The Art of Dredging

Dredging and shipping

Cristobal Colon, design philosophy

The newbuilding program within de Jan De Nul Group comprises two mega hopperdredgers.

The first vessel  “Cristobal Colón”, is already operational, with the second vessel “Leiv Eiriksson” in the outfitting phase at a Spanish shipyard. Both have a record-breaking  46.000m³ hopper capacity. Both vessels are in a lonesome region of dredging capacity, only shared with the lengthened vessels  'HAM 318' of Van Oord and the 'Queen of the Netherlands' of Boskalis.



Main Dimensions


The exact dimensions of these vessels can be found on the JDN-website.

The main design parameter is a hopper capacity of 46,000 m³, while e.g. the “Vasco da Gama” ( a year 2000 designed TSHD, generally seen as the first “mega” dredger)  packs 33.000m ³ in its hold.

This increase is primarily achieved from ship’s  length rather than width.  Width is increased for stability requirements. As a result, the “Cristobal Colón” is currently a post-panamax vessel.

 Jan De Nul has never had a major operational problem with  draught restrictions and did not take really any limitations in the design considerations this time for the new megadredgers. If a high-capacity shallow-draught vessel is necessary, their next newly built 30.000 m3 TSHD can take the job.

However, there was a distinct effort to improve the vessel’s speed.

This involved a number of issues, influencing design choices. The loaded speed is 18 knots and speed in ballast 20 knots. By comparison, the “Vasco Da Gama” makes 16 knots.

Although the hopper is much larger, the two suction tubes are slightly smaller. It is assumed that the mixture concentration is higher because of the submerged dredgepumps and that  loading times are the same if not better. The underwaterpumps virtually exclude vacuum problems.

The shore discharge system is comparable with the Vasco da Gama. Two of the largest available discharge pumps have been installed. Additional discharge capacity would require larger pumps or a third pump to be placed in line.

Hull Form


The choice for the submerged dredge pump gives a lot of freedom in the hull form, because the design is not tied to a particular configuration of the suction pipes inboard. Trunnion slides are put forward as far as possible, already in the flaring of the bow.  This layout allows dredge pipes to reach a depth of 155 meters and still be accommodated onboard without trouble.

Furthermore, because of the submerged dredge pumps, there is a possibility of designing the engine room differently. The pumproom is squeezed in between both main engines, thus allowing the hopper to be longer, in turn, avoiding a concentrated weight of the load and unacceptable high stress in the structure. As the weight is now distributed as long as possible, the construction is also slightly lighter, which in turn benefits payload.

Furthermore, a long ship sails at a higher speed. Speed is an important parameter for the total production of a hopper dredger, especially for hoppers designed to transport bulk sand over long distances.

 

There is a genuine twin skeg stern designed, in which the drive shafts are integrated; no shafts in the water anymore and no suspended struts. However there is a central skeg at the bottom, but that is only needed for strength in the stern while docking. If not, the unsupported length would be so long that the overhanging stern would sag.

 

 

 On the bow is an elegant bulbous bow design that is genuinely intended to improve the streamline. At loaded ship, there is a wave reduction and the streamlining of the empty vessel is also improved. 

 

 

 

 

 

 

 

 

 

These hydrodynamic improvements are mainly achieved by improving the computing capabilities in the design stage. Early in the design stage, MARIN modeled the potential flow with viscous flow models.

 

 

 

 

 

 

 

 

 

 The bottomdoors of the “Cristobal Colón” are in the plane of the bottom plates. This is much better for the streamline and reduces the resistance, but renders bottomdoors more vulnerable when the vessel runs aground.

The result is that the hopper capacity per ship weight for the “Cristobal Colón” is a lot higher than for previous vessels. Additional advantage with this hull design is that a speed reduction translates in a large reduction in fuel consumption.

Steel


For sailing at high speed and still carry as much cargo as possible, the ship has to be as light as possible. If you do not want to carry less load, you should carry less ship. To keep the ship strong enough, high tensile strength steels are applied at various places.

The biggest weight saver was made by not designing the ship as Jack-of-all-trades.

Previously, within JDN, ships were designed for all jobs, under all circumstances. With the new series of ships this philosophy was abandoned, some options have been left out deliberately.

The ships now under construction and commissioned are actually designed as a fleet, with the functions of the various vessels complementing each other as they work together on a project. The installations and facilities that JDN doesn not have on one boat, because it is on the other boats, saves a lot of weight on the first and is –simply- extra payload.

Machinery


Jan de Nul has finally arrived at the conclusion that electrical drives are the way to go, leaving behind the idea of gearbox driven dredgepumps.

 The electrical drives has given more freedom in the layout of the ships, for example locating the discharge pumps in between the main engines.


Hopper

In comparison with the Vasco da Gama, which has six doors, the Cristobal Colón now has nine. Partly because it was of the frame arrangement that lend itself for three times three doors. On the other hand, it is a fact that these hoppers indeed do very little soil dumping. If they are deployed on projects where their soil dumping capacities are needed, it is often difficult, sticky material that receives no further treatment. Experience (on the Glory Holes project) taught that clayish material with stones can create problems… such that it is desirable to have more dumping capacity in these cases.

The self-emptying system is simplified. Where “Vasco da Gama” has a classic arrangement with top doors where the material falls in the suction channels, the “Cristobal Colón” is equipped with a direct emptying suction system, where the suction mouths are directly in the hopper. This is a little less useful for difficult materials. But as said they are rarely discharged by pipeline, they are usually dumped.

 
Dredging Installation

In concept the ship has three configurations of suctions pipes.

One for standard depths, one for normal “deep” depths to 80 meters and one “ulta deep” to 155 meters. The difference between 80 and 155 meters makes that the lowest lower suction tube is extremely long. This could create problems with the stiffness of this tube. These problems were solved by integrating the jet water line into the structure of the pipe arrangemen, divide it into several lines along the tube and made in a lattice frame construction. This provides an extremely light and rigid construction.

Although there is hardly any work expected with two pipes to dredge ultra deep sand, where two pipes on the ground is necessary, Jan De Nul indicates that they themselves want to have both pipes available for immediately deployment. The background for this philosophy is that if they work offshore and have problems with one pipe, the penalties and consequential damages can be punishing. Now; a second pipe is at hand when the first pipe breaks down.
The drag head is, in relation to the diameter of the suction tube, extremely wide. Where the “Vasco da Gama” still has a ratio of 1 to 5, this ratio is higher for the “Cristobal Colón”. The largest draghead on the new dredgers is 9 meters wide.

 

Marc Van de Velde

october 2009

 

With thanks to  www.dredger.dds.nl

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