Dredging for aggregates and reclamation materials is generally perceived to be preferable environmentally to taking land sources from quarries. And the latest generation of dredging equipment is reducing the environmental impact of dredging, be it for winning material or deepening shipping channels.
Heavy rock excavators, for example, can now dig to exacting tolerances which allows greater environmental sensitivity through selective excavation. Until recently an operator was at best accurate only to within 0.5m, but now sophisticated sensors feeding into a central computer allow for precision as great as 50mm.
One such case is a new barge-mounted Liebherr P996 backhoe excav- ator operated by California-based Dutra Dredging Cor- poration which has just completed its first contract, deep- ening the port of Oakland in Cali- fornia.
The two year, $42M (pounds26M) contract for the US Corps of Engineers involved removal of 4.6M.m3 of sandstone. Clamshell dredgers were also deployed on the site. But the big hydraulic excavator - nicknamed 'the Beast' by its creator Dutch barge builder Shipyard De Donge - was the only piece of plant with enough muscle for the central task of gouging solid rock to create clearance for ships of 13m draft.
The operator's 'underwater eyes' include depth monitors, a north-seeking gyroscope and differential global positioning system (DGPS) which locate the excavator on longitude, latitude and vertical axes. Sensors monitor the angles of boom, stick and bucket in relation to the barge, and seabed contours. Data is displayed graphically on two screen, which defines the bucket position in a global x-y-z space.
Dutch firm Seatec, which manu- factures the Dipper- master information system says software allowing the oper- ator to 'see' the worked surface has only become suf- ficiently sophistic- ated in the last five years. The system is modular, allowing it to be reconfigured for different jobs, or for levels of functionality to be added. Because it can be programmed with up to 20
bottom profiles the system also allows semi-automatic operation, improving efficiency.
The barge-mounted 996 is the world's largest mining excavator, yet it consumes a modest 10,000 litres of fuel per 20 hour working day. According to Shipyard De Donge it can shift 570m3/h at 20m depth. It can work in a 2.5m swell when jacked up on its three 38.8m legs, and it can stand firm in a 15 knot current.
The 60.5m long, 2,050t unit (the excavator section alone is 470t) is controlled from the operator's cab, which is elevated to give a clear view over the 6m side of a scow. The operator moves the barge using the leg at the stern giving 11m movement per 'stride'.
Liebherr has modified the P996 for marine use with over 200 adaptations that seal the machine against the aggressive environ-ment and give a deeper dig geometry.
Such capabilities have secured Shipyard De Dong a second order for a floating P996, for Great Lakes Dredge & Dock in Chicago.
Great Lakes takes possession in April 1999 and is angling for harbour deepening jobs on the US east coast. The company's chief mechanical engineer Steve Becker believes the most profitable areas are hard materials and long haul 'where these machines really shine'. However, demand for this sensitive giant is global.