What's going on down there . . .

Dredging is similar to hydraulic systems in that you can't see what is actually happening. The exact operation of the dredging cutter is pretty much a mystery, and most of what you learn about the operation is by inference. This makes the specification, design and operation of what is basically a fairly simple machine into an interesting challenge.

At first thought the process seems similar to that of a dry bucket wheel excavator. However, the water makes a mockery of this as the material settles much more slowly in water than it falls in air - like excavating feathers, hard but light feathers.

The first and the biggest . . .

 The first dredge that I worked on, AMA Dredge 23 in Queensland, was recently retired from service after about 30 years of mining mineral sands. We installed new winch hydraulic power packs to replace the original unreliable units. Twenty years later, the same system was still in use, with modifications to take advantage of new technology such as proportional valves.

Bucket wheel mining dredge
Bucket Wheel Cutter Suction Dredge

The biggest dredge, and the most challenging set of problems, was much more recently with RGC's Dredge 26 in Western Australia. This was a new dredge that produced well below its budget output. An analysis of the operation showed that a good proportion of this was due to low availability from reliability problems. However, the study also showed that the production while it was working was well below specification. The reliability problem could obviously be solved by upgrading the equipment to an acceptable quality level, simply requiring some good engineering and money; but the low production was another problem, as it was a mystery.

Initial investigation of ground conditions and dredge operation did not find any factor that could explain the magnitude of the shortfall. An extensive trials program was initiated with computer data acquisition for trend analysis. Controlled test procedures were developed and carried out while varying all operational parameters - face material variations at different bench levels, different dredging techniques, cutter speed and torque, swing speed, cutter design, water velocity - leading to two identifiable trends; that specific production fell significantly with increasing cutter power and that there was consistently better production at higher swing speeds.

 At the end of the trials period it was planned to shut the dredge down as not being up to the job. For the last trial, a theory was developed that the problem was not with the cutter but in the suction mouth.

$100 worth of boiler plate was used to reduce the size of the suction to increase the suction velocity. This was trialled on the last day and gave nearly 50% improvement in production, substantially meeting the design specification, and restoring the feasibility of the operation. Talk about a last minute reprieve - less than 24 hours from closure!

 Mining dredge
Plan View of a Bucket Wheel Dredge

Mostly mining . . .

In between the first and the biggest, and since, I have worked on many dredges, mostly in mining, providing troubleshooting, hydraulic systems, mechanical design, operator and maintenance training, and specification.

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