Vasco da Gama about to start work

Vol. 1, issue #22, 3 March 2000

The world's largest trailing suction hopper dredger, Jan de Nul's Vasco da Gama, was launched on 05 November at German shipyard Thyssen Nordseewerke, and is about to start 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, Royal Boskalis Westminster's W D Fairway and Queen of the Netherlands, each of which has a capacity of approximately 24,000m3.

Intended primarily for use on large scale land reclamation jobs, the Vasco da Gama will also be used for offshore projects such as trenching, pre-sweeping, and rock dumping in deep water, but her size makes her particularly well suited to the land reclamation role.

In the last decade, tremendous changes have taken place in the dredging market. Traditionally a highly cyclical business, and one where demand has gone through regular peaks and troughs, the dredging market in the last 6-7 years has enjoyed a period of steady growth, fuelled in large part by the growing number of large-scale land reclamation projects around the world.

Several contractors now operate vessels in the 18,000-23,000m3 range, but the capacity of the Vasco da Gama is such that on a single voyage it can transport 2-3 times the load of most competing vessels.

Another key feature of the latest generation of 'jumbo' trailing suction hopper dredgers exemplified by the Vasco da Gama is the ability to work at great depths. As demand for clean sand for reclamation work has grown, so dredging contractors have had towork further and further offshore to obtain it.

For this reason, in addition to her very large capacity hopper, Vasco da Gama is also capable of working at depths that exceed those of any other dredger.

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 material at a depth of 125m, although, as currently configured, she is not yet equipped to do so. As the shipyard explains, however, 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. The keel of the enormous dredger was laid on 28 April 1999, and the Vasco da Gama is due to be delivered in March of this year.

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.

Salt water at very high pressure, and variable angles of operation from 10-60 degrees, present real challenges for the engineers who design submerged dredge pumps, as do issues such as lubrication, monitoring and control, maintenance, weight, and last but not least, watertight design.

A number of different drive systems have been proposed for submerged dredge pumps, which, in the case of the Vasco da Gama, are a modified version of an existing GIW MHD dredge pump with a new impeller.

Of the various drives that could have been used on board Vasco da Gama, Jan de Nul considered direct drives using a single low speed, frequency controlled three-phase electrical motor; direct drive with a pair of low speed motors in a tandem drive; usingone or two high speed, frequency controlled three-phase motors and a reduction gearbox; and opted for the direct drive approach in which the dredge pump was driven directly by the gearbox.

The dredging company says it has found that of the various pump drive options available, it has found that this solution is relatively trouble-free, and the robust execution of the direct drive principle results in a very reliable installation.

As Jan de Nul's engineering department point out, which type of drive system is the right one for a dredger depends very much on the specific requirements of the ship itself, but when it came to the design of drive for the Vasco da Gama there was very little experience of drive systems of in excess of 5,000kW on which to draw.

Hydraulically optimised for efficient pumping in a balanced range of head and flow conditions, the MHD-105 pumps on board the Vasco da Gama were also fitted to Jan de Nul's earlier jumbo newbuilding the Alexander von Humboldt.

This high efficiency range of pumps was developed to provide a flow rate of 1,800-23,400 m3/hr, maximum head of 95mwk, and designed to work with suction pipes of 455-1,200mm. They have impellers of 1,115-2,794mm diameter.

The pumps in the MHD series have a proven shaft and bearing assembly with spherical rollerbearings and separate thrust bearings, and make use of abrasion resistant white irons, ductile irons and special alloys. The impeller hydraulics are computed for high suction performance and a large solids passage.

Once the drive for the submerged pump for the trailer had been defined, the overall design of the pump room could also be addressed. In order to save space and avoid having too many bearings the output shaft of the gearbox is commonly used as the pump shaft and the pump is hooked up directly to the gearbox.

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 builders 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 new vessel's layout and the arrangement of the major equipment is based onthe hopper dredger concepts pioneered by Jan De Nul since 1992 with the 11,750m3 J F J De Nul, which was followed by the Cristoforo Colombo (7,000 m3), Gerardus Mercator (18,000m3) and Alexander von Humboldt (9,000 m3).

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.

The propellers, bow thrusters and stern thrusters were supplied by LIPS BV, and the main generators and thruster drives by VEM. STN Atlas was responsible for the ship's electrical installation.

To enhance manoeuvrability, two 1,650kW bow thrusters, and two 750kW stern thrusters are installed. The vessel is also equipped with two fully independent, highly efficient flap rudders. A Simrad Kongsberg Dynamic Positioning (DP) system has been installed.

The arrangement of the dredging installation aft requires the deckhouse and accommodation to be installed forward; the living quarters are thus located far away from the engine and pump rooms, and are therefore very quiet. The forward location of the wheelhouse also provides excellent visibility in all directions. The accommodation is arranged for 40 persons, most in single cabins.

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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