The Art of Dredging

Dredging is underwater architecture.

Dredgers position their massive tools (dragheads, cutterheads, crane buckets) on the seabed, with extreme accuracy, to create an underwater landscape

Trailer dredgers are giant vacuum cleaners, dragging suction pipes along the seabed. They are mostly conventional ships, twin-screw, CPP, high-lift-rudders and added bowthrusters.

Dredge display with underwater bathymetry; the dredger has one suction pipe on the seabed. This positioning system is often networked with DP and a dredge control system. The position of the draghead is perfectly known in three dimensions.  Accuracy of the work is often one or two metres, in the horizontal plane.

In the 1990's dredgers got equipped with DP/DT systems, to qualify for offshore jobs.

These were class-1 systems at best, more often class zero.

Once the DP-consoles were onboard, crews started to figure out how to put them to good use.

New trailer dredgers are often equipped with a DP/DT-installation, specially designed for dredging. ("IHC Systems" from the Netherlands is the only outfit who makes these dedicated DP/DT-systems).

DP-controls at the navigation desk of a dredger. 

These specialised systems focus on:

•  The suction pipe(s) often excert varying forces (in excess of 50 ton / pipe, pending on seabed soil conditions).  These forces can be measured, and put in the DP-model. The dynamic tracking -with suction pipes on the seabed- is a cut-throat affair, pushing the whole ship to the edge. A Kongsberg service-engineer said: "This DP-system has no mercy with the engines."
• The suction pipes and dragheads are not steerable things;  they are suspended from gantries with cables and winches. Dragheads on the seabed have lives of their own; sliding down slopes, and with a natural tendency to follow earlier draghead tracks. The only way to steer these dragheads is to adjust the ship's position, either heading or track offset. This is where a DP/DT system comes in very handy.
• Different algorithms for current prediction are used, adapted for shallow water effects.
• The dredge equipment takes up a lot of the power available in the vessel, often more than 50%. To avoid overload on power generators, a dedicated power management system -with safeguards- must be in overall control.
• The DP/DT system is often networked -or even integrated- with the dredge display system and dredge control system. 

TSHD "Gerardus Mercator" in the field with "Molipaq", off Sachalin island, 2005.

Mostly, trailer dredgers are not involved in high precision jobs, but in maintenance and capital dredging; deepening waterways and creating new land or ports.

DP/DT systems play an ad hoc part in conventional dredge operations; they are often used for station-keeping, or as a speed-control tool.

Ship's speed is the main parameter influencing dredge production, and the more accurate ship's speed is maintained, the higher the ship's output. DP can handle that task, while the skipper can focus on other things; navigation, lookout and dredging.

 

 TSHD "Alexander Von Humboldt" rainbowing. The ship keeps station on DP, countering the huge reaction force at the rainbow nozzle.

The dredging industry has been slow to pick up DP-technology; two decades after the offshore industry.

But nowadays, thanks to specialised DP-sets, dredgers reap full benefits: higher postioning accuracy, and higher production.

Rather than attract DPO's from the offshore industry, dredging companies allow their deck officers to train and get hold of limited DPO-certificates themselves, giving them one more trick up their sleeves.

december 2010

Marc Van de Velde