Dredging Up Dirt From Down Under

The dredge has an operational weight of 440 tonnes with a total installed power of 1400 kW and can excavate material as deep as 18 m below water level, with a production rate of 1500 tonnes/hr. It is designed for 24 hr/day continuous operation; therefore, the equipment was selected to provide a B₁₀ life of over 40,000 hr and a structural life of 25 yr.

The Neumann dredges have the advantage of high operational efficiency, high availability, and ease of operation and maintenance — reducing total operating costs.

Dredging using hydraulics

All dredge functions are controlled manually by a single operator. Selection and monitoring of each function is achieved through levers, switches, and instrumentation mounted in an operator's console-situated in the control cabin.

Interestingly, no electronic controls are incorporated on the dredge. This is done deliberately, to minimize the qualification requirements of technicians who must maintain the dredge. As most dredging operations are usually in remote locations, the availability of skilled maintenance personnel and the supply of spare parts are generally limited.

Excavation of material is done by lowering the bucket wheel below water level into the material. By the use of winches, the dredge swings through a 90° arc about a main mooring spud (a vertical pole mounted at the rear of the dredge and embedded into the river bottom).

All dredging functions are powered hydraulically. This includes providing power to the dredge pump, the bucket wheel cutter, and all winches and cylinders used to anchor and maneuver the dredge.

The dredge hydraulic system consists of three main circuits: one circuit for the dredge pump, one circuit for the bucket wheel, and one circuit for the winches and spuds. There is also a control circuit, as well as an emergency stand-by circuit for the winches and spuds. All circuits are of an open-loop design, drawing oil from a common hydraulic oil tank. The three main circuits and the control circuit are all protected by pressure-compensating controllers fitted to the hydraulic pumps, as well as a main pressure-relief valve fitted to each and every circuit.

Filtration of the hydraulic oil is achieved by high pressure filters installed at each hydraulic pump and return filters installed in the return line at the tank. All filters have a 10-µm absolute rating.

Excavation of the material is accomplished by a rotating bucket wheel cutter mounted on the front end of the ladder (the large frame-mounted structure shown between the side pontoons). Three Flender Hydrex radial-piston motors attached to a reduction gearbox power the bucket wheel. Maximum continuous operating pressure for this drive is 250 bar. At this pressure, the resulting bucket wheel output torque is 280 kNm. Supply of hydraulic oil to this drive comes from half (P16 + P16) of a quadruple Denison (Parker Hannifin) piston pump assembly (P16/P16/P05/ PV20) direct-coupled to a Deutz TBD 616 V-8 diesel engine.

Each of these pumps is fitted with a pressure cut-off controller, saving power and reducing operating costs. The P16s and the P05 are each fitted with hydraulic stroker controllers, which have externally adjustable minimum and maximum stroke stops. The through-shaft capacity of these pumps allows the staking of the pumps, thereby eliminating the need for a PTO splitter gearbox. This reduces maintenance and noise, and gives higher reliability, due to fewer mechanical parts in the drive line.

Excavated material is pumped away from the dredge by a hydraulically driven centrifugal dredge pump. Two Hydrocraft fixed-displacement motors attached to a gearbox drive this pump. Maximum continuous operating pressure is 315 bar; at this pressure, 800 kW of power is supplied to the pump shaft. This is accomplished by two tandem Denison piston pump assemblies, attached to a Deutz TBD 620 V-8 diesel engine. A tandem pump assembly is direct coupled to each end of the engine.

All five winches installed on the dredge are driven by SAI radial-piston motors through planetary reduction gearboxes direct coupled to the winch drums. Maximum continuous operating pressure for all winch drives is 250 bar. At this pressure,-the resulting base layer line pull of each winch is 11.5 tonnes of force. Here, a single Denison piston pump (P05), which is part of the quadruple pump assembly mounted on the diesel engine, is used. The functions of the five winches are as follows:
• one winch raises and lowers the ladder assembly, which has the bucket wheel excavator attached to the front end,
• two winches swing the dredge about the main spud, and
• two winches relocate the swing anchors.

Three hydraulic cylinders are fitted to the dredge. Two of these cylinders lifts and lowers the main and auxiliary mooring spuds. The third cylinder advances the dredge and the excavating bucket wheel into the face of the dredge cut, by pushing on the main spud carriage. When the advance cylinder has run out of stroke, the auxiliary spud is lowered to moor the dredge, the main mooring spud is then raised clear of the bottom and the advance cylinder is fully retracted. The main spud is then lowered into the bottom, the auxiliary spud is lifted and the swinging operation of the dredge is continued. The cylinders have a continuous operating pressure of 250 bar.

As well as being fitted with a pressure-compensating controller, each Denison variable-displacement piston-pump is fitted with a hydraulic stroker controller. This allows infinitely variable speed control of all dredge functions.

Why hydraulic drives?

Hydraulic drives were selected for their inherent ability to allow the various pieces of equipment to be easily mounted in the required position on board the dredge without the restrictions of electromechanical systems. Remember that some of the dredge equipment operates underwater. By using variable-displacement pumps and relief valves, full control over operating speeds and loads is achieved. Another important reason for the selection of hydraulic drives was their compactness.

One of the major benefits of hydraulic drives over other forms of drives is that the equipment requires minimal maintenance — apart from hydraulic filter changing. By appropriate selection of equipment, a service life in excess of 80,000 hr can be achieved.