Grab hopper dredgers can be operated extremely accurately and are often used for dredging works near constructions – in ports and waterways, for instance. The grab hopper dredger is equipped with a crane that has a grab or clamshell. In heavily contaminated areas, a special grab can be used to bring the soil up without causing turbidity. Special grab dredgers are also applied for mining sand and gravel, especially at greater depths.
Main features of a grab hopper dredger:
- mechanical – specialised in accurate dredging operations
- self-propelled and seagoing
- equipped with a hopper to store and transport the dredged soil
- suitable for dredging sand, gravel and clay, and pieces of rock or underwater objects
- can be used to dredge at larger depths
- dredges while stationary, either moored or by means of spuds or anchors.
Grab: a grab or clamshell collects the soil. Based on the type of soil, different kinds of grabs or clamshells can be used.
Crane: the crane brings the clamshell or grab into position.
Hopper: the dredged material is deposited in the hopper, which is later emptied using the grab by opening discharge valves or, when the vessel is equipped with a pump, via a self-discharge system.
Spuds: grab hopper dredgers can be fitted with a spud system to keep the dredger in position while dredging. However, this is not a standard piece of equipment aboard a grab hopper dredger. The dredger can also remain stationary by using anchors or by being moored to the quay.
How does a grab hopper dredger work?
The dredging method of a grab hopper dredger is simple. The crane lowers the opened grab or clamshell, and at the bottom the grab is closed, after which it is hoisted again. The crane then swings around to the hopper and the grab is lowered so that the soil can be released in the well by opening the grab. Then, the crane is spun around again and the cycle continues. Once the hopper is full, the dredger sets sail to the discharge location to empty the hopper, either by means of the grab, by opening the discharge doors or the soil is pumped to shore.
Grab dredgers can also be installed on pontoons. A pontoon equipped with a grab crane can perform a useful task in places that cannot be reached by other types of equipment. This type of grab dredger doesn’t have a hopper, so the dredged materials are stored and transported in barges or dumped directly into the deeper area of the channel where larger dredgers can reach it.
Initial investment costs of a grab hopper dredger
The grab dredger is a much-used dredger around the world. Grab hopper dredgers are generally custom-built and based on their size, installed power, dredging depth, type of grab or clamshell, the application of spuds, anchors or both, and accommodation requirements. Their costs can therefore vary widely.