Retrofit Degassing Lifts Dredger Efficiency

A DDE degassing installation.

The presence of gas in dredged mixtures attacks the efficiency of the dredge pump and therefore the entire dredging process.

Gas is usually present in layers of silt, which means contractors engaged in maintenance dredging operations are among those most likely to be affected by the problem.

One such company is Dragages Ports, the French government owned contractor whose primary activity is maintaining safe depths for navigation in the country's major ports and fairways.

Two of Dragages Ports' vessels, the 8,500m3 capacity trailing suction hopper dredger 'Samuel de Champlain' and the 5,000m3 capacity trailer 'Daniel Laval' were both suffering from gas problems. Damen Dredging Equipment (DDE) was brought in to design and install retrofit degassing systems for both vessels. Samuel de Champlain was to be equipped with a suction pipe mounted degassing system designed for an in situ gas content of 3% whilst for the smaller Daniel Laval the target was 3%. As the degassing system runs on the existing jetwater pumps, the situation called trouble shooting and redesigning the existing jetwater systems.

The degassing systems delivered were based on the vessels' specific working conditions as well as equipment currently on board such as the dredge pumps and jetwater pumps. The Damen degassing system makes sure that the gas trapped in the silt does not reach the dredge pump and therefore cannot affect the dredging process. The system is comprised of components to extract the gas from the dredge pump using jetwater driven ejectors in a separation tank, components to return sludge to the dredging process and those for separating the gas from the jetwater. Since the sludge is returned into the dredging process, the system is entirely closed and thus environmentally friendly.

Damen designed both systems, delivered and installed all components and trained the crews on board. Damen's challenge was to fit a degassing system on the suction pipe near the submerged dredge pump whilst remaining within the safe hoisting load capacity of the existing hoisting gear. Success was achieved by fitting only the separation system on the suction pipe and placing the entire vacuum system on deck. The two are connected by the existing jetwater piping, which could be reconfigured using a number of valves.

All parties were challenged to get the systems running before a seasonal peak in the presence of gas in dredged silt. As decomposing algae and biomass produce the gas that is trapped in the silt layers, the in situ gas percentage reaches its peak in late summer due to rising temperatures. Damen had both systems up and running by early autumn. Enhancements to efficiency were immediately apparent, with hopper density raised by up to 15%.

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