Dredger

Overview

A trailing suction hopper dredger is a self-propelled ship that excavates the seabed on the move. Unlike a stationary cutter-suction dredger, it lowers one or two suction pipes to the bottom and drags them along while steaming slowly, sucking up a mixture of sand and water and pumping it into its own large hopper. When the hopper is full it sails to the dump or reclamation site and unloads, then returns for the next load. This cycle of dredge, sail, discharge and sail back is the work of channel deepening and maintenance, beach nourishment and land reclamation.

The ship is built around the central Cargo Hopper inside the Hull Structure. Material is gathered through the Drag Arm suction pipes, lifted by the Dredge Pump, and later emptied through the Discharge System. The Propulsion Train train moves and positions the ship, the Electrical Power Plant feeds the heavy dredge drives, and the crew run everything from the Deckhouse.

The drag arm and draghead

Each Drag Arm is an articulated Suction Pipe hung over the ship side on a gantry and ending in a Draghead that trails on the seabed. The draghead is the working tool: its replaceable Drag Teeth rip up compacted sand and clay, high-pressure Jet Nozzle jets fluidise the soil ahead of the mouth, and a hinged Draghead Visor follows the bottom contour to set the cutting depth. A Draghead Grid keeps stones out of the pipe. Because the ship heaves in a swell while the head must stay in firm contact with the bed, each arm carries a Swell Compensator, a hydraulic cylinder that pays the arm in and out to hold roughly constant pressure on the bottom. Sealed Arm Joint connections let the arm articulate as the head follows the seabed.

The dredge pump

The slurry is pulled up the arm by the Dredge Pump, a heavy centrifugal pump built to survive a flow of abrasive sand and gravel that would destroy an ordinary pump in days. Its Pump Casing is wear-lined, its Slurry Impeller is hardened and thick-vaned to pass stones without choking, and replaceable Wear Plate panels on each side take the brunt of the abrasion. A clean-water flushed Gland Seal keeps slurry out of the bearings. On a deep-digging dredger a second submerged pump is fitted partway down the arm so the system can lift from far greater depth than a single inboard pump could manage.

The hopper

The Cargo Hopper is a large open tank that fills as the pump runs. Crucially, the pump delivers far more water than sand, so the hopper is allowed to overflow: water spills over an adjustable Overflow Weir weir while the heavier sand settles, which is how the dredger ends up with a hold of mostly sand rather than mostly water. A Density Meter tracks the settling cargo to estimate the dry tonnes loaded, and the operator stops loading at the economic optimum. The hopper walls carry a wear Hopper Liner, and the floor is fitted with hydraulically operated Bottom Door panels driven by heavy Door Cylinder rams.

Discharging the load

The dredger has three ways to empty its hopper, provided by the Discharge System. For disposal at sea it opens the bottom doors and dumps the whole load in seconds. For land reclamation it re-fluidises the settled cargo with a Jet-Water Pump and then either pumps it ashore through a floating pipeline coupled at the Bow Coupling using the Shore Booster Pump booster, or throws it onto the fill area through a bow Rainbow Nozzle, the high arcing plume that gives the technique its name. A set of Discharge Valve units routes the flow to whichever mode is in use.

Propulsion, positioning and power

Dredging demands very precise low-speed track-keeping, so the Propulsion Train train uses twin Controllable-Pitch Propeller controllable-pitch propellers for fine speed control and adds bow and stern Thruster units for lateral positioning over the dredge track. The dredge pumps, jet pumps and arm hydraulics draw enormous power, far more than the ship needs to sail, so the Electrical Power Plant carries large generating sets and a heavy switchboard feeding Dredge VFD Cabinet drives. Many modern dredgers are diesel-electric, sharing one set of engines between propulsion and dredging so installed power can be sent wherever the cycle needs it.

Materials, variants and use

The wetted dredge components are made from abrasion-resistant alloys and hard facings because sand wears steel fast; teeth, wear plates and pipe liners are treated as consumables and changed often. Trailing suction hopper dredgers range from small ships of a thousand cubic metres for harbour maintenance up to giants of forty thousand cubic metres used on major reclamation projects. They are valued for being self-contained and mobile: a single ship can deepen a channel, sail to a reclamation site, and build new land there, then move on to the next job without any fixed shore plant.