going on down there . . .
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
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
Wheel Cutter Suction Dredge
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!
Plan View of a Bucket Wheel Dredge
mining . . .
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.