Dredge knowledge

Dredging

Dredging is excavating under water or on water surface level and transporting the slurry by pipe line, buckets or barges to another location. This process can be done by cutter suction dredger, trailing suction hopper dredger, bucket wheel dredger, cutting wheel dredger, suction dredger or backhoe dredger. The purpose of excavating can be for land reclamation, using sand and stone of one area to build land in another area, or for deepening sea ways, rivers and harbors.

Dredger Types

• Cutter Suction Dredger
• Trailing Suction Hopper Dredger
• Bucket Wheel Dredger
• Wheel Suction Dredger
• Suction Dredger
• Backhoe Dredger

Cutter Suction Dredger

Cutter suction dredger is a dredger using a rotating cutter head to cut the sea bed and a suction pump placed inboard or underwater to suck and discharge the slurry from the cutter to the discharge location by a pipe line. Also instead of a pipe line a barge can be used to transport the slurry and dump it at location. The barges can be filled with a dredger using side discharge. Cutter suction dredgers are moved sideways around a center point formed by a spud lowered to the sea bed. After each cut the dredger is moved forward by its second spud pushed backward by a hydraulic cylinder. Some cutter suction dredgers are self-propelled. Others need to be towed to their destination.

Trailing Suction Hopper Dredger

Trailing suction hopper dredger is a ship with a full sailing capacity that can load sand in its hopper. The sand is picked up from the sea bed by a dredge drag head that is lowered to the seabed like a vacuum cleaner and connected to the hull of the ship by a side suction pipe. The sand is sucked into the ship by a dredge pump located inboard. In some situation a second pump is placed under water to support deeper excavations. The trailing suction hopper dredger can discharge the sand by:

• Bottom doors: doors located on the bottom plates of the dredger that can be controlled by hydraulic cylinders.
• Rainbow or spray nozzle: A Jet pump dilutes the sand in the hopper to make it able for the main dredge pump to suck the sand from the hopper and press it through a nozzle located at the bow of the ship.
• Discharge line: A Jet pump dilutes the sand in the hopper to make it able for the main dredge pump to suck the sand from the hopper and press it through a pipe line connect to the ship by a bow coupling.
• Split Hopper: Some hoppers can split the complete dredger into parts longitudinally to allow the sand to drop off in very short time. In this case the dredge is called Split Trailing Suction Hopper Dredger.

Bucket Wheel Dredger

Bucket wheel dredgers are the oldest principle of dredging but still used in many countries. It is mainly used for mining fields underwater. It is a ship with many buckets chained together on a ladder construction. The ladder is lower to the seabed and the chain is driven to allow the buckets to penetrate the seabed and fill the bucket with sand. On the top of the ladder construction the buckets are emptied when they are upside down to a side discharge installation to load another ship called a hopper barge.

Wheel Suction Dredger

A wheel suction dredger is almost the same as a cutter suction dredger but the rotating cutter head is replaced by a wheel provided with buckets. In some cases the buckets have replaceable teeth to allow the dredger to cut in harder soil. After the cutting wheel the slurry is sucked by a suction pump placed inboard or submerged to discharge the slurry through a pipe to its destination.

Suction dredger

A suction dredger is a dredger that stings a pipe in the seabed to suck the sand into other ship like a hopper dredger or to discharge through a pipe line. The dredge pump can be located inboard or submerged. The suction pipe can be provided by some bars or a hatch to prevent entering of big stones and logs. Suction dredgers can only be used when the soil is soft and not compacted.

Backhoe dredger

A backhoe dredger is the same as a dry excavator but placed on a barge. The barge is supported by spuds that are lowered to the seabed for the stability of the barge. Backhoes unload on hopper barges located sideways.

Dredge Components

• Dredge Drag Head
• Dredge Ball Joint
• Dredge Valve
• Dredge Spud
• Dredge Spud Carrier
• Dredge Ladder
• Dredge Turning Gland
• Dredge Hose
• Dredge Suction Mouth
• Dredge Anchor Booms
• Dredge Side Winch

Dredge Drag Head

A dredge drag head is a steel box used by a trailing suction hopper dredger to collect sand from the seabed into the hopper. It is connected to the ship by a suction pipe. The dredge drag head and the suction pipe are lowered by the dredger by gantries using hydraulic winches. The force on the dredge drag head due to its weight and the weight of the dredger is controlled by a swell compensator. Some dredge drag heads are provided with a visor to control more or less flow of water into the dredge drag head to control the water sand mixture. In some dredgers this control mechanism is hydraulically operated real time.

Dredge ball joint

A dredge ball joint is a flexible coupling between two pipes used to transport water and sand mixture from a cutter suction dredger or a trailing suction hopper dredger to the discharging area. Depends on the design of the dredge ball joint it can turn to all directions between 15 to 22.5 degrees. The dredge ball joint consist of at least three parts named casing, ball and gland. Some ball joints have some additional parts like case liner, ball liner or gland liner. Dredge ball joint parts are mostly made from cast steel and machined on vertical lath to the right dimensions. In some cases the liners are made from wear resistant materials with high chromium content. The ball and the case are welded to the pipe ends and assembled together. The ball is held in place by the gland that is locked in the case by a locking pin or a locking plate. Press here for more information about dredge ball joint

Dredge Valve

A dredge valve is a vertical valve used to control the flow of mixture of water and sand in suction and discharge pipes of a cutter suction dredger or a trailing suction hopper dredger. It is function is to block the flow of mixture of water and sand or to reduce the flow of the same mixture. The reason for blockage could be to divert the mixture from the cutter head or from drag head to the ship or from the hopper to the discharge pipe. The dredge valve consists of a blade normally lowered from the upper side of the pipe supported by two seals on both side of the blade. Nowadays the blade is controlled hydraulically from the wheel house. The biggest challenge for the design and operation of dredge valves are to keep the dredge valve clean of sand that might enter the valve body and cause damages. Also when closing or opening the dredge valve sand might block the blade. In the dredge valve body several water flushing canals are provided to keep the dredge valve body clean and to prevent blockage of the blade during opening and closing.

Dredge Spud

Dredge Spud is a steel welded spud with a sharp end used on cutter suction dredgers during dredging. The spud is lowered to the seabed in a free fall movement to penetrate the seabed and anchor itself in that same position. After positioning the dredge spud, the cutter suction dredger will swing around the dredge spud with the dredge ladder down to cut and suck the seabed. Most cutter suction dredgers are provided with two spuds. In some cases one spud is a so called traveling dredge spud and the other dredge spud is a fixed dredge spud. When a cutter suction dredger has a travelling dredge spud, the ship will push herself away from the traveling dredge spud to cut the next layer. To ensure that the dredge spud is stiff enough the wall thickness of each spud segment varies from thick at the middle to thinner at both ends of the dredge spud. For big cutter suction dredgers the dredge spud length can be around or even more than 50 meters long, 2 meters wide and weigh more than 100 ton.

Dredge Spud Carrier

A spud carrier is a floating pontoon located at the end of the cutter suction dredger that carries the so called traveling dredge spud, the dredge spud hoisting installation and the hydraulic cylinder to push the ship away from the dredge spud. The dredge spud carriers allows a cutter suction dredger to swing and move forward cutting sand layers by pushing the ship away from the traveling dredge spud. Each forward movement is pushed away by a hydraulic cylinder mounted horizontally on the dredge spud carrier. When this hydraulic cylinder reaches its final position, the fixed dredge spud is lowered to the seabed and the traveling dredge spud is hoisted and repositioned to the beginning of the dredge spud carrier.

Dredge Ladder

A dredge ladder is a steel construction used on cutter suction dredgers to support the dredge cutter head or dredge cutting wheel and other dredge components like hydraulic drive, bearing blocks, gearbox, submerged dredge pump, etc. The structure is longitudinal stiffened plates welded together. One end is connected to the ship by a pivoting hinge allowing the complete ladder to turn downwards to the seabed. On the other end the dredge cutter or a cutting wheel is mounted to cut the seabed. On the dredge cutter or cutting wheel hoisting installation is provided for hoisting and lowering the complete ladder. On big dredgers the weight of the dredge ladder reaches 1500 tons and a length of more than 50 meters.

Dredge Turning Gland

A dredge turning gland is a flexible connection between two pipes allowing one of the pipes to turn in radial direction. The dredge turning gland is mostly connected to the onboard pipes of a cutter suction dredger at the end of the ship to allow the free movement of the dredge discharge pipe. The dredge turning gland consist of two short pipes connected to each other by a split ring and a bearing. Both pipes are provided by flange to connect to the ship and to the dredge discharge pipe.

Dredge Hose

A dredge hose is rubber connection in dredge suction pipe or dredge discharge pipe. Dredge hose for the suction pipe is provided with steel rings to prevent the hose to close due to vacuum inside the pipe. For the discharge pipes the hose is provided with reinforcement according to the applied pressure to prevent the dredge hose from bursting.

Dredge Suction Mouth

Dredge suction mouth is special formed pipe made from casting steel or formed plates and welded together to connect to the suction pipe on one side and on the other side to enter the dredge cutter head. One side is round and provided with a flange to connect to the suction pipe with bolts and nuts. The other end is oval and twisted to match the shape of the dredge cutter head and to fit between the dredge cutter shaft and the dredge cutter ring.

Dredge Anchor Booms

A dredge anchor booms is a boom to carry the anchor and drop it as far as possible from the cutter suction dredger. The anchor will be used to pull the cutter suction dredger toward the anchor in a swing movement during dredging. The anchor is pulled by dredge side winch that is redirected through dredge winch sheave placed near to the dredge cutter head.

Dredge Side Winch

A dredge side winch is often a hydraulic driven winch to pull the ship toward the anchor that is placed as far as possible from the ship using the dredge anchor boom. The cutter suction dredger is pulled toward the anchor in a swing movement to allow the dredge cutter head to cut the seabed during dredging. A steel wire is used to pull the anchor rolled on the winch drum. The dredge side winch is placed parallel to the dredge ladder to lead the winch cable through a sheave placed near to the dredge cutter head.

Dredging equipment engineering

FEA – Finite Element Analysis

DREDGE YARD’s components and equipment are design using this method to optimize the design and ensure low stress forces in the material. In this way the life time of the equipment will be increased and the cost price will be reduced.

FEA – Finite Element Analysis is one of the most dominant numerical methods used to check how the structure will react to certain loading conditions. Theoretically speaking, FEA is using a system of points called nodes that together create a grid called mesh. The mesh contains the material and structural properties that shows how the structure will react to certain loading conditions. Depending on the stress levels anticipated of a particular area, the nodes are assigned at a certain density all over the material. Generally a higher node density will be found in the areas that will receive large amounts of stress. The areas that will receive little or no stress it will usually have a lower node density. So, FEM provide detailed visualization of where structures twist or bend and gives precise indications of the distribution of stresses and displacements.



DREDGE YARD’s components are tested in Finite Element Analysis as one part after that all parts are assembled and tested under pressure and maximum movements. Worst case scenarios are simulated and pick loads are applied. After analysis the design might be reviewed couple of time to find out the most optimal shape and lowest stress in material. For wear parts a Finite Element Analysis is run for worn out parts as well allowing DREDGE YARD engineers to design the dredging components for the longest life time by adding material where necessary.

In summary, FEA’s benefits include enlarged accuracy, intensified design and enhanced perception into critical design parameters, virtual prototyping, amplified productivity and increased revenue.