Trailing suction hopper dredger VASCO DA GAMA

According to HANSA June 2000

The trailing suction hopper dredger VASCO DA GAMA is designed to recover sand from the sea, haul it for long distances and place it ashore. With a massive hopper capacity of 33,000 m3, the vessel was the largest dredger in the world at the time of delivery.

- Dredging equipment

The vessel is equipped with two 1400mm trailing suction pipes. Her standard configuration provides for working to depths of 45m and 60m. However, the vessel has been prepared to enable the starboard pipe to be extended for operations at 80m or 125m dredge depths. Each suction pipe consists of an upper part with trunnion and a lower part with a draghead, moving freely with respect to each other and to the hull. For dredging, the pipes are lifted by three cranes each: one at the draghead, one at the trunnion and one at the intermediate cardanic joint of the upper and lower suction pipe part. To compensate for draghead movements due to uneven seabed and the ship motion due to waves, a swell compensator is installed. The installation compensates for relative motions of up to 8 metres.

Two dredge pumps are the heart of the dredging equipment, driven by the main diesel engines via three-speed gear boxes. They feature an extremely wear resistant alloy called “white iron metal” and a special sealing arrangement requiring water sealing only on the shaft side of the pump. During dredging, the pumps draw spoil from two side suction pipes, with a power of about 4500 kW.

- Loading

From the dredge pumps, the mixture is directed to the hopper and distributed through a common 1600mm pipe arranged centrally on top of the hopper. The load is distributed in the hopper through three loading boxes to facilitate settling of sand in the hopper. A poor mixture discharge line ensures that only the mixture with sufficient spoil content is directed to the hopper.

The hopper is provided with four overflows, hydraulically adjustable in their height by remote control for drainage of excess water from the hopper through the ship bottom. The hopper is open at the top and has sloped void wings tanks enhancing the ship stability. To prevent spillage when the hopper is fully laden, a splash coaming is provided.

- Discharging

The hopper is equipped with a central row of bottom doors for spoil dumping. The sides of the hopper bottom are sloped, to facilitate spoil inflow to the bottom doors or to the self-emptying channels located besides the bottom doors. To facilitate dumping in shallow water, where parts protruding beneath the ship’s bottom might be damaged, all bottom doors are placed in recesses.

Dredge pumps are used for discharging. They draw spoil from two self-emptying channels in the hopper bottom. The channels extend from a fore sea chest to the pump room aft, and are equipped with twelve upper doors each. The mixture is removed from the hopper by opening the upper doors which are hydraulically-operated and controlled from the wheelhouse.

For shore discharge a floating pipeline of 1100mm diameter can be connected to the ship mixture pipe system at a bow coupling on the foreship. From the hopper via the dredge pumps, the mixture is then directly pumped to a discharge site on shore through the floating pipeline. The pumps can be operated in series for self-unloading at 8000 kW each, with a maximum pressure of 20 bar at the second pump. In result, the material can be transferred over distances as great as 8km without the need for boosters. The mixture can be also jetted forward over the ship bow via a mixture jetting nozzle.

- Jet system

To fluidise the mixture in the hopper for discharging and for serving the draghead, three jet water pumps are installed, two high pressure pumps aft and one low pressure pump fore. The high pressure jet water pumps are driven directly by the auxiliary diesel engines, via two-speed gear boxes and couplings. The low pressure jet pump is electrically driven. The jet water system supplies pressure water of up to 18 bar to the draghead for loosening the soil. For this purpose, both high pressure pumps are working in series. The system also supplies water to the nozzles arranged in rows in the hopper bottom, around the bottom doors and adjacent to the upper doors of the self-emptying channels.

Length oa: 201m, Length bp:178m, Breadth mld: 36.20m, Design draught:13m, Dredging draught: 14.6m, Deadweight at 14.50m: 60,000t, Hopper capacity: 33,000m3, Main engines: 2 x 14V48160, each 14,700kW/500rpm, Dredge pumps: 2 x 8000kW, Bow thrusters: 2 x 1650kW, Stern thrusters: 2 x 750kW, Classification: Bureau Veritas 1 3/3 +E Hopper Dredger Deep Sea AUT-MS.

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