About Dredgers

DSB Offshore is active in the sale, purchase and charter of dredgers within the marine market.

History

Dredging has been in existence for centuries, starting when rowing boats “dredged” the sea for food in shallow estuaries. Since that time the requirements for vessels that “dredge” has evolved into the present day dredging systems of removing large quantities of materials from the sea bed.

Dredging is now a large industry encompassing vessels able to perform multiple tasks from fishing, to flattening, scouring harbours, to deepening harbour channels to winning spoil for construction projects.

Present day dredgers are large versatile ships with considerable power for high capacity removal of spoils from varied areas to be relocated as building materials or just moved from location to location.

Dredger Types

The major dredger types are as follows: -

Trailing Suction Hopper Dredgers (TSHD)
Clamshell Dredgers
Cutter / Wheel Suction Dredgers
Aggregate Hopper Dredgers
Special Dredgers including “walking Platforms, Semi-submersible ships, Barges, Ploughs” and various others, capable of moving sea bed materials.

Due to the size of the present day dredgers most have the facility to pump the dredged material ashore, up to 4 kilometres, depending on the “Flux” (constituency) of the material pumped. This is carried out either by semi-submersible pumps or a floor ejector slurry discharge system pumping through floating pipes to the shore or jet ejector to approximately 100m from the bow of the dredger. This facility on a dredger is called “Pump Ashore”.

Discharge Methods

Dredgers can be discharged by several methods. Each dredger in her specification will outline the method of discharge. The most sought-after dredgers are the ‘pump ashore’.  The varied methods of getting cargoes ashore are:

- Pump the sand / spoil through a pipeline when the dump site is at some distance
- Unload the sand and spoil through the hopper bottom doors when in deeper waters.
- “Rainbowing” through a Snout on the bow which jets water and spoil to ashore or to reclaim groins. This pump ashore method is similar to a fixed fire nozzle on the ships bow but instead of water column it a water and spoil column. Once the material is ashore the weight of the spoil retains it ashore and the water runs off back into the sea.
- A discharge pipe can be connected to the bow Snout to allow discharging of the cargo to up to 5,000m ashore. The shore facility must be able to allow the water to “run off”.
- the self discharge variety, which includes…

- “Bucket scrape and belt discharge arm”
- “Below hold belt discharge and discharge arm”

Most of the remaining discharge methods are by shore crane and grab facility. Most dredgers have a through hold dump facility to be able to dredge harbours and estuarial waters and dump at sea away from the dredged areas. To speed up the dumping process in this type of dredge work “Splitbarges” evolved where the hull splits rapidly dumping the cargo from the hull.

“Self Propelled Trailing Cutter Hopper Suction Dredger” with “Pump ashore facility”

Estuarial waters and channels are being deepened around the world as ship sizes are increasing. The dredgers have been designed with a drill cutting unit. These units will cut rock and hard spoil and usually have the facility to dump spoil cargo at sea or ashore and are usually self-propelled.

Self Propelled Trailing Cutter Hopper Suction Dredger vessels are a combination of the “Cutter Suction Dredger” and the “Trailing Suction Hopper Dredgers” where the trailing suction head is replaced by the cutting head. As noted there are few self propelled Cutter Suction Dredgers as the power requirements are excessive. However only 8 are in existence and several are under construction throughout the world.