BIRTH OF A CONCEPT
When the IHC group handed over the 12,888m³ trailer dredger “Xin Hai Long” to the Shanghai Dredging Corporation (SDC) in 2002, the new dredger went operational on the Yangtse river. However :the ship was unable to produce more than “colored water” during tests on certain area’s of that river.
Although these area´s were dredged by cutters before, for safety-of-navigation purposes, authorities preferred these channels to be dredged by hoppers instead.
The area contained fine (60 to 100 μm) clayey sand, hard compacted. The problem is well kown in the dredging industry: common dragheads perform badly in this material; pickpoints do not penetrate, jetwater does not erode, density of dredged material is often smaller than 1.1 t/m³; “colored water” indeed.
IHC devised an answer of sorts: the ‘Wild Dragon®’ draghead.
After the design phase the draghead was tested on board of the Xin Hai Long, in 2004. While systematically adjusting settings, the new draghead was tried at the known hotspots, containing densely compacted sand. Production rates shot up by 50 to 100%, compared with earlier performance in the same soils.
Production with Wild Dragon installed SB (left) and a common active draghead port side (right).
The Wild Dragon draghead has a double row of hollow teeth, in the visor, injecting jetwater deep into the seabed material.
The resulting dilation means that this densely packed fine uniform sand expands; the pores between the sandparticles become larger, and the material can be dredged hydraulically.
IHC claims that a jetwater pressure of 8 bar is sufficient for this effect. Most modern hopperdredgers can deliver the higher flow demand for a Wild Dragon at a pressure of 8 bar plus, with their existing jetwater systems.
Hard compacted fine sand is difficult to dredge with trailers. The soil particles do not erode well with commonly designed dragheads, equipped with chisels, jetwater, even not with active dragheads.
Soil is normally eroded with the use of chisels, pickpoints, knives, etc…
In many cases, it is more feasible to erode the seabed with jetwater, combined with knives, or pickpoints. The effectiveness of this jetwater is depending from –amongst others- the waterpermeability of the soil.
Pickpoints, chisels nor jetwater penetrate easily in a hard compacted mass of uniform fine particles. Sand with a high density (with few pories between the particles) needs a high energy level to cut.
Densely packed sand can practically deform only when the volume expands somewhat, making the sand looser. This is called dilatancy.
Because this dilation is an underwater process, and the pores between the sandparticles are filled with water, underpressure occur in the pores. For this reason, particles will “stick” together even closer, with more friction between particles, and become even harder to “loosen up”; to dredge, in fact. In extreme cases underpressures can become so high, that water vaporizes, and cavitation may occur. The soil particles become one cohesive mass.
A recent trend in the design of dragheads is to rely less and less on erosion (as in Californian type dragheads).
Instead one nowadays relies more and more on the controlled use of jetwater in combination with a carefully chosen cutting system (as in excavating ‘active’ type dragheads).
Traditional options toimprove the production of common dragheads are:
- increase jetpumppower
- increase the weight of the draghead
Practical experience proves that these options work well in medium sized sands.
However, in soils with an extreme low water permeability (like hard packed fine sands) the results of both options are disappointing.
Tests in a towing tank, filled with silty sand, mirroring the Yangstse seabed conditions. Excavating parts of dragheads are evaluated on their suitability to dredge this particular material.
(Courtesy Jan De Nul Group)
If you add a jetting system that jets water through the teeth into the sand, however it shows that cutting forces reduce significantly and that teeth can penetrate the soil. Instead of barely scratching the surface of the sand, the teeth are capable of penetrating the surface and excavating a layer of material.
DRACULA, same idea, but a different story
The Belgian dredging company DEME had a similar project,named “DRACULA®”-system, using higher jetwater pressures (up to 400 bar, water leaves the nozzles at close to Mach 1 !) through hollow pickpoints, to cut hard clay and cemented sand.
The DRACULA concept has been used on TSHD Lange Wapper (Cadiz, in cemented sand with UCS-value of 8MPa). It’s definitely another niche than the “Wild Dragon” concept, and cannot be confused with it.
It is mainly designed as a tool to cut harder materials.
This article gives an idea about the pioneering with DRACULA onboard TSHD Jade River. (Terra et Aqua nr. 89, dec. 2002)
Further tests with WILD DRAGON
During later trials with the “Wild Dragon” in coarser, more heterogenous sand (for which the Wild Dragon was not originally intended), production rates were increased as well. Very few of these tests were conducted, as very few Wild Dragons are sold up to this moment.
Some of these tests were conducted onboard TSHD “Gerardus Mercator” off Abu Dhabi, 2007, and I could witness -first hand- production increases of 30 % with a “fresh” Wild Dragon draghead. Production was closely monitored by datalogging, and this 30% production increase is genuine.
As the days wore on, the Wild Dragon got a lot of punishment on the borrow area, consisting of a thin sandlayer over caprock, sometimes just … caprock.
The hard sub-bottom ripped of the teeth of the Wild Dragon, degrading its performance. But the same happened to all the other dragheads on the rocky bottom. Caprock is at the outer edge of what a trailer dredger can handle.
The experience in this particular borrow area was: the more “difficult” the area, the more rock strewn, the more a Wild Dragon dominated over other dragheads. All the extra jetwater undoubtedly “washed out” the sparse sand from between the caprock.
The “Wild Dragon” achieves its higher production rates in fine, packed sand, by a combination of improved penetration, resulting in a low mixture velocity and higher density, leading to shorter loading times, less fuel consumption and lower wear and tear.
Whatever the advantages, IHC has never managed to make Wild Dragon a sales success. It remains a product on the sideline.
The major four dredging companies (VOA, JDN, Boskalis, DEME) have ongoing research programs into their own line of dragheads. So far for the top-end of the market.
Wild Dragon is originally an active draghead. Active draghead technology is still a long shot for most third- world and second-world dredging companies. This rules out the bottom end of the market.
Only very few units were sold, some to the Chinese market. We already got reports that copycats made local copies there; the usual story. Fabricating a draghead is –after all- no rocket science.
Another set of Wild Dragons was delivered “off yard” with TSHD ABUL, a low-budget 6000m³ TSHD, for Karachi Port Trust, launched 2006. This vessel is intended for port maintenance. Initial production results were encouraging.
Recently, IHC has been promoting Wild Dragon as part of a “total trailer design”, with the ship design adjusted to the demands of Wild Dragon dragheads, with regard to the different loads on suction pipes and gantries, tailored jetwater systems, hopper layout and power management.
Marc Van de Velde