Hydraulic Dredging: A cost effective solution for most dredging tasks

The vast majority of dredging jobs require removal of solids to depths of 30 feet or less. Several features enable hydraulic dredges to be the solution of choice for these applications.

First, what are hydraulic dredges and how do they compare with alternative mechanical dredge types?

Hydraulic dredges create slurries, combinations of solids and water, pump the slurries to the surface of lagoons, ponds, lakes, waterways, and canals, and then pump the slurries through floating and land based pipe to disposal sites. The hydraulic dredging process can be continuous, as contrasted to intermittent in the case of so-called clamshell dredges.

Basic components of hydraulic dredges include a cutterhead, pump, floatation system or platform, engine or motor, control system, and the discharge system. Cutterheads dig up settled solids in the waterway to be dredged and create slurries.  A powerful centrifugal pump mounted directly behind the cutterhead draws the slurry into a discharge piping system, pushes the slurry to the surface, and then to the discharge site, often thousands of feet away from the dredging site. The engine, typically diesel, or electric motor, by means of hydraulic motors and high-pressure hose, transfers the engine’s power to the cutterhead and pump, and in the case of self-propelled dredges, such as a Rotomite-6000, to the dredge’s propulsion system. Hydraulic dredges integrate the foregoing components into a balanced, highly productive and efficient system.

Hydraulic dredges are made in several configurations, all of which except for cutter/suction and spud dredges, and dredges that require a support vessel, are made by SRS Crisafulli:

• Horizontal auger hydraulic dredges
• Suction dredges that have no auger
• Free standing, self-propelled, steerable dredges with one or more onboard operators
• Remote controlled dredges typically controlled by a cable system within a well-defined area
• Dredges with varying degrees of automation
• Dredges that can operate safely in highly caustic applications
• Dredges that operate efficiently without a support vessel, or dredges that require a support vessel
• Cutter/suction dredges
• Dredges that are positioned with spuds (long, powerful spikes driven into the bottom of the waterway) and use hydraulic rams to apply forward force to the cutterhead
• Hydraulic dredges are defined also by the depths to which they can dredge, the volume of water and/or solid material they move, the height to which they can discharge the slurry, the power of the auger (or cutterhead), the hardness and abrasiveness of the slurry, the size of solids the dredge pump can pass, dredging speed, width of dredging cut, the cutting power (torque) of the auger, and the cost of the dredge.

The foregoing features all contribute to and determine the productivity of the dredge and the owner’s Return On Investment (ROI) – i.e., how much work the dredge can do in relation to the initial cost of the dredge and the ongoing operating, repair and maintenance expenses of the dredge.

Other types of dredges and solids removal systems are classified as mechanical and include:

• Clamshell dredges that typically must be operated with a crane
• Huge, ocean going vessels that can dredge huge volumes at great depths
• Drag lines that haul a large shovel through the area to be dredged

Each of the foregoing three dredge types provide advantages and disadvantages, including:

• With the possible exception of draglines, these systems are far more expensive to buy and operate than horizontal hydraulic dredges. For example, horizontal auger hydraulic dredges vary in cost from approximately $150,000 to $700,000. SRS Crisafulli’s most expensive horizontal auger hydraulic dredge can be purchased new in 2011 for approximately $350,000 plus the cost of accessories, spare parts, and options. A high capacity cutter/suction dredge can cost upwards of $1.5 million. An ocean going dredge can cost several million dollars.
• For the vast majority of applications these three alternative mechanical systems are “over kill”.
• Clamshell/crane dredging systems together with the large barges the systems are operated on can dredge to greater depths, and dispose of the dredged materials many miles away at dumpsites approved by the EPA. But these systems are bulky, noisy, and more costly to buy and operate.

Selecting the best dredge solution for any application requires careful analysis by experienced, well-trained people in collaboration with the customer’s staff. Frequently, such an analysis requires the efforts of engineers with the skills and experience to sort out the many variables.

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