International Dredging Review

International Dredging Review

This year Damen Dredging Equipment the dredging subsidiary of the Damen Shipyards Group and its DOP submersible dredge pump is celebrating a milestone anniversary. The DOP was the first pump of its kind designed by Damen a quarter of a century ago. It had a modest start: a contractor with a tunnel project was looking for a small sized dredge pump with plenty of power. Damen modified an existing dredge pump added a casing and the rest is history. Over the past decades the DOP submersble dredge pump has become a well-known tool in the dredging world – with hundreds of units sold over all continents.


A dredge fitted out with a DOP350L submersible dredge pump in the A frame.

Damen said the compactly built and wear resistant dredge pump is known for its durability and its many different uses. The whole range of heavy-duty DOP submersible dredge pumps with a variety of suction heads can be used in almost any dredging job.

The DOP submersible dredge pump has been designed as a true dredge pump. At the heart of this tool is a set of highly wear resistant Cr-Ni alloyed white cast iron wear parts such as the pump casing impeller and wear plates protecting the pump covers the bearings and drive. The impeller is directly mounted onto the pump shaft using a tension set. On this shaft a mechanical seal is fitted to avoid ingress of water and sand particles in the bearing housing. This mechanical shaft seal is a solid cartridge unit made of stainless duplex steel with sealing faces of silicium cabide. Its main advantages are that it does not require a supply of grease nor gland water. At the other end of the compactly built bearing block a hydraulic motor is mounted driving the dredge pump. Damen said all design details are focused on the harsh dredging environment. 

The complete wear resistant submersible dredge pump its bearing block and hydraulic drive are built into a protective steel casing. The strength of the casing is such that at the top end lugs are foreseen to attach the DOP submersible dredge pump unit to a crane boom using a chain spread. Another mounting meth-od is welding a crane’s counter couplings to the top plate. This is an excellent method for a quick and rigid mounting onto a crane boom. The limited weight of the DOP pump unit and the required hydraulic oil flow for its drive have been designed for use on hydraulic excavators. The operator simply moves the boom and thus the submersible dredge pump.

The lower end of the DOP submersible dredge pump has been fitted out with a mounting flange. Onto this flange a wide variety of suction heads can be mounted. The enormous versatility of the DOP pump is evident; due to the wide range of suction heads one DOP submersible dredge pump can be used in a multitude of different dredging jobs.

The type of suction head used depends on the dredging job at hand. In Fig. 1 a number of available suction heads are shown. The first from the left is a sand production head. This is at its best when used in a sand pit where bot-tom material is loosened using the jetwater supplied to the nozzles which surround the suction head. The high pressure jetwater optimizes the sand water mixture which can be pumped at a high concentration as the DOP submersible dredge pump is lowered well below the water line. The sand production head is an excellent mining tool.

The suction head (second from left on Fig. 1) is a compactly built flat bottom head. Due to the compact design of the DOP submersible dredge pumps it can be used perfectly in restricted areas. Examples of earlier use include cleaning up building pits for foundations of apartment blocks and the construction of underground car parks as well as bed leveling at the final dredging phase in tunnel construction. This suction head is also used for barge unloading. Hopper barges filled with soil can thus be emptied in an efficient way. During the barge unloading process the constant supply of a high volume of jetwater is vital for a smooth process. The DOP submersible dredge pump is non-self-priming; hence the dredge pump is to be lowered into the hopper’s cargo first using the supplied jetwater to create a pit.

The cutter unit is the third suction head from the left on Fig. 1. The unit consists of a cutter head which is wear resistant and replaceable. The cutter head is driven by a hydraulic motor mounted within the protective casing. The cutter unit is chosen in dredging projects where the DOP submersible dredge pump is to remove layers of hard packed sand. This bottom material has become extremely consolidated here pressurized jetwater would not create a sufficiently high mixture concentration. Using the cutter unit layers of sand are cut off continuously ensuring a high mixture concentration and thus an efficient process. When the cutter unit is used the DOP submersible dredge pump is to be mounted to the crane boom using a fixed coupling.

The suction head at the right hand side on Fig. 1 is an auger head. The hydraulically driven auger transports bottom material in a controlled way from both its sides to the centre where it is pumped away by the DOP submersible dredge pump. The auger unit is built into a closed steel construction. All design features are aimed at keeping the dredged mixture within the auger head’s casing and creating a high mixture concentration. Every detail is based on dredging with maximum accuracy for removing contaminated sediments and avoiding turbidity – reducing spill to the minimum. Damen said as environmental clean-up dredging projects have soared over the past few years the DOP submersible dredge pump fitted out with its auger head has become a sought-after tool.

These are not the only applications of the DOP submersible dredge pump. When needs be a new suction head is designed to tackle a specific problem. For dredging clay – a notoriously difficult material to handle – a disc bot-tom cutter has been designed. The remarkable cutter head was attached the usual way to a standard DOP submersible dredge pump – on the lower mounting flange. Other uses over the past years have seen the DOP submersible dredge pumps used as a mobile booster station. The DOP pump has been mounted in a ladder as well as into an A-frame of a dredge and it has been fitted out with an electric drive. The uses are countless and will continue to evolve due to the ongoing developments at the design department of Damen Dredging Equipment.

Focus points in the design of the DOP submersible dredge pumps have been the large spherical passage in the impeller – which goes up to 200 mm – and the high dredge pump pressure. The DOP submersible dredge pump creates a high head enabling it to pump dredged material over 1.000 meters (about 3.281 feet) using one single dredge pump. As the DOP pump only works in a submerged situation it will create a relatively high mixture concentration boosting the efficiency. The DOP  submersible dredge pump can do its job at important depths even working at -500 meters (abpit -1640 feet) below sea level.

All Damen dredge pump characteristics have been fed into a dredge pump calculation program which has been developed by the Damen R&D department to calculate a pre-diction of the production of a Damen dredge pump used in a specific situation. For the calculations project-specific soil data and suction and discharge piping data are used. Therefore the contractor gets a customized estimate of the working point of his Damen dredge pump. 

Damen can supply the DOP submersible dredge as a complete dredging package. This package typically includes matching discharge hoses and pipelines both floating and ashore drive components such as hydraulic hoses and a diesel-hydraulic power pack in case the DOP  pump cannot make use of the hydraulics of the excavator. Other functional additions may include another suction head to finish the dredging job a jetwater pump or process instrumentation. These packages are delivered from stock together with the DOP pump.