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