Pacific Maritime Magazine - Marine Business for the Operations Sector

By Jim Shaw 

Digging Deeper: the world of dredging

 

June 1, 2018

Floating bucket ladder dredges were used to dig both the Suez and Panama Canals but are now more routinely employed for aggregate production, and have outputs of between 250 tons per hour and 850 tons per hour. Photo courtesy of Aggregate Dredge Solutions.

Marine construction often calls for dredging or filling and a vessel known as the dredge has long fulfilled these requirements, although few know how they work. For the most part they are unappealing vessels of awkward-looking design and loaded down with what appears to be too much cumbersome equipment. Many are not even ranked as ships but come under the heading of barges or floating pontoons fitted with digging gear. This can consist of buckets, suction pipes, grabs, augers, cutter heads or mechanical shovels. A few very small dredges even use their own propellers as devices to move loose debris along the bottom rather than dig it using machinery. Nevertheless, dredges are something the maritime world could not live without, at least not for very long. They have opened up passageways between the oceans, allowed bigger and larger ships to be built by deepening navigation channels, and created new land for the construction of docks and terminals. Several dredges, disconnected completely from the normal maritime world, have even been used to dig for gold and other precious minerals far inland from the sea. As an engineering and excavating tool, dredges are being continually refined to boost their capacity and precision as well as the comfort and working conditions of their crews.

History

The history of dredging can be traced back through many centuries but today's mechanized industry is only about 150 years old. Prior to the advent of iron hulls and steam engines the deepening of harbors and channels was an arduous and slow process. At the Dutch National Museum of Dredging at Sliedrecht, The Netherlands can be found a model of an early wooden dredge, its underwater excavating wheel turned by a team of horses stabled on deck. Reclamation works in The Netherlands, in fact, and the digging of the world's two great inter-ocean canals, first gave the general public an idea of what mechanical dredgers could accomplish. In 1859 excavation at Suez was launched using human labor, a slow process, but by 1865 the canal's construction had been turned over to a fleet of new mechanical dredges. These machines, put into service by the French firm of MM Borel, Lavalley & Co, were able to complete a 26-foot deep channel across the isthmus separating Africa and Asia by 1869. French dredgers were later sent to Central America in an attempt to create a second inter-ocean canal across Panama. The first attempt failed but more modern dredges, acquired in both Europe and the US, completed the project by 1914. Even before the Panama Canal was finished the massive Zuiderzee Works in Holland had been started and was not completed until 1932. This lowlands reclamation project helped launched the Dutch and Belgian dredging industries, which remain world leaders today.

Dredge Designs

Creating machinery to remove material from under the water has resulted in a number of different excavating processes and equipment designs. Most dredgers are considered either mechanical or hydraulic, with a few other types, such as air-lift and jet-lift, used for specific dredging or disposal situations. Almost all modern dredges are equipped with a range of sophisticated electronic control and data-logging systems appropriate to their type. These systems assist with positioning, loading and recording during dredging operations. In a modern dredge's control cab, video displays will show the position and attitude of the digging equipment as well as the location of the dredger and its track and heading. Other monitors will show the depth of cut, dredged depth and angle of slope. All of these aids lead to increased efficiency of operation by minimizing over-dredging, reducing travel time and improving dumping precision. This is particularly important in localities where strict environmental regulations exist. At the same time, the type of material to be dug, the required productivity rate and the potential for interference with nearby shipping will determine what type of dredge should be used.

Bucket Ladder Dredge

The bucket ladder dredger, such as employed in the digging of the Suez and Panama canals, is one of the oldest types of mechanical dredge. It consists of a rectangular pontoon with a central well in which a heavy steel frame or ladder is suspended. The ladder supports an endless chain of buckets, each of which is equipped with a cutting edge. By rotating the bucket chain about flat-sided wheels, known as "tumblers" at each end of the ladder, underwater material is loosened and transported. At the top of the ladder the buckets overturn and the contents are discharged into drop chutes and on into disposal barges or scows moored alongside. Each bucket then returns empty on the underside of the chain to the bottom of the ladder where the cycle repeats itself. The size of a bucket dredge is usually described by the capacity of its buckets, which can range from 15 gallons to 320 gallons and larger. A small proportion of dredges of this type are self-propelled but the propulsion machinery is used only to move the vessel from site to site and is not used in dredging.

In operation a bucket ladder dredger is held in position by up to six moorings or anchors while the ladder and its chain of buckets is moved from side to side to excavate material. Much of the power of a bucket dredger is used to turn the chain, and the high inertia of this action can assist in overcoming localized hard spots. Because of this, bucket ladder dredges can dredge almost any type of material. If the buckets are fit with ripper teeth, even weak rock can be dredged. The maximum weekly output of a bucket dredge varies between 10,000 cubic meters and 100,000 cubic meters depending upon the size, location and type of material being dug. Because of their mechanical design, the maximum dredging depth for a ladder dredge is normally around 20 meters. They are complex and expensive machines to operate but can dig to a required depth very quickly and accurately. However, their mooring wires can obstruct other shipping and their rapidly rotating buckets create high noise levels. Bucket ladder dredges are not as common as they once were because of the increasing sophistication of other types of dredges.

Cutter Suction Dredge

Taking the place of bucket ladder dredges in most applications has been the cutter suction dredge, which incorporates suction rather than buckets to bring material to the surface. These vessels employ various means to achieve the initial loosening of the bottom material. If it is naturally very loose, such as sand, suction alone may be sufficient, but more solid material requires mechanical loosening. The most common method to accomplish this is with a rotating cutter, auger or bucket wheel mounted on the lower end of the ladder. Material loosened by the cutter then passes up the suction pipe, through the pump and on into a delivery line. From the delivery line it can be disposed of either via a floating or submerged pipeline running ashore, into barges, or even back onto the seabed for re-handling by another type of excavator. Cutter suction dredges are used mainly for new or "capital" dredging projects, especially when land reclamation is also being accomplished. By using a cutter suction dredge, harbors can be deepened while new space for additional berths and terminals is created.

Cutter suction dredges operate by swinging about a central working spud using moorings leading from the lower end of the ladder to anchors. By pulling on alternate sides the dredge clears an arc of cut, then moves forward by pushing against the working spud using a hull-mounted spud carriage. A generally smooth bottom can be achieved this way, and modern instrumentation allows channel profiles and side slopes to be cut very accurately. Some of the larger cutter suction dredges are self-propelled to allow easy movement from site to site. The size of a cutter suction dredge is measured by the diameter of the suction pipe, usually between 100mm and 1000mm, and by the installed machinery power.

One of the larger self-propelled cutter suction dredges is the 27,190kW J.F.J. De Nul, built for the Jan De Nul Group by the IHC Holland yard at Kinderdijk, The Netherlands. It can dredge from a depth of -6.50 meters to -35.00 meters using a support ladder that has 2 pivot points. The vessel's dredge pump power is rated at 15,800 kW and incorporates one submerged pump on the ladder and 2 pumps mounted inboard.

Trailing Hopper Dredge

Trailing suction hopper dredges, more commonly known in the dredging trade as "hoppers" or "trailers" have a hull in the shape of a ship thus are more seaworthy than other types. Because they dredge while moving, and they don't use any form of mooring wire, they also offer little obstruction to nearby shipping. Trailing suction dredges are equipped with either single or twin (one mounted on each side) trailing suction pipes fitted with a draghead and lifted and lowered using cables. The draghead forces material into the suction inlet and may incorporate a water jet system, blades or teeth to dislodge compacted material. Once in the suction pipe, material is lifted by pumps and discharged into the dredge's own hopper. The measure of the size of a hopper or trailer dredge is its hopper capacity and this can range from a few hundred cubic meters to well over 30,000 cubic meters. One of the world's largest suction hopper dredges, the 60,000-dwt Vasco da Gama, has a hopper capacity of 33,000 cubic meters, which can be filled in the space of one hour using suction pumps rated at 5,500kW.

A trailing suction hopper dredge operates very much like a floating vacuum cleaner, sailing slowly over the area to be dredged and filling its hopper by suction as it proceeds. On completion of loading the vessel navigates to a disposal area to dump its material, either by opening doors in the bottom of the hopper or by using pumps to move material ashore. This latter operation can be accomplished by pipeline or by blowing the material through the air using a special bow jet. The blowing technique is known as "rain bowing" and is commonly used for land reclamation or beach reconstruction. Some trailer hopper dredges are designed to split over their entire length to achieve a very rapid rate of discharge. Such dredges are more expensive to build than conventional types but the increased production rate is often worth the additional investment. Trailing suction hopper dredges are usually employed to maintain harbor and channel depths but they can also be used in new construction projects, underwater pipe trenching and land reclamation. A few are non-self propelled and are equipped with a forward pointing suction pipe that can be used in much the same manner as a cutter suction dredger.

Grab, Dipper and Backhoe Dredges

Hopper-type hulls, such as used by suction dredges, can also be fitted with mechanical digging equipment. This can include cranes fitted with various types of grabs, mechanically operated dipper shovels or hydraulically operated backhoes. The same type of equipment can also be mounted on deck barges or pontoon hulls. As the machinery is rotated on its mounting platform, dredged material is dropped into the dredge's own hopper, a dump scow moored alongside or even onto the nearby shore bank. With a grab dredge the dredging operation consists of lowering the open grab to the bottom, closing it, raising it back to the surface and discharging the contents. The size of this type of dredge is determined by the capacity of the bucket, which can vary from 1 cubic meter to over 20 cubic meters, depending upon available crane power. If the machinery is installed on a hopper hull, the size of the dredge may be expressed in terms of its hopper capacity, which can range from 100 cubic meters to around 2,500 cubic meters. Dipper and backhoe dredges are less common than the grab type but backhoes are increasing in popularity because of the digging precision they afford and their strong breakout force, which can exceed 100 tons. The size of a backhoe dredge is described by its bucket capacity. These can vary from .5 cubic meters to 15 cubic meters. Dipper or face-shovel dredges can also exert a powerful breakout force but they use an upward rather than downward motion. Grab, dipper and backhoe dredges produce an irregular bottom profile thus are mainly used for bulk excavating rather than channel deepening. However, they are highly efficient for removing material close to quay walls and near docks that would otherwise be difficult to access.

Other Types of Dredges

Backhoe dredges, such as this WASA spud-equipped unit, can be fitted with a range of boom and arm lengths to give a dredging depth of between 75 feet and 95 feet. Photo courtesy of WASA.

Beyond mechanical and hydraulic dredges are a small number of craft that use other means to move underwater material. Jet-lift dredges use the Venturi effect of a concentrated high-speed stream of water to draw the adjacent water, together with bed material, into a delivery pipe. The jet head has no moving parts thus blockage by wires and other bottom debris is minimized. These dredges are relatively small and some are positioned using spuds alone. Air-lift dredges are similar to the jet-lift type but the medium for inducing material flow is high pressure air injected at the month of the suction pipe. As with jet-lifters there are no moving parts in the flow system but this also means that hard bottom material cannot be dislodged. Pneumatic dredges work on the "evacuator" principle and are handy for lifting contaminated sediments with minimum bottom disturbance. A chamber with inlets for bed material is pumped out with the inlets closed. The chamber is then lowered to the bottom by crane where the inlets are opened, allowing water and material to be drawn in. The inlets are again closed and the mixture is lifted for disposal at the surface. The movement of bottom material can also be accomplished by dredges using water injection to loosen sediment, or even their own propellers. At the same time, an uneven bottom surface, such as left behind by a grab dredge, can be smoothed by a device known as a "bed leveler," which is towed by a tug or work-boat. Dredging, particularly for new construction, has become a rapidly growing industry and the technology being used is under continual modification and development.

 
 

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