Sucking up silt in France?s waterways

Toward the tail end of last year, Izar?s shipyard in Gijon, northern Spain, delivered a second dredger to the French national dredging company Dragages-Ports in Rouen. Named Samuel de Champlain, the 8,500m3 trailing suction hopper dredger is designed for harbour and channel entrance maintenance work in waterways controlled by the French State but mainly in the river Loire estuary. The Gijon yard had earlier, in February last year, delivered the smaller 5,000m3 dredger, Daniel Laval, to the same owner.

The hopper suction dredging equipment was designed and engineered by Vosta LMG Dredging Technology, an engineering and contracting company serving the worldwide dredging industry from offices in Amsterdam, Holland, and Lübeck, Germany. This company was also responsible for the supply of all the dredging components such as the trailing side suction pipe with underwater dredge pump and bottom valves for load dumping in addition to the appropriate computer hardware and software. One of the highlights of Vosta LMG?s achievements was its involvement with one of the largest custom built hopper dredgers, Vasco da Gama, for the Belgian dredging group Jan de Nul.

Vosta LMG supplied the shipyard with the basic design for the dredger complete with engine systems diagrams and steel classification drawings, hydrostatic and stability calculations, strength calculation, steel specifications and all dredging component specifications including assistance in installation and commissioning during quay- and sea trails in order to construct and deliver the dredger in accordance with the contractual building requirements.

Equipped with a 1,250kW submerged dredge pump, 1,100kW high pressure jet-water pumps, 700kW low pressure jet-water pumps and a suction pipe of 900mm diameter, the dredger is capable of dredging to a depth of 26m. The propulsion system is diesel-electric with power generated by two 3,200kW Wärtsilä CW16V200 diesels and one rated at 2,890kW. Two 3.4m diameter four bladed reversible and fixed pitch

propellers

give the vessel a service speed of 13 knots. A 650kW bow thruster and two high-lift flap rudders facilitate manoeuvring. The engine room can be operated

unattended.

The hull has been fitted with a twin skeg configuration which has proved successful for operating at varying draughts and, particularly, in shallow water. The lines of the dredger were further optimised as a result of the new way of construction developed by Vosta LMG where the hoppers are designed for bottom dumping through valves arranged in the centreline. These are operated by hydraulic jacks mounted on the hopper deck and enable the hoppers to have smooth walls to facilitate unloading.

The ship?s subdivision is influenced and dominated by its hopper compartments. The dredge pump and machinery rooms are located aft of the hoppers and contain main and auxiliary engines and other engine-related systems. In front of the hoppers, auxiliary engine rooms have been arranged with sea chests as well as smaller auxiliaries. The deckhouse is located above the forecastle deck for crew accommodation and provides enhanced visibility and effective operation from the bridge, which is designed to offer the personnel on the bridge a clear view in all directions. In addition, the ship is equipped with a dynamic positioning system with which the dredging routes can be programmed so that the vessel automatically follows pre-established routes. Not only is highly precise steering achieved in this way, but the system also serves to optimise the power outputs applied to the

propellers

Dredging equipment

Today?s hopper suction dredgers are called on not only to dredge and then dump their spoil but, also, increasingly to pump it over long distances to settling ponds and the like. For such applications, Vosta has developed new loading and unloading systems that, while dredging, direct the mixture into the middle of the hopper where it is split into two at half the original speed. This ensures better settling characteristics, reduces overflow losses and shortens the dredging cycle. For unloading, the mixture is fluidised further by pressure and diluting water. The fluidised load is discharged into a central outlet valve from on top. A special technique allows controlling the mixture concentration precisely.

The most important part of suction hopper dredgers is, not surprisingly, the dredging equipment of which the dredging pumps are at the heart. Samuel de Champlain is equipped with an integrated submerged dredge pump mounted at the intermediate section of the side suction pipe. This concept was chosen because it makes it possible to improve operations in very deep muddy water and, for that purpose, the pump is driven by an electric motor. The side suction pipes arranged on starboard side of the dredger are equipped with hydraulic active drag-heads of Vosta LMG-design with hydraulic cylinders to actuate the visors and the water flaps. The hydraulic power pack is an integrated part of these drag-heads. Advanced dredge control and automation systems allow the operator to adjust visor position and water flap.

The unloading equipment, with the Vosta LMG type bottom valves and the jet-water system for diluting the load, allows quick discharge, four minutes for silt, of the hopper even without conventional bottom flaps with wide openings. Owing to its geometry, the system of bottom valves also ensures better sealing of the hopper than bottom flaps.

The Dredge Automation Systems DCMS (dredge control & monitoring system) was also supplied by Vosta and uses various local interfaces. The ergonomically arranged operating panel includes the conventional operating elements as well as modern process control via screen, keyboard and trackball. Its server-client structure not only allows full control of the process from the main operating desk on the bridge, but also easy transfer of specific tasks reserved for the captain to the navigation console. This was a specific requirement of Dragages-Ports.

The automation takes over control, visualisation and reporting of the entire dredging process. It controls the pumps and valves to ensure operation with the maximum discharge rate. Among other parameters, the mixture concentration, mixture flow, and pressure on the suction side are taken into account for optimisation of the dredging process. Moreover, various automatic functions and sequences such as swinging out the side suction pipe, moving the valves into the required working mode etc. are of great help to the dredge-master to achieve quicker and safer operation.

Besides the process visualising system, a graphic side suction pipe position indicator shows the exact position of the side suction pipe. It is integrated into the entire system in the same way as the loading computer for exact monitoring of the loading condition of the dredges.

In addition, there is a customized survey system DOCS (dredge operations and charts system), integrated with the dredge automation and dynamic positioning and tracking systems. The system allows the operators to plan their dredging tasks, set out way points, monitor the ship within the displayed dredging area, and update bottom charts on-line during dredging. n