Drill & Blast

GPS positioning means accurate dredging

A Queensland sand and concrete supplier is saving $120,000 per year, thanks to Trimble GPS positioning equipment on its two sand dredges.

At Mooloolaba, on Queensland?s Sunshine Coast, Cemex Australia gets much of its sand from its own dredging operations on the northern river flats of the Mooloolaba River.

The sand extends to a depth of around 18m and is underlain by sandstone. Above the sand are topsoil and clays which are stripped with an excavator before the sand is accessed by dredges using a hydraulic cutter wheel and suction to bring the sand to the surface for screening. The dredges move across an area progressively, in a series of parallel passes, dredging the full depth of sand as they go.
Traditionally, a dredge operator would try to stay on course on each pass by lining up some sticks in the mud or other ?landmarks? as a guide and by feeling his way through feedback from the machine.

There was quite a bit of guesswork in positioning, and areas would often need a second pass, to pick up material left behind. In addition, it was only when the cutter started bouncing off the sandstone that an operator would know that the dredge head had reached the bottom. Neither of these scenarios was very satisfactory.

Earlier this year, Cemex Mooloolaba sand plant manager Peter Allitt was watching dredges fitted with Trimble?s HYDROpro GPS positioning system construct channels to tolerances as small as 1cm. He saw potential in introducing the system to his own business and approached Ultimate Positioning, the Trimble dealer in Australia?s southern and eastern states, for advice.

Mr Allitt?s idea was a good one. Cemex didn?t require 1cm horizontal accuracy in their operations so, for each dredge, the company had Ultimate Positioning fit a Trimble HYDROpro system that would use the GPS base stations of the Australian Maritime Safety Authority (AMSA) to correct satellite signals and provide the dredge with horizontal positioning to one metre or less.

The system is based around a Trimble marine modular GPS receiver which picks up satellite positioning signals and corrections to those signals transmitted by AMSA.

In the cabin of each dredge, the HYDROpro software runs on a computer hooked up to the Trimble receiver, a screen, an electronic compass and a tilt sensor on the dredge ladder carrying the cutter head and suction plumbing.

With the compass giving a dredge?s heading, the operator can zoom in on a site plan displayed on the screen to steer the dredge along perfectly parallel dredging lines. Information from the tilt sensor tells the operator the depth of the cutter head below water level, to within 50mm.

The software monitors and records the dredge?s progress, showing the operator the site coverage in real time on the screen. Cemex has the screen set to show the sites as a grid of cells or ?bins? of 1m2. As the cutter moves down in an arc from near horizontal to near vertical, the movement is represented on the screen by a change in the colour of each bin the head moves through, with different colours representing different depths.

The operator can ?see? what?s going on beneath him in either plan view or cross section.

In the Mooloolaba River flats, the base of the sand is at a reasonably consistent depth. The depth is programmed into the software so that when the dredge head is getting close to that depth the operator sees the colour of the relevant bins change to red. For the operator, it is a game of colouring in bins; when he sees all red bins on his screen he knows the dredge head has covered everything.

If required, the system can record the completed floor profile and export the data either to Trimble Terramodel or other processing software.

Immediately after fitting the Trimble HYDROpro systems, Cemex found that dredging productivity had increased by four per cent. Within 10 weeks, the systems had paid for themselves and over 12 months the company expects to have saved around $120,000.
According to Mr Allitt, the benefits of the system extend far beyond the dollar savings.

?If you bring a dredge back to dry land for smoko or to switch operators, it is very easy to take the dredge back to precisely the point you stopped, and start work again,? he said.

?The operators love it. They wouldn?t want to go back to the old way of doing things,? he added.

The system that Cemex has installed operates with Trimble?s HYDROpro Construction software which is designed for marine and waterway construction where precise positioning is required. It incorporates the hydrographic features of Trimble?s HYDROpro Navigation software, with features added specifically for marine rig construction, piling and dredging. Dredge Mode allows you to dredge (or fill) to a design shape if required.

Source: Trimble

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