Couplings drive dredge pump

Bolted together couplings form the drive shaft of the probably longest suction pump dredge in the world, allowing a large underwater sand mine to reach depths that had previously not been accessible to pump dredging

A large sand mine in Florida had reached a depth where the existing pump dredge was becoming ineffectual. Due to the depth that the sand deposit reaches, the mine's operators thought that they would have to abandon their preferred dredging method and adopt clamshell-style dredging in order to extend the mine from 21 to 60 metres; allowing for another 40 years of mining. They approached Dredging Supply Co (DSC), experts in the design and build of dredging technology, to supply the equipment.

Bob Wetta, President of DSC, comments: "During the design brief it was obvious that clamshell dredging was only being considered due to the depths that will eventually be required of the apparatus. We are always keen to supply our customers with their desired solution so suggested that a 60m dredge pump would be a possibility. We had already built a 36m pump successfully; we figured that this one is just a little longer."

Following a year of design, a 90m floating dredge was proposed that would be capable of operating uninterrupted for the next 40 years. The dredge would excavate, suction and pump to the surface 190 to 230m3 of sand per hour carried through 40cm OD high density, polyethylene piping in a slurry of water flowing at 38,500l/min.

The mining process begins by excavating the sand at the seabed using high pressure water cutting jets. The sand is then suctioned by an underwater pump which is driven by the longest drive shaft ever built in the dredging industry. Sourcing a drive shaft of this length, which was capable of operating in the harsh underwater conditions, was a major challenge of the project.

Damon Gonzalas, Director of Engineering for DSC, adds: "Any engineering project of this size involves a number challenges. One of the key issues we had was designing a drive shaft which could transmit the 450kW from the electric motor on the dredge's hull to the underwater pump on the sea bed. Not only would this be the longest ever drive chain built for a mining dredge; we also had to modify it for operation in the vertical position. In the end we worked closely with TB Woods to design a bespoke solution."

TB Woods, a brand of Altra Industrial Motion's Altra Couplings group, supplied six 6 metre long disc couplings which would bolt together to form the drive shaft. The Form-Flex couplings use a hollow tubular shaft, which reduced the overall weight of the solution. The hubs were customised with a unique flange disc pack and tapered bushing mounting to facilitate ease of installation and adjustment to the underwater pump impeller. Because of the abrasive material being pumped, axial adjustment of the impeller is necessary when it becomes worn down. The couplings are able to handle parallel offset and angular misalignment which is essential in an application that will be subjected to unavoidable external forces.

Damon Gonzales continues: "This project required us to build what is currently the deepest dredge pump in the world; it then had to operate in an abrasive, underwater environment until 2049. Any unscheduled maintenance would be very costly in terms of both man hours and downtime which means that a vital component, such as the drive shaft, needs to be reliable. Looking back on the project I am extremely grateful to Altra for providing us with a component that we could trust."