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    World's largest dredger starts work

    Vessel & ROV News // May 8, 2000
    The world's largest trailing suction hopper dredger, Jan de Nul's Vasco da Gama, has started work. With a capacity of 33,000m3 - equivalent to a deadweight of 58,000dwt - the Vasco da Gama has a hopper approximately 40 per cent larger and carrying capacity approximately 70 per cent greater than its nearest rivals.

    Intended mainly for large scale land reclamation jobs and offshore projects such as trenching and pre-sweeping, Vasco da Gama is also designed for rock dumping in deep water and a variety of other tasks.

    Apart from her enormous hopper capacity, Vasco da Gama also has the ability to work at great depths. With the hopper empty, and her twin 1,400mm diameter suction pipes inclined at an angle of 60 degrees, the Vasco da Gama is capable of dredging materialat a depth of 125m. Although not currently configured to do so, she was designed and built with foundations capable of accepting larger, heavier equipment at a future date.

    With a shortened suction pipe, inclined at an angle of 55 degrees she is well suited to working at depths of around 80m, but with hopper loaded she will also be capable of work as deep as 131.50m. The huge suction pipes - with integral submerged dredge pump and draghead attached - are raised and lowered using four massive gantries.

    Vasco da Gama's twin 1,400mm diameter suction pipes are also equipped with world's largest submerged dredge pumps (5,500 kW), enabling the enormous hopper of to be loaded with 33,000m3 of dredged material in a record time of just one hour.

    The vessel was ordered from a consortium consisting of German dredging equipment specialists Krupp Fordertechik, of Lubeck, and Thyssen Nordseewerke shipyard in Emden in January 1998. Her keel was laid on 28 April 1999.

    Given the huge size of the dredger, some aspects of the ship required particular attention, particularly those relating to its structural strength. The extremely high loading capacity required a very thorough investigation of the vessel's strength, a task that was carried out by the Krupp-Thyssen consortium and classification society Bureau Veritas (BV). BV in particular undertook detailed Finite Element Analysis (FEM) of the complete vessel.

    Stability was another key issue, since with a length of 90m there would be a very large free surface effect within the hopper. However, the wide beam (36.2m) and the large freeboard (the vessel's depth is 19m, while draught is only 14m) have resulted ina very stable ship, complying with all relevant regulations.

    Typically, when fully laden, a trailing suction hopper dredger has a negligible amount of freeboard; however, even when fully laden with 58,000 tonnes of spoil the Vasco da Gama will still have a freeboard height approaching 5m.

    At the heart of the deep dredging installation on board Vasco da Gama are the ship's submerged dredge pumps, which are based on a design developed jointly by Georgia Iron Works (GIW) in the US and Krupp Fordertechnik, and their drive system.

    Submerged dredge pumps have become more and more popular with operators of larger trailing suction hopper dredgers. Locating the dredge pump in the suction pipe positions it much closer to the seabed than a conventional dredge pump (which is housed in the hull of a ship), enabling it to work much more efficiently. However, doing so also makes new demands on the pumps and the associated drive system, which must work efficiently in the demanding underwater environment.

    Hydraulically optimised for efficient pumping in a balanced range of head and flow conditions, the pumps on board the Vasco da Gama were also fitted to Jan de Nul's earlier jumbo newbuilding the Alexander von Humboldt.

    The Vasco da Gama has a total installed power of 37,060kW, of which 16,000 kW is directed to the pump ashore system, enabling her to pump material ashore over a huge distance - more than 10km - without the need for booster stations.

    To load the huge hopper in a relatively short time, two suction pipes with a diameter of 1400mm are installed - the largest diameter suction pipes used in the dredging industry before Vasco da Gama had a diameter of 1,300mm.

    The diameter of the impellers in the dredge pumps - 2,800mm - makes the pumps the largest and most powerful ever installed in a dredger. Together, her twin 1,400mm diameter suction pipes and submerged dredge pumps enable the enormous hopper of to be loaded with 33,000m3 of dredged material in a record time of just one hour.

    When not pumping material ashore for land reclamation, discharge can also take place via a single row of bottom doors, which are located above the vessel's keel to enable her to dump spoil even in shallow water.

    The design of the hopper combines elements of the hopper design favoured by the owner in previous classes of vessel with some modifications based on the builder's standard hopper. Within the hopper there are a total of six hydraulically operated double-type bottom doors, 24 self-emptying doors, four hydraulically operated overflows, plus a high pressure and low pressure jet water sets.

    The pump room and the suction pipe installations are located aft, and the dredge pumps are directly driven by the main engines, via gearboxes. Using this arrangement makes it possible to increase the power on the dredge pumps to a massive 8,000 kW (16,000kW when working in tandem) in shore discharge mode.

    The main engines also drive shaft generators, each of 10,000kVA, 1,200rpm, 6kV 42-60Hz. The emergency generator is a 725kVA, 1,800rpm, 690V, 60 Hz machine, and the auxiliary generators, of which there are two, are 3,150kVA, 720rpm, 690V, 60Hz machines.

    This huge quantity of power is required to supply the dredge pumps and other large consumers of on board power, such as the jet water pumps, bow thrusters, stern thrusters, and the hydraulic system for the suction pipe winches.

    When not engaged in winning material from the seabed, the total power of the two main engines, MAN B&W machines each of 14,700kW, can be applied to the two main variable pitch 5.5m diameter, propellers, which work in nozzles and turn at 115rpm.

    Another important feature of the Vasco da Gama is its ability to operate in an environmentally friendly 'green' mode with as little impact on the environment as possible.

    When working in particularly sensitive areas the vessel can operate in 'zero-dump' mode, in such a way that nothing is discharged from the vessel. The main engines comply with the NOx emission standards specified by IMO, and solid waste and sludge can beincinerated on board.

    The dredging installation on board is controlled by means of a fully integrated system developed by Krupp Fordertechnik and STN Atlas in Hamburg and designed in close co-operation with Jan De Nul. The dredge operator controls the complete system by meansof touch screen VDUs, dedicated keyboards and a limited number of control levers.

    In terms of size, installed power and dredging depth, Vasco da Gama is an exceptional trailing suction hopper dredger, but her innovative design is also groundbreaking in other ways.

    Typically, a medium to large size trailer might have an unladen speed of 13-15kt - the massive Vasco da Gama, however, is capable of more than 16kt whilst fully laden with her cargo of dredged material.

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