DREDGERS

WHAT IS A TRAILING SUCTION HOPPER DREDGER?

Even though systems that allow dredgers to be described vary, in general three major classifications are recognised, depending on the extraction and operating method.
These are mechanical, hydraulic and hydrodynamic dredgers. Trailing suction hopper dredgers (TSHD) are categorised with the hydraulic dredgers. Hydraulic dredgers include all the dredging tools that use centrifugal pumps for at least one part of the process of transporting the dredged materials, either by lifting the materials out of the water or by transporting them horizontally to another site.

WHEN ARE TSHDs USED?

TSHDs are used in a great variety of maritime maintenance and construction projects. This ranges from dredging ports and access channels to remove sand in order to re-establish the necessary sediments, to deepening projects such as giant backfilling projects that require millions of cubic metres of sand. The effectiveness of a TSHD in terms of performance has a direct influence on the costs of a project. As a result, continuous efforts are put into research and development on TSHDs to improve their profitability.

WHAT ARE THE CHARACTERISTICS OF A TSHD?

TSHDs are self-propelled vessels that contain a hopper inside their hull. They are mainly used for the dredging of loose materials such as sand, clay or gravel. The principal characteristics of a TSHD are the strainers (also called dragheads), the suction pipes (or drag arms), the swell compensators and the davits. Generally speaking, a TSHD is fitted with one or more suction pipes, to which the dragheads are attached. A draghead is often compared to a giant vacuum cleaner. The suction pipes are lowered under the water and the dragheads are "dragged" along the seabed, sucking up the material, while the vessel moves slowly forward, that is, it dredges. The suction pipes and dragheads can be positioned depending on the needs of the specified dredging operation with regard to transporting the materials to the hopper. Thanks to a pumping system, the blend of sand and water, called the mixture, is conveyed into the vessel's hopper. Davits and winches enable the suction pipes to be handled overboard. A swell compensator is used to control the contact between the dragheads and the seabed if dredging when there is swell. Furthermore, the TSHD should have an overflow system to separate dense materials and unload surplus water. The efficiency of each of these elements will have a direct effect on the productivity of the TSHD.

WHAT TYPES OF DRAGHEAD ARE AVAILABLE?

Although all TSHDs have dragheads, these dragheads may be of different types. The function of the draghead is to extract the materials from the seabed and mix them with water to create the mixture. The dragheads is the first point of contact with the ground. Generally speaking, the force that enables the teeth of the dragheads to penetrate into the ground is the weight of the head and the suction pipe. However, when undertaking dredging work on hard grounds, if this weight is not sufficient, the dragheads will not penetrate far enough into the ground and will slide on the surface without cutting the ground. This is manifested by a low density of the mixture which reduces the production of the dredger.

The higher the density of the mixture created by the drag head, the better the performance. Research work have enabled dragheads to be developed that use high-pressure water jets and teeth in order to improve the process of forming the mixture at suction level and, as a result, the productivity of the dredger. In order to improve their efficiency, water injection is sometimes carried out at the end of the teeth in order to facilitate the process of cutting the materials. The force needed to penetrate into the ground is thus reduced, and this increases cutting efficiency. The dredging pump then sucks up the materials from the seabed and transports the materials hydraulically. The sediment is transported hydraulically into the hopper by the centrifugal pump through suction ducts. The dense material settles in the hopper awaiting transport and application.

WHAT IS A "RIPPER" DRAGHEAD?

A recently developed draghead is the "ripper", a drag arm fitted with teeth. Usually, rock is dredged using a cutter suction dredger (CSD) fitted with a special head that digs into hard materials. But when sea conditions are difficult or if a waterway is very busy, a CSD is not suitable. A "Ripper" draghead can be fitted onto a traditional TSHD and combines the cutting power of a CSD with the flexibility and stability of a TSHD.

WHAT DOES A SUCTION PIPE DO?

The suction pipes play several important roles. They are the conduit through which the mixture is transported to the hopper. In addition, the suction pipe, steered by the dredger operator, controls the movement of the drag head on the seabed. By transmitting the forces from the draghead to the vessel, the suction pipe maintains a good contact between the draghead and the seabed. By working with swell compensators, the optimum height of the draghead in relation to the seabed can be adjusted. If the draghead is too high, this creates a mixture with too much water, but if it is too deep or if its weight is pressing on it too much, this causes too much frictional force. The suction pipe and the swell compensators compensate for the vertical movement of the vessel as well as any irregularities in the seabed and help to maintain the right balance for the position of the draghead in relation to the seabed. The dredging operator is actually able to see and adjust all these actions through sophisticated instrumentation. If this work is done correctly, it makes a net improvement to performance.

WHAT IS AN OVERFLOW SYSTEM?

The mixture which is dredged by a TSHD is a mixture of water and solid matter, like sand. Given that the ultimate objective of the TSHD is to collect sand to reuse it or deposit it elsewhere, a TSHD must have a system that maximises the retention of the solid material and minimises the water remaining in the hopper. The surplus water must be separated and discharged overboard. The solid, sandy part of the mixture will settle at the bottom of the hopper, but this process takes time. An overflow system enables the solids to be separated from the water by reducing swirling in the mixture and allowing sufficient time for the solid part (sand, gravel) to settle at the bottom. Then, the surplus surface water is carried away overboard thanks to this overflow device.

HOW DO TSHDS DISCHARGE THE DREDGED MATERIALS?

TSHDs are very flexible and can operate independently of any other equipment and, since they are self-propelled, they are capable of transporting the dredged materials over long distances. Once completely loaded, the vessel is steered to the unloading or depositing site where the dredging backfill is discharged. Depending on the type of project, the backfill is discharged in one of the three following ways:

• the materials are deposited on the immersion site by opening the doors located at the bottom of the hopper;
• they can be tipped onto the ground through pipes, which may be submerged or floating; or
• the materials can be discharged into the air by the backfill pumps, a process called "rainbowing".

The discharge method is directly connected to the type of project.

WHEN IS DISCHARGING DONE THROUGH THE BOTTOM DOORS?DE FOND ? 

When the materials are dredged in a port or an access channel and if the materials are clean, the TSHD will sail to a designated place and discharge the dredged sediment by opening the bottom doors. Discharge through the bottom doors enables rapid, direct and total submersion of all the dredged materials at a chosen location. This is a reliable and efficient method, but only under special circumstances.

WHEN IS DISCHARGING DONE THROUGH A PIPE?

For significant hydraulic backfill and beach nourishment work, the TSHD will sail towards the selected extraction zone which may be some kilometres from the construction site. At the extraction site, the dredger will load its hopper with sand and then sail towards the site to be backfilled. These materials are then expelled onto the site by "rainbowing" or through floating or submerged pipes. A special connector, called a bows coupling, is needed to connect the pipe to the dredger. If the distance from the vessel to the shore is quite long, relay pumps can be added along the discharge line as an additional source of energy. The nozzle for "rainbowing" is also part of the "bows coupling" device.

A submerged discharge line is less sensitive to weather conditions and is not an obstacle for other vessels moving around in the area. It is usually assembled on shore, and then pulled until its open end is correctly positioned on the beach. Additional sections may be added if needed. Floating pipes, as well as being more sensitive to heavy seas, have the advantage of being visible above the surface of the water and can be easily reached if repairs are necessary.

WHAT IS RAINBOWING?

“Rainbowing” is the name given to the technique by which the TSHD projects the sand that it has dredged from the sea bed into the air by discharging in a direct jet onto the backfilling site. These sites can vary from a beach that needs refilling to prevent coastal erosion or for leisure purposes (or both) to an area for reclaiming land under the sea where new breakwaters or islands are built to extend a port, or for leisure or other purposes.

Initially, the TSHD creates what is called a mixture which, because of its liquid qualities, can be sent onto the beach or into the deposit site like a projectile in the air following a trajectory in the shape of the arc of a circle ("rainbow"). "Rainbowing" is often the best method for discharging enormous quantities of sand into shallow areas close to the shore, for backfilling or beach re-nourishment projects. Given that "rainbowing" does not require floating or submerged pipes, relay stations or pipes onshore, it is often the most economic method.

WHAT ARE THE FACTORS THAT INFLUENCE THE TSHD CARRYING OUT "RAINBOWING"?

Numerous factors influence the productivity of a TSHD carrying out "rainbowing", but above all, it is the characteristics of the nozzle which should be taken into consideration.

Firstly, the vertical angle of the nozzle must be taken into account. About 10 years ago, a 45° angle was considered to be the norm. Currently, the vertical angle of the nozzle is 30° because research has shown that this is the most efficient angle for projecting the mixture over a long distance. With such an angle, there is less return towards the dredger and the craters which form in the backfill area are smaller.

The diameter of the nozzle is also a key factor. The smaller it is, the lower the flow rate, which reduces performance, but because the speed when exiting is higher, the sand can be projected over a greater distance. Take, for example, modern jumbo dredgers: even though the discharge time is about 30% longer, they can discharge in a rainbow and have a range of 150 m. These jumbos can nevertheless discharge at peak production rates of 25,000 m³ per hour to start with. TSHDs recently climbed up on additional rung on the production ladder in relation to discharge distance by being fitted with two nozzles working simultaneously in order to optimise performance.

Another significant factor is the height of the projection nozzle in relation to the water line. The shape of the nozzle is also important, with the latest nozzles giving a better flow and higher sand exit speeds, which means more efficient rates of production.

WHEN IS RAINBOWING APPROPRIATE?

For "rainbowing" discharge, the load draught of the dredger should be such that the vessel can be brought close to the site to be backfilled. Shallow waters can limit access to the area to be backfilled. Usually, this can be compensated by discharging enormous quantities of sand in the first few minutes to reduce front draught, so that the vessel can then be pushed closer to the beach. Beaching of the vessel is one option, but this can damage the hull over time and the bows must be reinforced.

WHAT IS THE SIZE OF A TSHD?

TSHDs vary considerably in size. Their size is expressed by the capacity of the volume of the hopper, their length and their pumping capacity. This can vary by several hundred cubic metres up to 45,000+ m³. Recently, several international dredging companies have ordered very large TSHDs. For example, one of the largest in the world has a hopper capacity of 46,000 m³, a dead weight of 78,500 tonnes and measures around 223 m long, with a load draught of 15.15 m. It can reach a maximum dredging depth of 155 m with suction pipes that have a diameter of 1300 mm. Its pumping capacity when dredging is 2×6500 kW, its discharge pump capacity is 16,000 kW and its propulsion power is 2×19,200 kW. Its total installed capacity is 41,650 kW and it can sail at a speed of 18 knots. Conversely, one of the smallest TSHDs has a hopper capacity of only 3400 m³, a deadweight of 4800 tonnes with a length of only 93.3 m and a load draught of 5 m. Its maximum dredging depth is 26.5 m with a suction pipe that is 800 mm in diameter, a dredging pump capacity of 1250 kW and a capacity when pumping onshore of 2000 kW, and a propulsion power of 2×1000 kW. Its total installed capacity is 4,100 kW and it can sail at a speed of 11.5 knots.

WHAT ARE THE ADVANTAGES OF TSHDS?

TSHDs can be used for a great number of operations, because they are the most flexible dredging units that exist. This flexibility is evident in the types of material that they can dredge, the sites where these materials can be discharged and those where they can work. For example, they can dredge sand, clay, silt or gravel, and now even some types of rock. They can work in calm and protected waters or more heavy seas like port access channels all way out to see where the weather and marine weather conditions can be much worse. Unlike stationary vessels, TSHDs can work in very busy ports because they do not have any anchors or cables and are self-propelled, so they can therefore move freely.

Furthermore, they can work at great depths or in shallow waters. Large vessels have the economic advantage of being able to dredge materials at sites that are very far from the backfilling zone. They have relatively high output, even though this can vary depending on the types of materials, the depth of the sea bed and weather conditions.

WHAT IS THE ACCURACY OF A TSHD?

TSHDs are not particularly accurate and are therefore not particularly suitable for removing thin layers of sediment (contaminated). But because a TSHD is fundamentally specified to scrape away the seabed horizontally and not dig into it, only limited quantities of material are broken up with each pass. The quantity of materials that is broken up but not sucked up is generally low, and even if the water is added during the suction phase, this can also, these days, be limited and monitored.

WHEN IS A TSHD THE APPROPRIATE CHOICE FOR A DREDGING PROJECT?

Each project must be evaluated in relation to its advantages, and the decision on the equipment to be deployed depends on the type and quantity of the material to be dredged as well as the dredging site and the backfilling site. These days, large international dredging companies often order customised TSHDs which meet specific dredging needs. The vessels are completely computerised and often a minimum crew is needed because the dredging processes can be controlled from the bridge. Because TSHDs are self-propelled and can go out to sea, they are ideal for large backfilling projects or projects where enormous quantities of material are needed and where appropriate sand must be conveyed from extraction zones far away. This range enables them to dredge materials and to discharge them where needed without any other supporting equipment. This also means that they can be used in very busy ports where maritime traffic is a problem. For the same reason, they can be mobilised efficiently in any part of the world and can go there under their own steam. All of these qualities added together make these vessels particularly profitable and efficient dredging equipment.