretty much everywhere, where there is dredging. You have been warned…

Example of cemented sand forming sediment normally encountered in dredging (Credit: Wikipedia)

References

Lessons in Camping: Basic Soil Investigation

Pitching our tent at Bad Bear Campground, Idaho, USA

Oh the horror! An old salt like me had to go camping during our summer holiday. Our daughter had her birthday during our road trip in the USA and she wanted to celebrate it by camping in the woods. Complying to her wishes we pitched a tent and roasted marshmallows. Meanwhile my mind was frantically searching for familiar clues to connect to my maritime heritage. Hammering down the tent pegs, it dawned to me: putting up a tent is basically a simple Standard Penetration Test.

Standard Penetration Test explanation infographic

Standard Penetration Test is one of the easiest soil investigations you could do1. All you need is a pipe and a hammer. You count the number of blows to hammer the pipe down and you have an indication of the effort it takes to cut the soil. This method completely ignores sophisticated parameters as e.g. undrained shear strength, porosity or internal friction angle. It is very crude in its results. On the other hand, the basic principle of driving the pipe into the ground is very similar to the cutting action of the pick points on a cutter. As such, it is a very good indicator for the performance of a cutter head. This is also the reason, why for initial discussions about the performance of a CSD, the SPT is a good starting point to ask the client. He might have a report like this already available, or he can easily perform the tests. Also ‘Sandy’ accepts SPT values for an indication of the soil quality2.

Be aware, that SPT’s are often not very deep. Of course, a full soil investigation report with a Cone Penetration Test is much more valuable. We can always translate the results from a CPT report into a SPT value. But the SPT information is not covering all the parameters to translate this to a CPT. Sometimes even an SPT report fails. And then it might be useful to discuss with the client on a qualitative level about the soil condition. Usually people have actually touched the soil, or at least can paint a mental picture of the soil conditions and these criteria might help to use the same descriptive language.

Hammering a rod into the soil is a relative cheap and quick method to collect the soil consistency. It can be performed everywhere, anytime, under most conditions. That is why it was also selected for the soil investigation on one of the most remote locations imaginable. Although it is still on my wish list of dream destinations, the prohibitive price tag of the ticket will prevent it for me to pitch up my tent over there. I just have to revel in the camping adventures of Neil and Buzz.

Month: January 2018

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

References

See also

Hiking Through the Norris Geyser Basin and the Risks for Your Dredge Production

References

See also

Lessons in Camping: Basic Soil Investigation

References

See also